摘要:

GENERAL DISCUSSIONS OFTHE FARADAY SOCIETYDate1907190719101911191219131913191319141914191519161916191719171917191819181918191819191919192019201920192019211921192119211922192219231923192319231923I92419241924192419241925192519261926192719271927SubjectOsmotic PressureHydrates in SolutionThe Constitution of WaterHigh Temperature WorkMagnetic Properties of AUoysColloids and their ViscosityThe Corrosion of Iron and SteelThe Passivity of MetalsOptical Rotary PowerThe Hardening of MetalsThe Transformation of Pure IronMethods and Appliances for the Attainment of High Temperatures in aRefractory MaterialsTraining and Work of the Chemical EngineerOsmotic PressurePyrometers and PyrometryThe Setting of Cements and PlastersElectrical FurnacesCo-ordination of Scientific PiiblicationThe Occlusion of Gases by MetalsThe Present Position of the Theory of ionizationThe Examination of Materials by X-RaysThe Microscope : Its Design, Construction and ApplicationsBasic Slags : Their Production and Utilization in AgriculturePhysics and Chemistry of ColloidsElectrodeposition and ElectroplatingCapillarityThe Failure of Metals under Internal and Prolonged StressPhysico-Chemical Problems Relating to the SoilCatalysis with special reference to Newer Theories of Chemical ActionSome Properties of Powders with special reference to Grading byThe Generation and Utilization of ColdAlloys Resistant to CorrosionThe Physical Chemistry of the Photographic ProcessThe Electronic Theory of ValencyElectrode Reactions and EquilibriaAtmospheric Corrosion.First ReportInvestigation on Oppau Ammonium Sulphate-NitrateFluxes and Slags in Metal Melting and WorkingPhysical and Physico-Chemical Problems relating to Textile FibresThe Physical Chemistry of Igneous Rock FormationBase Exchange in SoilsThe Physical Chemistry of Steel-Making ProcessesPhotochemical Reactions in Liquids and GasesExplosive Reactions in Gaseous MediaPhysical Phenomena at Interfaces, with special reference to MolecularAtmospheric Corrosion. Second ReportThe Theory of Strong ElectrolytesCohesion and Related ProblemsLaboratoryElutriationOrientationVolumeTrans. 33678999101011121213131314141414151516161616171717171818191919191920202020202121222223232GENERAL DISCUSSIONS OF THE FARADAY SOCIETYDute1928i9W1929192919301930153119321932193319331934193419351935193619361937193713981938193919391940194119411942194319441 94519451946194619471947194719471948194s1949194919491950195019501950195119511952195219521953195319541954SubjectHomogeneous CatalysisCrystal Structure and Chemical ConstitutionAtmospheric Corrosion of Metals.Molecular Spectra and Molecular StructureOptical Rotatory PowerColloid Science Applied to BiologyPhotochemical ProcessesThe Adsorption of Gases by SolidsThe Colloid Aspects of Textile MaterialsLiquid Crystals and Anisotropic MeltsFree RadicalsDipole MomentsColloidal ElectrolytesThe Structure of Metallic Coatings, Films and SurfacesThe Phenomena of Polymerization and CondensationDisperse System in Gases : Dust, Smoke and FogStructure and Molecular Forces in (a) Pure Liquids, and (6) SolutionsThe Properties and Functions of Membranes, Natural and ArtificialReaction KineticsChemical Reactions Involving SolidsLuminescenceHydrocarbon ChemistryThe Electrical Double Layer (owing'to the outbreak of war the meetingThe Hydrogen BondThe Oil-Water InterfaceThe Mechanism and Chemical Kinetics of Organic Reactions in LiquidThe Structure and Reactions of RubberModes of Drug ActionMolecular Weight and Molecular Weight Distribution in High Polymers.(Joint Meeting with the Plastics Group, Society of Chemical Industry)The Application of Infra-red Spectra to Chemical ProblemsOxidationDielectricsSwelling and ShrinkingElectrode ProcessesThird Reportwas abandoned, but the papers were printed in the Trunsnctions)SystemsThe Labile MoleculeSurface Chemistry.(Jointly with the Sociktk de Chimie Physique atColloidal Electrolytes and Solutions Trans. 43The Interaction of Water and Porous Materials Disc. 34Lipo-Proteins 6Heterogeneous Catalysis 8Physico-chemical Properties and Behaviour of Nuclear Acids Trans. 46Spectroscopy and Molecular Structure and Optical Methods of In- vestigating Cell Structure Disc.9Electrical Double Layer Trans. 47Hydrocarbons Disc. 10Radiation ChemistryThe Physical Chemistry of ProteinsThe Reactivity of Free RadicalsThe Equilibrium Properties of Solutions of Non-EIec trolytesThe Physical Chemistry of Dyeing and TanningThe Study of Fast ReactionsCoagulation and FlocculationBordeaux.) Published by Butterworths Scientific Publications, Ltd.The Physical Chemistry of Process iMetallurgyCrystal Growth 5Chromatographic Analysis 7The Size and Shape Factor in Colloidal Systems 1112131415161718Volume2425252526262728292930303131323233333434353535363737383940414242 A42 BDisc. 1GENERAL DISCUSSIONS OF THE FARADAY SOCIETYDate Subject Volume1955 Microwave and Radio-Frequency Spectroscopy 191955 Physical Chemistry of Enzymes 201956 Membrane Phenomena 211956 Physical Chenlistry of Processes at High Pressures 221957 Molecular Mechanism of Rate Processes in Solids 231957 Interactions in Ionic Solutions 24I958 Configurations and Interactions of Macromolecules and Liquid Crystals 251958 Ions of the Transition Elements 261959 Energy Transfer with special reference to Biological Systems 271959 Crystal Imperfections and the Chemical ReactiVity of Solids 281960 Oxidation-Reduction Reactions in Ionizing Solvents 291960 The Physical Chemistry of Aerosols 301961 Radiation Effects in Inorganic Solids 311961 The Structure and Properties of Ionic Melts 321962 Inelastic Collisions of Atoms and Simple Molecules 331962 High Resolution Nuclear Magnetic Resonance 341963 The Structure of Electronically-Excited Species in the Gas-Phase 351963 Fundamental Processes in Radiation Chemistry 361964 Chemical Reactions in the Atmosphere 371964 Dislocations in Solids 381965 The Kinetics of Proton Transfer Processes 391965 Intermolecular Forces 402966 41 The Role of the Adsorbed State in Heterogenous CatalysisFor current availability of Discussionvolumes, see back cover

ISSN:0366-9033    

DOI:10.1039/DF966410X001

出版商:RSC    

年代:1966

数据来源: RSC

摘要:

Chemisorption on Single Crystal Planes *BY GERT EHRLICHGeneral Electric Research and Development Center,P.O. Box 8, Schenectady, N.Y., 12301, U.S.A.Received 3 1st January, 1966Chemisorption on well characterized crystal planes, as well as on atomic clusters, can now beroutinely observed. Results so far obtained on tungsten are reviewed, with special emphasis on(i) bonding of metal atoms to their own lattice, (ii) the localization of distinct binding states on in-dividual crystal planes, and (iii) atomic and electronic displacements arising from the act of chemi-sorption.The relation between surface structure and adsorption has persisted as one ofthe important but unsolved problems in surface chemistry. This has again beenbrought to the forefront of attention by the realization that even in the chemisorptionof simple diatomic gases, bonding occurs in discrete states with distinct bindingenergies, surface dipoles, and chemical properties.1 Are these states adsorbed oncrystallographic features differing in atomic arrangement, and therefore chemicalreactivity, or are changes in the surface environment, brought about by the actof adsorption itself, responsible ? Experiments on polycrystalline samples suggestthat both effects may operate in one and the same system; they do not, however,permit an unequivocal assignment of a given binding state to a particular surfacefeature.Adsorption studies on well-defined single crystal planes are now feasible andcan in principle establish both the atomic configuration of the chemisorbed com-plex as well as the variations in its behaviour at different surface features.Thereare two ways of executing these : one is to make observations under high resolution,as is possible in the field ion microscope,2 to reveal the individual crystal planespresent on any specimen ; the other is the brute force approach of preparing macro-scopic single crystal planes, which can then be examined by conventional means.The former is easier and may yield information on processes at surface imperfections,as well as on the individual atomic events. Examination of large scale single crystalsurfaces, however, suffers from fewer of the uncertainties which hedge about theinterpretation of high field experiments. Moreover, it is only by learning to controlsurface structure on a large scale that we shall be able to take advantage of theinformation on structural specificities now being uncovered. Here we presentsome of the more interesting structural effects found for individual adsorbed atomsand molecules in both types of studies.SELF-ADSORPTION OF LATTICE ATOMSMetal atoms adsorbed on their own lattice constitute a simple example ofchemisorption; they are also of interest for their possible role in chemisorptionand catalysis.Despite this, experimental information on the behaviour of atomsheld on different planes has not been available. Such information can, however,*supported in part by the U.S. Air Force Office of Scientific Research, Office of AerospaceResearch.8 CHEMISORPTION O N SINGLE CRYSTAL PLANESbe derived in a straightforward fashion in the field ion microscope.A single atomis deposited on a given crystal plane at a low temperature. The mean square dis-placement (r2) at a more elevated temperature can then be determined by directobservation of the atom movements, and this in turn yields the diffusion coefficientD. The usual relationshipbetween the mean square displacement (r2) along a given direction and the period ofobservation z in a random walk is not directly applicable; boundary effects put acertain amount of correlation into the otherwise random motion of the migratingatoms. This can be simply accounted for, however, provided the separation of theboundaries a is sufficiently large so that a2> 2002.For example, in a one-dimensionalrandom walk the mean square displacement becomes 3a(r2) = 2 0 2 (1)both for boundaries that reflect as well as for those that adsorb a colliding atom.Quantitative measurements of the movement of individual tungsten adatomsat different,, temperatures have been carried out 3b on the (1 lo), (21 1) and (321)planes of the tungsten lattice and yield the activation energies for surface diffusionlisted in table 1 ; for the latter two planes, these values pertain specifically toatomic motion along close-packed atom rows. In moving over the surface thetungsten atoms serve as a convenient probe, mapping out the potential acting uponan atom at different points at the surface. Estimates of this potential have beenmade by Drechsler 4 on the assumption of pairwise additive interactions, andactivation energies derived in this way are also listed in table 1.It is apparent thatTABLE ENERGETICS OF SINGLE TUNGSTEN ATOMS ON TUNGSTEN SURFACES(kcal/mole)diffusion barrier, YDexpt. theoryplane diffusion direction(1 10) random 22 31(21 1) parallel 13 41(321) parallel 20 43desorption barrier, xexpt. theory - 50 137178- 176apart from quantitative differences, pair interactions give the wrong qualitative orderof mobilities on these three planes. Of particular interest are the differences be-tween the almost identical (21 1) and (321) planes. Although the much smaller valueon the (211) is not consistent with simple pair interactions, it is in qualitative accordwith the behaviour of conduction electrons at a surface outlined by Smoluchowski.~The electron cloud tends to concentrate in the atom troughs, leaving atoms at alattice step partly denuded.This must be expected to weaken bonding at suchsites. The (321), however, is less steeply stepped than the (211), and should there-fore be less affected by an electron redistribution, in agreement with experiment.Most important for the moment is the indication that pair interactions fail to describesurface bonding even qualitatively.Diffusion data reveal differences in the energy of an atom bound at differentsurface positions. Equally interesting but more difficult to measure are the absolutebinding energies themselves. In principle the desorption energy of a metal atomfrom a given crystal plane can be determined from measurements of the electricfield2 required for evaporation at very low temperatures (TI20"M).For aG. EHRLICH 9atom on its own lattice, this field E is related to the desorption energy x (in eV)throughHere n is the charge of the ion evolved in field desorption, In the energy for ionizinga gas-phase atom to the charge state n, and # the work function of the area fromwhich the atom if removed. The term cE2 formally resembles a polarizabilitycorrection ; in fact it also hides other effects such as field penetration into the lattice,and is best considered an empirical correction term. For tungsten atoms at a growthposition, i.e., at kinks in a lattice step, Muller and Young 6a determined an evapora-tion field of -5.4 V/A at 77°K.A value of 5.6 V/A has recently been found at20°K by a more direct method, in which the relation of applied voltage to the fieldis determined from the slope of the Fowler-Nordheim plot in field emission. Thesemeasurements suggest that the polarizability term in eqn. (3) is relatively minor,amounting to less than 15 % of the observed desorption energy of the atom.So far, only tungsten atoms held on the (1 10) plane have been studied ; theseare desorbed by fields as low as 4-4 V/A. In view of the high work function of the(1 10) plane (5.96 eV),6b it might be most favourable for tungsten atoms to evaporateas triply charged ions, and eqn. (3) then yields a desorption energy of -50 kcal/mole.These results are still preliminary; they do reveal a sizeable disparity be-tween experiment and estimates based on pair interactions only. It will be especi-ally interesting to extend these observations to atoms held on other planes, andto correlate variations in binding energy with the atomic environment.x+ I,, - n# + cE2. (3) 3,79n3/2E1/2 =SURFACE STRUCTURE AND INDIVIDUAL BINDING STATESQuantitative determinations comparable to those for tungsten atoms self-adsorbed on their own lattice are not yet available for the chemisorption of diatomicgases. However, even though essentially qualitative, observations on diatomicgases have revealed a fascinating selectivity. In the ion microscope, nitrogeninteracting with tungsten at 300°K is found to leave the (1 10) plane bare, even thoughall the surrounding areas are heavily covered by a strongly bound material.Atlow temperatures (T< 150°K) gas can be discerned on this plane. In its heat ofadsorption (9 kcal/mole) this material resembles the y state previously identified 1on polycrystalline samples. The fact that despite its low heat of adsorption theadsorbed gas withstands the field required for imaging (E24V/A) is significant.If nitrogen were held on the (110) as molecules, its field desorption should resemblethat of CO of comparable desorption energy. Carbon monoxide is removed atE13.5 V/A,8a whereas a field of 4.3 V/A is required to desorb nitrogen. Assumingdissociative adsorption, however, the binding energy of a nitrogen atom amountsto 117 kcal/mole ; at 20°K the adatoms are immobile and it is their stability thatdetermines the behaviour in the applied field.Despite the very small heat of ad-sorption from the molecular gas, nitrogen must therefore be held as atoms on the(110). These findings have been confirmed and enlarged by observations onlarge-scale single crystal planes, using a combination of contact potential and flashdesorption techniques.9 At 300°K the work function of the (1 10) remains unchangedunder continuous exposure to nitrogen at pressures as high as lO-4mm. At lowertemperatures ( T I 130"K), or higher pressures, the work function is found to dropand the material desorbed on flashing follows second-order kinetics, again indicatingthe participation of adatoms.The absence of nitrogen adsorption on the (110)at room temperature has also been verified 10 by measurements in the field emissionmicroscope10 CHEMISORPTION ON SINGLE CRYSTAL PLANESThe specificity of the (110) plane of tungsten toward nitrogen is extreme. Hereis a clean surface capable only of relatively weak interactions and therefore un-populated under ordinary conditions of temperature and pressure, under whichplanes of different orientation are saturated. Surfaces other than the (1 10) also reveala distinct though less dramatic localization of different binding states, which is indic-ated by the experimental data 9 in table 2. The a state of intermediate binding energyhas been identified only on the (1 1 1). The (100) appears unique in having two differentlow temperature y states, with dipoles of opposite sign.A split of the high tem-perature p state has so far been found only on the (1 11) plane. This highly selectivenitrogen adsorption is at least in part due to the very high dissociation energy andvalence requirements of the nitrogen molecule. To be thermodynamically profitable,dissociative chemisorption requires sites that exceed the very strong interactionsbetween nitrogen atoms held to each other in the molecular gas. This require-ment can be met at only a limited number of surface sites. However, even in themolecular chemisorption of carbon monoxide pronounced structural specificitieshave been established.TABLE 2.NITROGEN ADSORPTION ON INDIVIDUAL TUNGSTEN PLANESdipoleD plane T"K state sticking energy association(1 10) 110 Y -9 atoms > -04desorptioncoefficient kcal/mole- 10 molecules >O- -11 molecules <O*25 75 atoms *4- -9 molecules -1(100) 110 Y+300 B-(1 11) 110 Y220 a (-005 16 molecules (-1300 p1+p2 <*a -75 atoms - -02That differences in binding on different crystallographic features of a tungstensurface were responsible for the distinct states of CO was already inferred 1 frommeasurements on polycrystalline filaments, in which four distinct states have beenisolated at 300°K: an a state, forming only at high coverages, and three p sub-states. A detailed assignment of these states to a specific surface feature has notyet been accomplished.However, examination in the field ion microscope hasrevealed several interesting structural effects.As is apparent in fig. 1, adsorptionof CO at 300°K leads to a fairly uniform distribution over the higher index planes.This is in distinction to the behaviour of nitrogen,7 for which the (1 11) and its en-virons proved comparatively unreactive, and indicates that the rate of adsorptionof CO is much the same on the different atomic configurations. Again unlikenitrogen, CO does chemisorb on the densely packed (1 10). The (1 10) plane, how-ever, appears to fill up later than its surroundings, and in this respect resembles therate behaviour already established for the p1 and a states.1The nature of the material held on the (1 10) plane is clarified by experimentsat lower temperatures.In fig. 2d, CO has been adsorbed at 77"K, after which thesurface was warmed to 190°K and then recooled for another picture, shown in fig. 26.Warming removes four of the five bright spots initially present on the (110), estab-lishing that the CO is moderately labile. This is in agreement with the observationsat room temperature. Carbon monoxide impinging upon the (1 10) would migratetoward the periphery, where it can be held more strongly, leaving the (110) itselfless densely populated. Either the a or the state could conform to the generaFIG. 1.-(a) Field ion micrograph of clean tungsten, shaped by field desorption at 20"K, imageexposure to CO while at 300°K; image, taken at 15.9 kV and 20°K to minimizearrows indicate triplets.FIG.2.-(a) View of tungsten surface after adsorption of CO at 77°K ; box indicates locationlost all but one CO after warming to 190°KFIG. 3.-(a) Clean tungsten. (b) View of surface in (a) after heavy oxygen adsorption atFIG. 4.-(a) Tungsten atoms deposited on clean tungsten at 20°K. (b) Same surface after warmingarrow denotes displaced tungsten atom on (1 10) ; no nitrogen is apparenG. EHRLICH 11behaviour observed for carbon monoxide on this plane. Again the stability underthe conditions of observation can be used to good advantage. The a state has adesorption energy of -20 kcal/mole or less. For CO bound with this energy onthe high index planes of tungsten, it follows from the work of Swanson and Gomer 8athat desorption should be complete in a matter of minutes at 3-7 V/A and at 20°K.On the (110), with a work function 1.5 eV greater than the average, the ionic stateis stabilized and desorption should occur at even lower fields.The gas held on the(1 10) at moderate concentrations, as in fig. 1, must therefore be PI.Evenafter warming to 700°K this plane is not cleared of adsorbed material; desorptionis completed only at E 2 5 V/A. An unequivocal interpretation of this change isnot yet possible; it is, however, consistent with the formation of a CO on the(110). During observation of the surface the adsorbed material is subjected tocontinuous electron bombardment, which is known to fragment the a state intocarbon.8b The lability of the material on the (1 10) at low concentrations indicatesthat such fragmentation does not yet occur. The change toward an extremelystable state at high concentrations cannot just be due to filling up of the periphery-this is substantially populated even when material on the (1 10) has been found tobe labile.Rather the stability of the high concentration layer on the (1 10) pointsto the presence of decomposition products on the surface. This in turn suggeststhat a CO is formed on the (110), but only when it is heavily populated by carbonmonoxide in the j31 state.In its interactions with carbon monoxide, just as with nitrogen, the (1 10) planeof tungsten appears capable of only relatively weak interactions. Since adsorptionof CO is molecular, however, there is not the sharp distinction between planes foundfor nitrogen and it still remains to determine the surface features on which p 2 and j33states are formed.The weak interactions of the (1 10) in general must be assignedto an electronic, rather than to any strictly geometrical effect. For nitrogen thelatter cannot enter at the low concentrations at which observations have beenmade. A satisfactory explanation for the low binding energy of atoms and mole-cules on the (1 10) has not yet been offered ; qualitatively this can perhaps be cor-related with the high degree to which tungsten atoms within this plane are sur-rounded by other lattice atoms, and therefore to the saturation of their valence.Perhaps the most important feature to emerge from observations of chemi-sorption on single crystal planes is the complexity of the effects.Even the (1 10)plane appears capable of accommodating a variety of different binding states.These evidently arise as a result of the heterogeneity induced in a given plane by thepresence of adsorbed material: one state of binding changes the environment inits proximity, and therefore creates sites for an entirely different type of interaction.The nature of these induced sites differs from one plane to the next, as do the intrinsicbinding properties of the bare surfaces themselves.At higher concentrations the behaviour of CO on the (1 10) is different.STRUCTURAL CHANGES I N ADSORPTIONIt has long been recognized that the atomic arrangements and spacings at a crystalsurface may differ from those in the bulk.Chemisorption involves an energychange of much the same order as is expended in creating a surface site from the bulk ;it should therefore also be capable of altering the structure of the surface layer.This is especially true when chemisorption is a precursor to the formation of adistinct new phase, such as an oxide. The extent to which rearrangement does infact occur has not been clear in the past. Recently, however, low energy electro12 CHEMISORPTION ON SINGLE CRYSTAL PLANESdiffraction measurements have been wildly interpreted as indicating a surface re-arrangement in chemisorption,ll involving large scale movements of surface atoms.Even though the field ion microscope is not capable of detecting bond elongationsamounting to only a few per cent of the normal spacing (changes which in principlecan be measured by diffraction methods) long-range displacements should be clearlyobservable. Such observations have been made for tungsten interacting with hydro-gen.12 In this system the act of observing the ion image removes all of the adsorbedmaterial.Despite this the tungsten lattice itself appears undisturbed by the adsorp-tion: ion micrographs before and after adsorption, both at 20 and 300"K, can besuperposed, atom for atom, without showing any removal of atoms from their normalsites.Even more interesting are measurements for oxygen, recorded in fig. 3. Thesurface layer is perturbed during observation, but the adsorbed material is not com-pletely eliminated; this complicates interpretation of the pattern as a whole.Theedge of the (1 lo), however, has remained unmoved, proving that there is no corrosion,no significant reconstruction of this plane either by the adsorption of oxygen or theobservation itself. Subject only to the proviso that small changes in lattice spacingscannot as yet be detected, this is also the conclusion to be drawn from close examina-tion of the dilute layers of carbon monoxide in fig. 1, as well as for small amounts ofnitrogen adsorbed on a clean tungsten surface.7So far, only interactions with well-developed crystal planes have been considered.On real surfaces, especially on thin films and on catalysts, single metal atoms or smallatomic clusters are likely to be important. That these may react differently fromatoms embedded in the surface is revealed in fig.4. Three tungsten atoms weredeposited on the (I 10) plane of tungsten at 20°K ; this was then exposed to nitrogenwhile at 263°K. Photographs taken after this cycle give no indication that anynitrogen has adsorbed at the tungsten adatoms on the (1 10). Although further testswill be desirable, it appears that at room temperature the absence of nitrogen fromthe (110) cannot be simply related to the high co-ordination number (9 of tungstenatoms within this plane. An adsorbed tungsten atom has at most three nearestneighbours, yet does not appear capable of forming a strong enough bond withnitrogen either. We are now able to create tungsten clusters containing an arbitrarynumber of atoms on the surface.It should be possible to determine the aggregatesize required to establish strong bonding to nitrogen, and therefore also the intimatestructural dependence of bonding.Of primary interest at the moment is the fact that, as a result of the exposure tonitrogen, the tungsten atoms in fig. 4 have been displaced. On warming to 263°Kthe adatoms undergo a mean square displacement of 0.01 &/min. When the (110)is brought to the same temperature in the presence of nitrogen, the mean squaredisplacement is increased by a factor of ten. The detailed mechanism of this dis-placement has not yet been established. However, we know that at this temperaturenitrogen can be dissociated on the (110), even though re-evaporation is so rapid thatthe steady state concentration is too small for observation at pressures below 10-3 mm.If dissociation occurs in the vicinity of an adsorbed tungsten atom, part of the heatof adsorption may be imparted to the tungsten, propelling it to a new site. Whateverthe mechanism, we have here a clear example of the displacement of an isolatedlattice atom by a chemisorbing gas.Again it will be of interest to determine thesize of the atom cluster beyond which these effects are negligible.There are indications that even in a well-formed, relatively smooth plane, neigh-bouring lattice atoms may be affected by the presence of adsorbed material. Thepicture of a tungsten surface after exposure to CO shown in fig. Ib reveals the presencG . EHRLICH 13of triplets : a large bright central atom, and on either side of it, in a straight line, asmaller emitting spot brighter than the surroundings.This configuration is nbtspecific to carbon monoxide. It has been observed with adsorbed oxygen and nitro-gen, as well as with tungsten. These triplets do not represent a complex with tungstenatoms pulled out of the metal. Within the limits to which quantitative comparisonshave been made, the two satellites coincide with atom sites of the bare lattice and atnormal picture voltages are indistinguishable from their neighbours. The separationof the two satellites is quite large-ca 13 A on the (21 1) plane. This, as well as thelinear configuration, eliminates the finite size of the helium atoms used in imageformation as a possible cause of these triplets. It is known, however, that the screen-ing charge around an impurity in a metal does not decay monotonically. Instead, itoscillates, falling to zero as the inverse cube of the distance.13 At present, it is mostreasonable to attribute the appearance of triplets to such an electronic perturbationaround the adsorbed material. To what extent this could affect chemical processesremains to be established.The examination of adsorption on single crystal planes is still in its very earlystages ; however, even the few results outlined here suggest a diversity of interestingeffects that should help in coming to a better qualitative understanding of morecomplex reactions on less clearly specified surfaces.1 for a review, see Ehrlich, Ann. N.Y. A c d . Sci., 1963, 101, 722.2 Miiller, Science, 1965, 149, 591.3 (a) Ehrlich, J. Chem. Physics, 1966, 44, 1050 ;4 Drechsler, 2. Elektrochem., 1954,58, 327.5 Smoluchowski, Physic. Rev., 1941, 60, 661.6 (a) Miiller and Young, J. AppZ. Physics, 1961, 32, 2425;(b) Young and Muller, J. AppZ. Physics, 1962, 33, 91.7 Ehrlich and Hudda, J. Chem. Physics, 1962, 36, 3233.8 (a) Swanson and Gomer, J. Chem. Physics, 1963,39,2813;(b) Menzel and Gomer, J. Chem. Physics, 1964, 41, 3329.9 Delchar and Ehrlich, J. Chem. Physics, 1965, 42, 2686.10 van Oostrom, Thesis (University of Amsterdam, 1965).11 for review, see May, Ind. Eng. Chem., 1965, 57, 18.12 Ehrlich and Hudda, Phil. Mag., 1963, 8, 1587.13 Friedel, NUOVO Cim. Suppl., 1958, 7, 298.(b) Ehrlich and Hudda, J. Chem. Physics, 1966, 44, 1039

ISSN:0366-9033    

DOI:10.1039/DF9664100007

出版商:RSC    

年代:1966

数据来源: RSC

摘要:

Different Adsorption States on Single SubstrateBY ROBERT GOMERDept. of Chemistry and Institute for the Study of Metals,The University of Chicago, Chicago, Illinois 60637Receiued 3 1st Jdnuary, 1966The adsorption of potassium and of CO on tungsten is examined with a view to defining thesimilarities and differences between metallic and covalent adsorption on the same substrate. Itis concluded that metallic adsorption shows no distinctly different adsorption states, variationsresulting from differences in substrate geometry and work function being quantitative rather thanqualitative, while covalent adsorption shows definite, discontinuous changes in adsorption type,which can be correlated with differences in geometry and binding mode. In both cases, however,substrate-adsorbate geometry appears to be important even though the effect in metallic adsorptionbecomes obvious only after some dissection of the factors entering the heat of adsorption.It is the purpose of this paper to examine two systems more or less at the oppositeend of the chemisorption spectrum, CO on W, and K on W, in order to see whatcan be learned from the different ways in which the same substrate is able to interactwith these different adsorbates.In particular, we shall attempt to see how andin what way substrate structure appears to affect adsorption in these systems, andwhether any generalizations may be drawn from this. This task is complicated bythe fact that only averages obtained on polycrystalline substrates are available forthe COfW system, so that interpretation must proceed with caution.In thefollowing we shall attempt first, to summarize briefly the salient results for eachsystem, how they were obtained, and their interpretation in terms of adsorptionstates. Finally, we shall turn in a qualitative way to the question of a coherenttheoretical interpretation.POTASSIUM ON TUNGSTENWe discuss first the K on W system because the contrast between measurementsof averages on polycrystalline or poly-faceted substrates with measurements onsingle crystal planes emphasizes where and to what extent caution is required ininterpreting the CO results. The average contact potential, heat of adsorption andsurface diffusion behaviour of K on W were investigated as a function of (absolute)average coverage by Schmidt and Gomer,l using field emission.The method con-sisted of evaporating K on to a tungsten field emitter, determining the absoluteflux by means of an auxiliary surface ionization detector, and then carrying outwork function and Arrhenius measurements in the usual manner. More recentlythese authors have looked at K adsorption on the 110, 211, 100 and 1 1 1 regions ofa field emitter using a probe hole, Faraday cage and rotatable emitter, which per-mitted measurements of a small portion of the total beam, corresponding to emissionfrom single crystal planes. This work is discussed in detail elsewhere?Measurements were carried out both for immobile adsorption (substrate at78°K during and after adsorption) and after thermal equilibration. After deter-mining the impinged K flux, the former measurements yielded work function againstatom density on individual planes.The flux calibration was carried out by de-1R. GOMER 15positing a given number of standard doses, equilibrating this layer by heating to400"K, obtaining its average work function 6 from the total emission, and finallycomparing the latter with the 6 against Pt (average absolute atom density) curveestablished in ref. (1). The value of E per dose thus found could then be con-verted by a simple calculation into the actual flux per dose impinged on the regionunder study. Measurements of 6 and of # on single planes after thermal equi-libration yielded values of # against i, so that combination with the # against ndata yielded values of n/Z against i.For convenience, we also use a non-dimen-sional coverage given in terms of an arbitrarily defined monolayer density,l no =3.9 x 1014 atoms/cm2, as 8 = n/no. Fig. 1 shows curves of # against 8 ; aboveFIG. 1.-Work function 4 againstK atom density n and fractionalcoverage 8 (defhed as 4 3 . 9 ~ 1014)for various planes of thermallyannealed tungsten field emitter.1.20 (monolayers)LLl.*III""n x 10-14 (atoms cm-2)1.0 20 30 40 508 = 0.6-0.8 immobile adsorption leads first to plateaus and then to monotonicdecreases in the apparent work function. This suggests incipient multilayer forma-tion with a very disordered adsorbate structure and consequent microscopic fieldenhancement as well as the possibility of genuinely decreased work functions.Inany case, the results are not considered trustworthy in this region. However, then/% against 8 values shown in fig. 2 indicate that the n/Z values for all regions except100 approach unity above 8-0.6. Consequently the 4 against 6 curves shownin fig. 3 must be close approximations to the correct # against n curves for 050.6,and have been so interpreted in computing dipole moments. These are definedin terms of the contact potential A#, the adsorbate charge q and its distance dofrom the image plane byA 4 = 2npn = LCnqdon, (1)i.e., asp = 2qdo16 DIFFERENT ADSORPTION STATESFIG. 3.-Work functions of individualplanes against average coverage 3 foran equilibrated K layer on tungsten.FIG.2.-Ratios of actual to averageK atom densities as - function ofaverage coverage B (defined as7 3 . 9 x 1014) for a K layer equili-brated at 400°K. I I tI 1 1 1 I.I0 20 .30 -40 .50 *cOe' (monolayers)1 3 0.2 0.4 06 0.8 1.0 1.2 -8 (monolayersR. GOMER 17Fig. 4 shows the dipole moments so computed as function of coverage, and alsothe fictitious average dipole moment fi, obtained from $. The latter is much lowerthan any of the real moments at low coverage because the adsorbate is then con-centrated on the high work function regions, which do not yet contribute appreci-ably to emission. Consequently the observed A+ values, which correspond toemission from the intrinsically low work function regions are in fact caused by con-siderably smaller adsorbate concentrations than the average Ei used for computing $.4 a 2 ' A 4 I -6 ' I -8 I ' I.0 ' ' 1-2coverage 8actual (not average) K coverage for several planes of a tungsten emitter.FIG.4.-Dipols moments p &do (q adsorbate charge, do surface-adsorbate distance) againstThe n/Z values may also be used to compute anisotropies in the heat H of ad-sorption, entropy effects presumably being small at 400°K where equilibrium is as-sumed to be frozen in. Since adsorption predominates on 110 at low 8, it is reason-able that the average heat of adsorption be equated with H110 so that absolute Hvalues for the other planes j can be obtained from njjrzllo at low 6. Since the largen anisotropies turn out to correspond to fairly small H anisotropies, it is permissibleto neglect the latter altogether as 0 increases and the n anisotropies become small.Thus, N against 8 curves over the entire coverage interval 0 1 can be constructedfrom the l7 against 0 curve, after converting 8 to 0 by means of the data of fig. 2and by taking H anisotropies into account at low 8.The results, together withthe previously 1 determined II against B curve are shown in fig. 5.Although the adsorbate charges cannot be determined in the absence of dovalues, certain bounds can be placed on them from the fact that the heat of ionicadsorption is given by(3) Hioni, == 4e -1,e + e2j4d, - Rep,where Rep is a repulsive ion core interaction and 10 the ionization potential of theadsorbate atom. Since the heat computed from eqn.(3) must be less than or equalto the actual H value, it turns out for the (I 10) plane at 8 = 0 that do = 2.85 A ifRep = 0, leading to a value of q = 0.58 electron charges, or that Rep = 0-9 eV ifq = 1 and do = 1.6A. The limits on the do values obtained in this way can thenbe used for the other planes to get estimates of the adsorbate charge and its variation18 DIFFERENT ADSORPTION STATESwith coverage. The results, shown in fig. 6, are that y, whatever its assignmentat 8 = 0, decreases rapidly with increasing coverage as is obvious from the dipolemoment curves.1 --T"0 -2 *4 -6 -8 to0FIG. 5.-Heats of K adsorption against actual (not average) coverage for several planes of tungsten.The average H against average coverage curve is also shown.I0FIG.6.-K atom charges as function of coverage for the limiting cases,do = 2.85 A, and Rep = 0.9 eV.To summarize the experimental findings, adsorption appears to be polar (possiblywholly ionic on (110) at low coverage), maxinium adsorbate charge occurring onthe highest work function region. As coverage increases, adsorbate charges de-crease markedly but smoothly on all regions. The heats of adsorption appear tR. GOMER 19be relatively insensitive to substrate structure or work function, but decrease fromthe initial values of 243-2.5 eV by over 1 eV as O+l.It is fairly clear from these findings that K adsorption on W must be essentiallymetallic, the large decrease in adsorbate charge with increasing 8 resulting from theincreased filling of the very broadened and split adsorbate level 1, 3, 4 as the latteris lowered by the decrease in work function.It is possible to set up a quantummechanical mode1,2 which utilizes system wave functions constructed from linearcombinations of one-electron metal wave functions and the appropriate K atomicorbitals (corrected for actual adsorbate charge), and, at least in principle, to deter-mine the coefficients and calculate the energy. If the nature of K adsorption hasbeen correctly diagnosed, this model would then indicate that system states far inenergy from that of the (corrected) adsorbate atom would be just metal states,with negligible atomic coefficients, while states lying fairly near the atomic levelwould have appreciable atomic wave function coefficients.Thus, the electroniccharge at the adsorbate consists of small contributions from a large number ofelectrons, the exact charge being fixed by the fact that all system levels below theFermi energy must be filled. For our present purposes it suffices, and is in someways more illuminating, to accept the general validity of the statements just madeand to consider the sharp adsorbate level so split by interaction with the substrate,that the adatom can be regarded as a tiny piece of metal, which we shall call a“ metallet ”. The heat of adsorption relative to substrate and isolated atom canthen be found by considering the energy changes in the following steps.(i) The atom far from the surface is ionized, the electron transferred to the metaland the ion allowed to approach to a distance do with the restriction that no electrontransfer back into the atom is as yet allowed.The energy decrease is that givenby eqn. (3), #e - loe + e2/4do - Rep.(ii) Since the sharp atomic level at -10 has now been broadened into a bandof metallet states we fill the latter to the equilibrium Fermi level I(q) by transferringamounts of electronic charge dq- from the metal into the metallet levels. Theenergy decrease is - #dq-+ I(q-)dq-- qdy-/2d, where the last term arises from 1: S6 c the fact that work must be done against the (to the electron, repulsive) image potentialq/2do in moving a charge dq- to the adsorbate. Since the net positive adsorbatecharge q = e-q- we obtain as the sum of steps (i) and (ii)H = +q + 1 eI(q)dq + q2/4d, - Ioe - Rep.4(4)Eqn. (4), which reduces to eqn.(3) for q = e casts the heats of adsorption largelyin electrostatic terms except for the integral which represents the energy of electronsactually in the metallet, so to speak. Eqn. (4) makes the variation of q and # withcoverage explicitly responsible for a large part of the decrease in H, the compensatingfactor being the increase in the integral as q decreases. # must be taken as thezero coverage work function plus the portion of the contact potential effective atdo, which can be obtained from the measured A# and the appropriate dipole sums,l’ 4since the absolute coverage n is known. Since q varies with coverage, it is moreconvenient to recast eqn. (4) asH = 4q + q2/4d0 - 1 ‘l(q)dq +A - Rep,0whereA = I(qjdq-f,e.J20 DIFFERENT ADSORPTION STATESSince I(q)dq is the average energy of the metallet levels (or better its absolute value,measured from the zero of field free space) A is simply the amount by which thisenergy has been shifted downward from the value -1oe for the sharp level by inter-action with the metal.The numerical value of 1(q) can be obtained from the equality of Fermi levelsat equilibrium, taking due account of electrostatic potentials,We are thus in a position to determine I(q) as a function of q. If the adsorbate-substrate geometry did not change with increasing coverage (which is the way inwhich q is varied) it would be legitimate to integrate the resultant curve from 0 toq and thus to obtain A from the actual H values and eqn.(5). This procedurehas been used rather than the more legitimate one of obtaining I(q)dq from Hand eqn. (4) to make the comparison between different planes easier, since A in-volves the same range of integration in all cases. Changes in adsorbate substrateinteraction then show up in A, although the numerical values will be incorrect ifbased on the distorted Z(q) against q curve obtainable from experiment. The resultof the Z(q) determination is shown in fig. 7 and 8 for the two extreme assumptionscI(q) = 4 +@do. (7)c-Ii -14+9/2do, e vFIG. 7.-K atom charge against Fermi level $+q/2do for several planes of tungsten, calculated onthe assumption of do = 2~85A.I , 110; 0,211 ; 0, 100; A, 111.d = 2.85 A, Rep = 0 ; and Rep = 0.9 eV, do = 1-6-24 A. The resultant +-widths ofthe metallet band are -3 eV in each case. The values of A obtained for the twoassumptions are shown in fig. 9. While the numerical values differ for the twoassumptions, the trends are very similar and do show some dependence on struc-ture and also on coverage. The behaviour can be rationalized by postulating thatA is greatest where the adsorbate can embed itself most deeply into the substrate,i.e., greatest on the (111) plane, less on the (211) plane and least on the (110) plane.The same qualitative results are obtained when the net delocalization energy isdetermined by means of the detailed model referred to earlier, by subtracting fromthe observed H values the explicitly electrostatic interactions given by that model.The reason why the curves seem to show maxima can be rationalized on the basiR.GOMER 21of fit with the substrate and lateral, attractive adsorbate-adsorbate interactions.At low coverage, optimal fit to the substrate is possible. As 8 increases, but notenough to cause departure from best fit, metallic adsorbate interactions (all repulsiveinteractions have already been taken into account by using the proper value of#) can slightly increase A. However, since the binding energy of pure K metal ismuch less than the heat of adsorption on W, this effect is soon overcome by thefact that further increases in coverage force departure of all adatoms from optimalsites, in order to accommodate more adsorbate in the first layer, leading to anoverall decrease in A.4+9/2do, evFIG.&-Data as in fig. 7, with the assumption Rep = 0.9 eV.I 1FIG. 9.-A against coverage for the two cases do = 2.85 A, and Rep = 0.9 eV.Thus, the present model rationalizes the fact that differences in substrate workfunction and geometry lead to only small heat anisotropies by casting the resultsinto a torm where the greater electrostatic contribution from the high work function,close-packed regions, is largely compensated by the increased resonance or exchangeeffects on regions where intrinsically lower work function is balanced by a morehospitable geometry22 DIFFERENT ADSORPTION STATESAlthough there is no obvious reason why adsorption on 100 should be less favour-able than on 211, the intrinsic work functions being almost equal, it is possible toinvoke differences in geometry and to point out that the nearest neighbour contacton 211 is 5 as compared with 4 on 100, while 111 can accommodate a close-packedlayer of K.However, it would be stretching the argument to go much beyondthe general trends just discussed. For present purposes it suffices to note that evenfor metallic adsorption substrate structure plays a role apart from explicitly electro-static effects, in determining adsorption properties.CARBON MONOXIDE ADSORPTIONThe adsorption of carbon monoxide on tungsten has been studied intensivelyby a number of authors in recent years and there is now evidence from flash fila-ment desorption,S'-*o field emission 11, 12 and electron impact desorption 12 for theexistence of at least three types of chemisorption states, which differ not only in theirheats of adsorption and electron desorption cross-sections but even in the sign oftheir dipole moments. Briefly some of the evidence for this Fehaviour is thefollowing.When CO is adsorbed at low temperature on a clean field emitter aweakly bound electronegative (4 increased) virgin state results, characterized byfairly uniform dipole moment and surface concentration. This is shown by theappearance of the pattern which indicates little change in emission anisotropyfrom the clean emitter, and also by the fact that the average work function increasesfairly linearly with amount adsorbed.The result of heating such a layer to >400"Kor of exposing it to low energy electron impact 12 is partial desorption, as will beshown presently,l3 and creation of a more tightly bound electronegative p layercharacterized by a high-temperature flash desorption spectrum showing considerablesfructure,~-7~ 10 and by a very low electron desorption cross-section.12 When COis re-adsorbed on a p layer the virgin layer is not restored, but a third type of ad-sorption is observed, characterized by a very high electron desorption 12 cross-sectionand by a positive dipole moment 8 , 9, 11- 12 (i.e., the work function is reduced).This layer can be desorbed by reheating 11, 12 to 300-400°K or by electron impact 12without affecting the underlying p layer.Menzel and Gomer12 were able to show, by means of electron desorption,that this picture is somewhat oversimplified.Since the electron desorption cross-sections for the different adsorption types differ by orders of magnitude, it is possibleto dissect the composition of a given layer by noting the observed rates and extra-polating the corresponding A& to zero desorption. In this way it was found thatthere is a small amount of a-CO on the virgin layer even before heating, and byimplication a corresponding amount of P-CO, since the formation of a-COseems to hinge on the presence of p states. Further, heating a virgin layer to 270°Kleads not only to p formation, but to some a formation as well, indicating that virgindesorption may proceed through an intermediate a-state.Also, in addition tocc-CO a small amount of virgin GO can be adsorbed on a full p-layer.Although all of these results refer to averages over the entire emitter there isless reason to suspect their basic validity than would be the case with electropositiveadsorbates, in large part because adsorption is immobile below 600°K and becausethere is less reason to suspect severe coverage anisotropies as shown by the factthat the intrinsic high work function regions stay dark, although the overall workfunction (i.e., that of the intrinsically low 4 regions) is raised to 54-56 eV at 8 = 1.On the other hand, this means that no information is obtained on the 110, 21 1 and100 regions since they do not contribute to emission at any coverage.MeasurementR. GOMER 23on single planes may force some modification of these assumptions as well asproviding information on the high cf, regions.While a quantitative corrclation of work function changes with amounts adsorbedand a determination of the relative abundances of the various adsorption statesrequires measurements on single crystal faces, it is already of considerable interestto obtain these quantities averaged over the surface. To this end, Bell and Gomer 13measured average work function changes as function of the relative amount ofCO impinged and then converted the amounts impinged into amounts adsorbed bydetermining sticking coeffcients in a separate experiment. The method used forthe latter consisted of “illuminating” a tungsten ribbon with a CO sublimationsource.A moveable field emitter serving as detector could be raised in front ofthe ribbon in such a way as to receive a CO deposit only by reflection (or desorption)from the ribbon. In this way the amount reflected per dose impinged on the ribboncould be determined quite accurately acd step desorption spectra could be obtainedas well. The resultant sticking coefficients are shown in fig. 10. 4 as functions ofSticking Coefficientson Initially Clean W2 4 6 8 10 12 14 16number of doses adsorbedFIG. 10.-Sticking coefficients of CO on initially clean tungsten against relative coverage; the 500and 780°K curves coincide.doses impinged and doses adsorbed (converted to the latter by means of the stickingcoefficient results) are shown for virgin, /3 and a adsorption in fig.11-13. The cicurve is linear, while the virgin and /3 curves show two essentially linear segments.For the /3 layer formed at 400°K this may mean that the initial portion of the layercorresponding to a high sticking coefficient has an intrinsically lower dipole moment.For the virgin curve where the sticking coefficient is unity throughout, the higherdipole moment is observed at lower coverage, and it might therefore be supposedthat the terminal section of the 20°K layer consisted of the initial portion of thep layer, i.e., that the low temperature layer is a composite of virgin and p states.This is improbable, however, for a number of reasons : (i) only a small amount ofM-CO is detectable on a low temperature layer by electron desorption; since theratio of a to fl is close to unity (see later), this implies the presence of only a smallamount of P-CO.(ii) The total amount of the low dipole 20°K deposit is roughl24 DIFFERENT ADSORPTION STATEStwice the amount of the low dipole B deposit obtainable on a saturated layer.(iii) It is improbable that under conditions of unit sticking coefficient virgin sitesshould be occupied first, and that only after the completion of the virgin layer a---- 3 --6I 2 3 4 5 6 7 8 9 10 II 12 13 14 15 16 17 18 19 2021 22 232425number of doses impingedBFIG. 11.-Work function against number of 30 sec CO doses adsorbed at 20°K. The curve markedB represents the change in the logarithm of the Fowler-Nordheim preexponential term.7 - 1 I.I I5 2 0 400'K Adsorption-460 -450 I 2 3 4 5 6 7 8 9 1 0 1432 I'I0number of doses impingedFIG. 12.-Work function against number of 60sec CO doses impinged (open circles) and dosesadsorbed (full circles) for adsorption at 400°K. The B curve refers to the change in the logarithmof the Fowler-Nordheim pre-exponential term.fl deposit should be formed. It therefore seems more likely that the decrease indipole moment at higher coverages for the low temperature layer is simply theresult of less optimal adsorption under conditions of crowding, as well as of the forma-tion of some a-CO at high coverage.Fig. 14 shows the step desorption spectrum of a complete low-temperaturelayer, and superimposed on it an a desorption spectrum.The low-temperaturelayer shows a main desorption peak at 320°K with desorption of this part completR. GOMER 25- I4 I I 5.4 0Alpha Adsorptionon 600°K Beta 15Layer5.20 16-5.10 '\5.004 4.90%'.0 I 2 3 4 5 6 7 8 9 10number of dosesFIG. 13.-Work function against number of 30 sec CO doses impinged and adsorbed at 100°K ona B layer prepared by heating a virgin layer to 600°K. Where the impinged and adsorbed dosesdiffer the latter are shown by open circles. The In A curve represents the logarithm of the Fowler-Nordheim pre-exponential term.I 11 Virgin, Alpha and Beta SpectraI -I -!i <,/fp&q$ i0 100 200 300 40C 700 800 900 1000 1100 1200 1300 1400T"KX4s s aLFIG.14.4uperimposed step-desorption spectra for virgin and alpha layers. Virgin ordinatesare shown by vertical bars, alpha ordinates by circles. The virgin spectrum was obtained byheating a full 20°K layer in steps to 450°K. The emitter was then redosed at 20°K and the alphaspectrum obtained. Continuation of heating then gives the beta spectrum shown. Two pointson the latter (x ) have been normalized to the average T interval. Scale for beta spectrum givenon right side of figure. A V/Vo represents relative simal size of detector26 DIFFERENT ADSORPTION STATESat 450"K, corresponding to formation of the p layer. The latter shows desorptioncommencing at 700°K. The a spectrum is remarkably similar to the low-temper-ature portion of the virgin spectrum, despite the fact that the y-states have a dipolemoment of opposite sign.The desorption results leave the possibility that virgin and j? states are adsorbedsimultaneously at low temperature, with virgin desorption simply leaving behinda pure p layer.This is ruled out, however, by the adsorption hysteresis, i.e., thefact that re-adsorption leads to a rather than to virgin adsorption, and also by thedata shown in fig. 15 where the ratio of CO desorbed below 500°K to the total amount*I '2 ' 3 - 4 -5 -6 -7 -8 *9 to' 38 (fraction of full 20°K layer)FIG. 15.-Ratio of virgin CO desorbed (i.e., CO desorbed below 500°K) to total amount depositedon the surface at 20°K against coverage (in terms of the full low temperature layer).adsorbed at 20°K is plotted against the initial coverage, expressed as a fraction ofthe 20°K layer maximum coverage.If the ratio of virgin to p-CO adsorbed at20°K were fixed, this ratio should be constant for immobile adsorption. The S-shape actually observed is consistent with the assumption that most of the CO isadsorbed as virgin, and that heating leads to desorption and conversion to p, de-sorption being rate-controlling at high coverage.If this interpretation is accepted the work function data can be used to determinerelative dipole moments. The results relative to the dipole moment of the initialportion of the virgin deposit (p,) are as follows: ,u, (at high coverage)/p, = 0.7;pb (low coverage)/p, = 0-68 ;Relative abundances can be obtained from the desorption data, if the presentinterpretation of the layer composition is accepted.As already pointed out, electronimpact measurements indicate that there is a small amount of a-CO on the lowtemperature layer, and by implication a similar amount of p-CO. If the corres-ponding work function increments obtained from electron desorption are convertedinto relative coverages by means of the dipole moment ratios just obtained, thecorrections turn out to be -12 % each of the total low-temperature coverage.It further turns out from electron desorption that redosing a P-layer leads to a smallamount of virgin adsorption, corresponding to -7 % of the total low temperaturedeposit. With these corrections it is possible to calculate the ratio of total p tototal virgin as well as the convertible virgin to converted p. It is also possible to(high coverage)/p, = 1.0 ; pa/pv = -0.68R.GOMER 27correct the amount of o! obtained in desorption for the small amount of re-adsorbedvirgin CO. The results of these manipulations are shown in table 1.TABLE 1 .a unwrr. as fraction of: 01 con. function of: virgin and fi (corr.)0.61 0.69 1.4 0.78 1.2 0.68 0-60 1.8 2.1Summary of abundances of virgin, /3 and cc CO on a polycrystalline tungsten surface. cc un-corrected refers to the total amount desorbed after redosing a b layer. 0: corrected is this amountminus the amount of re-adsorbed virgin CO, as determined from electron impact desorption and thedipole ratios. The virgin and /3 data have been corrected for residual ct and j? on a low-temperatureI ayer .While it is apparent that these results are only averages over a polycrystallinesurface, it is nevertheless probable that valid qualitative conclusions can be drawnfrom them, because there is no evidence for special anisotropies from field emissionor electron desorption.These conclusions are the following. The dipole momentsof virgin and p-CO are negative and of approximately equal magnitude. Thedipole moment of a-CO is positive androughly 70 % of that of virgin CO. If amean value of 6 x 1014 molecules is taken as the density of the monolayer, the dipolemoments are -0-44 D for the high dipole forms of virgin and p-CO, -0-3 D for thelow dipole forms of virgin and p, and + 0.3 D for a-CO.For every molecule desorbedfrom a virgin layer, roughly one p state can be formed and one molecule re-adsorbedas a, i.e., the ratio of virgin to p sites is roughly 2 and the ratio of /? to a sites isroughly 1.Even if these numbers have to be modified severely on certain regions of thesurface not yet accessible to measurement, the conclusion seems very strong thatthe different adsorption states correspond to different geometric substrate-adsorbateconfigurations. Thus, adsorption at low temperature may occur via a weak, un-activated mode utilizing sp2 carbon orbitals ; since these are half-filled it is reasonablethat adsorption should be electronegative. Heating or electron impact may leadto desorption and to activated rearrangement of the remainder to tightly boundstates utilizing partly filled molecular orbitals and thus having negative dipolemoments.If this is a lying-down mode, the occupancy of two virgin sites by each/J molecule would be explained. On a full /? layer, single substrate atom sites wouldstill be available for M adsorption, presumably through a doubly occupied sp carbonorbital, with some electron transfer from CO, thus accounting for the positivedipole moment. It seems reasonable, but is by no means proved, that this generalscheme applies in varying degrees to all regions of the surface and that the structurein the p spectrum, for instance, is due to variations of configuration, either fromregion to region or possibly within a given region as well.A UNIFIED PICTURE OF CHEMISORPTION?Even if the particular scheme advanced for CO is incorrect, it is apparent thatstrong chemisorption of molecules with high ionization potentials is markedlydifferent from metallic adsorption.In its outward manifestations covalent ad-sorption shows a much greater sensitivity to substrate structure but less sensitivityto work function, a greater variety of discrete adsorption states, but much less leewayfor gradual change within a given adsorption type. It is not difficult to understandthe reasons for this in a qualitative way. In metallic adsorption the relevant energ28 DIFFERENT ADSORPTION STATESlevels fall within the conduction band of the total system, and a very large numberof levels are involved in binding, i.e., the basis wave functions contain in additionto the relevant adsorbate orbitals a very large number of substrate orbitals, andmany electrons participate in adsorption.In electronegative adsorption of thetype just considered it may be concluded that the system states involved lie belowthe conduction band (even when adsorption is weak). Consequently they can beconsidered as composed of the appropriate adsorbate orbitals and interband surfacestates of the substrate, which are not utilized in metallic adsorption. Since thesurface states are highly localized it may be concluded that (a) the number of elec-trons involved in adsorption is small, so that the adsorption complex is much closerto a conventional chemical compound than for metallic adsorption, and (b) thatthe types of surface compounds so formed will be much more sharply defined, withdiscontinuous changes, as the configurations and hence the orbitals involved change.Since the number of electrons involved in bonding is small, the polarizability ofthe ad-complex, and its ability to vary its charge significantly will be very smallcompared to metallic adsorption. Further, the net electron transfer involved inbonding is also likely to be much less, since binding must be covalent. The dis-parity in 10 and 4 precludes electropositive polar binding, and the relatively lowelectron affinity of the adsorbate precludes very polar electronegative binding.So far the emphasis has been on differences between metallic and covalent ad-sorption, but significant similarities exist. In both cases, exchange integrals whosenumerical values depend sensitively on configuration play a role, even though thelatter is less pronounced in metallic adsorption. To a reasonable degree the con-cept of geometric fit, i.e., of hard-sphere models, would appear to have the significancewith which intuition has long credited it.1 Schmidt and Gomer, J. Chem. Physics, 1965, 42, 3573.2 Schmidt and Gomer, J. Chem. Physics, in press.3 Gurney, Physic. Rev., 1935, 47, 479.4 Gomer and Swanson, J. Chem. Physics, 1963,38, 1613.5 Ehrlich, J. Chem. Physics, 1962, 36, 1171.6 Redhead, Trans. Faraduy Soc., 1961, 57, 641.7 Rigby, Can. J. Physics, 1964, 42, 1256.8 Eisinger, J. Chem. Physics, 1957, 27, 1206.9 Gavrilyuk and Medvedev, Fiz. Tver. Tela, 1962, 4, 2372. [English trans., Soviet Physics-Solid10 Madey, Yates and Stern, J. Chem. Physics, 1965, 42, 1372.11 Swanson and Gomer, J. Chem. Physics, 1963, 39,2813.12 Menzel and Gomer, J. Chem. Physics, 1964, 41, 3329.13 Bell and Gomer, J. Chem. Physics, 1966, 44, 1065.State, 1963, 4, 1737.

ISSN:0366-9033    

DOI:10.1039/DF9664100014

出版商:RSC    

年代:1966

数据来源: RSC

摘要:

Chemisorption and Surface Corrosion in the Tungsten +CarbonMonoxide System, as Studied by Field Emission and FieldIon MicroscopyBY A. A. HOLSCHER AND W. M. H. SACHTLERKoninklijke/Shell-Laboratorium, Amsterdam, The Netherlands(Shell Research N.V.)Received 5th January, 1966A new interpretation of the occurrence of different adsorption states of carbon monoxide ontungsten has been achieved by combining field emission and field ion microscopy techniques. Itis shown that the adsorption of CO on a field-evaporated tungsten surface causes a re-arrangementof the surface atoms. The activation energy required decreases with increasing coverage and issmallest for the less densely packed crystal faces. At 78°K re-arrangement takes place to a verylittle extent only.Two adsorption layers can be distinguished : a chemi-adsorption layer and aphysisorbed layer on top. At 300°K re-arrangement leads to complete surface corrosion, creatingvacancies in the layers beneath the corrosive chemisorption layer. On top of the latter layer, addi-tional CO can be weakly adsorbed.Hydrogen does not cause any re-arrangement of tungsten. However, CO adsorption on tungstenprecovered with hydrogen, again gives rise to re-arrangements with removal of hydrogen. Theresults, together with the evidence obtained with other systems, justify the conclusion that surfacere-arrangement can be an important factor in adsorption, even at low temperatures and on metalswith a high cohesive energy.Recent experimental work 1, 3 on single crystal faces has revealed that crystallo-graphic specificity in adsorption is an important factor in understanding the simultan-eous occurrence of different adsorption complexes on one and the same metal.Buteven when allowance is made for this finding, the complexity of many compositeadsorption layers is still not completely resolved, because even on one single facevarious complexes can often be distinguished.In the discussions on the nature of such different complexes it has in most casesbeen assumed that the positions of the surface metal atoms remain unchanged, thephenomena being ascribed to either different types of bonding or different adsorptionsites on the surface. However, it has been shown4 that chemisorption causes ade-metallization of the surface atoms.In those gas-metal systems where the corres-ponding bulk compounds are stable, it is logical to expect that eventually a re-arrangement of substrate atoms will take place, as it is the first stage to bulk compoundformation.Experiments with the low-energy electron diffraction (LEED) technique haveshown 5 9 6 that in several cases the formation of chemisorption complexes involves adrastic re-ordering of substrate atoms. These experiments were confined to adsorp-tion temperatures of about 300°K and above. A major restriction of LEED is thatsurface re-arrangements can only be detected if they give rise to new surface structureswhich exhibit long-range order.The question arises whether any re-arrangement can be observed at temperaturesbelow 300°K even on metals with a high cohesive energy, like tungsten, and beforetwo-dimensionally ordered structures appear.The most promising experimentaltool for studying these phenomena is the field ion microscope,7 because it allows230 F.E.M. AND F.I.M. OF co ON wobservation of metal surfaces in atomic detail. The effect of gas adsorption canbe investigated by comparing the ion images of the same surface before and afteradsorption. By applying field evaporation, ideally ordered clean surfaces can beobtained to start with.So far, gas adsorption on ideally ordered surfaces has received little attention.George and Stier 8 studied the adsorption of oxygen on an ordered tungsten surfacewith the field electron microscope. They used the ion microscope mainly to character-ize the structure of the initially clean surface (either highly ordered or thermallyannealed) and concluded that the field emission results are independent of the degreeof initial surface order.After adsorption at 20°K on a field-evaporated surface andremoving the oxygen by field evaporation they obtained an ion image of a surfacelittered with out-of-place atoms. Ehrlich and Hudda have used the ion microscopefor studying the adsorption of nitrogen 9 and carbon monoxide 10, 11 on tungsten.They conclude that the metal surface is only negligibly disturbed by the adsorption.Ln the present work we have combined the ion microscope and field emissiontechniques in a more comprehensive way to study the adsorption of carbon monoxideon clean ordered tungsten surfaces and on tungsten precovered with hydrogen.Inthis way we obtained different kinds of information on one and the same state ofadsorption. First, the change in work function caused by adsorption is derived frommeasurements of the change of electron emission and applying the Fowler-Nordheimrelation. Although the exact meaning of changes in average work function is not asclear-cut as those obtained from measurements of individual crystal faces on accountof the complicated averaging inherent in the method, the average work function canstill be used as an important parameter to describe the adsorbed state.A second useful parameter is provided by the change in the pre-exponential factorof the Fowler-Nordheim relation.It is often found that adsorbed states characterizedby the same work function differ markedly with respect to the change in the pre-exponential factor.The field emission results can be fitted into a more detailed picture of events atthe surface by making use of the observations with the ion microscope. Theseessentially fall in two categories. First, we may compare the ion image of the cleansurface with the one obtained after adsorption, the adsorption state being characterizedby the Fowler-Nordheim data. Secondly, the procedure may be reversed by measur-ing the electron emission current after taking ion pictures of successively advancedstages of field desorption and field evaporation following adsorption. The lattertype of experiments provides information on the effect of adsorption on atom layersbeneath the outermost surface layer and constitutes a unique feature of the ionmicroscope.Observations made with the ion microscope are purely visual ; conclusions areobtained by comparing photographs of ion images.The reproduction of the photo-graphs in the present article has been restricted to those necessary for illustratingthe most important points.EXPERIMENTALFig. 1 shows the microscope tube. An outer Dewar, normally filled with liquid nitrogen,shields an inner Dewar, which can be filIed with a suitable coolant to adjust the tempera-ture of the tip. The tip assembly is mounted on tungsten rods, which are ledthrough the inner Dewar. The distance from the tip to the flat conductive fluorescentscreen (1 1 cm dim.) is about 9 cm.A polished stainless steel cylinder, closely fitting the lower end of the Dewar and keptat the same potential as the screen, serves two purposes. First, it prevents spurious electroA.A. HOLSCHER AND W. M. H. SACHTLER 31emission from the conductive coating during field desorption and helium-ion experiments.Secondly, it effectively helps in accommodating the helium atoms before they are ionizedat the tip surface, thus improving the intensity as well as the resolution of the ion image.External electric contact with the conductive coating is made via a Kovar tube, sealed witha glass bead.The vacuum system is schematically shown in Fig. 2. It consists of an ultra-high vacuumpart with an ion pump and a fore-vacuum line with an oil-diffusion pump backed by a two-stage rotary pump.The ultra-high vacuum part can be baked out to 400°C, all valvesexcept A1 being in open position. After the normal bake-out procedure and closing valveA2, the pressure was of the order of 10-10 torr.I/ f 1, I,L - - -- - - - - - - - - -FIG. 1 .-Field electronand ion microscope.FIG. 2.-Schematic representation of vacuum apparatus.T, working table ; V, 75 l./sec VacIon pump; 0, 10 l./sec oildiffusion pump ; R, 2-stage rotary pump ; M, microscope tube ;C , cold trap ; P, Penning gauge ; I, ionization gauge ; A 1, 2, 3,4, 5 , 6, bakeable u.h.v. valves Granville Phillips type C ; A 7,ditto type L; B, magnetically operated ball joint valve; G, gassupply ampoules or diffusion thimbles.‘The gases used in our studies were of an ultra-pure grade and stored in glass bulbs Gprovided with a break seal, except hydrogen, which was purified by diffusion through anickel thimble.In the field electron emission experiments, use was made of a highly stabilized 0-5 kVpower supply.The output voltage could be regulated in steps of 10 V, while a fine regulationprovided for intermediate values. As the accuracy of the dial setting was better than 0.2 %,no separate voltage measurements were carried out. Emission currents were determinedwith a vibrating reed electrometer with an accuracy of about 2 %. The high voltagesrequired for field desorption, field evaporation and ion microscopy came from a stabilized0-50 kV power supply and were measured with a precision potentiometer (accuracy aboutPictures of the electron and ion images were taken with a Nikon camera (1.4 opening)on Gevaert spectroscopic G film.To increase the speed and lower the contrast of thisfilm, we pre-exposed it just to the point such that after developing a slight fog became visible.Unless otherwise stated, we started all adsorption experiments with a clean tungstensurface, field-evaporated at 78°K. The process of field evaporation was checked by visualobservation of the electron image and measurement of the emission characteristic afterreversing the polarity of the tip with respect to the metal cylinder and the screen. Thesewere always kept at earth potential. When after prolonged field evaporation successivelyhigher emission voltages were required to give a constant emission current of 0.5 PA, it was1 %>32 F.E.M.A N D F.I.M. OF co ON wassumed that the field-evaporated end form had been obtained. Ion images of the surfacesprepared in this way invariably showed perfect order, which allowed us to omit imaging ofthe clean surface when this was less convenient.A desired amount of gas was dosed on to the metal tip by manipulating valves A4, A5and A6, respectively, the ball joint shutter and valve A7. Valve A3 was normally kept inopen position and closed only when the main system had to be exposed to air for replacementof the metal tip. Thus it could be arranged that A4, A5 and A6 never experienced atmo-spheric pressure once the break-seals of the gas storage bulbs were broken.Before introducing helium into the microscope tube, valve A7 was closed and the requiredimage-voltage applied to the tip to prevent any contaminant in the helium from beingadsorbed. All ion pictures were taken at a helium pressure of about 2 x 10-3 torr and mostlyat 78°K.Though lower tip temperatures, down to 20"K, would improve the intensity aswell as the resolution of the images, the image quality at 78°K was sufficient for our purposes.After taking pictures the main part of the helium was removed via the fore-vacuum line byopening valves A1 and A2. When a pressure of 10-5 torr was reached (in about 5 min),valve A1 was closed. Upon opening valve A7 the ion pump quickly restored a pressure ofless than 10-9 torr.In those experiments where the electron emission was to be measured after taking anion picture, the tip was kept at the high voltage required for image formation when pumpingoff the helium.Under these conditions a clean surface remains clean, adsorption beingprevented by the presence of the high field, which ionizes all the reactive gas moleculesapproaching the surface.When the pressure was again in the 10-9 torr range we quickly changed the polarity anddetermined the electron emission characteristic. The constancy of the emission current at agiven voltage was found to be a criterion that no re-adsorption took place either from thegas phase or by diffusion from the shanks of the tip.Field electron emission is described by the Fowler-Nordheim relation,' which in asimple approximate form can be written as I = A Wexp (- B#%/ V).( A and B being constants.)By comparing the slopes of the linear plots of loglo (IIV2) against l/V for the clean andcovered surfaces, the relative work functions of covered surfaces may be calculated. SuchFowler-Nordheim plots were determined in the current range of 10-8-10-~ A by varying thevoltage stepwise. The experimental data were processed on a computer to calculate slopesand values of loglo A . The accuracy of the results, although differing slightly in the variousexperiments, was always better than 0.4 % for the derived work functions and better than1 % for the values of loglo A. For the average work function of the clean surface we assumeda value of 4.50V. All the Fowler-Nordheim plots of gas-covered tips were taken afterreducing the gas pressure to 5 x 10-9 torr.RESULTS AND DISCUSSIONCARBON MONOXIDE ON TUNGSTENELECTRON MICROSCOPYWe have studied the adsorption states at 300 and 78°K.The results obtaindewith saturated surfaces are summarized in the following scheme (see top of page 33).The data are averaged from several experiments.(a) The adsorption state at78°K depends on the way it is obtained. Direct adsorption at 78°K (step 1) leads toa large increase in #, while adsorption at 300°K and subsequent saturation at 78°K(steps 3 and 4) give a much smaller increase in #. The same result can be obtainedby temporary heating of the 78"K(I) state to 300°K (steps 2 and 4).Thus, theattainment of the stable 78°K (10 state involves an activated process, which has theeffect of decreasing #,(b) The same work function and electron pattern result for the 300°K state, itbeing immaterial whether this state is reached directly (step 3) or via the 78°K (I) stateSome of these results need additional comment.Step 4 was found to be completely reversible (step 5)FIG. 3.-(a) Clean tungsten (field-evapor ated at 9.6 kV) ; image at 78°K and 8.05 kV ; (b) samesurface after CO adsorption at 78°K (partial coverage) ; image at 78°K and 8.05 kV.[To face page 32FIG. 4.-Tungsten surface after saturated CO adsorption at 78°K; image at 78°K and 7.20 kV.FIG. 5.-Tungsten surface after saturated CO adsorption at 300°K; image at 78°K and 9.6 kVFIG.6.-Tungsten surface after CO adsorption at 300°K (partial coverage) ; image at 78*K and9.2 kV.FIG. 7.-Tungsten surface after CO adsorption at 78°K (partial coverage) and heating to 280°Kfor 2 min ; image at 78°K and 9.7 kVFIG. %--Field desorption and field evaporation of surface layers after saturated CO adsorptionat 300°K ; for experimental data and conditions, see table 2A. A. HOLSCHER AND W. M. H. SACHTLERA 4 = 0.73 V3 Aloglo A == -1.4 1A# = -0.31 VA loglo A = -0.3 A 4 = -0.47 V4 v A loglo A = 0.033A 4 = 1-20VA loglo A = - 1.55(steps 1 and 2). This also holds for the stable 78°K (XI) state, which can be obtainedvia the 300°K state. Although there is a slight difference in the decrease in loglo Afor both routes, it seems that the 300°K state is not greatly influenced by pre-adsorp-tion at lower temperatures.(c) Adsorption at 300°K as well as at 78°K causes a large decrease in the pre-exponential factor A .In contrast to this, relatively small if any changes in A occurupon heating the 78°K (I) state to 300°K and redosing at 78°K.Although the 300°K state is independent of the way it was reached, it is not atrue equilibrium. We observed that the emission current at 300°K rose slowly withtime, even in the absence of gaseous CO. No current rise was noticed at 78°K.The rise could be accelerated by temporarily heating the tip to 330°K. Afterheating for 5 min, the work function (measured at 300°K) was dropped by 0.03 V,while loglo A had increased by 0.3.After redosing such a tip with CO at 78°K thework function returned to the 78°K (11) value, but loglo A was lower by 0.3 comparedwith the 78°K (11) value. From the temperature dependence of the current rise anactivation energy of 8 kcal was derived for this process.ION MICROSCOPY(a) ADSORPTION AT 78"K.-We have compared the ion images of a clean, ideallyordered surface with those obtained after adsorbing various amounts of CO at 78°K.In each of these experiments, ultra-high vacuum conditions were restored and thesurface subjected to field evaporation at 78°K before the desired amount of CO wasadmitted. The adsorption states were characterized by their F.-N. plot, the para-meters of which are given in table 1. Up to a surface coverage characterized byA 4 = 0.35 V not the slightest difference was observed between the ion images takenbefore and after adsorption of CO.This striking result proves that adsorbed CO molecules cannot be observed in theion microscope. Apparently, they have been removed from the surface by the highfield without disturbing the metal surface.This field desorption is probably pro-moted 12, 13 by the impact of helium atoms, which, due to polarization, acquire akinetic energy of about 0.2 eV.When with increasing coverage A# becomes equal to about 0-5 V, the first smallchanges of the ion picture occur. This is seen from fig. 3, where 3(a) is the image ofthe clean surface and 3(b) the one obtained after adsorption. The disordering of theoriginal array of bright spots is most pronounced in the regions between the (21 1) and(100) faces.The four atoms visible on the (100) face of the clean surface are absen34 F.E.M. AND F.I.M. OF co ON wfrom the ion image taken after adsorption. Closer inspection reveals that also inother regions atoms have disappeared and " holes " are left behind. At a coveragewhere A$ = 0.65 we observed a more severe disordering, still concentrated in theareas between (21 1) and (100).TABLE 1 .-FOWLER-NORDHEIM DATA AND OBSERVED CHANGES IN ION IMAGES AFTERADSORPTION OF INCREASING AMOUNTS OF co ON A FIELD-EVAPORATEDTUNGSTEN SURFACEchange in ion image comparedwith image of clean surface4-59 - 0-40 no change at all4.61 - 0.24 no change at all4.85 - 0.43 no change at all5.03 - 0.55 very slight disordering between (21 1) and(100) see fig.35.15 - 1.27 increased disordering, also around (1 1 1)5.70 - 1.52 generally spread disordering see fig. 4Upon saturation (A$ = 1.2 V) the disordering is more generally spread over thewhole surface, as shown in fig. 4. Similar images obtained from tips less sharp thanthe one of fig. 4 showed slightly greater effects of disorder, but the symmetry of thepattern always remained clearly visible.How are these changes to be interpreted? The new dots are not due to CQ mole-cules adsorbed on an undisturbed surface as such molecules would have been tornoff by the combined action of the high field and impinging helium atoms. There isno reason either to assume that field desorption is incomplete at high coverage.On the contrary, it is well known that the heat of adsorption decreases with coverage.We therefore propose identifying the changes with a disordering of the tungstensurface itself.The following model can then be set up to fit the experimentalobservations. With increasing coverage, metal atoms move out of their originallattice sites into more favourable positions surrounding adsorbed CO molecules.Imaging of such a re-arranged surface in the ion microscope will result in a completeor partial field desorption of the adsorbed complexes (including both CQ moleculesand tungsten atoms), leaving behind the remnants of a disordered surface structure.The re-arrangement, and thus the observed changes depending on the surface cover-age can be understood for two reasons.First, it is reasonable that the re-arrangedstructure is thermodynamically more stable only above a certain minimum coverage.Secondly, re-arrangement of surface atoms is an activated process. It is conceivablethat the activation energy decreases with increasing coverage owing to the weakeningof the intermetalIic bonds upon formation of the chemisorption bonds.4 Moreover,it is to be expected that the activation energy required for re-arrangement is differentfor the various crystal faces and smallest for the atomically rough areas, as e.g., (41 1)and (61 1). These are in fact the regions where the first changes are observed.A further verification of this interpretation is provided by experiments reportedbelow, (b) and (c).(b) ADSORPTION AT ca.30O0K.-Fig. 5 shows a surface which had been saturatedat 300°K ( A 4 x 0.73 V, A loglo A = - 1.4). The symmetric pattern of the cleansurface (not shown) has completely changed into a random array of dots all over thesurface. The disorder is far more intense than was observed after direct adsorptionat 78°K (fig. 4). An exactly similar pattern is obtained after adsorption at 78°Kfollowed by heating to 300°K and redosing at 78°K ( A 4 = 0.42 V ; A loglo A = - 1.7)A. A. HOLSCHER AND W. M. H. SACHTLER 35These experiments prove that the disordering as observed in the ion picture of fig. 5involves an irreversible activated process. This may be expected when metal atomsparticipating in a chemisorption layer have to re-arrange in order to achieve a morestable configuration.The occurrence of an activated process in going from the low-temperature state 78°K (I) to the 300°K state was already concluded from fieldemission experiments alone. The ion microscope studies have now revealed thenature of this process.Dosing experiments, similar to those reported under (a) showed that the disorderincreased with increasing coverage. Although a completely unchanged pattern wasnever obtained after adsorption at 300"K, the changes at low coverage were small.A more intense disordering, but still much less than at saturation, is shown in fig. 6,for a surface for which after adsorption at 300°K A 4 = 0.39 V and A log10 A = - 0.52.There is a marked parallelism between the degree of disorder and the decreasein loglo A .To illustrate this, a tungsten tip was dosed at 78°K up to a coveragewhich, at this temperature, involved little or no re-arrangement, but would havecaused severe disorder had dosing taken place at 300°K. The tip was then heated toabout 280°K for 2 min and a F.-N. plot recorded. The characteristic parameterswere A 4 = 0.51 V and A loglo A = -0.42 with respect to clean surface. Aftercooling to 78"K, the ion image showed that only very small changes had occurred(fig. 7). Apparently, the rate of the activated re-arrangement on a partially coveredsurface is low at 280°K.A comparison of the F.-N. data and the ion images of fig. 6 and 7 shows that themore pronounced changes of fig. 7 are accompanied by a larger decrease in loglo A ,but a smaller increase in work function.A surface saturated at 300"K, giving ahighly disordered ion-image, is characterized by a work function of 5.23 V, whichis only 0.2 V more than the value obtained in the 280°K case. However, the valueof A loglo A decreases from -0.4 to - 1.4. These results suggest that at a givencoverage re-arrangement is accompanied by lowering of both the work function andof log10 A .(C) FIELD DESORPTION AND FIELD EVAPORATION OF SURFACE LAYERS.-The resultspresented under (a) and (b) suggest that, under the conditions of image formation,the greater part of the adsorbed CO is removed from the surface by field desorption.To investigate this point in more detail and to clarify the nature of the surface layersleft behind, we carried out experiments in which ion pictures were taken of successivelyadvanced stages of field desorption and field evaporation after a saturated COadsorption at 300°K. After taking an ion picture we reversed the polarity of themetal tip and determined a Fowler-Nordheim plot.Table 2 summarizes the experi-mental data and conditions. The corresponding ion images of a representative partof the surface are shown in fig. 8. All the ion pictures were taken under exactly thesame conditions. All A 4 and A loglo A values are referred to the clean surface afterfield evaporation at 78°K in the presence of helium. This circumstance might beresponsible for the relatively low work function increment after CO adsorption.Examination of the F.-N.data and ion pictures reveals many remarkable facts.In the successive stages of field desorption the work function passes first through aminimum value of 4.50 V and then through a maximum (see later).It is clear that the mere application of the required image voltage of 7-8 kV removesa major part of the chemisorbed layer and that the field desorption effect is markedlyenhanced by the presence of helium. After field desorption at 8.4 kV the surface isregarded as clean, as from this stage the values of 4 and loglo A are no longer signifi-cantly different from those of the initially clean surface36 F.E.M. AND F.I.M. OF co ON wThe ion image of fig. 8(b) represents the characteristic disordered pattern whichis always obtained after adsorption at 300°K.Relatively small changes have occurredafter field desorption at 8-2 kV (8(c)). Field desorption at 8.4 kV (8(d)) involvesmore pronounced changes, particularly visible in the (1 1 1) region. Although accord-ing to the F.-N. data the surface is now completely clean, the arrangement of brightspots, showing strong differences in intensity, is still disordered. We also observerelatively large dark areas which are not imaged at all, in contrast with the same areasTABLE 2.-EXPERIMENTAL DATA OF FIELD DESORPTION AND FIELD EVAPORATIONOF SURFACE LAYERS AFTER SATURATED CO ADSORPTION AT 300°Kpicture A loglo A = conditions after whichnumber * ' log10 -4co,e,d-h310 dclean F-N plot and/or picture8a8b8c8dSe8f8s8h8i4.504.954.504-614.624.534.564.5 14.550- 1-52- 0.66+0*17 + 0.21- 0.03 + 0.090.00+ 0.07field evap.at 8-8 kV, 78°K with HeCO adsorption at 300°Ktip 5 min at 7.8 kV without Heafter taking 8btip 1 min at 8-2 kV with Hetip 1 min at 8.4 kV with Hetip 1 rnin at 8.6 kV with Hetip 1 rnin at 8-7 kV with Hetip 1 min at 8.8 kV with Hetip 1 rnin at 8.9 kV with Hetip 1 min at 9.0 kV with He* Ion pictures taken at 2 x 10-3 torr He, 78"K, 7.8 kV, and 15 min exposure time.on photograph 8(a) of the clean surface. An interesting phenomenon can be noted bycomparing 8(d) with 8(b). The latter photograph shows in some regions a slightfuzziness, which is absent from 8(d). As the experimental conditions for all photo-graphs were the same, the observed effect is significant ; it may be attributed to smallamounts of chemisorbed CO, which in view of the F.-N.data were still present on thesurface depicted in 8(b), but not on the surface of 8(d).Photographs S(e), (f), (g), (h) and (i) show the effect of field evaporation byapplying successively higher voltages : surface atom layers are torn off, the disordergradually decreases and atoms " fill up " the dark areas. After about four layers havebeen removed, a completely ordered surface is again exposed. The number of layerstorn off can most easily be determined by comparing the structure around the (211)and (1 11) faces on the various photographs.That the removal of surface layers can occur already below the voltage initiallyrequired to give the field evaporated end state shown in fig.8(u) can be explained asfollows. When, during the formation of the chemisorption layer, metal atoms moveout of their original lattice positions, vacancies are left behind. By diffusion processes,these vacancies are partly distributed among deeper lying atom layers until anequilibrium is established. As a consequence, the surface which becomes exposedafter the chemisorption " crust " is field-desorbed, shows a " hill and valley " structureon an atomic scale, the valleys being either single vacancies or vacancy clusters. Anion image of such a surface reveals only (part of) the atoms on the hills, because abovethe valleys the ionization probability for the helium atoms is comparatively small.Field desorption from such a disordered structure proceeds more easily, i.e., at alower applied voltage, than from a completely smooth surface.The attack commencesat the relatively protruding atoms and edges where the field strength is highest andcontinues until the increase in the (local) radius of curvature causes the field strengthto drop below the value required for desorptionA. A. HOLSCHER AND W. M. H. SACHTLER 37Eventually atomlayers become exposed which have no more vacancies than are normally observedand the surface is again ideally ordered.From these experiments we conclude that : (i) the disorder observed in the ionimages obtained after CO adsorption cannot be attributed merely to adsorbed COmolecules. Even after CO has been removed from the surface the disorder persistsand should therefore be identified with disurbances in the initially perfect arrangementof the metals atoms; (ii) there is evidence that chemisorbed CO molecules, only avery small part of which can escape field desorption during image formation, lowerthe image contrast instead of showing up as bright spots ; (iii) the adsorption-inducedre-arrangement of metal atoms strongly influences deeper lying atom layers by in-jecting vacancies into these layers.We think that these conclusions and those obtained from the experiments under (b)confirm the correctness of the interpretation put forward at an earlier stage to explainthe phenomenon observed in the ion microscope after adsorption of CO on field-evaporated tungsten surfaces.At consecutively higher voltages the whole process is repeated.DISCUSSIONIn the foregoing section we have given a description of the adsorption of CO ontungsten which satisfactorily explained the results of both ion microscope and fieldemission experiments.It implied a re-arrangement of surface metal atoms, whichwas dependent on coverage and, being an activated process, was far more pronouncedat 300°K than at 78°K.We now complete this description with further details, including a discussion ofthe changes in 6 and loglo A , and arrive at the following picture of the adsorption ofCO on tungsten. Adsorption at 78°K up to a fairly high coverage leads to a chemi-sorption layer uniformly spread over the surface, while no re-arrangement of substrateatoms occurs.The dipole moments of the surface bonds have their negative endpointing outwards, which results in a high work function increment. Upon formationof the chemisorption bonds the intermetallic bonds are weakened and this eventuallyallows a restricted surface re-arrangement to take place, particularly on the lessdensely packed crystal faces. The re-arrangement causes a tighter bonding of a partof the adsorbed CO molecules by embedding them in a two-dimensional tungstencarbonyl structure. New adsorption sites are thus created, which we visualize asbeing located on top of tungsten atoms that have penetrated through the adsorbedCO layer. CO molecules adsorbed on these sites are less strongly bonded thanthose in the carbonyl layer, the latter interacting with more than one tungsten atom.Upon saturation, an additional very weakly bonded layer may be formed on top of thechemisorbed layer.In assessing the effect of this complex adsorption process upon the work function,it is conceivable that any re-arrangement in the chemisorbed layer has the effect ofdecreasing the effective dipole moment vector perpendicular to the surface by intro-ducing components which point with the negative pole parallel to the surface or eveninwards.However, the rearrangement at 78°K takes place to a very limited extentonly. Therefore the work function is decreased only slightly, as compared to thehypothetical case when no re-arrangement takes place. The physisorbed layer mostprobably lowers the work function.Upon heating the 78°K state to 300°K drastic changes occur.First, the physi-sorbed layer is desorbed. If no other processes took place the work function wouldincrease. However, instead, there is a considerable decrease in work function o38 F.E.M. AND F.1.M. OF co ON wabout 0.5 eV, which now, from the ion microscope results, can be correlated with astrongly increased re-arrangement in the surface layer. The degree of this re-arrangement and the ultimate surface structure is probably different for the variouscrystal faces. In some areas it presumably takes place to an extent which justifiesthe term “ surface corrosion ”, i.e., CQ molecules are completely incorporated in thesurface structure.By analogy to what is observed in the formation of oxide layers, we assume thatthe corrosion layer is mainly formed by metal atoms diffusing outwards. Such asupposition is strongly sustained by the field evaporation experiments which provethe existence of vacancies underneath the chemisorption layer. As CO moleculesin this layer are more strongly bonded than those at the surface, different statesbecome distinguishable with respect to the energy required for desorption.Moreover,the states become discernible with respect to the surface potentials, as re-arrangementschange the component normal to the surface of the bond dipoles.After peeling off one or more surface layers, the work function drops to thevalue of the clean surface, although there is still some CO present.This can beroughly explained by visualizing that the surface now contains tungsten atoms withCO molecules underneath. Further field desorption reverses the situation again ;this results in an increase in work function.Upon re-dosing the 300°K state at 78°K a weakly bonded adsorption layer is formedon top of the corroded surface, which is accompanied by a further decrease in workfunction.The critical coverage where re-arrangement sets in, is much lower at 300°K thanat 78°K. This result proves that the critical coverage is determined by the pre-requisite that metal-metal bonds must be sufficiently weakened by chemisorptionsuch that the activation energy for re-arrangement becomes sufficiently low.Adsorption both at 78°K and at 300°K greatly decreases the pre-exponentialfactor A of the Fowler-Nordheim relation.Such a decrease has also been foundwith other adsorption systems; it is certainly not simply caused by a reduction ofthe emitting surface. Two other explanations have been put forward in the literature,both being concerned with the large decrease in loglo A at low temperature. In thefirst model, adopted by Menzel and Gomer 14 for CO on tungsten, the high electro-static field polarizes the adlayer, inducing an extra work function increment, whichto a first approximation does not alter the slope of the F.-N. plot, but decreasesloglo A. In the second model proposed by van Oostrom 15 for nitrogen on tungsten,the highly polarizable adlayer modifies the shape of the potential barrier throughwhich the electrons tunnel.One of the models, or a combination of both, might explain the observed changesin loglo A upon direct CO adsorption at 78°K.Neither model, however, accounts forthe decrease observed at temperatures where no weakly bonded polarizable layer canbe present. The relevant mechanism is in this case related to the rearrangement ofthe surface at these temperatures.We tentatively propose the following qualitative picture. The corroded layer andthe induced vacancies are expected strongly to disturb the periodic potential at thesurface. As a consequence, the number of electrons arriving at the potential barrier,i.e. the supply function, may be drastically decreased by an increased scattering inthe surface layers.16 If, in the small range of electron energies relevant in fieldemission, the scattering is assumed to be independent of the electron energy, it hasthe same effect as a decrease in emitting area, i.e., in loglo A.A weakly bonded layer on top of a corroded chemisorbed layer, formed at lowtemperature, gives rise to a relatively small change in loglo A .In terms of Corner’A. A. HOLSCHER AND W. M. H. SACHTLER 39model, CO molecules in such a layer are apparently less polarized by the field thanthose on top of what may be called a chemi-adsorption layer.Our view on the different types of adsorption and the specific changes in Cp andloglo A they induce, can be summarized in the following scheme.weak adsorption physisorptionA#<O 1 AlogloA$Ocorrosive - chemi-adsorptionchemisorp tion4A loglo A < 0 1 A#>OA loglo A $0clean cleanThe slow rise in emission current subsequent to CO chemisorption at 300”K,which rise has an apparent activation energy of 8 kcal, might tentatively be ascribed toan ordering of either the corrosion layer or of the disturbed tungsten layers beneath it.We may now review the bearing of our results on those obtained previously byothers.CO on tungsten was studied by many authors with a variety of techniques.For a survey of this work we refer to articles by Swanson and Gomer,l7 and byEhrlich.18 In general our experimental observations agree well with previous work.However, the preser,t combination of field electron and ion microscope studies hasresulted in a new interpretation of the phenomena observed.The new picture is atvariance with one of the conclusions by Ehrlich,los 11 who interpreted the disorderlyarranged bright spots in the ion images after CO adsorption as individual moleculeswhich had remained unaffected by the severe conditions of ion image formation.He concluded that the tungsten surface itself had been changed only negligibly.The same difference in interpretation holds for the adsorption of nitrogen on tungstenwhich we have also investigated.Our experimental results, although obtained on field-evaporated tungsten tips,are in fzir agreement with those of Gomer 7 9 1 7 s 19 et al., if we identify virgin layers,/?-states and cc-states of their terminology with what we have called the chemi-adsorptionlayer, CO molecules incorporated in corrosive chemisorption layers, and the moleculesmore weakly adsorbed on sites created on top of the corroded layers.The rearrangement Gomer proposes is “ a flipping of CO into a more tightlybound configuration locally ”.However, it seems unlikely that the potential barrierbetween the unstable and stable sites (in this model at a distance of about 2 A fromeach other) is so high that a molecule hitting the surface on an “ unstable” sitecannot immediately “ flip over ”, upon making an initially highly activated surfacebond.From their electron impact desorption experiments Menzel and Gomer 20 con-cluded that a thermal conversion of virgin states to p- and cr-states (the latter occurin their and in our model only when there is p-CO) starts at about 270”K, in completeagreement with our findings. A careful examination of other results derived withthe electron impact technique reveals that most conclusions fit well in the modelproposed in the present work40 F.E.M.AND F.I.M. OF co ON wH2CARBON MONOXIDE ON TUNGSTEN PRECOVERED WITH HYDROGENAfter saturation with hydrogen both at 78 and 300°K on a field-evaporated tung-sten tip, the ion image had not changed compared to that of the clean surface. Thus,hydrogen does not induce any re-arrangement of tungsten atoms. If, however, ahydrogen-covered tip was exposed to 10-6 torr CO for about 5 min both at 78 and300°K we obtained the same type of pattern as was found for CO adsorption onclean tungsten at these respective temperatures (see fig.4 and 5). Apparently COcan displace adsorbed hydrogen, and subsequently induces a re-arrangement oftungsten atoms. The field emission results are summarized in the following scheme.A# = 0.48VA loglo A = -0.7field-evaporated at 78°K# = 4.50, loglo A x -5.5clean clean300°K 78°K4300°K 4 = 4-98 V€32 A$ = 0.80VA loglo A = -0.4 ! 78°K 4 = 5-30VAt 78°K CO adsorption lowers the work function of a hydrogen-covered tip.This can be understood by assuming that CO is mainly present in a physisorbedlayer, while the major part of the strongly bonded chemi-adsorbed hydrogen layeris not removed. In complete agreement with this assumption is the large drop inloglo A , characteristic of a higher polarizable layer.Upon heating to 300°K the workfunction and loglo A further decrease. Presumably, the greater part of the hydrogenis now replaced by CO and a corrosive chemisorption layer is formed. The samestate can be achieved by direct CO adsorption at 300°K on a precovered hydrogen tip.Displacement of hydrogen by CO is accompanied by a relatively small increase inwork function and a large decrease in loglo A , this being again in agreement withthe conversion of a chemi-adsorbed hydrogen layer into a carbonylic corrosion layer.The 300°K state of the mixed adsorption is characterized by a slightly lower workfunction than that of the pure CO adsorption state at 300°K. This is most probablydue to hydrogen still present in the corrosion layer.After adsorption at 300°K orheating the 78°K state to 300°K the same steady increase in current was observed asafter CO adsorption on clean tungsten. After intermediate heating to 330"K, thework function had changed little (A# = - 0.03 V), but loglo A had increased markedly(A loglo A = 0.35). The activation energy of the process was found to be about12 kcalA. A. HOLSCHER AND W. M. H. SACHTLER 41Hydrogen admitted to a tip which was precovered with CO and temporarilyheated to 330"K, caused no further change in work function and loglo A. However,when intermediate heating to 330°K was omitted, a decrease in both work function( - 0.3 V) and loglo A (- 0-3) was observed. Heating to 330°K restored the originalvalues of 4 and loglo A characteristic of the pure CO chemisorption layer.This behaviour qualitatively resembles the results found by Siddiqi and Tompkins,21who studied the adsorption of hydrogen on nickel precovered with CO at 78°K bymeans of a contact-potential technique.They observed a large reversible decreasein work function caused by the adsorption of hydrogen on the CO-covered surface.CONCLUSIONSThe field emission and field ion experiments reported in the present work haveallowed us to set up a detailed picture of the adsorption of CO on clean tungstensurfaces and on tungsten surfaces precovered with hydrogen. The most remarkablefeature of the proposed model is a re-arrangement of substrate atoms, which evenoccurs, although to a slight extent, at very low temperatures.We now formulatetentatively some conclusions which may be of more general importance, in thatthey have implications for other adsorption systems as well. Also, the possibilitiesof the electron emission and ion microscope techniques deserve some further comment.(i) The formation of chemisorption bonds involves a weakening of the intermetallicbonds in the metal surface. The degree of weakening increases with the coverageand with the strength of the chemisorption bonds formed.(ii) Owing to weakening of intermetallic bonds the activation energy required formetal atom re-arrangement of a covered surface can be much less than for the cleansurface. The stronger the chemisorption bond and the smaller the cohesive energyof the metal, the more readily surface re-arrangement takes place.Since the strengthof chemisorption bonds goes parallel with the bond strength in the corresponding bulkcompounds,4 thermodynamic data of bulk phases can give a rough indication whetherand, if so, how easily surface re-arrangement will occur.(iii) The possibility of surface re-arrangement even at low temperatures and onmetals with a high melting point, should be taken into account in any interpretationof chemisorption phenomena and of composite layers in particular. It can no longerbe assumed d priuri that the surface consists of an assembly of adsorption sites ofwhich the configuration is not altered upon chemisorption. Chemi-adsorption maybe followed by the formation of corrosive chemisorption layers already at low cover-ages. Molecules or atoms incorporated in the corrosion layers will be more stronglybonded than those on top of these layers. Also, it is to be expected that the newsurface structures will be different for the various crystal faces, this giving a furtherdifferentiation in the " adsorbed states " on one and the same metal.(iv) From an experimental point of view the present work justifies the conclusionthat the ion-microscope technique offers many possibilities for studying chemisorption,particularly when applied in combination with field electron emission techniques.Ion microscopy can provide, at least in part, the atomic detail which is needed for abetter understanding of chemisorption phenomena. Apart from the fact that only" frozen-in " states can be observed, this work clearly shows that a severe limitationis set by the phenomenon of promoted field-desorption, which prohibits theobservation of the chemisorbed species themselves. Possibly the application ofother image gases, such as neon or argon, which give images at lower electric fieldsthan helium does, might prevent field desorption during imaging42 F.E.M. AND F.I.M. OF co ON w1 Ogurj, J . Physic. Soc. Japan, 1964, 19, 83.2 I-Iolscher, J. Chem Physics, 1964, 41, 579.5 Delchar and Ehrlich, J. Chem. Physics, 1965, 42, 2686.5 Lander, Surface Sci., 1964, I, 125.6 Farnsworth, Adv. Catalysis. 1964, 15, 31.Slachtler and Van Reyen, J. Res. Inst. Cat., 1962, 10, 87.Gomer, Field Emission and FieM Ionization (Harvard University Press, Cambridge, Massa-chusetts, 1961).8 George and Steer, J. Chem. Physics, 1962, 37, 1935.9 Ehrlich and Hudda, J. Chem. Physics, 1962, 36, 3233.10 Ehrlich, Trans. N. Y. Acad. Sci., 1963, 101, 722.11 Ehrlich, Adv. Catalysis, 1961, 14, 255.12 Nishikawa and Miiller, J. Appl. Physics, 1964, 35, 2806.13 Ehrlich and Hudda, Phil. Mag., 1963, 8, 1587.14 Menzel and Gonier, J. Chem. Physics, 1964, 41, 33 1 1 .15 van Oostrom, Thesis (Amsterdam, 1965).16Toya, J. Res. Inst. Cat., 1962, 10, 236.17 Swanson and Gomer, J. Chem. Physics, 1963,39, 2813.18 Ehrlich, Proc. 3rd Znt. Congr. Catalysis (North-Holland Publ. Co., Amsterdam, 1964), vol. 1,19 Gorner, J. Chem. Physics, 1958, 28, 168.20 Menzel and Gomer, J. Chem. Physics, 1964,41, 3329.21 Siddiqi and Tompkins, Actes du 2 i h e conqrG.7 international de Catalyse (Editions Technip,p. 113.Paris, 1961), part 2, p. 1767

ISSN:0366-9033    

DOI:10.1039/DF9664100029

出版商:RSC    

年代:1966

数据来源: RSC

摘要:

Field-emission Studies of the Adsorption of Chlorine and theDissociation of Carbon-Chlorine Compounds on TungstenBY M. 3. DUELL, B. J. DAVIS AND R. L. MossWarren Spring Laboratory, Stevenage, Herts.Received 17th January, 1966The adsorption/desorption of chlorine and the dissociation of carbon tetrachloride and chloro-form on tungsten were studied in a field-emission microscope observing the pattern variations andchanges in work function derived from the Fowler-Nordheim equation. Chlorine from the gas-phase and from dissociated carbon-chlorine compounds gave rise to positive surface potentials atlow coverages (oxygen was examined for comparison), accompanied by a reduction in emittingarea. This was ascribed to chlorine penetration below the metal surface ; final surface potentialswere about - 1 eV.The adsorption of methane and its attack on a chlorine-covered surface werealso examined. Under a continuously applied field tungsten whiskers were formed in carbontetrachloride or chloroform vapour, believed to arise from the migration of surface complexesinvolving both carbon and chlorine.Previous field-emission studies of the adsorption of halogen gases on tungstelr haveshown that while iodine 1, 2 is sensitive to surface topography and produces a smallincrease in work function, chlorine 3 and bromine 2 increase it by 1 eV or more,at constant coverage. At low bromine coverages, the work function decreases toa minimum accompanied by a reduction in emitting area, which was explained interms of the partial burial of the adatom in the metal surface as a result of the migra-tion of tungsten atoms.A corresponding study of chlorine adsorption is reportedhere, with emphasis on observations at low coverages together with some resultson oxygen, in order to compare their respective corrosive effect on the tungsten sur-face. Also, as background to work on carbon-chlorine compounds, methane wasexamined ; carbon has little effect on the work function 4 compared with chlorine,although carbon patterns 5 on heating are particularly characteristic. The field-emission technique could then be applied to study the dissociation of carbon-chlorine compounds, i.e., carbon tetrachloride and chloroform, on a clean tungstensurface with respect to both the adsorbed species formed and their action on themetal surface itself.The reaction between adsorbed chlorine and gas-phasemethane was also examined.EXPERIMENTALThe techniques used in field-emission microscopy have been described 6.7 and alsoexperimental details particular to the present work, including an indication of the pre-cautions necessary to obtain a sufficiently pure supply of the gas being studied.2 In experi-ments where the field-emission tip was flashed before admitting the gas ((ii), below), specialcare was taken to flush the metal valve, through which gases were admitted to the field-emission tube, to avoid desorbing unwanted gases at the start of the experiment. Thesame electrical arrangements 2 were used to measure the applied voltages and field-emissioncurrents from which Fowler-Nordheim plots were derived.Plots corresponding to theearly stages of adsorption moved rapidly in the direction of increasing voltage for constantemission current in some cases (e.g., fig. 3 for chlorine). Since individual voltage-currentcharacteristics required at least 30sec to record, work functions derived from the slopes444 FIELD-EMISSION OF CI2 A N D C-CI COMPOUNDSof such Fowler-Nordheim plots would be in error. Accordingly the actual time was re-corded at which each single voltage-current observation was made, adjusting the currentin turn to one value in a set of chosen values. From these results, sets of interpolationplots were constructed of applied voltage against time at constant emission current, andhence Fowler-Nordheim plots obtained from current-voltage values referred to the sameelapse of time from the start of the experiment.RESULTSThe basic observations, i.e., field-emission patterns and voltage-current data,were made in various ways : either (i) the gaseous compound under study was passedcontinuously through the field-emission tube before starting the experiment byflashing the tungsten tip at 2500°K; or (ii) since the early stages of interaction withthe clean tip, particularly of interest, might be affected by adsorption during theshort time required to cool to room temperature, the tip was flashed in ultra-highvacuum, before admitting the gas.Also the field was applied: either (iii) inter-mittently, but nevertheless for a substantial fraction of the total time to derivevoltage-current data, as described above ; separate experiments were made to checkfor the effect of applied field; or (iv) continuously in the presence of chloroform orcarbon tetrachloride vapour which produced unusual field-emission patterns andvoltage-current data.The Fowler-Nordheim equation 6 describes the relationship between the currentdensity j (total current Ilemitting area a) in A/cm2 and the applied field F thus :1.54 x 10-6P2 4%- 4t'CYI F j = exp -6.83 x 107--v[y],where the functions t[vJ and o l y ] have values given in ref.(6). The field F is givenapproximately bywhere Y is the applied voltage, r is the tip radius and K-0-2.7 A plot of log (J/V2)against l / V will be approximately linear with a slope M given bywhere s b ] lies between 0.833 and 1.000.The value of (r/K) for clean tungsten wasobtained from the Fowler-Nordheirn plot, taking the accepted work function,4 = 4.52 eV, and making a series of successive approximations in eqn. (3), startingwith s[y] = 1. Assuming ( r / K ) to remain constant, the value of the work function,after adsorbing a particular gas for a known time was also calculated, Usingeqn. (I) the quantity log A given byF x KVfr, (2)M = - 2.96 x 1074t(,lK)~[y], (3)1 * 5 4 ~ 1 0 - ~ Klog A = log [ ($1 4t"YI(4)was found from the magnitude of log (I/V2) at the mid-value of (1/V) in the Fowler-Nordheim plot.In the following sections variations in field-emission pattern (at constant emissioncurrent) and changes in work function and log A are reported for chlorine, carbon-chlorine compounds, and, for purposes of comparison as discussed later, oxygenand methane.CHLORINEFig.1 (a)-(c) shows some patterns observed, starting with clean tungsten whenchlorine was adsorbed, initially at 5 x 10-9 torr ; the pressure was then reduced anM. J. DWELL, B. 3. DAVIS AND R. L. MOSS 45the chlorine desorbed, (d)-(f), by heating the tip at a series of temperatures. Thetypical granular appearance of the adsorption patterns was evident after 3 min,and after 9 min the pattern continued unchanged up to the longest time for whichchlorine was adsorbed (36 min). The patterns were photographed at constant emis-sion current and the applied voltage had to be increased to produce this currentthroughout the adsorption period.However, since the dark (1 12) planes (cf. fig. 2)remain a constant size, the decrease in the dark area of the central (011) and corres-ponding " corner " planes in fig. l(c) is probably a real effect. The work functionwas approximately constant between 20 and 36 min at 56-57 eV in a final pressureof 2 x 10-9 torr and gave an indication (4 = 5-26 eV) of desorption on heating for1 rnin at 1170"K, fig. l(d). The work function decreased on further heating (at2100°K for 30 sec, q5 = 443 eV ; fig. l(e)) to a minimum at 2200°K (4 = 3-86 eV;fig. l(f)). Flashing at 2400°K produced an apparently clean tungsten surface but4 at 4.36 eV was still below the expected value, 4.52 eV.FIG.Z.-Disposition of the p h e s exposed on tungsten tip with (011) plane central, based onstandard stereographic projection.Fig. 3 shows Fowler-Nordheim plots observed for chlorine adsorption at1.5 x 10-9 torr, derived from interpolation plots as described above for times up tot = 250 sec, after which the normal method of taking a voltage-current character-istic could be used. Although the plots change position rapidly in the early stagesof adsorption, nevertheless satisfactorily linear plots were obtained. Values oflog A are recorded in table 1 and work function values calculated from these plotsare shown in fig. 4 (open circles). A " spot " value was obtained, without using theinterpolation procedure, by admitting chlorine at 1.5 x 10-9, then closing the gasinlet valve immediately (half-filled circle). At t = 30 min, the work function wasTABLE 1 .-Loglo A VALUES FOR CHLORINE ADSORPTIONtime (min) 0 0.8 1.7 2-5 3.3 4.2time (min) 7 10 15 20 25 30h 9 0 A -7.98 -8.33 -8.65 -8.98 -9.00 -8.51log10 A -88.27 -8.22 -8.13 -8.12 -8.00 -77.946 FIELD-EMISSION OF Cl;! AND c - c l COMPOUNDS5.44eV; on raising the pressure tow5 x 10-8, holding at 2 x 10-8 and finally re-ducing to 1 x 10-9 ton, 4 = 5.40 eV was found.In another adsorption experi-ment the field was only applied briefly to photograph the pattern to confirm thatoperating according to (iii) above had not increased field effects above the essentialminimum.Ih 2 G a b -&- ,W14.0-I-I2-5 3.0 3.5 4.01041 vshown.FIG. 3.-Fowler-Nordheim plots for chlorine adsorption at 1.5 x 10-9 torr after elapse of timesi? L5 .0 0 rI i4.75 c iI~ _ ~ ~ L10 15time (min)FIG. 4.Variation of work function with time (lower scale) during chlorine adsorption at roomtemperature : 0, calculated from plots in fig. 3 ; (3, chlorine " dose ", cf. text ; a, effect of heatat low coverage ; 0, removal of adsorbed chlorine with methane, upper time scale.A characteristic feature of the change in work function during chlorine adsorptionwas the initial decrease at low coverages, and this was examined further in thefollowing experiments. Chlorine at 1 x 10-9 torr was admitted to the microscopM. J. DUELL, B. J. DAVIS AND R. L. MOSS 47and the tip Sashed, the inlet valve was then fully closed and also the chlorine wasremoved from behind the valve.In the falling chlorine pressure, the work functionwent slowly through the minimum (fig. 4, filled circles) to 4-64 eV. The temper-ature of the tip was then increased at intervals of -70" and at 1320"K, 4 fell backto 4.39 eV, again less than the clean tungsten value. Chlorine was also adsorbedaccording to procedure (ii) above, i.e., the tip was flashed in - 10-10 torr background,allowed to cool for a short time, voltage-current data recorded and then 1 x 10-9 torrchlorine admitted. Work function values observed in the early stages were 4-52,4.50, 4-54, 4-59, 4.62, 4.62 eV for t = 0, 50, 100, 150, 200,250 sec respectively.The activity of hydrogen and methane towards the chlorine-covered tip was alsoexamined.As a result of experiments described above, it was thought that anychanges observed at 920°K or less could reasonably be ascribed to the hydrogenor methane rather than desorption with temperature. Chlorine was adsorbed for60 min (# = 5.79 eV), and 2 x 10-8 torr hydrogen admitted to the field-emissiontube and the temperature raised in stages to 920°K with little effect. A more positiveresult was obtained using methane at 6 x 10-8 torr at room temperature whichsteadily reduced the work function of a chlorine-covered tip from 5-46 to 4-86 eVover 50 min (fig. 4). Field-emission patterns observed during the removal of ad-sorbed chlorine with inethane are shown in fig. 1 (g)-(j).METHANE AND OXYGENShort studies were made of methane and oxygen for comparison with the resultsreported above for chlorine and carbon-chlorine compounds.With methane, itwas of interest to examine how far the patterns and work function changes ob-served with chloroform and carbon tetrachloride might be attributed to carbonresidues. The occurrence of a minimum in work function and other evidence forthe corrosion processes which are believed to occur when chlorine is present, weresought using oxygen; such effects were expected to be absent during methaneadsorption.Fig. 5 shows that a small increase in work function occurred, without a minimumat low coverages, when methane was adsorbed at 2 x 10-8 torr ; at t = 20 and 50 minthe patterns shown in fig. I@)-(Z) were observed. At a pressure of 5 x 10-9 torrmethane, # was 4.52, 4-57, 4.60, 4-63 and 4.67 eV after 0, 95, 260, 470 and 650 sec.In contrast, with oxygen at 4 x 10-9 torr, there was an initial minimum, followedby a sharp rise in work function (fig.5) ; in a lower oxygen pressure, 1 x 10-9 torr,the observed minimum was 3.97 eV. Fowler-Nordheim plots are not reproducedbut values of log R are summarized in table 2.TABLE 2.-LOglo A VALUES FOR METHANE AND OXYGEN ADSORPTIONtime (min) 0 0.8 1.7 2.5 5 10 15 20 25oxygen -8-30 -8-91 -9.41 -7-99 -8.62 -8.79 -8.84 -8.74 -8.82methane - 8-61 -8.24 -8.18 -8.21 -8.36 -8.53 -8.55CARBON TETRACHLORIDE AND CHLOROFORMCarbon tetrachloride at a pressure of 3 x 10-9 torr was passed continuouslythrough the field-emission tube, the tip flashed and the field applied intermittentlyto obtain the patterns shown in fig.6(a)-(d) (and also to record voltage-current data).At 2 min, the (1 11) planes appeared relatively bright and there was also emissionfrom the (001) and (010) positions (cf. fig. 2). The bright granular band whichdeveloped around the central (011) plane after 5 min, linked to diamond-shape48 FIELD-EMISSION OF Clz AND C-CI COMPOUNDSbright areas centred on the (001) planes, fig. 6(c), is associated with adsorbed chlorine.This pattern then evolved gradually into fig. 6(d) as the diamond-shaped areasdarkened ; at 8.5 min the circular dark areas centred on (001) and (010) thus producedhad a reasonably clear outline. After 30 min the pattern was essentially similarbut more granular in appearance.0 5 10 15 20 25time (min)during methane adsorption at 2 x 10-8 torr, both at room temperature.FIG. 5.-Variation of work function with time: 0, during oxygen adsorption at 4 X 10-9; 0,The tip was then heated after removing carbon tetrachloride vapour from thefield-emission tube.With increasing temperature up to 1050"K, the circular darkareas became brighter and heating at 1170°K for I min (fig. 6(e), q5 = 5.35 eV),restored the bright diamond-shaped areas observed during the adsorption sequenceat 5 min. Further changes in pattern occurred at 1370 and 1465°K with a reduc-tion in the work function to 4-65 and 4.58 eV respectively, but the tip was stillpartly covered with adsorbed residues (fig. 6 (f)-(g)), apparently completely removedby heating for 1 min at 1860°K.The work function values corresponding to the above adsorption sequence areshown in fig.7 (open upright triangles); the interpolation procedure was not usedand the exact depth of the minimum, but not the later values, is uncertain. A furtherexperiment is also shown (open inverted triangles), where the inlet valve was closedafter admitting 1.5 x 10-9 torr carbon tetrachloride vapour to the continuouslyevacuated field-emission tube and the tip flashed. The slower adsorption shouldenhance the accuracy of the data in the region where the minimum occurs, but alsoincreases the possibility of contamination by residual gases. Fig. 8 shows Fowler-Nordheim plots obtained with a reasonably large carbon tetrachloride pressure(final pressure 5 x 10-9 torr), admitted to a cold tip, but the interpolation procedurewas used to increase the accuracy.The occurrence of the minimum is apparentfrom the slope of the plot for t = 50 sec; the work function values derived fromthese plots are also shown (filled triangles) in fig. 7 and log A values in table 3.TABLE 3.-Loglo A VALUES FOR CARBON TETRACHLORIDE ADSORPTIONtime (min) 0 0.8 1.7 2.5 3.3 4.2 5 6.7log10 A -8.68 -10.67 -9.56 -8.99 -9.00 -8.68 -99.00 -8.8FIG. 1 .-Field-emission patterns for chlorine adsorption/desorption : (a) clean tungsten ; (b) ad-sorption in 5 x 10-9 torr chlorine for 3 rnin ; (c) after 9 rnin ; ( d ) chlorine desorbed by heating for1 min at 1170°K ; (e) for 30 sec at 2100°K ; (f) for 30 sec at 2200°K.Reaction of adsorbed chlorinewith methane at room temperature : (9) chlorine covered tip ; (h) after 7.5 rnin in methane ; (i)after 24 min ; (j) after 40 min. Methane adsorption at 2 x 10-8 torr on clean tungsten : (k) after[To face page 48.20 rnin ; ( I ) after 50 minFIG. 6.-Field-emission patterns for carbon tetrachloride adsorption/desorption : (a) clean tungsten ;(b) adsorption at 3 x 10-9 torr for 2 min ; (c) after 5 rnin ; ( d ) after 8.5 min ; ( e ) after heating at1170°K for 1 min in vacuum ; (f) at 1370°K for 1 rnin ; (9) at 1465°K for 1 min ; (h) chloroformat 2 x 10-9 torr adsorbed for 38 min on clean tungsten ; (i) after heating in chloroform vapour at1170°K for 1 min; ( j ) at 1260°K for 1 min; (k), (I) at 1370°K for 1 min, after which the field wasapplied continuouslyM.J. DUELL, B. J. DAVIS AND R. L. MOSS 49The work function at these low pressures of carbon tetrachloride apparently increasesto a steady value of 5.7 eV.Similar adsorption experiments were performed with chloroform, producingfield-emission patterns which closely resembled those from carbon tetrachloride,6 . 0 j.- i A,/< A / . - -' +'* /' /------- 5.5 c-I01 1 I 110 20 30 40time (min)FIG. 7.-Variation of work function with time during carbon tetrachloride and chloroform ad-sorption at room temperature : A, carbon tetrachloride corresponding to series of patterns shownpartly in fig. 6(a)-(d) ; v, gas " dose ", cf. text ; A, gas admitted after flashing tip, calculated fromFowler-Nordheim plots in fig.8 : 0, chloroform adsorption at 2 x torr.I I2 . 3 2.5 3'0 3.5104117FIG. 8.-Fowler-Nordheim plots for carbon tetrachloride adsorption at 5 x 10-9 torr using inter-polation procedure, cf. text.e.g., fig. 6(h) is the " steady " pattern, 4-53 eV (cf. fig. 7). However, experimentswere also carried out where chloroform vapour was reacted over the heated tip.Increasing temperature did not produce the gradual brightening of the circulardark areas around (001) and (010) positions observed when the tip was heated inthe absence of vapour. Instead at 1120"K, the circular dark areas were sharper in50 FIELD-EMISSION OF (212 AND c-c1 COMPOUNDSoutline and at 1170°K showed fine structure, fig. 6(i). The presence of carbonindicated by these patterns was shown clearly by heating to 1260"K, fig.6(j), whichresembles closely the carbon pattern,5 with its characteristic (334) planes.Operating the microscope with the field applied continuously, procedure (iv)above, in the presence of carbon tetrachloride or chloroform vapour can give riseto patterns such as fig. 6(k) where intensely emitting " spots " are " superimposed "on the expected pattern. In this case, the tip had been heated at 1370°K in chloro-form vapour, continuing the experiment described above. The " spots '' also showedfine structure which changed rapidly between various simple forms, doublets, quadru-lying pattern was suppressed, fig. 6(2). It is believed that these intensely emittingspots are due to the occurrence of projections or whiskers of tungsten on the originaltip.They are not formed by chlorine or carbon alone, nor indeed by oxygen, andcan be formed on the unheated tip after adsorption at room temperature of thecmbon-chlmine compunds.I#&%, p,tr,. ; C%n-t>PJJ.$3 tJxy d4Lwi'TPJbtd tb2" ewi%%kYA &a.wJb!i**iG% Q& that& thfz WXkT-DISCUSSIONSurface potential values for the adsorption of nitrogen on individual tungster iplanes vary not only in magnitude but also in sign,8-10 and average values for all[planes exposed on the emitter tip must be treated with caution. However, thc:observation of a positive surface potential, i.e., the initial work function decreascto a minimum shown in fig. 4, with a halogen, in this case, chlorine, merits furthei-consideration.Observations were started by flashing the tip to produce a clean surface, after.switching off the ionization gauge used to regulate the chlorine supply through the field-.emission tube.This avoids any unusual species produced by the gauge 11 and minim-.izes contamination of the tip by foreign gases desorbed in the early stages of the:experiment. Under these conditions, adsorption started on a tungsten surface:which had been subjected to thermal disordering and was still cooling to roomtemperature. When chlorine was admitted after allowing the tip time to cool,,the minimum was only just apparent. Heating the tip at low chlorine coverage:produced an unambiguous, if small, positive surface potential (cf. fig.4), and a larger.one, +0.66 V during desorption from a well-covered tip.Carbon tetrachloride, and also chloroform, are believed to dissociate on tungsten,,yielding adsorbed chlorine, as discussed later ; again positive surface potentialswere observed initially (fig. 7), but of greater magnitude. The minimum was present.even when carbon tetrachloride was admitted after the tip had been allowed to cooldown. Results with these compounds and also with chlorine suggest that reductionin pressure delays the eventual increase in work function after the minimum andprobably also increases the positive surface potential observed.There are many references 12 to enhanced photoelectric emission or to positive.surface potentials when a small quantity of oxygen is admitted to clean surfaces.The effect of halogens on the work function of metals is less well explored but positivesurface potentials have been observed at low coverages of chlorine on nickel13and titanium 14 films and in field-emission studies of bromine on tungsten? Thesize of the halogen is also important, thus, fluorine (and also oxygen) on iron at20°C produce positive surface potentials, unlike chlorine or iodine.15It now seems generally agreed that electronegative gases can penetrate below themetal surface producing a positive surface potential, providing 16 a certain criticaltemperature is reached.Direct evidence of the penetration of oxygen below M. J. DUELL, B. J. DAVIS AND R. L. MOSS 51tungsten surface is available from field-ion micrographs observed after the removalof successive metal layers by field evaporation.17 Penetration was found two layersdeep at 20°K within a few minutes and as deep as five layers if the tungsten surfacewas exposed to oxygen at room temperature.Attack began at lattice steps butpreferred oxidation at lattice imperfections could not be discovered. The detectionof positive surface potentials by field-emission using the procedures described, wasconfirmed by adsorbing oxygen which showed an initial work function minimum,and by adsorbing methane which as expected, did not (fig. 5). Again, the observedpositive surface potential was greater at a lower oxygen pressure.Therefore, the work function values observed when gas-phase chlorine wasadsorbed on tungsten or when carbon tetrachloride or chloroform dissociated togive adsorbed chlorine are believed to arise from two processes.(i) Penetration ofchlorine below the tungsten surface, as discussed later, causing a reduction in workfunction. The process is enhanced when the temperature of the tip is raised, eitherduring cooling after flashing in the gas, or by deliberate resistive heating. (ii) Ad-sorption of more chlorine above that which has penetrated the surface, i.e., essentially" chloride " formation, or adsorption on areas which it cannot penetrate. Sinceemission is primarily from low work function areas, further adsorption over theareas initially penetrated would appear to be responsible for the rise in work functionafter the minimum.Reducing the pressure would be expected to slow down thisadsorption process and hence the work function minimum due to (i) becomes morereadily observable. Further, the lower work function at the minimum observedwith the compounds compared with chlorine can be understood in terms of therelatively slow release of chlorine from, say, carbon tetrachloride. In effect, thisis the equivalent of a very low chlorine pressure.As a preliminary to examining process (i) further, the basis of the work functioncalculation from the Fowler-Nordheim plots, i.e., the constancy of ( r / K ) during anexperiment, might be considered briefly. Certainly, before the start of each ad-sorption sequence and after flashing clean, (r/K) is reasonably constant for a giventip.This indicates that the macroscopic dimensions of the tip were little changedby the adsorption process, but protrusions could have been formed leading to fieldenhancement and an apparent reduction in work function. However, the reductionwas observed at low coverages and disappeared on further adsorption which wouldbe expected to follow the surface contour.Penetration of the surface by chlorine, observable in the initial stages, appearsto be accompanied by a reduction in emitting area, derived from further analysisof the Fowler-Nordheim plots (fig. 3 and 8). For a partly covered surface, theFowler-Nordheim equation should be modified for polarization of the adsorbateby the field.where N is the number of adsorbed species per cm2 and a is the polarizability of theadatom.Hence the ratio of the pre-exponential terms for clean tungsten, Ao,and after a given period of adsorption, A , is given by 2, 10Polarization produces a work function change A# given by7A+ = 4nNaF ( 5 )=--- '' A , a0 (4+A+) t"Y1'"yo' exp (--$4;nNor#* x 6.83 x lO'v[y]).A a -At low coverages, say 8 = 0.1 ( N = 5 x 1013 atoms/cm2), a = 3.6 x 10-24 cm3,184 = 4.45 eV (i.e., the minimum for chlorine) and u[y] = 0 6 , the exponential termis evaluated as - 1-3. Since A+ -0.1 eV, and also t[yo] and tb] are sensibly unity,the ratio (A/&) is a reasonable guide to the variation in emitting area. For example,352 FIELD-EMISSION OF C12 AND C-C1 COMPOUNDSat the observed minima for chlorine, oxygen and carbon tetrachloride, the emittingareas have decreased by approximately 10, 10 and 100 times (tables 1-3).Examination of fig.6(b), corresponding to the minimum work function withcarbon tetrachloride, shows clearly that substantial areas, compared with the cleanpattern, were still brightly emitting. Hence it would appear that electrons wereemitted from small areas distributed over the surface where chlorine has penetratedand decreased the work function. The relative brightening of the (111) planes,the areas around the (001) planes and the ring around the central (011) plane, mayindicate preferential attack on these highly stepped regions ; initial attack at stepswas noted above for the corrosion of tungsten by oxygen.17 A further point ofinterest is the subsequent increase in log A, when the surface was covered, to ap-proximately the value observed at the start of chlorine or carbon tetrachloride ad-sorption. This indicates that emission from the covered tip was occurring gener-ally, whereas as low coverages it occurred through “windows” of lower workfunction where chlorine had penetrated the surface.DISSOCIATION OF CARBON-CHLORINE COMPOUNDSIt has been assumed in the discussion above that carbon tetrachloride and chloro-form dissociate on clean tungsten at room temperature producing adsorbed chlorineand field-emission observations on this point are considered first.(a) In additionto the work function minimum observed with both chlorine and the carbon-chlorinecompounds, the final work function values were similar, i.e., 5-6-57 eV for chlorine,5.4 eV at lower pressure, compared with 5.7 and 5.5 eV for carbon tetrachlorideand chloroform respectively. Further, carbon has little effect on the work function 4and as shown in the present work, adsorbed methane has only a small effect.(b) Patterns observed early in carbon tetrachloride adsorption and after heating acovered tip (fig.6(c) and (e)) and also in chlorine adsorption (fig. l(c)) are closelysimilar. The pattern shows a bright granular band around the central (011) plane,linked to diamond-shaped bright areas centred on the (001) planes and is associatedwith adsorbed chlorine. (c) There was an indication of chlorine desorption at1170°K (heated for 1 min, Q) = 5-26 eV) and with carbon tetrachloride, the patterndescribed above was restored (4 = 5.35 eV).Thus the presence of adsorbed chlorine from the dissociation of carbon tetra-chloride and chloroform shows up reasonably well on the patterns and in the workfunction measurements.However, the continued dissociation of these compoundson tungsten at room temperature eventually produces patterns with one readilyobservable difference with respect to adsorbed chlorine. This difference is thesubsequent darkening of the bright diamond-shaped areas producing large circulardark regions also centred on the (001) planes (cf. fig. l(c) and 6(d) or (h)). Thesedark areas were not observed when a chlorine covered surface was attacked by gas-phase methane (fig. l(g)-(j)) which clearly removed the chlorine, i.e., the character-istic chlorine pattern disappeared, to be replaced by the methane pattern (cf.fig.1 (I)). Also the work function decreased substantially, levelling out towards thevalue for adsorbed methane, i.e., 4.8 eV (fig. 4). Since hydrogen had little effecton adsorbed chlorine, it would appear that chlorine was removed as a compoundwith carbon.There is evidence, however, that at room temperature, carbon tetrachloride andchloroform yield in addition to chlorine, and possibly carbon, fragments of thetype CCl,. Under a continuously applied field, intensely emitting spots believedto be due to the formation of tungsten whiskers are produced. Since whiskerM. J. DUELL, B. J. DAVIS AND R. L. MOSS 53cannot be formed by carbon or chlorine alone, it would seem that a complex involv-ing both of these and surface tungsten atoms is required. This complex migratesto the end of the whisker where it dissociates and deposits tungsten.1 Moss and Kemball, Trans. Faraday Soc., 1960,56, 1487.2 Duel1 and MOSS, Trans. Faraduy Soc., 1965, 61, 2262.3 Silver and Witte, J. Chem. Physics, 1963, 38, 872.4 Klein, J. Chem. Physics, 1953, 21, 1177.5 Muller, in Physical Methods in Chemical Analysis, ed. Berl, (Academic Press, New York,. 1956),6 Good and Muller, in Handbuch der Physik (Springer-Verlag, Berlin, 1956), 21, 176.7 Gomer, Field Emission and Field Ionization (Oxford University Press, London, 1961).8 Oguri, J. Physic. SOC. Japan, 1964, 19, 83.9 Holscher, J. Chem. Physics, 1964, 41, 579.10 Delchar and Ehrlich, J. Chem. Physics, 1965, 42, 2686.11 Archer and Gobeli, J. Physics Chem. Solids, 1965, 26, 343.12 Klemperer, J. Appl. Physics, 1962, 33, 1532.13 Anderson, J. Physics Chem. Solids, 1960, 16, 291.14 Anderson and Gani, J. Physics Chem. Solids, 1962, 23, 1087.15 Burshtein and Shurmovskaya, Surface Sci., 1964, 2, 210.16 Riviere, Brit. J. Appl. Physics, 1965, 16, 1507.17 Muller, in Structure and Properties of Thin Films, ed. Neugebauer, Newkirk and Vermilyea18 Syrkin and Dyatkina, Structure of Molecules and the Chemical Bond (Butterworths, London,3, 135.(Wiley, New York, 1959), p. 476.1950)

ISSN:0366-9033    

DOI:10.1039/DF9664100043

出版商:RSC    

年代:1966

数据来源: RSC

摘要:

GENERAL DISCUSSIONDr. A. A. Holscher (Amsterdam) said : Ehrlich’s interpretation of the changesobserved in the ion images after adsorption of carbon monoxide and nitrogen ontungsten differs completely from the one presented in our paper. In his picture thedisorderly arranged bright dots are identified with individual atoms (N) or molecules(CO) on an undisturbed metal surface, whereas we concluded that the surface structureis highly disturbed by the adsorption, the bright dots being tungsten atoms. Thepresence of adsorbate is marked by a diffusiveness in some parts of the ion image.In studying the behaviour of the (1 10) face Ehrlich compared ion images obtainedafter adsorption at 20°K with those obtained after heating the imaged surface to roomtemperature or higher.By measuring the electron emission after imaging we showedthat most of the adsorbed nitrogen is removed from the Iront surface of the tip by theimaging procedure. However, the shanks of the tip and the support remain denselycovered. Heating such a locally cleaned tip assembly will most probably lead toreadsorption on the front surface, either by surface diffusion or via the gas phase.This readsorption may drastically alter the original array of bright dots, and may leadto a disappearance of the dots on the (1 10) face. An example of this effect is shown infig. a and b. Fig. a shows a clean field-evaporated tip, but by accident the zone“ decoration ’’ is also present on the (1 10) face. Fig. b shows the same surface afternitrogen adsorption at 300°K.The three tungsten atoms on the (1 10) face now havedisappeared, apparently due to field desorption of a nitrogen-tungsten complexSimilar changes can be noted in Ehrlich’s fig. 4 and 5. I think therefore that it isdoubtful whether from heating experiments as described one can decide on thenature of the bright dots on the (1 10) face of tungsten as seen after low-temperatureadsorption of nitrogen.The fact that after adsorption of nitrogen on tungsten at 300°K usually no brightdots appear on the (110) (however, we have sometimes observed them!) seems noconclusive proof that adsorption did not take place. The same conclusion would thenalso hold for the adsorption of oxygen on the (1 10) face of tungsten at 300°K. In thiscase, too, the (1 10) remains “ bare ” according to Ehrlich’s fig.3b. If a particularcrystal region does not show up in the ion image, neither before nor after adsorption,it seems difficult to draw any conclusion as to what actually happened in this region.To explain the appearance of bright dots on the (1 10) face of tungsten after adsorp-tion of CO and high coverage Ehrlich proposes a decomposition of the a-state byelectron bombardment leading to carbon formation ; the bright dots are thus identi-fied with carbon atoms. Objections are (i) It is required that after adsorption of p1and a-CO, upon imaging, ionization takes place above the (1 10) face, whereas thisplane remained “ dark ” in the clean state. (ii) In view of the low bond strength ofthe a-state it is most likely that it is completely torn away by the high field before anyconversion can take place.(iii) If any electrons are produced by field ionization abovethe (1 10) they will have no or little energy above the Fermi energy and it seems unlikelythat they should be able to achieve the proposed conversion of the a-state, even if thisspecies were still present at the surface.Dr. Gert Ehrlich (Schenectady, New Yurk) said: In our work with nitrogen thepossibility that the additional emission centres formed by adsorption were actuallydisplaced tungsten atoms was examined at length, but had to be rejected,l’ 21 Ehrlich and Hudda, J. Chem. Physics, 1962, 36, 3233.2 Ehrlich, Adu. Catalysis, 1963, 14, 255.5[To face page 54FIG. l(a).-Three tungsten atoms deposited on (1 10) of tungsten kept at 20°K.(b) Same surface after adsorption ofnitrogen. Two additional emission centres formed on (110). (c) (110) upon warming to 263°K. Tungsten atoms re-main undisturbed ; nitrogen additions have disappeared. All pictures taken at 70 % of the desorption field for W withTo facepage 55.112 sec exposureGENERAL DISCUSSION 55For the (110) plane of tungsten, our F.I.M. observations revealed that after inter-action with nitrogen, either at 300 or 190"K, this surface remains bare. At 145, 77,and 20"K, however, additional emission centres appear ; these could again be elimin-ated by heating to room temperature. The material formed at low temperaturesdiffers from tungsten atoms in being less resistant to field desorption, both in theabsence and in the presence of the image gas, as well as in its thermal stability.Moreover, in our experiments tungsten atoms interacting with nitrogen do notreproduce the behaviour of the centres observed after nitrogen adsorption.Thus, in fig.4a of our paper, three tungsten atoms are present on the (1 10) planeitself prior to nitrogen adsorption. Exposure to nitrogen at 263°K has not swept thesurface clean in fig. 4b; precisely three atoms are still present after this treatment,which has only resulted in a significant shift of one. Adsorption at 20°K is even morerevealing. In fig. a, three tungsten atoms have been deposited on the (I 10). Exposureto nitrogen at 20°K yields two additions to this plane (in b) without displacing thetungsten atoms. After warming to 263°K (c), the three tungsten atoms are still in theiroriginal locations, but the additions produced by nitrogen adsorption are gone. Thereis no indication of significant complex formation by which one might rationalize thecharacteristic properties of the emission centres found after adsorption.It is on these grounds, and not merely based on warming experiments, that weidentify these emission centres as nitrogens.Holscher's comment that it is difficultto draw conclusions about adsorption on a plane if no changes are observed on it is,however, quite appropriate. Indeed, this difficulty was already noted some time ago,in our work on nitrogen, where we state that: " although there are no changesapparent on the (110), adsorption on this plane is not necessarily ruled out.. . . Theact of forming the helium-ion image may denude this particular region of adsorbedmaterial without exerting such a strong effect elsewhere. This possibility can bereadily eliminated, however. . . . If nitrogen is allowed to interact with this sample at20"K, adsorbed material is immediately apparent on the (1 10). This clearly establishesour ability to detect adsorption on the (1 10) when it occurs, despite the high field ; thelack of change in the appearance of the (1 10) at 300°K must therefore be interpreted asindicating the absence of nitrogen adsorption." 1 Our conclusions about theoccupation of the (1 10) by nitrogen, both at 300°K and in the low-temperature range,have been verified by contact potential measurements 2 on a macroscopic (1 10) plane,as well as by independent field emission studies.3as bAs regards carbon monoxide, the intensity of an image point depends upon thelocal supply of helium, as well as on the rate constant for field ionization.An adatomwill influence both. The supply of He depends among other things upon the efficiencywith which a colliding atom transfers its kinetic energy to the lattice. Adsorbed gasincreases the accommodation coefficient of helium and should thereby raise theimage intensity. Adatoms also affect the probability of field ionization, as thisdepends upon the local electric field and the detailed shape of the barrier confrontingthe valence electron of the image gas.There is therefore nothing unusual in beingable to image adsorbed material on a plane which, at the same voltage, was quite darkprior to adsorption.The extent to which adsorbed material fragments during imaging depends upon therate of dissociation by electron impact under imaging conditions, compared with therate of field desorption. Neither the energy required for dissociation, nor the effi-ciency of this process has been measured under high fields. Without this quantitative1 Ehrlich and Hudda, J. Chem. Physics, 1962, 36, 3233.2 Delchar and Ehrlich, J. Chem. Physics, 1965, 42, 2686.3 (a) van Oostrom, Philips Res. Rep. Suppl., 1966, no. 1 ; (b) private communication56 GENERAL DISCUSSIONinformation, the interpretation of image changes observed on the (110) at high COconcentrations will remain equivocal, as already indicated in my paper.However,in flash desorption measurements from a (110) plane of tungsten, both the a and P1states have been isolated.1Apart from differences in interpretation, our experimental results, both for CO andfor nitrogen, differ significantly from those reported by Holscher and Sachtler (H & S).Although the reasons for these discrepancies are difficult to establish with certainty,the following must be kept in mind: (a) Even spectroscopically pure gases maycontain COa and other impurities at levels up to 0.2mole %. For CO, furthercontamination is introduced by disproportionation within the storage bulb duringbaking. Oxygen thus liberated is particularly dangerous, since it has been establishedthat surface rearrangement occurs during observation of oxygen covered tungsten.In our own work with CO, contaminants were therefore specifically removed justprior to the adsorption studies by selective adsorption and passage of the gas over acold finger at 63OK.2 With nitrogen, difficulties are less and purification over nickelfilms is sufficient.(b) The measurements recorded by H & S were done at 78°K.Successful operation of the F.I.M. at this temperature depends upon the use of veryfine emitters,3 for which the best image field approaches that for evaporation of thelattice atoms.4 In our studies all images were obtained with a liquid hydrogen cooledemitter, at -20°K. This by itself reduced field desorption.Judging from the workof Swanson and Gomer,s at a constant field of 4-71 V/A the rate of field desorption ofPCO diminishes by a factor of 10 just in going from 60 to 20°K. Furthermore, inour studies operation at 20°K made possible observations on larger tips, not as subjectto the deleterious influence of the field. (c) In the course of preliminary studies withCO, surface rearrangement became apparent during prolonged observations. In oursubsequent work, the first picture has therefore been taken considerably below the bestimage field, at less than 2/3 the desorption field ; the field itself was imposed for theshortest interval consistent with successful photography. In contrast, the imagefield of H & S ranges from 84 to 89 % of that for tungsten desorption, with at leasttwice the exposure to promoted field desorption.That there are severe difficulties in observing adsorbed gases, and that the partial(or sometimes complete) removal 2 , 6 of adsorbed gas is brought about by promotionof field desorption through the image gas, was clearly established some time ago.7The work function measurements by H & S provide an interesting quantitative accountof these, but do not introduce qualitatively new information requiring a re-evaluationof previous studies.Dr. D.Brennan (University of Liverpool) said : Displacement of surface atoms asconsidered by Holscher would be expected to influence the heat of adsorption.However, we underline the fact that most of the gas increments were adsorbed with acharacteristic heat, which is not as might have been expected on the basis of a largedegree of surface damage.The lower heats sometimes observed at low coverage areinteresting, but like Cernk, we believe that a more systematic study of this region mustprecede any useful discussion of the effect.Dr. J. Volter (Inst. anorg. Katalyseforschung, Berlin) said : Ehrlich has demon-strated the pronounced selectivity of individual crystal planes in chemisorption. We1 May and Germer, J. Chem. Physics, 1966, 44, 2895.2 Ehrlich, Adv. Catalysis, 1963, 14, 255.3 Drechsler and Wolf, 4th Int. Conf. Electron Microscopy, (1958), p . 835.4 Miiller and Young, J. Appl. Physics, 1961, 32, 2425.5 Swanson and Gomer, J . Chem. Physics, 1963, 39, 2813.6 Mulson and Miiller, J.Chem. Physics, 1963, 38, 2615.7 Ehrlich and Hudda, Phil. Mag., 1963, 8, 1587GENERAL DISCUSSION 51have studied various catalytic reactions on individual crystal planes of Cu and Ge.1We have found considerable selectivity, too. Now we have studied the p-H2-conver-sion of individual crystal planes of Ni and Cu single crystals in a static system. Theactivation energy on Ni varies from 3-2 kcal on the (1 11) plane to 5.2 kcal on the (1 lo),and 6.4 kcal on the (100) plane. On copper the corresponding values for the (1 11)plane are 9.1 and for the (100) plane are 13.5 kcal. The catalytic activities on thedifferent planes of Ni as well as of Cu vary by the factor of 2. These results mayshow some correlations with those of Ehrlich.(i) The accord of chemisorption pro-perties of a definite crystal plane on the small tip in the field ion microscope and on thelarge-scale single crystals give further evidence that it is possible to compare themacroscopic surface with the corresponding lattice plane. (ii) As expected fromchemisorption, in catalysis there is a specificity of the crystal planes, too. (iii) Ehrlichfinds on tungsten, that the closest-packed plane gives no or only very weak interactionwith nitrogen or CO respectively. He assigns this to an electronic rather than to a geo-metric effect. Eley 2 postulated that on the closest-packed plane the heat of hydrogenadsorption reaches a minimum, and that this plane must exhibit maximum activityin p-H2-conversion.He studied this reaction on polycrystalline metal films.We studied the reaction on single crystals and could verify Eley’s prediction. Onthe closest-packed planes of nickel and copper the activation energy of the conversionwas a minimum. This accord is not fortuitous and our results may be explainedtoo by energetic rather then geometrical factors.The importance of the energetic factor may be supported by Ehrlich’s result thatchemisorption of nitrogen on the (1 10) plane is not influenced by additional adsorptionof 3 tungsten atoms on this plane, i.e., he can vary the geometrical arrangement of thetungsten atoms to a considerable extent without altering their chemisorption properties.It might be possible to explain all specific effects of crystal planes by difference indislocation density or quality.We compared in several reactions the catalyticactivity of forms polycrystalline and monocrystalline of Cu and Ni. Considering thegrain boundaries as a special type of dislocation, we have in polycrystalline materiala much higher dislocation density. But we never found a specific positive or negativecatalytic effect of polycrystalline material compared with monocrystals. Thereforewe think the dominating factor is the lattice plane and not the dislocation.Dr. Gert Ehrlich (G.E., Schenectady, N.Y.) said: Dr. Volter’s findings thatdislocations have little effect on the catalytic reactivity of metals should hold gener-ally. A dislocation constitutes an atomic arrangement of higher energy than thatof the ideal lattice; as such, its termination at a surface can be expected to affectthe chemical reactivity of that particular crystal plane.However, after annealinga reasonably pure metal, the dislocation density is only of the order of 108/cm2or less compared with a total of - 1015 atoms/cm2. Reaction at the more numerousordinary sites should therefore overwhelm any contribution from the dislocations.Only in systems with extreme differences in reactivity, for which a reaction cannotoccur at ordinary surface atoms, are dislocations likely to play a significant role.Dr. Z. Knor (Inst. Physic. Chem., Czechoslovak Acad. Sci., Prague) said: First, Iwould like to know, if Dr. Oostrom (whose Thesis was quoted in Ehrlich’s paper)used for the study of nitrogen adsorption on the (1 10) plane of tungsten the fieldelectron or field ion emission.If the former, then was it possible to study the adsorp-tion on this plane which has such a high work function?1 Volter and Kordel-Kriiger,Z. anorg. Chem., 1964,329,261.2 EJey and Shooter, J. Catalysis, 1963, 2, 259.Volter and Schon, 2. anorg. Chem.,1963, 322, 202. Rienacker and Volter, 2. anorg. Chem., 1959, 302, 295 and 29958 GENERAL DISCUSSIONWe have used the usual type o€ F.E.M., in which the tungsten tip was treated athigh temperature in the presence of high electric field (which was the reverse of thatused for electron emission and thus the surface is protected against contamination and/or ion bombardment). The details will be published elsewhere.1 In this way,starting with the clean surface of tungsten (fig.l), we have obtained surfaces, coveredwith clusters of tungsten atoms (see fig. 2a, 3 4 , which can be converted again to theoriginal shape by mere heating to about 2300°K. During this procedure the diameterof the tip slightly increased in the ratio 1 : 1-4 (estimated by the method of Drechsler 6 ) .These clusters or microtips emit electrons at much lower voltage than the originalsurface, because of the increased intensity of the electric field in their neighbourhood(due to their small dimensions) (compare the legend to fig. 1 and 2 or 3 respectively).This behaviour of metal tips is well known, e.g., ref. (2)-(5). When hydrogen oroxygen was adsorbed on these microtips, no geometrical change was observed (fig.2 4 b and 3a, b, c). The emission current decreased in the same manner approximatelyas for the adsorption on the original surface (e.g., the change of the work functioncaused by oxygen adsorption on microtips was - 1.0 eV compared with the valueN 1.5 eV for the original surface, both estimated from the change in the slope ofFowler-Nordheim plot).From these results we conclude that probably no rearange-ment of these large clusters proceeds during these processes. This conclusion is inagreement with the results of Dr. Vernickel presented at the 3rd Int. Congr. VacuumSci. Tech. (1965 Stuttgart), who prepared the microtips by means of ion bombardment.With an adsorbed layer of hydrogen we have found a strong influence of this layeron the course of the subsequent high temperature treatment in absence of the fieldDr.Gert Ehrlich (Schenectady, New York) (communicated) : Dr. van Oostrom’smeasurements on the (1 10) plane were made in a field emission microscope equippedwith a Faraday cage, to which electrons from only a single crystal plane were admittedthrough a narrow probe hole. The work function of the clean plane was thendetermined by measuring both the slope of the Fowler-Nordheim curve and the half-width of the total energy distribution of the emitted electrons. Changes in the workfunction on adsorption were obtained as usual from changes in the Fowler-Nordheimslope.Prof. J . Oudar (University of Paris) said : During a study of reversible chemisorp-tion of sulphur on silver we have shown that this phenomenon is strongly affected bythe crystalline orientation of the metal.The heats of adsorption determined on thethree low index surfaces (1 11) (100) and (1 10) were as follows. (Values for 1 mole ofS 2 for a degree of coverage 50 % of the maximum) ; AH(ll1) = - 54 kcal, AH(100)= - 58 kcal, AH( 1 10) = - 66 kcal. These values show that the adsorbed atoms arefixed less strongly when the surface has a greater density of metallic atoms. On themost dense faces, (1 11) and (loo), the heat of adsorption is approximately constant asa function of coverage whereas it decreases for the (1 10) face. This suggests that forthe temperature range studied, 3O0-45O0C, a single absorption state exists for (1 11)and (100) while several states exist for the atomically rougher (1 10) plane. We havealso found that the structural defects associated with atomic steps are sites morereactive than the sites normalIy present on the low index planes.(fig. 24.1 Knor and Lazarov, Czech.J. Physics, 1966, 16b (in press).2 Drechsler, Z. Elektrochem., 1957, 61, 48.3 Benjamin and Jenkins, Proc. Roy. SOC. A , 1940,176, 262.4 Meclewski, Nicliborc and Wojda, Acta Physica Polonica, 1962, 22, 525.5 Vanselov, Phys. stat. SOL, 1964, 4, 697.6 Drechsler 2. Elektrochem. 1954,523, 340FIG. 1 .-Original surface of tungsten ; voltage U = 8,000 V.[To face page 58FIG. 2.-Tungsten tip treated at T = 2,300"K ; U' = 10,000 V : (a) clean surface at 78°K ; voltageU = 3,000 V ; (6) surface covered with adsorbed hydrogen at 78°K ; voltage U = 3,000 V ; (c) tipafter high-temperature treatment, T = 1,200"K; in absence of the electric field at 78°KFIG.3.-Tungsten tip treated at T = 2,300"K ; U' = 10,000 V ; (a) clean surface at 78°K ; voltageU = 4,000 V ; (b) surface covered with adsorbed oxygen at 78°K ; voltage U = 4,200 V ; (c) thesame surface as (6) ; voltage U = 5,800 VGENERAL DISCUSSION 59Finally, evidence of attractive forces between adsorbed sulphur atoms leads to theconclusion that the adsorbed layer consists not simply of sulphur atoms but of areconstructed mixed sulphur and silver layer. This hypothesis, compatible with themean separation of sulphur atoms in the layer, is in accord with recent results onother systems by low energy electron diffraction.Dr.H. Mykura (Glasgow Uniuersity) said: I would like to discuss briefly certainaspects of Ehrlich's and Gomer's papers, which they did not elaborate upon. Thisconcerns the adsorption at steps on vicinal surfaces and an experimental technique bywhich adsorption on steps can be differentiated from adsorption on singular crystalsurfaces. On the conventional terrace-and-ledge model of a surface (fig. 1) a vicinalFIG. 1.-The geometry of a vicinal surface : section perpendicular to the ledges.surface consists of" terraces " with monatomic steps (" ledges ") of height a, separatedby a distance s, so that a, the angle between the singular surface and the vicinal surface,is given by sin a = a/s. If the surface free energy of the terraces of singular surface isyo and the line free energy of the step /I, then the surface free energy of the vicinalsurface at inclination a isya = yo cos a+(B/a) sin I a I.Dividing by yo and differentiating with respect to a one obtainsPYo aa YOU1 = -sin a+- cos 1 a 1 ,which reduces to"91 = - PY o ax a=O you(3)in the limit a+O.1 a YY acthTow under suitable experimental conditions the value of - - (the " Herring torqueterm ") is directly measurable from surface-interface intersections.2,3 At present thetorque term measurements can only be done easily at high temperature and on face-centred-cubic metals by measuring the shape of ' 6 thermal etching " grooves at twin-boundary/surface intersections.In principle, grain-boundary/surface intersectionscan also be used.3 As a the step height is known from crystallographic data, the ratioP / y o , which is the relative step energy, can be evaluated.On performing the measure-ment first for a clean surface and then for a surface in equilibrium with adsorbate, thechange in the /3/ yo ratio can be measured.4 An increase in the p l y 0 ratio with increas-ing adsorption indicates preferred adsorption on the terraces, a decrease preferredadsorption on the steps.When the average surface free energy can also be measured as a function ofadsorbate concentration (using, e.g., the zero creep technique),s then the variation in1 Gjostein, Acta Met., 1963, 11, 957.2 Mykura, Acta Met., 1961, 9, 570.3 Shewnion and Robertson in Metal Surfaces, (A.S.M., Metals Park, Ohio), 1963, p.67-98.4 Robertson and Shewmon, J. Chem. Physics, 1963, 39, 2330.5 Buttner, Funk and Udin, J. Physic. Chern., 1952, 56, 65760 GENERAL DISCUSSIONadsorbate concentration with crystal orientation can be evaluated using the Gibbsadsorption equation.1The earlier experimental results obtained by this technique dealt with oxygenadsorption on Ag and Cu. For these metals on both (1 11) and (100) vicinal surfaces?oxygen adsorbs preferentially on the low index surface 2,3--the ratio increasedwith partial pressure of 0 2 in the annealing atmosphere. Other systems, however,behave differently ; sulphur on copper (1 00) vicinal surfaces showing preferred ledgeadsorption.2 Recent results on platinum heated in air at 1100°C 4 show that whileFIG.2.-Orientation dependence of surface free energy of platinum at 1100°C. Upper curves :platinum in vacuo ; middle curves : in air ; lower curves : calculated differences in surface coverageof oxygen (I?, is surface coverage at angle a from the low index orientation) ; result evaluated fromexperimentally determined torque terms.c: .- U-c,l- d, IN RA3IANS.1021- Ii 0 1 c2 0 3 0 4I1 0 2 1 1.02 11 0 0 L loo 1 0.1 0.2 0.3FIG. 3.-Torque terms and relative surface free energy for copper annealed in H2+0.12 %HzSmixture at 830°C.p / y o increases for (100) vicinal surfaces, it decreases for (1 11) surface and increasesmarkedly for (1 10) surfaces. Fig. 2 shows the surface free energies and the relativesurface coverage of oxygen derived from these measurements-while the absolutevalues of surface energy are based on debatable assumptions, the surface coverageresults are considered reasonably reliable.1 Rhead and McLean, Acta Met.1964,12,401.2 Robertson and Shewmon, J. Chem. Physics, 1963. 39, 2330.3 Buttner, Funk and Udin, J. Physic. Chem., 1952, 56, 657.7 M. McLean, Thesis (Glasgow University, 1965)GENERAL DISCUSSION 61If adsorption on the ledges is much stronger than on the terraces, then it is possiblefor the ratio to become negative. This has been observed for the system Ag/S 1and also CufS. Fig. 3 shows the torque terms and the orientation dependence ofsurface free energy obtained by graphical integration of the torque terms for the lattercase : copper heated at 830°C in a hydrogen atmosphere containing 0.12 % H2S.The negative torque terms for vicinal(100) surfaces and the surface energy maximumat (100) are very marked and imply a strong preferred adsorption of sulphur on theledges.In conclusion, relative surface energy measurements can yield much information onorientation dependence of adsorption and-particularly for vicinal surfaces-theadsorption on the different types of site on a surface of given orientation can beidentified and measured separately.Dr.Gert Ehrlich (Schenectady, New York) said: The inability to isolate discretebinding states for potassium on tungsten need not imply a qualitative difference in thedependence of bond strength on structure between metallic and covalent adsorption.The ratio of the heat of adsorption for potassium on the (110) to that on the (100)plane of tungsten ( N 1-4) is comparable to the ratio of desorption energies for the p 2and p1 states of CO, which are easily resolved.The absence of separate states is morelikely a consequence of the rapid equilibration of potassium over the surface and of itsmore continuous adaptation to a changing chemical environment as the surfacebecomes increasingly populated with other potassium atoms.At low temperatures, the adsorption of carbon monoxide itself no longer seems asvirgin as originally postulated and is now similar to the behaviour of nitrogen.1 Forthis system, in adsorption at T< 150"K, the y state successfully competes for surfacesites with the more stable p state.The population in the former as a function oftotal surface population is well described by an S-shaped curve. Adsorption at lowtemperatures, after prior exposure to nitrogen at 300"K, leads to a much lower ypopulation, with a different surface dipole from that obtained by continuous adsorp-tion at low temperatures. For nitrogen the observations are consistent with a model 2in which the strongly held state makes neighbouring sites less attractive to occupa-tion by other /3 nitrogens. Instead, a more weakly held state is formed at low tempera-tures; by its presence this interferes with strong bonding in the immediate vicinity.Inasmuch as the low temperature state now depends upon a cooperative effect (uponsite creation by adsorption itself) its concentration as a function of total coverageshould be given by an S-curve, as experimentally found. Furthermore, adsorption atlow temperatures must be irreversible-the high population of y cannot be achievedby redosing a surface saturated at higher temperatures. Without attempting aquantitative analysis of the data for CO, it appears that much the same model isapplicable to the low-temperature behaviour of this system and that a distinct virginentity may not have to be invoked.Prof. R.Gorner (Univ. of Chicago) (conmunicated) : Ehrlich is right in enumeratingsome of the reasons why discrete binding states are not observed in alkali adsorption.As my paper points out, metallic adsorption is structure-sensitive. However, there isalmost certainly a difference in the relative importance of structural and " purely ''electrostatic factors (in a sense the distinction is arbitrary since both depend on struc-ture) in covalent and metallic adsorption, because the two bond types are different, atleast in their extremes.1 Rhead and Perdereau, Acta Met., 1966, 14,448.2 Ehrlich, J . Chem.Physics, 1961, 34, 29.3 Ehrlich, Structure and Properties of Thin Films (John Wiley & Sons, N.Y., 1959), p. 42362 GENERAL DISCUSSIONWith regard to the separate existence of virgin states in CO adsorption, there is nodoubt from thermal and electron impact results that three distinct adsorption types,viz., virgin, beta and alpha exist. The virgin states may be the analogue of Dr.Ehrlich’s gamma states, but nomenclature is not the issue.The point is whether : (i)the low-temperature population is initially mostly virgin, with a relatively small fractionof beta states (possibly to be found only on certain crystal planes), with the percentageconverted to beta on heating or electron impact a function of coverage, as proposedby Menzel and Gomer,I and Bell and Gomer,2 and restated in my paper ; or whether :(ii) the amount of beta seen after heating or electron impact is on the surface ab initio,in amounts determined by the initial coverage, as suggested by Ehrlich. I favour thefirst hypothesis for the following reasons. Our results seem to show that alpha COis only formed when there is beta CO on the surface.For electron impact on a virgin(20°K) layer, it was shown by Menzel and Gomer 1 that the amount of alpha tenableby the surface went up at a rate equal to that of virgin desorption/beta conversion,while in the thermal case the results of Swanson and Gomer,3 and Bell and Gomer,2indicate that alpha is formed in appreciable amounts only after sufficient heating todrive off some virgin and (by our hypothesis) convert the rest to beta. If Ehrlich’shypothesis is correct these results can be explained only if it is further postulated thatalpha is tenable only if there is no virgin CO on the surface. On the other hand, ifthe formation of beta CO does not involve an activated rearrangement as postulatedin the desorption/conversion hypothesis, it is difficult to see why readsorption aftervirgin desorption should not again produce virgin CO.The point could be settled by electron impact experiments if the dipole moments peradmolecule in the virgin and beta states differed sufficiently ; it would then be possibleto determine from the conversion or desorption rate of virgin whether the initial depositwas virgin or beta, even if there is no desorption at all.Unfortunately the differencein dipole moments appears to be too small to make this practical. A good idea of thesituation could still be obtained by determining the amount of alpha CO tenable onpartial virgin layers before and after thermal or impact desorption ; this can be doneby electron impact desorption rate measurements.1 An increase in the amount ofalpha tenable on low 0 layers after thermal or electron impact treatment, would bestrong evidence for the formation of beta by conversion, rather than for its initial,masked presence.Dr.J. W. Geus (Staatsmijnen, Geleen, Netherlands) said: The distinction betweenmetallic and covalent adsorption made by Prof. Gomer is confirmed by the effects ofadsorption on the electrical conductance of evaporated tungsten films.4 It appearsthat caesium, which is bonded to tungsten surfaces in an analogous way to potassium,increases the conductance. Chemisorption of nitrogen, oxygen, and carbon monoxide,on the other hand, decreases the conductance. Adsorption of hydrogen brings aboutonly a small decrease of the conductance, although the amount of hydrogen adsorbedper cm2 of tungsten surface is of the same order of magnitude as that of the other gasesmentioned. The latter fact points to the chemisorption bond of hydrogen on tungstenbeing more or less of a metallic character.Dr.A. A. Holscher (Anzsterdum) said: The experimental results of Gomer on thelow-temperature adsorption suggest that part of the stable 0-state is formed in the laststage of adsorption. The model as proposed by us, invoking an activated rearrange-ment of tungsten atoms, might give a satisfactory explanation of this and fits in withmany other results on the CO/W system obtained by Gorner and co-workers.1 Menzel and Gomer, J . Chem. Physics, 1964, 41, 3329.2 Bell and Gomer, J. Chem. Physics, 1966, 44, 1065.3 Swanson and Gomer, J.Cliern. Physics, 1963, 39, 2813. 4 Geus, Surface Sci., 1964, 2, 48GENERAL DISCUSSION 63Prof. R. Gomer (Wniv. of Chicago) (communicated) : In reply to Holscher, I do notthink that it is possible to tell from our results at what stage of coverage beta CO isformed. (Probably, most of the eventual beta layer is formed only on heating.)Further, it is not yet known how the ratios for the various species vary with crystal-lographic orientation, so that a discussion of Holscher’s question seems premature.However, there is some indirect evidence from electron desorption that the beta layerobtained at low temperatures by electron impact differs from that obtainable byheating1 This evidence is summarized in fig. 38 of ref. (1) and can be interpreted tomean that heating leads to considerable surface rearrangement of the kind postulatedby Holscher and Sachtler at low temperatures.Dr.Z. Knor (Inst. Physic. Chem., Czechoslovak Acad. Sci., Prague) said: Wehave strong experimental evidence (L.E.E.D., F.I.M.) that the structure of the surfacelayer of a metal is in most cases rearranged during the chemisorption of gas particles.Therefore it seems to me improbable that the spatial distribution of the electrons onthe surface of a metal would not change during this process, even in the adsorption ofalkali metals. If the spatial distribution of the electrons on the surface changesduring the chemisorption, then we can divide the experimental value A 4 into twoparts :where 4 0 and +ads are the work functions of clean metal and a metal covered with theadsorbed layer respectively, A+e is the contribution due to the change of spatialdistribution of the electrons on the surface and A& is the part due to the dipole layer(image, permanent, induced dipoles, etc.), for which we can writewhere ,u is the dipole moment, n is the surface concentration of dipoles and a has thevalue 4n or 271, depending on the type of dipoles.Unfortunately we know nothingabout the contribution of A4e which need not be a negligible one.A4 = 40 - $ads = AA? + Ad%?A4d = a w ,Furthermore it seems to me that in the equationP = 2do4,both do (distance from image plane) and q (adsorbate charge) depends on the kind ofcrystallographic plane on which the adsorption process takes place.I therefore thinkthat the values of p calculated from the equation(as found frequently in the literature) or the values of q calculated for a special casefrom p obtained in this manner, represent only a crude approximation to p or q intheir exact physical meaning (e.g., p = dipole moment of the adsorbed particle -permanent or induced dipole, etc.).Dr. D. A. King (Imperial College) (communicated): Using two independentmethods, we have evaluated sticking coefficients s for the nitrogen + tungsten filmsystem.2 In the temperature range 78-150°K, s was high (0.9), and essentially in-dependent of temperature and also of coverage over the range 0-8 x 1014 moleculescm-2 geometric area of the film. A theoretical interpretation of this invariance wouldbe simplified if s was in fact unity, and the value 0.9 was due to some unsuspectedexperimental artefact.For this reason a cell (fig. 1) was constructed specifically to testwhether or not the sticking coefficient was unity.@o - 4ads) = a w1 Menzel and Gomer, J. Chem. Physics, 1964, 41, 3329.2 Hayward, King and Tompkins, Proc. Roy. SOC. A , to be published64 GENERAL DISCUSSIONWhen gas flows into the cell with a film deposited on the walls (but not up thegauge tubulation, a nickel disc being suspended across the mouth of the gauge duringdeposition of the film 1) it is so directed that it can only enter the gauge after collisionwith the film. Thus, the observation of a pressure increase in the gauge unambiguouslyindicates that s < 1.With the lower portion of the cell immersed in liquid nitrogen andan inflow rate of nitrogen of 1015 molecules sec-1, a pressure increase of 2 x 10-10 torrabove background was recorded in the gauge, confirming that, for N2 on W at 78"K,traps andFIG. 1.s < 1. The possibility that the observed pressure increase was due to a non-adsorbableimpurity in the gas supply was eliminated by isolating the cell from the pumps : thegauge pressure was unaltered by this procedure, whereas such an impurity would haveaccumulated in the cell, resulting in a pressure increase.The high sticking coefficients reported by Gomer for the adsorption of carbonmonoxide on a tungsten ribbon, which are completely at variance with values publishedelsewhere for tungsten filaments 2 9 3 and films,4 has prompted us to investigate theCO + W film system with the present cell.With the lower portion of the cell cooled to78"K, CO was allowed to flow into the cell at a rate of 1015 molecules cm-2; noincrease above the background pressure in the gauge was observed over a period of3 min, indicating a sticking coefficient of unity. It is estimated that the lowest pressurechange that could be detected was -2 x 10-11 torr, so that, taking into account thepressure increase for the Nz + W system where s = 0.9, we can confidently report asticking coefficient of 1.00+0-01 for CO on W at 78°K. It should be noted thats = 1 for films requires that s is also unity for an ideally smooth surface. In ourexperiment the temperature of the incoming gas was -290"K, compared with - 50°Kin Gomer's experiments ; thus, the suggestion 5 that his high values of s can beattributed to the low temperature of his molecular gas beam appears to be invalid.1 Hayward, King and Tompkins, Cltem.Comm., 1965, p. 178.2 Ehrlich, J . Chem. Physics, 1961 , 34, 39.3 Ustinov, Ageev and lonov, Soviet PhysicJ-Tech. Phys., 15165,10, 851 ; (Zh. Tekhn. Fiz., 1965,5 Bell and Gomer, J . Chem. Physics, 1966, 44, 1065.35, 1 106). 4 Ricca and Saini, Gazz. Chim. Ital., 1965, 95, 636GENERAL DISCUSSION 65Dr. L. J . Rigby (Stand. Telecomm. Lab. Ltd., Harlow) said: We have studied theadsorption and replacement of hydrogen, nitrogen and carbon monoxide on poly-crystalline tungsten wires at room temperature by means of desorption spectra.1-4It was found that two types of replacement occurred : (a) a slow replacement of thephases which were reversibly adsorbed at room temperature, e.g., the a phase ofcarbon monoxide and the phase of hydrogen were replaced by nitrogen.In thisprocess, the thermal desorption of a weakly bound phase was followed by adsorptionof the replacing gas on the vacated sites. (b) The p2 phase of hydrogen which wasirreversibly adsorbed at rooin temperature was rapidly replaced by p1 carbon monoxide.The sticking probability of carbon monoxide on a clean tungsten surface was onlyreduced by 36 % when that surface was saturated with hydrogen. This rapid replace-ment suggests that the sites for carbon monoxide adsorption are not covered by phydrogen atoms.Rapid replacement of hydrogen by carbon monoxide has now beenobserved at 78°K. The rate of surface diffusion of p hydrogen atoms at this tempera-ture must be extremely low and supports the suggestion that p2 hydrogen and p1carbon monoxide are not adsorbed on identical but on adjacent sites. This type ofreplacement is a very efficient process and may be responsible for many catalyticreactions.Prof. S . 25. Roginskii (Moscow) said : Direct electron-ionic techniques for studyingmetal surfaces, so much advanced by Muller and Gomer et al, are very efficient forinvestigation of the interaction between gases and metal surfaces in high vacuum andof the relations between the structure of surfaces and chemisorption.The paperspresented today by Ehrlich and Gomer have shown the great diversity of adsorptionspecies even for simple diatomic molecules under very pure conditions, and thepecularities of their interaction. The specificity of the techniques discussed makesthem applicable only for the simplest systems and under conditions far from thosetypical for routine catalysis on metals. True chemisorption during catalysis usuallyis considerably more complicated owing to the complexity of catalysts, to the contactwith mixtures of gases, and the greater complexity of the adsorbate molecules.Moreover, there will be considerable changes of the metal surfaces under the actionof catalytic corrosion.5 Such researches are of greater help for understanding theprocess of formation of real catalyst surfaces, rather than for establishing themechanism of the primary steps in catalysis.Also, the investigation of chemisorption by means of field emission microscopesat a coverage lower than a monolayer (8< 1) the effect of adsorption of many quitedifferent compounds on electron emission (more than forty compounds in ourexperiments) is similar.It is somewhat more specific at 1 < 8 < 2, when there appearlight spots (" molecular pictures ") of a nature that is yet insufficiently clear.6 Athigher coverage the field emission microscopes and the diffraction techniques of lowenergy are not efficient.Dr. D. W. Bassett (Imperial College) said: The way in which information aboutthe adsorption of carbon monoxide on tungsten has been deduced by Holscher andSachtler from field ion micrographs, in spite of apparently complete desorption of theadsorbate during imaging at 78"K, is encouraging.Desorption of the ad-layer is alsovirtually complete with oxygen layers, which I have attempted to examine. This had1 Robins, Trans. Amer. Vacuum SOC., 1962,9, 510.2 Redhead, Vacuum, 1962, 12,203.3 Rigby, Can. J. Physics, 1964, 42, 1256; 1965, 43, 1020.4 Holscher and Sachtler, this Discussion.5 Roginskii, Tretyakov and Shechter, Z h r . Fiz. Khim., 1955, 29, 1921.6 Shishkin and Roginskii, Dokl. Akad. Nartk S.S.S.R., 1962, 143, 37366 GENERAL DISCUSSIONforced me to conclude that using helium ion microscopy one was unable to obtaininformation about the atomic details of oxygen adsorption, and that one was restrictedto the investigation of rather more gross changes in surface structure, such as thosecaused by heating an oxygen-covered specimen.However, it may not be legitimateto extract information from ion micrographs by the methods of Holscher and Sachtlerif metal atoms are also removed from the surface during desorption of the adsorbate.Such surface damage occurs with both nitrogen, and carbon monoxide,l and is veryextensive with oxygen as the adsorbate, as is shown in fig. 1. This photograph was6 0 C>c?I2 3oc2CIHe B.I.V.I'1 I IFIG. 2.-Promoted field desorption of oxygen from tungsten at 78°K indicated by the reduction ofthe voltage required for 0.3 FA field emission, following the application for 1 min of a series ofincreasing desorption voltages (shown relative to the voltage needed for tungsten evaporation) ; (a)desorption in vacuum, (b) desorption in helium at 10-3 torr, (c) desorption in neon at 10-3 torr forthree oxygen coverages, A'p = -0.4, -0.9, and - 1.4 eV.printed from a superposition of a positive of the helium ion image of the clean surface,and a negative of the image of the same surface following the adsorption of a smallamount of oxygen.The many black spots show vacancies in the tungsten latticepresent only after the oxygen was desorbed in the act of imaging. Almost all theimage spots that appear in the second image only, white spots, can be associated withatoms of the tungsten lattice made visible by the removal of nearby atoms.The desorption of oxygen ad-layers during imaging has been examined in moredetail by using field emission measurements to characterize the amount of oxygenremaining on the surface after subjecting the layer to progressively higher appliedfields.This has been done for desorption in ultra-high vacuum, and in the imaginggases, helium and neon. The extent of desorption is greater, the higher the tiptemperature, but the general nature of the effects is indicated by some results fordesorption at 78"K, fig. 2. Vacuum field-desorption for 1 min at the best image1 Mulson and Miiller, J. Chem. Physics, 1963, 38, 2615FIG. 1 .-Vacancies, black image spots, present in a tungsten surface after the addition of a low oxygencoverage and its subsequent desorption during helium ion imaging of the surface.[To face page 66GENERAL DISCUSSION 67voltage for helium drastically reduces the oxygen coverage, and removal of the ad-layer is apparently complete if helium is present.For the purpose of chemisorptionstudies, the results for neon are more encouraging, since considerable fractions of thead-layer survive after 1 min at the best image voltage for neon. However, under theconditions of these experiments, the neon ion image was too faint to be observed orrecorded in 1 min, and increasing the neon pressure or the exposure time merelyincreased the extent of desorption. Nevertheless, the results suggest that with a high-gain image intensifier, neon imaging of the ad-layer should be possible. The tempera-ture dependence of field desorption found in these experiments, and in carbonmonoxide desorption,l suggests that imaging at 20"M, rather than 78"K, would alsobe advantageous.While it is true that field emission monitoring of the adsorbatecoverage gives little or no information about the regional variations of the extent ofdesorption or the situation in the (1 10) region, it would be desirable if field emissionevidence such as that given here for oxygen were presented in discussions of thevisibility or otherwise of adsorbates in the field ion microscope.Dr. D. Brennan (University of Liuerpool) said: I find it difficult to accept that COmolecules are able to penetrate into the subsurface region, or, alternatively, thattungsten atoms can diffuse out past adsorbed CO molecules and present new adsorp-tion sites to the gas phase.The CO molecule is large relative to the tungsten atomand carbonyl formation involves the creation of an even larger species. Qne canaccept perhaps the idea that, for example, oxygen or halogen is able to penetratebelow the surface ; in three-dimensional phases, both these species can form inter-penetrating lattices with metal atoms. But carbonyl formation, or incipient carbonylformation involves particles whose size is very different from that of the metal lattice,and surely such species are to be found only on the surface proper. Molecules of COwill be expected to surround tungsten atoms, rather than vice-versa. Is there anyinformation about the number of CO molecules adsorbed and the number of vacanciesleft after field desorption?Caution is necessary in assessing the disturbance to a surface caused by adsorption.The idea that some disturbance will occur is very reasonable having in r i n d the strengthof the bonds between adsorbate and the surface, and the large amount of energyavailable at the instant of adsorption.However, to assess the damage by the numberof layers which must first be field desorbed before a perfect surface is restored, ispossibly unreliable. The field greatly weakens the interactions between metal atomsbelow the surface, and, under these conditions diffusion of vacancies into the layerbelow might not be too difficult. That is the field itself may propagate the damage tothe subsurface regions to some extent.Prof.W. M. H. Sachtler (Amsterdam) said: Textbooks of inorganic chemistry 2state that tungsten hexacarbonyl is formed in macroscopic quantities by interaction ofcarbon monoxide and elementary tungsten at, e.g., 225°C. In the W(CO)6 moleculethe tungsten atom is octahedally surrounded by six CO ligands. Intermetallicbonds are completely broken under these circumstances. In view of the size of COmolecules and tungsten atoms, a W atom can only be lifted from the tungsten surfaceto the gas phase as a W(CO)6 molecule, if some CO groups have been attached to thisW atom from below. It follows that either CO molecules are able to penetrate intothe surface of a tungsten crystal or surface W atoms can jump above chemisorbed COmolecules. In view of this is is not surprising that similar rearrangements shouldbecome already noticeable at lower temperatures, where no macroscopic production1 Gomer and Swanson, J.Chem. Physics, 1963,39,2813.2 Remy, Lehrbuch der anotyanischen Chernie (Akademische Verlagsgesellschaft, Leipzig, 1959),11, p. 14968 GENERAL DISCUSSIONof UT(CO)6 takes place. At not too low temperatures the atomic vacancies created bythis process migrate also to deeper layers. The latter process is not influenced by anexternal electric field which, in a metal, cannot penetrate below sub-surface layers.Field effects during the act of adsorption were excluded in our work, since no field wasapplied when the gas was admitted to the tube.Dr.Gert Ehrlich (Scheizec:ady, New Yurk) said: It has often been assumed thatchemisorption of simple gases such as H2, N2, and CO on refractory metals involves aprofound disturbance of the lattice, even at room temperature. This assumption,however, appears to be only tenuously related to experimental fact. Thus in inter-preting their F.I.M. studies, Holscher and Sachtler (H & s) propose that tungstenatoms are displaced from their normal positions as a result of the interplay with CO.Even if we were to accept this, the ion images would only prove that the rearrangementand its variation with the temperature of adsorption occur for a surface under theinfluence of extremely high electric fields. During the observations by H & S atungsten atom at a kink site sits in a field ( >4 V/A) which amounts to 84-89 % ofthe field for rapid evaporation.A decrease in the binding energy of such an atom byas little as - 1.5 eV would bring about evaporation, yet in the absence of an appliedfield - 3 eV are required 1 ’ 2 just to move a tungsten atom from a kink site on to aflat, and N 8.7 eV are necessary for evaporation. The effect of the high field is furtherenhanced by promotion of field desorption through the image gas. For electro-negative materials chemisorbed on a metal this lowers the desorption field by as muchas one third the vacuum value.3 A small change in the bonding of tungsten atoms tothe lattice by an adsorbed gas can therefore be greatly magnified under observation inthe field ion microscope.Even if severe perturbations are unequivocally established,they are likely to be unique to the special conditions under which the measurementsare made and do not provide a definitive guide to the behaviour under ordinarythermal conditions.That chemisorption under ordinary thermal conditions may result in a reconstruc-tion has also been deduced from the appearance of additional intense spots in low energyelectron diffraction (L.E.E.D.) patterns of surfaces exposed to a chemisorbing gas. Intheir interpretation it has usually been assumed that scattering from the adsorbed layeritself is negligible. Additional diffraction spots therefore were rationalized as indicat-ing changes in the lattice itself. The fundamental assumption underlying this inter-pretation is not firmly based on either theory or experiment, however.Indeed, forCO on platinum, Tucker 4 has demonstrated the contrary. Probably the simplestexample of surface reconstruction deduced from L.E.E.D. has been reported for the(1 10) plane of Ni after chemisorption of hydrogen. Germer and MacRae 5 observedpatterns indicative of a lattice with twice the ordinary spacing in the [loo] direction ;this they ascribe to a reconstruction under the influence of adsorbed hydrogen, withevery second close packed row of nickel atoms missing. The interpretation of thesechanges, which require long range migration of Ni atoms, is not unique, however. Achemically more reasonable picture can be achieved on the assumption that pairs of[loo] rows are displaced towards each other (according to Tucker by -10 % theirnormal spacing).Even for hydrogen the assumption that the adatoms do not contribute directly toscattering may not be valid.In hydrogen adsorption on the (100) of tungsten,1 Sokolskaya, Soviet Physics-Tech. Physics, 1956, 1, 1147.2 Barbour, Charbonnier, Dolan, Dyke, Martin and Trolan, Physic. Rev., 1960, 117, 1452.3 Ehrlich and Hudda, Phil. Mug., 1963, 8, 1587.4 Tucker, Appl. Physics Letters, 1962, 1, 34.5 Germer and MacRae, J. Chem. Physics, 1962, 37, 1382GENERAL DISCUSSION 69Estrup and Anderson 1 find that at low coverages the diffraction patterns are typicalof a f.c.c. lattice with unit edge twice the tungsten [loo] spacing. Following Bauer,zwho pointed out that an adatom may rescatter electrons diffracted from the lattice,Estrup and Anderson interpret all their patterns as due entirely to hydrogen atoms,without any rearrangement of the underlying tungsten up to and including saturatedlayers.The same interpretation appears appropriate to L.E.E.D. studies of carbonmonoxide as well as nitrogen on the (100) of tungsten.3 For carbon monoxide uponthe (100) of nickel, Park and Farnsworth 4 also observed additional diffraction spots,corresponding to a surface mesh of twice the normal spacing. From the high intensityof these new reflections the authors concluded that either " CO produces a reconstruc-tion of the (100) nickel surface, or that diffraction intensity is not an adequate criterionfor assuming a reconstruction ".Hence, the appearance of new diffraction spots doesnot establish surface rearrangement and L.E.E.D. does not yet provide an unequivocalindication of surface reconstruction in the chemisorption of carbon monoxide,nitrogen or hydrogen on Ni and W.At the moment there is no unequivocal evidence to indicate a reorganization of thesurface, involving large-scale relocation of surface atoms, during low temperaturechemisorption ; there are, in fact, strong indications that this does not occur. Fromour F.I.M. studies we decided against any gross atomic displacements in dilute layersof nitrogen and carbon monoxide on tungsten, for which the interpretation of imagesis not yet too complicated; for CO, H & S agree with this conclusion.In principle,it should also be possible to deduce surface rearrangement from desorption measure-ments. Formation of a very strong bond between adatom and lattice, with severeweakening of lattice cohesion, should lead to desorption of an adatom together withan atom from the lattice during thermal evaporation. This would be detectable massspectrometrically, or indirectly, through differences in the amounts adsorbed deter-mined from ad- and desorption runs in flash filament experiments. At pressuresbelow 10-6 torr such an effect has not been observed for any of the systems consideredhere. 5-8Even if lattice atoms are only displaced, without forming a compound stableenough to manifest itself in the gas phase, differences should appear in the desorptionenergy, derived from kinetic studies, and from heats of adsorption, measured calori-metrically. Consider a hypothetical atom, which on chemisorption pulls a latticeatom from a kink and-on to a flat.At low temperatures the calorimetric heat ofadsorption would measure the enthalpy change in going from the gas phase to thisparticular configuration of the surface. Since it is unlikely that the extraction energyE, (liberated in restoring the displaced lattice atom to its original site) will be coupledto the desorption event, the heat of desarption would exceed the calorimetric value byEe ; if the gas is evolved in a bimolecular event, the energy discrepancy will amount to2E, per mole of gas for a one to one rearrangement. For tungsten the extractionenergy is several eV.However, the desorption energies determined from kineticmeasurements for nitrogen9 and carbon monoxide 10 on tungsten are in goodagreement with calorimetric heats for the same gases on evaporated films up toEstrup and Anderson, J. Chem. Physics, in press.2 Bauer, Physic. Reu., 1961, 123, 1206.4 Park and Farnsworth, J . Chem. Physics, 1965, 43,2351.5 Baker, Ado. Catalysis, 1955, 6, 164.6 Ehrlich, J. Physic. Chem., 1956, 60, 1388.3 Estrup, private communication.Redhead, Trans. Furaday Soc., 1961,57, 641.Ustinov, Ageev and Ionov, Soviet Physics-Tech. Physics, 1965, 10, 851.9 Ehrlich, J. Chent. Physics, 1962, 36, 1171.lo Brennan and Hayes, Phil. Trans., 1965, 258, 347.70 GENERAL DISCUSSIONmonolayer coverages, suggesting that rearrangement is not important.Finally, theagreement between the amounts of these gases adsorbed on films and their capacityfor rare gases,l gives no hint of site creation by adsorption. Although it is prudent tokeep the possibility of severe surface reconstitution in mind, at the moment theredoes not seem any necessity to invoke such an effect for the chemisorption of thesegases (Hz, N2, CO on W or Ni) under ordinary thermal conditions.Dr. A. A. Holscher and Prof. W. M . H . Sachtler (Amsterdam) (communicated) :We agree with Ehrlich’s remarks that the irr,age conditions influence the surfacestructure after adsorption. In fact, one of OUI main points concerns field effects,namely, where we have proved the occurrence of promoted field desorption ofcarbon monoxide molecules, which thus escape observation in the FIM.But! thereseems no reason for such induced effects not being equally present in Ehrlich’s ownFIM experiments on the adsorption of nitrogen and carbon monoxide on tungstenperformed in the same imaging field as in our work.While there thus are no essential differences between the experimental conditionsor the observations reported by Ehrlich and ourselves, there are differences in theinterpretation. Our conclusion of the observed disordered structures being dueto displaced tungsten atoms rather than to CO molecules is based on a number ofmutually consistent experimental results given in our paper, e.g., (a) electron emissionresults showed that under the conditions of FIM, most CO molecules are field-desorbed ; (b) field evaporation experiments revealed that the disorder had penet-rated several atom layers below the adsorbing surface ; (c) at 78°K disordered struc-tures were observed by FIM only after a high degree of CO coverage had beenobtained.The difference in temperature at which the FIM pictures were taken(Ehrlich, 20°K; our work, 78°K) seems cf little importance, as we conclude fromwork on the Nz+ W system. Here we used the same imaging temperature (20°K)as Ehrlich did, and still arrived at conclusions similar to those we obtained for theCO + W system.Once it is accepted that the disordered structures observed in the FiM are dueto displacements of tungsten atoms on a surface largely denuded of adsorbed en-tities, the question arises whether these displaceinelits are induced by the field itself.From our evidence we feel that the observed displacements predominantly takeplace already during the adsorption process; i.e., under conditions where in ourexperiments no field was applied.An important argument for this view is the ob-servation that the extent of disorder largely depends on the temperature of ad-sorption. Moreover, the disorder penetrates into deeper lying atom layers whichare shielded from the field.We realize that in the process of field desorption during imaging some atom dis-placements might be induced in addition to those already brought about by theadsorption itself. Being aware of these possible additional field effects we haverefrained from advancing too detailed an interpretation of the observed patterns,as the present knowledge of field effects and of the image formation does not justifythis.We do not see why there is so much reluctance in accepting the possibility ofsurface rearrangements at room temperature or even below it, when practice presentsso many examples of surface corrosion extending over macroscopic depths.Inany system where the bulk compound is known to be stable, chemisorption eventuallyhas to be followed by rearrangement processes. For example, for the N2+Wsystem it can be calculated from thermodynamic data 2 that at 300°K the nitrogen1 Brennan and Hayes, Phil. Tram., 1965, 258, 347.2 Kubaschewsky and Evans, Metal Physics and Physical Metalliirgy (1958)GENERAL DISCUSSION 71pressure in equilibrium with bulk W2N is about 10-11 torr.If, therefore, in theadsorption experiments at higher nitrogen pressures no bulk nitride is formed thisis due to kinetics only and not to thermodynamics.An intuitive feeling that reorganization of the surface in the presence of ad-sorbate is unlikely at room temperature because metal-metal bonds are so strongmight be misleading because the same intuitive reasoning would also fail to predicta dissociative adsorption of nitrogen at low temperatures, which requires the breakingof a bond of 226 kcal/mole. In both cases, nature apparently has at its disposala reaction path requiiing a low activation energy.Activation energies for desorption of nitrogen and carbon monoxide are deter-mined from kinetic measurements at high temperatures (TE 1000°K) where themobility of surface atoms even on a clean tungsten surface becomes appreciable,as has been shown by Muller 1 and by Ehrlich and Hudda.2 Thus, if adsorptioncauses a rearrangement of the suIface metal atoms it is most likely that upon de-sorption at high temperature the metal surface is reconstructed.The measuredvalues of the heats of adsorption and desorption then refer to the same initial andfinal states and therefore should be equal. Incidentally, we do not believe thatjumping of a tungsten atom from a surface site into a position between the adsorbedentities, where mutual co-ordination will be more favouiable, must be an endothermicprocess as Ehrlich seems to visualize.It seems to us that the inclusion of LEED and mass spectrometric results is oflittle help to the clarification of the present discussion because (a) results obtainedwith the CO + W system by either technique have not been published ; (b) the inter-pretation of LEED results still seems a controversial matter, as was pointed out byEhrlich ; ( c ) if a chemisorption complex decomposes rather than evaporates athigh temperature, mass-spectrometric analysis is of no relevance in elucidatingthe structure of the original surface complexes.As regards the question of the necessity for invoking a rearrangement of sub-strate atoms, in our view it gives a fair explanation for the irreversible conversionfrom the low-temperature into the high-temperature state in the cases of carbonmonoxide on tungsten (Gomer’s virgin states++u states) and of nitrogen ontungsten, as well as for many other experimental observations with these systems.Dr.J. M. Thomas (Univ. Coll. North Wales, Bangor) (partly commuiiicuted): Dr.Ehrlich has expressed some scepticism concerning the evidence, revealed by low-energy electron diffraction and field ion microscopy, for surface rearrangements duringor following adsorption. Gwathmey and his coworkers 3 , 4 have shown, by ordinaryoptical and replica electron microscopy, that extensive surface rearrangements takeplace when { 1001, (1 103 and certain other faces of copper single crystals function ascatalysts for the reaction of gaseous hydrogen and oxygen. From the studies carriedout by Meelheim et aZ.,5 it would appear that the surface rearrangement is initiatedand sustained primarily by the catalysis of the heterogeneous reaction.It does notappear to be identical with the phenomenon of thermal faceting (which takes placcwen in soft vacua) described by recent auth0rs.6~7 Surface rearrangement of copperatoms occurs freely during catalysis at temperatures as low as 400°C, whereas thermalMiiller, Z . Pl?vJik, 1949, 126, 642.2 Ehrlich and Hudda, J. Chern. Physics, 1961, 35, 1421,3 Gwathmey and Benton, J. Physic. Chem., 1940, 44, 3 5 .Leidheiser and Gwathmey, J. Amer. Chem. Soc., 1948, 70, 1200.Meelheim, Cunningham, Lawless, Azim, Kean and Gwathmey, Proc. 2nd Int. Cungr.Catalysis(Paris, 1960) (Technip Press, Paris, 1961), 1, 2005.6 Moore, in Metal Sitrfaces, Energetics atrd Striicture, (ASM-AIME Seminar, 1962), (A.S.M.Ohio, 1963). 7 Robertson, Acra Met., 1964, 12,24172 GENERAL DISCUSSIONfaceting, which is believed to involve 1 evaporation of the solid, requires a tempera-ture in the region of 1000°C.Ds. G. Ertl (Techn. Hochschule, Munich) said: We have studied with L.E.E.D.the interaction between N20 and a Cu( 100)-surface at 5OO0C, which leads to the forma-tion of gaseous nitrogen and adsorbed atomic oxygen. In the diffraction pattern ofthe clean (100)-surface (large circles) extra spots are visible after a short reaction timewith a twelvefold symmetry (small open circles), which correspond to the formationof two equivalent kinds of (1 1 1)-surfaces parallel to the (100)-surface, rotated by 90".These extra spots are divided into three points close together.This observation isascribed to multiple scattering between the (100)- and (1 1 1)-plane. Extra spotsresulting from multiple scattering can only be observed if there are differences in thelattice geometry. After a longer reaction time, some weak extra spots are visible inaddition (dark circles) with a nearly doubled lattice constant. These spots are attribu-ted to another surface structure with (1 1 1)-geometry.00 0 00 0 008O Q0 60004 0 0040 0 0000FIG. 1.The results can only be interpreted by the assumption that a rearrangement of thesurface copper atoms OCCUIS by the influence of atomic oxygen.Dr.T. A. Delchar and Prof. F. C. Tompkins (Imperial College) (communicated):Dr. Ertl is undoubtedly correct that rearrangement of surface copper atoms occursfollowing the chemisorption of oxygen during the N2Q decomposition. Some fiveyears ago we measured the change of work function of annealed copper (and nickel)films when oxygen was chemisorbed at 78°K and the temperature raised to roomtemperature. The negative surface potential (s.P.) due to the oxygen adatoms becamemore positive (with no desorption of oxygen) with increase of temperature; weinterpreted this as a " tunnelling " of the adatoms into the underlying copper to forman " ionic " species ; the newly exposed copper sites could chemisorb more oxygenadatoms, but the change of s.p.with number of oxygen atoms adsorbed was muchlarger showing that the electron distribution at the surface had substantially changed,and some reconstruction had probably occurred.1 Moore, in Metal Surfaces, Energetics and Structure, (ASM-AIME Seminar, 1962), ( A S M .Ohio, 1963GENERAL DISCUSSION 73Dr. A. A. Holscher (Amsterdam) said: The decrease in loglo A by a factor of 2,observed at low coverage in the carbon tetrachloride experiments by Duell et al., isascribed to a decrease in emitting area. As stated, the image (fig. 6b) does not showsuch a reduction, which was explained by assuming the emission to take place throughsmall windows with low work function. These windows, however, cannot beextremely small, because within an area with a length and width of the tunnel-barrier(10-1 5 A) a non-uniformity of the work function cannot be defined. With a resolutionof the image of about 20 A it is thus possible to have a checkerboard arrangement ofemitting and non-emitting patches which give a reduction of emitting area by a4actorof about four without this reduction being observed in the image.I think thereforethat it is more likely that another mechanism is responsible for the large drop inloglo A . It might be connected with the adsorption-induced disturbance of theperiodic potential at the surface, in the same way as we proposed for the CO/W system.Dr. D. W. Bassett (Imperial College) (partly communicated) : On the basis of fieldemission evidence given by Duell, Davis and Moss, the interaction of chlorine withtungsten surfaces is sufficiently similar to what has been observed by other workersfor oxygen on tungsten, that it should be valid to compare particular aspects of thebehaviour in these systems.However, the case for adsorbate penetration into thetungsten substrate in the initial stages of adsorption of these electronegative adsorbatesis weakened, rather than strengthened by the work function decreases claimed to occurduring oxygen chemisorption at 300°K.Considerable experimental evidence suggests that the work function does notdecrease during oxygen chemisorption at 300°K. For example, in a field ion micro-scope study of some aspects of oxygen adsorption at 300"Kl I found that work func-tions of oxygen-covered surfaces deduced from Fowler-Nordheim plots were alwayshigher than that of clean tungsten, no matter how small the coverage, or at what ratethe gas was added.The gas was added as small doses of arbitrary size, and the fieldemission data was taken under conditions of stable emission. In fact, at both 300°Kand 78°K the variation of the work function with the voltage required for a particularemission current was almost linear. Over the work function range, 4.5-5-5 eV, thepre-exponential factor of the Fowler-Nordheim equation decreased monotonicallyas the work function increased, loglo A changing from -9.55 to -10.15, and did notshow the large fluctuations given in table 2 by Duell et al. These observations areconsistent with the general features of oxygen chemisorption on tungsten as revealedin other field emission studies.2The conclusion that the work function rises steadily during oxygen adsorption ontungsten is further supported by the retarding potential measurements of workfunction made by Zingerman and Ishchuk.3 They found that when oxygen was addedto a clean tungsten surface from a molecular beam, there was initially a monotonicrise in work function at 300°K and also at 850°K.At the higher temperature, theirresults, low energy electron diffraction studies, and field ion microscopy all indicatethat extensive rearrangement of the tungsten surface accompanies adsorption. Onheating oxygen covered field-emitters, this rearrangement of the surface leads to fieldenhancement, especially over the (1 1 1) regions, that causes apparent decreases in workfunctions deduced from Fowler-Nordheim plots.Fig. Id given by Duell et al.indicates that this also happens with chlorine-covered surfaces, so that the workfunctions obtained in such heating sequences are not unambiguous evidence thatchlorine adsorption may lead to positive surface potentials. In view of the evidence1 Bassett, unpublished.2 Gomer and Hulm, J. Chem. Physics, 1957, 27, 1363.3 Zingerman and Ishchuk, Fiz. Tverd. Tela., 1964, 6, 117274 GENERAL DISCUSSIONwith oxygen adsorption, it also seems doubtful whether the interpolation procedureused by Duell et al. to deduce work functions from field emission measurements madeunder conditions of unstable emission is reliable, or whether the work functiondecreases obtained are real effects.These comments are not intended to imply that lattice penetration by electro-negative adsorbates does not occur. Oxygen certainly penetrates into tungsten at300°K in a slow activated process after completion of a saturated ad-layer, and thisuptake has been followed volumetrically,l and by field emission.2 In fact, it was thisprocess that Muller observed in the field ion microscope3: the observations of" surface corrosion " after exposure of a tungsten surface to oxygen at 20°K are notrelevant, since this was almost certainly due to damage caused in the imaging process.Dr. M. J. Duell, Mr. B. J. Davis and Dr. R. L. Moss (Warren Spring Lab.) (com-nzunicated) : In reply to Bassett, the point at issue is not whether electronegative gasescan " penetrate " the tungsten surface at 300°K but if this can occur at low coverages, inparticular with oxygen, as indicated by the present work function measurements.There are fewer field-emission studies than suggested of oxygen adsorption directlycomparable to those reported in the present paper. For example, work functions havenot been obtained from Fowler-Nordheim plots in some cases but from the emissioncurrent at constant voltage which is unreliable. Bassett reports that his results areconsistent with the general features of oxygen chemisorption on tungsten found byGomer and Hulm.4 The latter studied the spreading of oxygen deposited initiallyat very low temperatures and also work function changes on heating oxygen ad-sorbed at 300°K and " relatively high pressures '' such that A+ - 1.6-2.0 eV aftershort exposure times. In Bassett's work reported above the amounts of oxygenadmitted were not measured.5 We would emphasize the necessity for controlledadmission of oxygen or halogen compounds at very low pressures to reveal positivesurface potentials. In addition to our experiments under conditions of " unstableemission ", we used gas " doses " of chlorine or carbon tetrachloride (cf. fig. 4 and 7)and again found a reduction in work function at low coverages.The pre-exponential term of the Fowler-Nordheim equation has often beenneglected but in recent studies of nitrogen adsorption on tungsten it was found todecrease substantially from loglo A = -5.66 to -7-41.6 In this case the pre-exponential term decreased monotonically but on the (41 1) plane specifically,7 itincreased initially, followed by a decrease until A loglo A N 2. Apart from changesin emitting area, fluctuations of the pre-exponential term arise from the influenceof the applied field on the adsorbed layer depending on the polarizability of theadsorbate. It has further been suggested that a modification of the potential barrierby a corrected image-force potential should also be taken into account.71 Rideal and Trapnell, Proc. Roy. SOC. A , 1951, 204,409.2 Gomer and Hulm, J. Chem. Physics, 1957, 27, 1363.3 Miiller, in Structure and Properties of Thin Films, ed. Neugebauer, Newkirk and Vermilyea(Wiley, New York, 1959), p. 476.4 Gomer and Hulm, J . Chem. Physics, 1957,27, 1363.5 Bassett, personal communication.6 Ehrlich and Hudda, J. Chem. Physics, 1961,35, 1421.7 Van Oostrom, Philips Res. Reports Suppl., 1966, no. I

ISSN:0366-9033    

DOI:10.1039/DF9664100054

出版商:RSC    

年代:1966

数据来源: RSC

摘要:

Adsorption and Desorption of Hydrogen by EvaporatedMolybdenum Films at Low TemperaturesBY D. 0. HAYWARD, N. TAYLOR AND F. C . TOMPKINSDept. of Chemistry, Imperial College of Science and Technology, London, S.W.7Received 3 1 st January, 1 966The adsorption and desorption of hydrogen on evaporated molybdenum films has been studiedin the temperature range below 250°K. Sticking probabilities have been measured using a flowtechnique. The initial value (07) is higher than found in flash filament experiments due to themultiple collisions with the surface. At 78°K the adsorbed layer builds up on the outermost regionsof the film and comes into pseudo-equilibrium with the gas phase whilst the less exposed parts arestill bare. Above a coverage of 8 x 1014 molecules per geometric cm2 of film surface, cessation ofthe flow of hydrogen results in a slow decay of pressure P such that 1/P is linear with time.Thisis shown to be due to a slow redistribution of adsorbed hydrogen, via the gas phase, between theconcentrated outer layers and the interior surface of the film. The desorption of hydrogen fromfilms has been studied on warming from 78 to 300°K. Two, and in some cases three, pressurepeaks have been observed, but the spectrum is complicated by surface diffusion and rearrange-ments in the temperature range 170 and 200°K. It is concluded that there is good evidence for onlyone low-temperature state of adsorption of hydrogen on molybdenum; this is largely desorbedbelow 150°K.Much interest has been shown recently in the existence of weakly held statesof adsorption of elementary gases on clean metal surfaces, as demonstrated by suchtechniques as flash desorption, work function measurements and field emissionmicroscopy.Such states, significantly populated only at low temperatures, maywell play an important role in catalytic reactions, especially those which are knownto proceed readily at low temperatures, such as H2/D2 equilibration.The primary purpose of the present work was to investigate the nature of theselow-temperature states for the adsorption of hydrogen on evaporated molybdenumfilms. Experimental data have been obtained on the kinetics of adsorption andredistribution over the surface of the film, the nature of the isotherm at low tem-perature, and the ‘‘ desorption spectra ” of the hydrogen layer.EXPERIMENTALThe apparatus employed was a conventional u.h.v.system capable of an ultimatevacuum of about 7x 10-11 torr. Hydrogen was obtained from the British Oxygen Com-pany in sealed-off ampoules and was purified by passage through a palladium thimble.The apparatus was designed primarily to measure sticking probabilities, although it provedto be suitable for the other measurements. However, as the absolute magnitude of thesticking probability is not of major concern in the present paper, only a brief descriptionof the technique employed in its measurement will be given here. Further details willbe published elsewhere.The method is basically that of Wagener 1 in which a steady stream of gas flows intothe vessel containing the film.The sticking probability is obtained as the ratio of therate of adsorption to the rate of collision with the surface, the latter being calculated fromthe pressure at the film surface. The main problem arises in obtaining a meaningfulvalue for the ambient pressure in a system which is rapidly gettering the incoming gas.The problem has been solved using the arrangement shown in fig. 1 in which the gas is776 ADSORPTION OF HYDROGEN O N MOLYBDENU-Mintroduced at the centre of a bulb on which the film has been deposited. The method issimilar in principle to that used by Clausing,2 although deveIoped by us independently.The molybdenum film is evaporated on to the walls of the reaction vessel from the filamentF, with the diffuser D retracted into the vertical side-arm, and the nickel disc ND heldM.S.FIG. 1.-Cell used for measuring stickingprobabilities.magnetically over the orifice leading to theionization gauge IG.The latter precaution isessential to prevent deposition of film in theionization gauge sidearm otherwise spuriouslyhigh values of sticking probability (sometimesgreater than unity) may be obtained. Afterevaporation, the diffuser D is lowered intoposition and gas from a constant pressurereservoir is introduced through the inlet I.The tubing P leads to the pumps via a Deckervalve. Normally this valve was closed, al-though little difference in the pressure readingswas obtained if it was open, except when thesticking probability was extremely low (< 10-4).MS is a compact 180 degree mass spectrometer,built in this laboratory and is similar in designto that of Goldstone.3The cathodes in both IG and MS wererhenium filaments electrophoretically coatedwith lanthanum hexaboride to reduce theoperating temperature below that at whichsignificant atomization of hydrogen takesplace.Both gauges were operated with anelectron emission current of 10pA, at whichion pumping is insignificant. The massspectrometer and the ionization gauge could be operated simultaneously or separately.When the system was being pumped, all partial pressures were below the detection limit ofthe mass spectrometer (5 x 10-11 torr), and on isolating the system only helium could bedetected within a period of several hours.Desorption spectra were obtained by monitoring the pressure as a function of timeafter the liquid-nitrogen bath around the cell had been removed and the cell allowed towarm up in the atmosphere, with or without lagging around it. The temperature of thefilm was measured approximately by thermocouples fixed to the external wall of the cell.Molybdenum films, weighing between 30 and 50 mg, were usually deposited on the cellwall maintained at about 50°C.The total surface area, as calculated from the total uptakeof hydrogen, was, on average, about ten times the geometric area.RESULTS AND DISCUSSIONSTICKING PROBABILITY DATAFig. 2 shows the variation of sticking probability with coverage when the filmis maintained at 78°K.The initial values are considerably higher than those foundusing metal filaments 4 (-0-3), presumably because incoming molecules may makemore than one collision with the rough surface before being either adsorbed orreflected back into the body of the cell. In the particular experiment shown in fig. 2,the gas supply was stopped for 10-20 min at the points marked A. Up to a coverageof about 8-0 x 1014 molecules per geometric cm2 of film surface this cessation of gasinflow resulted in a very rapid decrease in pressure to the background value ( N 7 x10-11 torr) as shown diagrammatically in fig. 3a. On resumption of flow, the pressureincreased abruptly to its previous value, This behaviour is to be expected when thD. 0. HAYWARD, N. TAYLOR AND F.C. TOMPKINS 77film itself is pumping rapidly and the number of molecules in the gas phase isnegligible. Above this coverage, however, the pressure decreased only slowly tothe background value on stopping the gas supply, despite the high sticking proba-bility, as shown in fig. 3b; and on resumption of gas flow, the sticking probabilitywas initially much higher than before the stoppage, but fell to near the previousvalue on addition of a further 1013 molecules per cm2 (see fig. 2). At 78°K the1 6 3 0 L 2 10 2 0 ' 30 . ' 40 . ' 5 0 ' 'coverage (molecules per cm2)for 10-20 min at the points marked A.FIG. 2.-The variation of sticking probability with coverage at 78°K. The gas supply was closedtime 3FIG. 3.-Diagrammatic representation of pressure changes on closing and opening gas supply to(4 (b)film.A, gaq supply closed ; B, gas supply opened ; PO, background pressure.adsorbed hydrogen is immobile, and the more accessible (" outer ") regions of thefilm are covered first. The slow decay in pressure arises when some of these moreaccessible regions have become saturated and are in pseudo-equilibrium with thegas phase. On stopping the gas supply, the pressure falls, hydrogen desorbs fromthese saturated regions and is transferred via the gas phase to those regions of theinner surface that remain unsaturated. On restarting the gas flow the outer regionsof the film are able to adsorb further quantities of hydrogen and consequently highsticking probabilities are observed. However, these regions rapidly become satur-ated again and the sticking probability falls.On the basis of this model, stickin78 ADSORPTION OF HYDROGEN O N MOLYBDENUMprobabilities calculated from AP2 (see fig. 3b) should remain constant providedAPl B AP2, because the same, virtually saturated surface is exposed to the gas phaseafter each cessation of gas flow. This is found experimentally, as shown in fig. 2.Also, the onset of the slow process corresponds approximately to a break in thesticking probability curve, as is best seen in the inset to fig. 2. This is predicted bythe model outlined above since at the start of the slow process the most exposedregions of the film are no longer adsorbing gas.ISOTHERM AT 78°KThe molybdenum films could not readily be saturated by adsorbing hydrogenat 78°K due to a persistent slow uptake of gas, as is often found with evaporatedmetal films.However, by cycling the temperature of the film between 78 and300°K in the presence of hydrogen, this slow uptake was reduced to negligibleproportions, and it was possible to obtain an isotherm that was effectively reversible.coverage (molecules per cm2)FIG. 4.Temkin isotherm at 78°K.This is shown in fig. 4, where logP is plotted against coverage. The linear plot,which corresponds to a Temkin isotherm, indicates that the heat of adsorptionvaries linearly with surface coverage within the range of the data.KINETICS OF THE REDISTRIBUTION PROCESSFor the slow process, following cessation of the gas supply, the reciprocal ofthe pressure varies linearly with time as shown in fig.5. This relationship is wellobeyed both near the onset of the slow process when the pressure decay is stillfairly rapid, and also when the film is nearing saturation and the process takes 10-20min. Such a relationship can be derived theoretically if two assumptions are made :that the film consists of a system of pores of uniform cross-section, and that thesaturated areas of the film are at all times in equilibrium with the gas phase. Thesituation is shown diagrammatically in fig. 6 D. 0. HAYWARD, N. TAYLOR AND I;. C. TOMPKINS 79The saturated layer, each element of which is in equilibrium with the gas phaseimmediately above it, covers the outer surface and extends down the pore to thepoint E. The equilibrium has been found experimentally to obey the Temkin isotherm,In P = aN,+k,where NA is the number of molecules adsorbed per cm2, and c( and k are constants.4- 7time (sec)FIG.5 - 4 4 and @).-Plots of reciprocal pressure against time after closure of gas supply to film.l iPO D‘ EFIG. 6.-Diagrammatic representation of the redistribution process down a pore.The arrows indicate the direction of net gas flow.Differentiating with respect to time,This equation applies provided that 8N~Iat is small compared with the rates ofadsorption and desorption, i.e., a pseudo-equilibrium is maintained.By application of Fick’s first law of diffusion to the net flow of gas across theline AB,8 In Plat = adN,/dt. (280 ADSORPTION OF HYDROGEN ON MOLYBDENUMwhere ( a N ~ / d t ) , is the total number of gas molecules flowing across AB per sec,A is the cross-sectional area of the pore and D is the diffusion coefficient, assumedconstant.Similarly, for the flux across the line CD,The total rate of desorption from the saturated walls of-(dNA/dt)Ldx, where L is the circumference of the porethenrate of desorption =the element ABCD iswall.Using eqn. (2),(5)The equivalent of Fick's second law may now be obtained by considering massbalance in the element ABCD. The rate of change in the number of gas phasemolecules due to changes in pressure may be neglected, since it is very small com-pared with the rate of flow and the rate of desorption from the walls. Thereforeor, substituting eqn.(3) and (4), and rearranging, (Z)=&(y).A solution of this equation, applicable to the present problem, isL ( a - b x - x 2 )2aAD ( t + c ) 'p = -(7)where a, b and c are constants, the values of which are determined by the particularboundary conditions imposed. The pressure that is measured is Po, the value of Pat x = 0. Thus,This equation gives a linear relationship between reciprocal pressure and time, asfound experimentally, and gives support to the model of the redistribution processproposed.Po = aL/2aAD(t+c). (9)DESORPTION SPECTRAThe normal procedure in obtaining " desorption spectra " was to admit a knownquantity of hydrogen to a molybdenum film maintained at 78"K, record the pressurechanges on warming to room temperature, re-cool and repeat the process withfurther doses of hydrogen until the film was saturated. The hydrogen added at78°K is concentrated on the outer regions of the film, but, during the warm-up,redistribution takes place, some hydrogen being desorbed from this concentratedlayer, and re-adsorbed on the internal surface, except when the whole film is nearingsaturation.Redistribution of hydrogen also occurs by surface diffusion atsufficiently high temperatures.Fig. 7 and 8 show how the hydrogen pressure varies as the temperature of a filmis increased for a series of warm-ups at increasingly higher hydrogen coverages.In table 1 is recorded the quantity An of hydrogen adsorbed at 78°K prior to eacD. 0. HAYWARD, N. TAYLOR AND F.C. TOMPKINS 81warm-up, the total quantity TZT of hydrogen on the film, and an approximate valuefor the quantity n of hydrogen in the concentrated outer layer, each expressed inmolecules per geometric cm2 of film surface. n is evaluated as (An+ni/R), wheretemp. (“K)FIG. 7.-Desorption spectra between 78 and 300°K for a single molybdenum film,The numbers refer to desorptions at successively higher coverages.3temp. (“K)FIG. 8.-Desorption spectrum with film isolated from the pumps.Desorption from the same film as in fig. 782 ADSORPTION OF HYDROGEN ON MOLYBDENUMn> is the total amount of gas adsorbed up to the previous warm-up, and R is the ratioof real area to geometric area of the film (about 7.5 in this case). Below n = 6.8 x 1014molecules cm-2 no desorption is observed on raising the temperature of the film.This is in good agreement with other runs on 10 films in which approximately6-5 x 1014 molecules of hydrogen cm-2 must be added to a clean film at 78°K beforea desorption spectrum is observed.The slow redistribution of hydrogen via the gas phase at 78°K obeys the kineticspreviously discussed, and is always associated with occurrence of a desorptionspectrum. The discrepancy in the coverages at which the two phenomena are firstobserved arises because of differences in the quantities of weakly held hydrogenthat can be detected by the two methods.TABLE 1"T AntotaI HZ adsorbed(molecules cm-2 x 10-14H2 increment at 78°K(molecules cm-2 x 10-14)HZ concn.i: outer layer,(molecules cm-2 x 10-14)spectrum Of the desorption4.579.2 113.4418.6223.4527.8332.0745.574.574.644.235-1 84.834.384.2413.504.575.2 15-386.867.167.327.72no desorption3,9 912345In desorptions 1 to 4 the film was pumped for most of the time, although periodicisolation from the pumps made no detectable difference to the pressure-time curves,presumably because pumping by the inner areas of the film was much greater thanthat through the exhaust port. In desorption 5 the whole surface area of the filmwas close to saturation, and the warm-up was carried out with the system isolated.Pressure peaks or shoulders can be observed in fig.7 and 8 at 130, 170 and 220°K.The sharp drop in pressure between 170 and 200°K is attributed to the onset omobility within the adsorbed layer.This results in weakly held hydrogen, presentat 78°K in the concentrated outer layer of the film, migrating to strongly adsorbingsites on the inncr bare surface. This hydrogen would otherwise have desorbed attemperatures greater than 170°K. Thus, on this basis, the pressure maximum at170°K is not attributed to the existence of a separate adsorbed phase, because thepressure-temperature profile does not reflect the variation of population densitywith bond energy at this point.That mobility becomes important at 170°K is shown by the following experi-ment. Desorption data were obtained as before except that the liquid-nitroger,cooling bath was replaced around the cell at various stages during the desorpiionprocedure.In addition, sticking probabilities were measured during the adsorp-tion of hydrogen at 78"K, aiid they are displayed in fig. 9. After each teinperaturecycle there is an increase in the sticking probability measured at 78"K, due to theredistribution of the hydrogen that was concentrated in the outer regions of the film.The addition of mere gas at 78°K causes a fall in the sticking probability, and thenumber of molecules required to reduce it to its value prior to the temperature cyclegives a measure of the extent of the redistribution process. However, the redistributionis partly via the gas phase, and partiy via the surface. The contribution of the formerATJG can be calculated approximately from the pressure-time profile during the warm-up, and that of the latter ANs by subtracting the gas phase contribution from the totalD.0. HAYWARD, N. TAYLOR AND F. C. TOMPKINS 83The figures in table 2 are calculated in this way for the run shown in fig. 9. It maybe seen that at 170°K only a small number of adatoms have migrated, whereas at 200°Kthe majority have done so.coverage (molecules per cm2)FIG. 9.-The effect of redistribution on sticking probability.Open circles, sticking probabilities measured at 78°K ; filled circles, sticking probabilities measuredat 300°K; A, cooled to 78°K; B, warmsd to 300°K; C, warmed to 190°K and recooled; D,warmed to 200°K and rzcooled ; E, warmed to 170°K and recooled.However, complete equilibration does not appear to be readily achieved evenat higher temperatures, as evidenced by curve number 3 in fig.7. i n this run thetotal coverage of hydrogen on the film was defiritely below that required for satura-tion, even at 300"K, and yet desorption was still occurring at 230°K and above.TABLE 2total number of H2molecules adsorbed(cm-2x 10-14)18.1823-6431.9237.2339.0342.2550.0453.4154.82AN gas phase(molecules cm-2x 10-14)0.0420.120-290.320-970.650.940.472.3AN surface2.76.04-20.93-06.02.00-52.0(moleculzs cm-2 x 10-14)highest temp. reachedduring warm-up (OK)190300200170190300190170190If equilibration were complete at 200"K, only the high energy sites would be coveredabove this temperature, and desorption would not commence until the film wassaturated under the prevailing conditions of temperature and pressure.In runnumber 4, where the total coverage is higher, saturation appears to have been achievedabove about 240°K as the pressure is rising steeply. In fig. 8 the film is close tosaturation at 78°K and is apparently in pseudo-equilibrium with gas phase between140 and 170"K, above which the layer starts to rearrange. As the film is isolatedfrom the pumps and the number of molecules in the gas phase is negligible com-pared with those adsorbed on the film, the last part of the pressure-temperatureplot in fig. S should correspond to an isostere, provided equilibrium is achieved.It is found that this is not so below 250°K but, from other desorption spectra84 ADSORPTION OF HYDROGEN ON MOLYBDENUMisosteres were successfully constructed for temperatures between 250 and 270°K givingheats of approximately 17 kcal/mole.Fig.10 shows the desorption spectra for two films, both of which had beeneffectively saturated with hydrogen at 78"K, the rate of adsorption at this tem-perature having decreased to a negligible value. Part of the difference in the twodesorption spectra is due to the fact that in run (a) an attempt was made to equilib-rate the adsorbed layer by cycling the temperature between 78 and 220°K duringthe adsorption, whereas in run (b) the film was saturated with hydrogen at 300"K,and the remainder of the hydrogen was added with the film maintained at 78°K.The sharp decrease in pressure between 170 and 200°K in run (b) is again attributedto a rearrangement within the adsorbed layer, so that more hydrogen is accommodatedon the surface.I 1x id210 -5 -I Itemp.("K)too I 5 0 200 2 5 0FIG. 10.-Desorption spectra for two films.(a) cell lagged, adsorbed layer equilibrated ;(b) cell unlagged, adsorbed layer not equili-brated. Both desorptions were carried out withcontinuous pumping.150 200 2 !temp. ("K)3FIG. 11 .-Number of molecules desorbed perOK against temperature. (a) and (b) refer tothe same desorptions as in fig. 10.However, fig. 10 (and to a lesser extent, fig. 7) is somewhat misleading becausethe rate of temperature rise is continuously decreasing and this has a spurious effecton the shape of the pressure-temperature profile.Further, in run (b) the cell wasunlagged and the warm-up was rapid, whereas in run (a) the cell was lagged. It canbe seen from fig. 11 that if the number of molecules desorbed per cm2 per degreerise in temperature is plotted (instead of the instantaneous pressure), the shape ofthe desorption curve is considerably altered. This is especially true at the highesttemperatures where the rate of rise of temperature tends to zero. Since the systemis being pumped, it follows that the pressure must also tend to zero, irrespective ofthe population distribution of the various binding energies.NUMBER O F LOW TEMPERATURE STATES OF ADSORPTIONIt has been common practice recently to identify each maximum or shoulderon a pressure-time desorption profile with a separate state of adsorption.Theprevious discussion suggests that this may not always be a valid deduction, evenwhen using metal filaments or sheets which expose the whole of their surface directlyto the gas phaseD. 0. HAYWARD, N. TAYLOR AND F. C. TOMPKINS 85The present investigation provides evidence for, at most, two states of adsorptionin the low temperature range : one desorbing below about 150"K, and anotherdesorbing around 220"K, although the latter is doubtful, since it is not observedin all runs. Two studies4Y5 of the adsorption of hydrogen on molybdenum fila-ments or ribbons have been made, but, unfortunately, neither provides informationon the temperature region below 200°K.Pasternak and Wiesendanger 4 foundevidence for two states of adsorption when desorbing from 225"K, but since theirsystem contained no means of chemical identification, the possibility that one ofthe states was due to CO or some other contaminant cannot be ruled out (seeHickmott,6 and Moore and Unterwald 5). However, valid comparisons can prob-ably be made with the related H2/W system ; Hickmott 6 found one state of ad-sorption above 195"K, designated j?, and another desorbing below this temper-ature, designated a. Recently, Ricca, Medana and Saini 7 have obtained desorptionspectra for hydrogen adsorbed on a tungsten sheet that show six pressure peaks,labelled y1, 7 2 , al, UZ, p1 and p2, the y peaks being identified with Hickmott's a phase.However, the differentiation of the desorption above 190°K into a and p peaks maybe artificial due to the fact that the desorption was carried out in two stages, 78 to300°K and 300 to 600°K.No such differentiation is observed by other workers 5 9 6who desorbed in a single flash.In neither of the two investigations discussed above was an attempt made toachieve equilibrium in the adsorbed layer at 78°K by cycling the temperature duringadsorption of hydrogen, although Ricca et aZ.7 saturated the surface at 300°K beforecooling to 78°K in some of their experiments. It is relevant, therefore, to comparetheir desorption spectra with curve (b) in fig. 11, rather than curve (d). This com-parison would suggest that the distinction between Hickmott's a and p states, andbetween Ricca et al.'s y2 and a2 states, arises largely because of a surface rearrange-ment, which enables weakly held hydrogen to migrate to more strongly adsorbing sites.We conclude, therefore, that there is good evidence for only one low-temperaturestate in the adsorption of hydrogen on Mo or W, and that it is largely desorbed by150°K. This state can probably be identified with the y1 peak of Ricca et uZ.andwith the onset of the a peak in Hickmott's work. This should not be understoodto preclude the many adsorbed complexes observed by Rootsaert, van Reijen andSachtler 8 in the field emission microscope, but merely to indicate that the heats ofadsorption of these complexes are not sharply defined, but vary with coverage andoverlap one another.NATURE OF THE LOW-TEMPERATURE STATE OF ADSORPTIONThis state is responsible for the slow redistribution process that occurs via thegas phase at 78°K when the flow of hydrogen to the molybdenum film is stopped.The lowest coverage at which it is observed is about 6-5 x 1014 molecules per geo-metric cm2 of film surface adsorbed at 78°K.However, if the film is first saturatedwith hydrogen at 300"K, this weakly held state can be detected after very smallquantities of hydrogen have been adsorbed at the low temperature. This behavioursuggests that the weakly held hydrogen can only exist above a layer of the stronglyheld p state. One possibility is that it is molecular and is held over single vacanciesin the atomic layer. This would explain why this state is less pronounced whenthe surface layer is equilibrated by temperature cycling (see fig. ll), as the singlevacancies would be largely eliminated. Similar behaviour was observed by Riccaet aZ.7 who found that their y1 peak was considerably reduced if the surface wassaturated at 300°K prior to the adsorption at 78°K86 ADSORPTION OF HYDROGEN ON MOLYBDENUMWe gratefully acknowledge the support of the Ministry of Aviation in carryingout this work and also in providing one of us (N. T.) with a grant, and other financialassistance from Shell Research N.V., Amsterdam.1 Wagener, Brit. J . Appl. Physics, 1950, 1, 225.2 Clausing, Trans. 8th Vacuum Symp., 1961, 1, 345.3 Goldstone, Rev. Sci. Instr., 1964, 35, 1265.4 Pasternak and Wiesendanger, J. Chem. Physics, 1960, 34,2062.5 Moore and Unterwald, J. Chem. Physics, 1964, 40,2626.6 Hickmott, J. Chem. Physics, 1960, 32, 810.7 Ricca, Medana and Saini, Trans. Faraday SOC., 1965, 61, 1492. * Rootsaert, van Reijen and Sachtler, J. CataZysis, 1962, 1, 416

ISSN:0366-9033    

DOI:10.1039/DF9664100075

出版商:RSC    

年代:1966

数据来源: RSC

摘要:

Calorimetric Studies of the Chemisorption and Desorptionof Hydrogen on Nickel Films under Ultra HighVacuum ConditionsBY F. J. BROCKER AND G. WEDLERInstitut fur Physikalische Chemie und Elektrochemie der TechnischenHochschule Ilannover, GermanyReceiced 1st February, 1966Heats of chemisorption and desorption of hydrogen on evaporated nickel films have been de-termined at 273°K with an improved Beeck-type calorimeter. The heats of chemisorption aresignificantly lower than those previously reported in the literature. They are independent of thecoverage up to nearly a complete monolayer. When the adsorption became reversible, differentialheats of desorption could also be measured. The change of the electric resistance of the nickel filmswith increasing hydrogen coverage was simultaneously measured.The resistance passes through amaximum. The differential heat of chemisorption is not altered markedly at this coverage. Repeatedadsorption and desorption processes at high coverages indicate a further irreversible sorption ofhydrogen by the nickel films.The chemisorption of hydrogen on evaporated nickel films has been the subject ofnumerous investigations. Volumetric measureinents,l 2 studies of the work function,3the surface potential,49 5 or the contact potentia1,eS 7 investigations of the change ofthe electric resistivity 3 , 8-10 and the heat of adsorption 11-14 yielded much informa-tion on the interaction of hydrogen with nickel surfaces.The results obtained by different investigators and by means of different methods,however, are frequently not in agreement with one another. This may be due to theinfluence of the conditions, under which the nickel films had been prepared.Furthermore it is often difficult to compare the dependence of different adsorptioneffects on surface coverage, since this quantity is not unambiguously defined by theauthors.In order to explain the change of the electric resistance of nickel films due tochernisorption of hydrogen, additional measurements of the heat of adsorption anddesorption were carried out simultaneously on the same film.An improved Beeck-type calorimeter 15 was used, which could be operated under ultra high vacuumconditions. This is essential, since both the change of resistivity 8 as well as the heatof chemisorption *j are strongly affected even by small surface contaminations.EXPERIMENTAL‘The ultra high vacuum apparatus was the same as that described earlier.15 I t wasconstructed of Duran glass without stopcocks or greased joints in the high-vacuum section.The whole apparatus could be baked out at 380-400°C in order to achieve a vacuum betterthan 2 x 10-10 torr.The calorimeter differed from the original Beeck calorimeter in the use of a sphericaladsorption vessel and of a resistance thermoineter that consisted of only 10 p thick tungstenwires, embedded in glass sealed directly to the thin walled bulb.This shape of the calori-meter is more advantageous than a cylindrical tube, because a more uniform distribution of888 CHEMISORPTION OF HYDROGEN ON NICKEL FILMSthe hydrogen is achieved when the gas is admitted to the film.16-18 The sealed-in themo-meter guarantees a very good heat contact.The maximum sensitivity of the calorimetricdevice is a deflection of 6.4 mm on the recorder chart for a temperature change of 1 x 10-5 deg.at room temperature and of 2-6 mm at 77°K.At two spots opposite to one another, platinum foils, 5 ,u thick, were sealed to the innerwall of the calorimeter bulb in order to measure the electric resistance of the films evaporatedfrom the small coil of spectroscopically-pure nickel wire M.R.f~A -. I 1.- *FIG. 1 .-Calorimeter. F, outer jacket ; Thy resistance thermometer ; Rth, connection terminal ofresistance thermometer ; Pt, platinum contact foils ; Rf, connection terminals of the platinum foils ;My nickel wire.During the evaporation of the films the calorimeter was surrounded by a bath of liquidnitrogen.A hydrogen pressure of about 10 torr was maintained in the outer jacket F of thecalorimeter in order to keep the temperature of the films below 150°K. The evaporationof the films was carried out at a pressure less than 2 x 10-10 torr with a rate of about 10 Almin.After the evaporation the films were sintered at 60°C for 1 h, until no further decrease of theresistance of the film occurred due to ordering effects.The adsorption experiments were carried out at a temperature of 273"K, while thecalorimeter was surrounded by a bath of crushed ice, and the outer jacket had been pumpedto high vacuum.The temperature drift was usually about 10-5 deg./min.The hydrogen was purified by diffusion through a palladium tube. It was added in smalldoses. During each adsorption process the change of the resistance of the evaporatednickel film, the change in temperature (resistance thermometer) and the gas pressure bothin the storage bulb as well as near the calorimeter could be measured simultaneously bymeans of four Wheatstone bridges, amplifiers and four recorders. Xt can be estimated, thatthe change in resistivity of the film is only due to the electronic interaction and not to theheat evolved during adsorption.The calibration of Beeck-type calorimeters has been the main difficulty in exact determina-tion of differential heats of adsorption.123199 20 The heat capacity of the calorimeter wasdetermined by means of two methods recently described.15 The " stationary method " issimilar to the '' equilibrium method " used by Wahba and Kemball,l2 the " pulse method "is similar to that used by Bagg and Tompkins.2F. J.B R ~ C K E R AND G . WEDLER 89The stationary method makes use of the equilibrium established between the heat gener-ated by the electric current passing through the evaporated film and the loss of heat by radia-tion. The heat capacity is evaluated bysE2aC =4-18 x IO-~R,A,~/C'where s = deflection of the recorder, when varying the resistance Rth of the thermometerby 10-4Rth, E = potential drop along the film resistance Rf, a = temperature coefficientof the resistance thermometer, A , = deflection of the recorder at temperature equilibrium,Y = Newton's cooling constant, r/C is obtained from the logarithmic plot of the heating orcooling curve.When using the pulse method the electric current is switched on only for a short timeinterval t.With the assumption that the rate of increase of temperature dATldt is pro-portional to the sum of the rates of heat production dQ+/dt and heat loss dQ-/dtdAT --- - l(,Q+ - +- df;) ' (2) dt C dtC can be calculated bywhere A is the deflection of the recorder, when the current is switched off at time t.The values of C calculated by the stationary and the pulse method are slightly different,if only short pulse times are used. For instance, the value obtained with pulse times ofabout 10 sec was 0-573 cal/deg., whereas the value obtained with the stationary method is0-607cal/deg.Thus the pulse method results in a capacity of the calorimeter, which isabout 6 % smaller than the value calculated by means of the stationary method. This isdue to the fact, that eqn. (3) is only valid, if heat equilibrium in the system metal filmlglasslthermometer is established. This seems not to be achieved during short pulse times.Nevertheless, the value obtained by means of the pulse method was used, since the processof adsorption also occurs within such short periods.The heats of chemisorption AU were determined by means of a numerical method 1 5 ~ 2 1taking the kinetics of gas adsorption into account. Frequently the rate of adsorption iscontrolled by the diffusion of the gas resulting in a first-order law.In this case AU isgiven byCNL10-4 k,-riC dAUS k,n dD______ AU = (4)where NL = Avogadro's number, k, = rate constant of the gas adsorption, which can bedetermined from the change of the resistance of the evaporated film, n = number of moleculesadsorbed, dA/dD = slope of the straight line obtained, when A is plotted against{exp (-(r/C)t)-exp (--k,,t)). Eqn. (4) may only be used, if the kinetics of the adsorptionis of first order. Otherwise no such simple numerical evaluation can be carried out. Thisis the case, e.g., when the gas is desorbed by pumping.Following a proposal by Wittig22 the heats of adsorption and desorption were alsocalculated by means of the equation,(dQ+/dt)dt = CdT'+ r(Tk- T0)dt.(5)On integration between the time limits tl and t2 one obtainswhere Tk is the temperature of the calorimeter and TO the temperature of the environment.Eqn. (6) is only valid, if the calorimeter has again attained its original temperature duringthe interval tl to t2 (quasi-isotherm)90 CHEMISORPTION OF HYDROGEN ON NICKEL FILMSQ+ is either the heat generated by the electric current, the adsorption or the desorptionprocess, i.e., the heats of adsorption and desorption may be determined by comparison ofthe areas on the recorder chart. Since this method is independent of the underlying kinetics,it was used to determine the heats of desorption, for which the kinetics are not known.RESULTSFig. 2 shows a typical recorder diagram for the temperature change due to chemi-sorption (a) and desorption (b).The amount of hydrogen chemisorbed was 6-74 x 1015molecules, corresponding to 8.64 x 1013 molecules per cm2 or about 1/20 of a completemonolayer. The monolayer capacity is defined as that coverage, at which eachnickel surface atom is occupied by one hydrogen atom. The maximum increase oftemperature in fig. 2d was 2.9 x 10-4 deg., the heat of chemisorption 15.6 kcal/mole.In the desorption, 8.4 x 101s molecules were pumped off, the temperature was loweredby 2.9 x 10-4 deg., and the heat of desorption was 19 kcal/mole.3Ln 3 '0m0Wtime (min)u% O-00(d - I - .m .c1-3 s-2 -b-3t 1 I I I I I0 I 2 3 4 5 6time (min)FIG. 2.-Variation of the temperature of the calorimeter during the adsorption (a) and desorption (b)of hydrogen.The variation of the resistance of the nickel film and of the heat of chemisorptionwith coverage is shown in fig.3. The doses of hydrogen admitted were about1.1 x 1014 molecules per cm2 of surface.The range, in which the adsorption is reversible is of particular interest. Forequilibrium pressures of higher than 10-5 torr, the heat of adsorption decreases.The residual hydrogen in the gas phase is not adsorbed even after a few hours. Ifthe gas, however, is pumped off, further doses of hydrogen may be adsorbed with ahigh heat of adsorption, as is to be seen from fig. 3. The full lines refer to adsorption,the dotted lines to desorption. The coverage at which the heat of adsorption beginsto decrease is shifted to higher coverages with each new admission.The heats ofdesorption (triangles) lie very well on the expected desorption curves (dotted lines).The electric resistance of the hydrogen-covered film increases when the gas isdesorbed. Further additions of hydrogen result in a new descrease of resistanceF. J . BROCKER AND G. WEDLER 91After pumping for a very long time (about 10 h) the film resistance nearly attains thevalue of the resistance maximum. The adsorption curves of the resistance are shiftedparallel to higher coverages with each new gas admission. At a given hydrogenpressure in the gas phase both the resistance of the film as well as the heat of adsorptionhave a certain value, although the amount of adsorbed hydrogen increased with eachadsorption-desorption cycle.0 0 - 5 1.0 1.5coverage (lo15 molecules/cm2)FIG.3.-Variation of resistance and of heat of chemisorption with coverage.The good reproducibility of the calorimetric measurements is seen in fig. 4, inwhich the results obtained on three different films are compared. The heat ofadsorption is always the same within the experimental error; the only differencebeing that the adsorption capacity of film (a) is slightly higher.DISCUSSIONThe properties of the nickel films prepared as described have been thoroughlystudied in earlier papers. The films exhibit a fairly small internal surface, theirroughness factor being dependent on the thickness. For films used in this paper(about 100 .$) the roughness factor is about two.16s23 No proportionality betweenthe thickness and the adsorption capacity 9 was found, as has been reported byKlemperer and Stone 13 who prepared their films in a different way.The films arefairly homogeneous, consisting of small crystals of nearly equal size.23 The latticeconstant of the f.c.c. crystals is slightly smaller than that of bulk nickel.24 Theelectric conductivity of the films may be explained by the theory of Fuchs andSondheimer,25 showing that they have no island structure26 and that the tunneleffect plays no important role in the conductivity.The change of the film resistance with increasing hydrogen coverage observed inthese experiments is the same as that described in earlier ~apers.3~ 8-10 Thepartial reversibility of this effect at high coverages, however, has so far not beenstudied in detail and will be discussed in connection with the heats of adsorption.The heats of chemisorption, as well as their dependence on coverage, are differentfrom those reported in the literature.For comparison, the results of Beeck, Coleand Wheeler,ll Wahba and Kembal1,lz Klemperer and Stone,l3 Brennan and Hayes,l92 CHEMISORPTION OF HYDROGEN O N NICKEL FILMSRideal and Sweett,27 and of this paper are shown in fig. 5. It has to be consideredthat surface coverage is defined in a different way by the various authors, thereforethe abscissa of fig. 5 has merely a qualitative meaning. The characteristic feature ofthe curves, however, is not effected.The initial differential heats of chemisorption range from 18 to 42 kcal/mole.Since in all cases the differential heat of chemisorption is about 20 kcal/mole, whenthe monolayer is completed, the change in the differential heats with surface coverageis the larger the higher the initial heats of adsorption are.coverage (1015 molecules/cm2)FIG. 4.-Variation of heats of chemisorption of hydrogen with coverage as obtained on three differentnickel films (A, X , e) of about 100 A thickness. The arrows indicate the position of the maximumin the resistance curves.Two factors may be responsible for the different results illustrated in fig. 5, thestructure of the evaporated films and the vacuum conditions.The structure isdependent on the temperature of deposition, the rate of evaporation and the handlingafter the evaporation, i.e., the temperature and time of tempering.The films usedby Beeck, Klemperer and Stone, Wahba and Kemball, and by Brennan and Hayes,were deposited at room temperature. Their structure and their properties will besomewhat different from those of the films used in this work. It is, however, not tobe expected, that the difference is solely due to such a structural effect. This can beconcluded from the fact that the differential heats of adsorption of hydrogen on anickel film deposited at 77°K and annealed at room temperature were the same as thosein fig. 4, referring to nickel films tempered at 60°C. On the other hand it has beenclearly demonstrated by measurements of the photoelectric work function 28 and theelectric resistivity,29 that the properties of nickel films sintered at room temperatureand at 60°C are significantly different.The vacuum conditions are known to have a marked influence on the heat ofadsorption and its dependence on the surface coverage.Recently, this has beenclearly shown by calorimetric studies of hydrogen chemisorption on evaporatedtitanium films.15 For films evaporated and sintered under a pressure smaller than2 x 10-10 torr the heat of chemisorption is 27.5 kcal/mole and is independent of surfacF. J. BROCKER AND G . WEDLER 93coverage up to half a completed monolayer. If the pressure, however, was 1 x 10-9torr, the initial heat of adsorption was 35 kcal/mole and decreased on further hydrogenadsorption.Further experiments have to be carried out in order to study the influence of thestructure of the films and of the vacuum conditions on the heat of adsorption.Fromthe results in fig. 3 and fig. 4, however, it may be deduced, that the films prepared in thedescribed manner, are energetically fairly homogeneous with respect to the adsorptionof hydrogen at room temperature. It would appear that the constancy of the heatof chemisorption is not due to an immobile layer, as suggested by Beeck.11 Thisconcept would not be consistent with the finding that the hydrogen can be pumpedoff to an appreciable extent.‘(1 o l , , , , I0 ol* 5 1.0coverage (monolayer : 8 = I)FIG. 5.-Dependence of heat of chemisorption of hydrogen on coverage for evaporated nickel films.1, Klemperer and Stone 13 - - -; 2, Beeck, Cole and Wheeler 11 -; 3, Wahba and Kernball12 - - - . - ; 4, Rideal and Sweett z7 .. . ; 5, Brennan and Hayes 14 - . . . - . . . - ; 6, this paper - - -.In earlier papers 3 the existence of the resistance maximum was attributed to thepresence of two or more species of adsorbed hydrogen, one of which should increasethe resistance (hydrogen polarized towards H-), the other decrease the resistance(proton +electron). It would be expected that these species would give different heatsof adsorption, i.e., that the heats of adsorption should be altered at a coverage nearthe resistance maximum. The heats of adsorption, however, are practically in-dependent of surface coverage.The slight minimum in fig. 3 may be within theexperimental error. * Although the existence of different species of adsorbed hydrogenhas been experimentally verified,s. 30 the reason for the appearance of the maximumin the resistance curve cannot be given before the nature of these species is known.Both from the behaviour of the resistance as well as of the differential heats ofchernisorption and desorption at coverages beyond the resistance maximum it may b94 CHEMISORPTION OF HYDROGEN ON NICKEL FILMSconcluded that part of the hydrogen is irreversibly sorbed. As mentioned above theresistance as well as the heat of adsorption are functions of the hydrogen pressure.For the heats of adsorption, this has also been observed by Klemperer and Stone.13On the other hand, the coverage is not a function of the gas pressure. Consequentlythat part of the hydrogen, which is irreversibly adsorbed at high coverages does nottake part in the equilibrium between the gas phase and the adsorbed phase.Thesame conclusion has been reached for hydrogen adsorption on palladium films atroom temperature 31 using isotope techniques.The authors thank Prof. Dr. R. Haul for his interest and for numerous discussions,Dr. H. Strothenk, who studied the titanium + hydrogen system, and Dipl. Phys.,P. Wii3mann for their assistance. The support of the Deutsche Forschungsgemein-schaft, the Verband der Chemischen Industrie and the Max-Buchner-Forschungs-stiftung is gratefully acknowledged.1 Gundry and Tompkins, Trans.Faraday SOC., 1956,52, 1609.2 Gundry and Tompkins, Trans. Faraday Soc., 1957,53, 218.3 Suhrmann, Mizushima, Hermann and Wedler, 2. physik. Chem., 1959,20, 332.4 Culver, Pritchard and Tompkins, 2. Elektrochem., 1959, 63, 741.5 Crossland and Pritchard, Surface Sci., 1964, 2, 217.6 Mignolet, Disc. Farday SOC., 1950, 8, 105.7 Mignolet, Rec. trav. chim., 1955, 74, 685.8 Sachtler and Dorgelo, Bull. SOC. chim. Belg., 1958, 67, 465.9 Mizushima, J. Physic. SOC. Japan, 1960,15, 1614.10 Ponec and Knor, Coll. Czech. Chem. Cornm., 1960,25,2913.11 Beeck, Cole and Wheeler, Disc. Faraday SOC., 1950, 8, 314.12 Wahba and Kemball, Trans. Faraday Suc., 1953,49, 1351.13 Klemperer and Stone, Proc. Roy. SOC. A, 1957,243, 375.14 Brennan and Hayes, Trans. Farraday SOC., 1964, 60, 589.15 Wedler and Strothenk, Ber. Bunsenges. physik. Chem., 1966,70, 214.16 Wedler and Fouad, 2. physik. Chem., 1964,40, 12.17 Suhrmann and Wedler, 2. Elektrochem., 1959,63, 748.18 Brennan and Jackson, Proc. Chem. SOC., 1963, 375.19 Brennan, Hayward and Trapnell, Proc. Roy. SOC. A , 1960,256, 81.20 Bagg and Tompkins, Trans. Faraday Soc., 1955, 51, 1071.21 Wedler, 2. physik. Chem., 1960, 24, 73.22 Wittig, private communication.23 Suhrmann, Gerdes and Wedler, 2. Naturforsch., 1963, I&, 1211.24 Suhrmann, Wedler, Reusmann and Wilke, 2. physik. Chern., 1960,26,85.25 Sondheimer, Adv. Physics, 1952, 1, 1.26 Wedler and Fouad, 2. physik. Chem., 1964,40, 1.27 Rideal and Sweett, Proc. Roy. SOC. A , 1960,257,291.28 Suhrmann and Wedler, 2. angew. Physik., 1962, 14, 70.29 Wedler, Brocker, Kock and Wolfing, in Grundprobleme der Physik dunner Schichten (Gottingen,30 Rootsaert, van Reijen and Sachtler, J. Catalysis, 1962, 1, 416.31 Suhrmann, Schumicki and Wedler, 2. physik. Chem., 1964,42,187.1966), in press.* Addedinproof: Further experiments have shown that the minimum in the heat of chemisorp-tion in figs. 3 and 4 is a reproducible effect

ISSN:0366-9033    

DOI:10.1039/DF9664100087

出版商:RSC    

年代:1966

数据来源: RSC

摘要:

Heats of Adsorption of Oxygen on Evaporated Films ofMolybdenum, Tungsten, Cobalt and Nickel at 77, 90 and273'K, and Nature of Adsorbed LayersBY D. BRENNAN AND M. J. GRAHAM"Donnan Laboratories, University of LiverpoolReceived 24th January, 1966Calorimetric measurements are reported which show that oxygen adsorbed to saturation onmolybdenum and tungsten at 77°K has the same energy as oxygen adsorbed on these metals at273°K to the same coverage. The additional coverage at 273"K, either for a surface initially satur-ated at 77°K or for a surface maintained at 273°K throughout the adsorption, is effectively thesame in both cases and is associated with the same energy which is lower than that of the initialstate and falls with increasing coverage. It is argued that the final state of the adsorbed layer onthese metals at 273°K is independent of the temperature path.A model is proposed for each stageof the adsorption.For cobalt and nickel at 77"K, despite the similarity of the saturation coverages to those ob-tained at 273"K, the heats of adsorption are lower than those at 273°K. This is explained interms of the formation at 77°K of a chemisorbed layer confined to the surface proper of the metal,whereas, at 273°K incorporation of oxygen into an oxide layer occurs. Adsorption of oxygenat 273°K on a surface previously saturated at 77°K results in a further uptake amounting to 30-40 %of the initial adsorption at 77°K. It is argued that, on certain planes, the oxygen initially adsorbedat 77°K is to be found at 273°K sandwiched between the first and second layers of metal atoms,and that the metal atoms again exposed in the surface proper are able to adsorb further oxygen.Models for the adsorbed layers are proposed.Low temperatures can give rise to states of adsorption not observed at highertemperatures.In the present work, the energy of the adsorbed state and the sur-face concentration are used to establish the existence of temperature-dependentstates of adsorbed oxygen and to aid the elucidation of possible models.EXPERIMENTALMaterials, gas handling and vacuum technique have been described previously.1CALORIMETRYThe design of the calorimeter and methods of measurement are essentially the same as thepreviously described.132 Two calorimeters were used, viz., calorimeter A, which was thesame instrument as was employed for our previous measurements, and calorimeter 33.The windings of thermo-pure platinum wire (Rloo/Ro> 1.3923, supplied by Johnson,Matthey and Co.Ltd.) on the egg-shell tube of calorimeter B had a smaller pitch (0.05 in,)than hitherto.The values of (Cla)~ (see ref. (1)) for the new calorimeter at different temperatureswere determined by two methods, both of which relied on the dissipation of known amountsof electrical energy in tungsten films laid in the calorimeter, but which diflered in themethod of achieving electrical contact with the films. In one method, electrical contactwas made by brushes similar in shape to those used previously, but instead of several strandsof piano wire, four arms of spiralled steel wire, each terminating in a blob of soft solder,* present address : Division of Applied Chemistry, National Research Council, Ottawa, Canada996were used.In the other method, two bands of 43 s.w.g. steel foil, 2mm in width, wereemployed to give an even larger area of contact with the film. As pointed out previously,2the calorimeter tube is generally at a higher temperature than that of the bath. Measure-ments of (C/~)B were made for various heat inputs and rates of heating; the values ob-tained at the various temperatures are given in fig. 1. The close agreement between thedifferent measurements supports the conclusion that the quantity measured is accuratelyC/a and that there is no significant systematic error due to a contribution to the heatcapacity from the contact assembly, nor difficulty arising from the possible presence ofhigh contact resistances.The low temperature values for (C/C()A used to derive results inthis paper, have been based on the ( C l a ) ~ values given in fig. 1. This is necessary becauseADSORPTION OF OXYGEN ON METALS110100-96ao706 05031------77% 90°KI I I I35 40 45 5 0FIG. 1.-Values of (C/& as a function of the resistance R of the calorimeter. The figure givesthe results for measurements at low temperatures (0, spring contacts ; 0 , strip contacts). At195"K, (C/O~)B = 490klO cal, R = 112 D and, at 273"K, (C/CC)B = 926fl0 cal, R = 162 8;the standard deviations have been obtained from six measurements in each case.a series of (Cla)~ values is not available in the entire temperature range 77-90°K and ispermissible because the materials used in the construction of the two calorimeters wereequivalent ; calorimeter A is unfortunately no longer available for further heat capacitymeasurements to be made on it.RESULTSThe heats of adsorption at the various temperatures are presented in fig.2 and3. The parameter p is defined by the ratio, p=N(O)/N(Kr), where N(Kr) is thenumber of krypton atoms present in the monolayer at 77"K, and N ( 0 ) is the numberof oxygen atoms adsorbed by the same surface at the temperature specified. Eachpoint corresponds on the coverage scale to the mean coverage due to a gas incre-ment. The results obtained with the two calorimeters are in good accord.ADSORPTION ON CLEAN SURFACESAlthough values for the heats of adsorption of oxygen on films of these metalsat 300°K are available,l measurements were made at 273°K in order to confirmearlier work and to strengthen the characterization of calorimeter B.Satisfactoryagreement with the previously published integral heats is obtained (see fig. 2 and 3) ;correspondence between the two sets of maximum heats is not quite so close in everycaseD. BRENNAN AND M. J. GRAHAM 97The data of fig. 2 and 3 show that (a) for molybdenum and tungsten, while thesaturation coverage at 77°K is smaller than at 273"K, the heats of adsorption areeffectively the same, and (b) for cobalt and nickel, despite the similarity betweenthe saturation coverages at the high and low temperatures, the heat of adsorptionat 77°K is significantly smaller than at 273°K.(a) molybdenum0 0 .5 2.0 2.5 3.0(b) tungstenFIG. 2.-The heats of adsorption of oxygen on molybdenum and tungsten films at 273,90 and 77°K.(a) Molybdenum : a, 42.4 mg, 273"K, 192 kcal mole-1 ; A, 13-9 mg, v, 10.9 mg, 77"K, 170kcal mole-1. (b) Tungsten : 0, 28.0 mg, 6 , 28.7 mg, 273"K, 183 kcal mole-1 ; 0, 17.9 mg,90"K, 184 kcal mole-1 ; A, 23.8 mg, 77"K, 184 kcal mole-1. The open points refer to calorimeterA and the filled ones to calorimeter B ; the associated heats in each case are the average integralheats of adsorption. Previously reported 1 integral heats of adsorption at about 300% are :molybdenum, 168 kcal mole-1 and tungsten, 180 kcal mole-].ADSORPTION AT 273°K ON SURFACES SATURATED AT 77°KSurfaces were saturated with oxygen at 77°K and excess oxygen was then pumpedfrom the system.Calorimetric and coverage data for the further adsorption ofoxygen on the resulting surfaces are given in table 1 ; for comparison, the saturationcoverages obtained for the clean surfaces at 273°K are also included in the table.The values of p for cobalt and nickel are uncertain to at least 5 %.TABLE VALUES OF p FOR CLEAN SURFACES SATURATED AT 77 OR 273"K, AND FOR SUR-FACES SATURATED FIRST AT 77°K AND THEN AT 273°K; AND THE HEATS OF ADSORPTION INTHE LATTER CASEmetalp for saturation atp for saturation of the cleail 273°K of surfacespreviously saturated increments admitted77% 273°K at 77'K to surfaces previouslyheats of adsorption(kcal mole-1) atsurfaces at which had been 273°K for gassaturated at 77°Kmolybdenum 2-3tungsten 2.1cobaltnickel7.46.33.0 3.2 1 043.3 3.1 122, 120, 120,121, 114, 857-8 10-8 78, 77, 71, 666-4 8.2 798 ADSORPTION OF OXYGEN ON METALSMOLYBDENUM AND TUNGSTEN.-Table 1 shows that the saturation coveragesat 77°K are appreciably smaller than at 273"K, even though the heats of adsorptionare effectively the same. The coverage obtained on saturating at 273°K a surfacealready exposed to oxygen at 77°K is effectively the same as obtained at 273°K forsaturation in one step.Also, for tungsten, the heat of adsorption of the additionaluptake at 273°K is very similar to the heat of this portion of the coverage at 273°K.(a) cobalt100 ' * O h0 1 2 3 4 5 6 7(b) nickelFrG.3.-The heats of adsorption of oxygen on cobalt and nickel films at 273, 90 and 77°K.(a) Cobalt : e, 22.4 mg, 273°K 98 kcal mole-1 : A, 16.4 mg, V, 30-3 mg, 77"K, 84 kcal mole-1(b) Nickel: 0, 45.5 mg, 0 , 31.5 mg, 273"K, 107 kcal mole-1 ; 0, 31.4 mg, 0, 45.2 mg, 90"K,71 kcal mole-1 ; A, 29.0 mg, V, 57.3 mg, A, 35-5 mg, 77"K, 70 kcal mole-*. The open points referto calorimeter A and the filled ones to calorimeter B ; the associated heats in each case are theaverage integral heats of adsorption. Previously reported 1 integral heats of adsorption at about300% are : cobalt, 101 kcal mole-1 and nickel, 105 kcal mole-1.For molybdenum, no measurements of heat were made for adsorption at 273°Kfor coverage greater than p = 2, but it has already been found 1 that the heat fallsat these higher coverages.The heats of adsorption at 273°K on surfaces saturatedat 77°K show an analogous decrease. It is concluded, for niolybdenuin and tungsten,that two stages in the adsorption can be distinguished. The first stage is commonto the adsorption at both temperatures and terminates at about p = 2. The secondstage does not occur at 77"K, but does so rapidly at 273°K. The adsorbed layerat 273°K is the sane whether it is formed by adsorption on the clean surface at273"K, or by adsorption first at 77°K and then at 273°K.COBALT AND NICKEL.-The adsorption at 273°K on a surface saturated at 77°Kis appreciable and occurs with a heat whieh is comparable to the heat associatedwith the initial adsorption at 77°K.'Thus, the adsorbed layer obtained in the two-stage adsorption contains more oxygen, but is less stable than the layer obtainedby saturation of a surface kept at 273°K throughout the adsorption and, therefore,it is concluded that the two states are differentD. BRENNAN AND M. J. GRAHAM 99DISCUSSIONIn a consideration of the adsorbed state of oxygen, there immediately arisesthe problem of the polarity of the surface bond and with it the effective size of theadatom. The relatively small surface potential due to adsorbed oxygen was origin-ally interpreted 3 to mean that adsorbed oxygen was essentially atomic in character.However, MacRae4 has argued that the surface potential data can also be used insupport of considerable charge transfer to the adsorbed oxygen.Magnetic studiesyield conflicting evidence.5 Park and Farnsworth 6 consider obedience of photo-electric data to the Fowler curve for metals as strong evidence for adsorbed oxygenbeing mainly atomic in character. We incline to the opinion that chemisorbedoxygen is not very highly polarized and that, in considering the size of the adatom,the atomic diameter of 1.32 A is the best guide.For molybdenum and tungsten, thep values at saturation are small enough forthere to be little doubt that the adsorbed oxygen is confined to the surface proper.The values of p for cobalt and nickel are larger and raise the question whether allthe oxygen is on the surface, or whether some incorporation has occurred.Foradsorption at 273"K, the adsorbed layer must closely resemble oxide and the heatof adsorption is similar to the appropriate heat of oxidation, in keeping with thisview.1 The argument that an oxide is formed at 77"K, but that it is different fromthe oxide formed at 273°K meets with difficulties. If this were so, it would beexpected that the final state at 273°K should not depend on whether the adsorptiontakes place first at 77°K; also, a smaller value of p would be expected at 77°Kthan at 273'K and, possibly, a higher heat of adsorption, but such is not the case.The surface potential change when oxygen is adsorbed on nickel at 273°K undergoesa relatively slow increase following the initial virtually instantaneous decrease 7and this is evidence for reorganization of the adsorbed layer.No similar slowsurface potential changes were observed for nickel at 77"K, suggesting that theadsorbed layer formed initially does not undergo any subsequent change. Theseseem good arguments for supposing that oxygen adsorbed at 77°K is confined to thesurface proper, and this proposal will be examined below.MOLYBDENUM AND TUNGSTENUsing the configuration for adsorbed krypton proposed by Brennan and Graham 8and the restriction that only one oxygen atom is adsorbed per surface metal atom,the theoretical values of p for the most probable planes 1 are ~(100) = p(110) = 2 ;for the (211) plane, the metal atoms immediately below the surface are sufficientlyexposed to be counted as part of the surface proper, as far as oxygen adsorptionis concerned, and p(211) = 4.It is proposed that the adsorption up to saturationat 77°K or, at 273"K, up to the coverage at which the heat of adsorption begins tofall, viz., p = ca. 2, is confined to the surface proper. The adsorption occurringafter warming the surface saturated at 77"K, or accompanied by a falling heat at273"K, is attributed to adsorption of oxygen atoms on metal atoms immediatelybelow the surface of the (100) face (the (1 10) face does not offer sub-surface sitesof sufficient size to accommodate an oxygen atom). This description requiresadsorption on the sub-surface sites to be activated to an extent which debars occupa-tion at 77°K and this is reasonable having regard to the confined nature of thesesites.The description is also compatible with an increasing energy of activationand a decreasing energy of adsorption with increasing coverage, and with a limitingcoverage of p = 3 at 273°K. Surface potential measurements lend further suppor100 ADSORPTION OF OXYGEN ON METALSto the model. Quinn and Roberts7 have reported that there is little change in thesurface potential due to oxygen adsorbed on molybdenum when the temperatureis raised from 77 to 298°K in keeping with a common state of adsorption on thesurface proper at the two temperaturss; nor is there a significant change in surfacepotential on adsorption of oxygen at 298°K on a molybdenum surface previouslyexposed at 77"K, in keeping with the view that this extra adsorption is confined tothe sub-surface.COBALT AND NICKELTheoretical values of p similar to those measured for cobalt and nickel can beobtained for chemisorbed oxygen if it is supposed that the surface atoms of the mainfaces 1 can each account for two oxygen atoms, and, further, if the sub-surfaceatoms of the (1 10) and (100) faces can each account for one ; the (1 11) face, beingclose packed, does not offer sub-surface sites. Again using the configurations ofadsorbed krypton proposed by Brennan and Graham,s the values of p predictedfor these conditions are : p(110) = p(100) = ~(111) = 6.These values are a littlelower than the observed values, especially for cobalt, but having regard to the im-perfect nature of evaporated films, the agreement is fair.An adsorbed layer ofthis kind would undoubtedly be less stable than an oxide layer, in keeping with thesmaller heat of adsorption observed at 77°K. Additionally, such a layer wouldbe expected to result in a much larger surface potential change than an oxide layerand this accords with observation.7On raising the temperature from 77 to 273"K, it is envisaged that the adsorbedlayer undergoes a transformation in which oxygen atoms take up positions betweenthe surface metal atoms and the underlying atoms. This change is best exemplifiedby reference to the (110) plane as illustrated in fig. 4. The presence of an oxygenatom occupying a spacious sub-surface site would be expected to facilitate thereorganization.As a result of the change, metal atoms will again be exposed inthe surface plane and will be capable of adsorbing additional oxygen with a heatsimilar to that observed initially at 77°K. The state envisaged in fig. 4(b) has a vacantposition between the first two layers of metal atoms for each metal atom in thesurface proper. Such a state could have interesting catalytic properties, particularlywith respect to hydrogen. Further adsorption of oxygen on the surface shown infig. 4(b) would probably result in the state shown in fig. 4(c) and the accompanyingincrease in coverage would have Ap = 4 (cf. table 1). In the change from the staterepresented by fig. 4(a) to that by fig. 4(b), the surface potential would be expectedto increase very considerably and in fact does s0.7 Further the additional adsorp-tion resulting in the state represented in fig. 4(c) would be expected to change thesurface potential to a value nearer to that found initially (fig.4(a)), but not quiteso negative; this is again as observed.7 Similar changes in the adsorbed layercan be envisaged for the (100) plane, but are not feasible for the close-packed (1 11)plane, which is likely to remain unchanged by increase in temperature. Presumably,if the temperature were taken high enough, structures of the type shown in fig. 4(b)and (c) would collapse into the normal oxide structure, but the required activationenergy is too high for this to happen at room temperature. The act of adsorptionis accompanied by the liberation of an appreciable quantity of energy which mightbe expected to cause transformations impossible with the aid of the normal thermalvibrational energy alone.Equally, at sufficiently low temperatures, even the sumof the energy of adsorption and the thermal energy may be inadequate to bringabout a given transformation. In the present case, oxide formation is possibleat 273°K by virtue of both the energy of adsorption and the relatively great thermaD. BRENNAN AND M. J. GRAHAM 101energy of the metal; at 77"K, the difficulty of moving metal atoms prevents oxideformation and results in a genuine chemisorbed layer. Only on warming cansurface transitions occur, but not to the oxide state, since such transitions as nowoccur are due only to thermal excitation and lack the extra boost due to the energyof adsorption.FIG. 4-Proposed configurations for oxygenadsorbed of the (1 10) face of the face-centredcubic metals; (a) at 77"K, starting with theclean surface; (b) on warming state (a) to273°K; and (c) on adsorbing oxygen on state(b) at 273°K. 0, metal atoms in the plane ofthe surface ; @, metal atoms immediately belowthe plane of the surface ; 9 oxygen atoms.The authors are grateful to the former Department of Scientific and IndustrialResearch for a grant to one of them (M. J. G.).1 Brennan, Hayward and Trapnell, Roc. Roy. SOC. A, 1960,256, 81.2 Brennan and Hayes, Trans. Faraday SOC., 1964, 60, 589.3 Gundry and Tompkins, Quart. Reu., 1960, 14, 257.4 MacRae, Surface Sci,, 1964, 1, 319.5 Culver and Tompkins, Adv. Catalysis, 1959, 11, 67.6 Park and Farnsworth, Surface Sci., 1965, 3, 287.8 Brennan and Graham, Phil, Trans. A, 1965, 258, 325.Quinn and Roberts, Trans. Faraday Soc., 1964, 60, 899.

ISSN:0366-9033    

DOI:10.1039/DF9664100095

出版商:RSC    

年代:1966

数据来源: RSC

摘要:

GENERAL DISCUSSIONDr, Gert Ehrli& (Srhsnerjady, New York) said ; The question of how well pressurepeaks reflect the state of gas adsorbed on a surface arises quite generally in flashdesorption experiments. For filamentary samples, artifacts can be caused by diffusioninto the bulk or by conversion of a weak binding state to a more stable form duringwarm up. Conversion is usually concomitant with evaporation. Under theseconditions the desorption trace prescnts a qualitatively correct picture of the bindingstates. However, it will not give a quantitative account of the amount of materialpresent prior to observation, or of the correct desorption kinetics. If during evapora-tion of a state, conversion starts but terminates (possibly through saturation of acces-sible sites) before the weaker state is conipletely evolved, then the desorption tracemay show a peak which is not indicative of a separate entity on the surface.Ineither situation the rate of heating plays an important though often neglected role.Evaporation, having the highest activation energy, will be favoured over competingsurface processes by heating the sample at the highest possible rate. With oscillo-graphic recording the only real limit is set by the requirement that the time interval forevolution should be long by comparison with the time of flight through the cell(usually -500 psec). From desorption spectra over a wide range of heating rates,the extent to which the original distribution is perturbed can easily be determined,provided that at low temperatures the rate of conversion does not already predominate.This contingency can be simply explored by adsorption measurements at differenttemperatures. Finally, the peaks in a desorption trace depend both on the rate ofheating and of pumping ; comparison of peak temperatures therefore are useful onlyif the other variables are properly accounted for.Of particular interest in wnnecl;ian with the work of Hayward et al.are the demsp-tion studies of Mimeault and Hansen.1 After hydrogen adsorption at 100°K and-5 x 10-8 mm, they find an additional weak state of binding on tungsten. Theresolution of their detector is unsufficient to distinguish any fine structure ; however,in experiments with deuterium they were able to show that this state does not exchangewith hydrogen and is presumably molecular.On rhodium and iridium, a low tempera-ture state was also isolated. In contrast with tungsten, this did undergo isotopicmixing, suggesting the presence of adatoms.Dr. D. 0. Hayward (Imperial CoZZege) (communicated): It has been pointed outin our paper and also in the comments of Dr. Ehrlich that spurious pressure peaks mayoccur in desorption ‘‘ spectra ” which are not related to separate states of binding ofthe gas on the metal surface. The pressure profile necessarily reflects the state ofsurface occupation in a quantitative manner only when the adsorbed layer hasreached equilibrium prior to the desorption, i.e., there is no possibility of conversior,from weak to strong binding states during the warm-up.Work with many gases onevaporated metal films shows that it is extremely difficult to achieve equilibriumat low temperatures and this probably applies, although to a lesser extent, to filaments.Hence, we agree with Dr. Ehrlich that desorption “ spectra ” obtained by heatingfrom low t5mperatures should be treated with caution.Dr. S. Cerni (Inst. Physic. Chem., Prague) said: In connection with the paper ofBrocker and Wedler, we have also measured the heat of hydrogen adsorption onevaporated nickel film? The evaporation took place at a vacuum of 5 x 10-8 mm Hg1 Mimeault and Hansen, J. Chem. Physics, 1966, in press.2 Cern9, Pontx and Mhdek, J. Catalysis, 1966, 5, 27.10FIG. 1.-Interaction of oxygen with rhodium, as observed in the field emission microscope.(a)Clean Rh ; voltage V = 13.2 kV for emission of 5 x 10-9 A ; (b)-(e) tip at 79°K exposed to oxygenstream at p < 10-8 mm for increasing time intervals. (b) V = 14.1 kV ; (c) 14.7 ; ( d ) 14.8 ; (e)15.6 ; (f) surface warmed to 300°K ; V = 13.9 kV ; (9) clean Rh at 300°K ; V = 13.8 kV ; (h) ex-posed to 0 2 , V = 15.1 ; (i) same surface after continuing interaction at 79°K ; V = 16 kV.[To face page 102GENERAL DISCUSSION 103and at room temperature of the calorimeter wall. The weight of the film was 44 mgand its area was approximately 1000 cm2. The initial heat of adsorption was 25-26kcal/mole and the heat-coverage curve gradually decreased, in agreement with resultof Brennan and Hayes 1 and at variance with that of Brocker and Wedler. In Wedler’spaper the fundamental difference of the results from those of all the other authors whohave studied calorimetrically the Ni-HZ system is accounted for by two factors : thedifference in the vacuum and the wall-tsmperature during the evaporation.I do not think that the difference of our and Wedler’s result is due to contaminationof our films and cleanliness of his : (a) the amount adsorbed on 1 cm2 was in bothcases the same, viz., about 3 x 10-3 pmoles.(b) The most significant source of residualgases above the nickel film is during the evaporation of the filament and therefore thepresence of hydrogen, carbon monoxide and inert gases in the residual gas phaseshould be considered. The pressure did not change with time after the film depositionin my calorimeter; this can be explained neither by the equilibrium pressure ofhydrogen, nor of carbon monoxide (the number of adsorption sites for M2 and COon nickel probably does not differ substantially 2).Therefore in my system the resi-dual pressure of 5 x 10-8 mm Hg was caused most probably by inert gases. ( c ) Thisalso suggests the fact that the nickel filaments used by Brennan and Hayes and by uscame from various sources, so that composition of residual gases probably differed ;the measured heats, however, agreed. (d) Further arguments are available 3-5 infavour of the statement that the film contamination under conditions used in ourcalorimetric work as well as in that of the authors mentioned in the Wedler’s paper,was of no essential significance, not exceeding 1 % of surface metal atoms.The size of nickel crystals doubtlessly depends on the wall temperature during thefilm deposition. However, it is difficult to understand why nickel films deposited at77°K should expose to the gas phase crystallographic planes different from thoseexposed by films deposited at room temperature.Moreover, their 5lms were afterdeposition at 77°K sintered and the heats of hydrogen adsorption were the samewhether the nickel films were tempered at room temperature or at 60°C. ThereforeI think that the differences between the results reported here by Wedler and thosepublished by other authors are not due to a different crystallographic structure of therespective nickel films caused by different wall-temperatures during deposition.Evaporated films are always polycrystalline.It can be expected that with nickelthe heat of adsorption on crystallographic planes with various indices differs, thoughthis effect is with f.c.c. metals less marked than with b.c.c. metals.6 At 273°K theadsorbed hydrogen is most probably mobile.19 7 Under these circumstances theheat-coverage curve of hydrogen on nickel should exhibit a slightly decreasing course,as has been found by several authors. I do not think that the constancy of the heatof hydrogen adsorption reported in his paper can be accounted for by the homogeneityof the film as he suggests and it is difficult to find any plausible mechanism by whichthe reported constancy could be explained.The same applies to the reported increaseof the adsorption capacity of the film in each adsorption-desorption cycle.En conclusion also for oxygen adsorption, which is different in character, the heats of1 Brcnnan and Hayes, Trans. Fara&y SOC., 1964, 60, 589.2 Gundry and Tompkins, Trans. Faraday SOC., 1957, 53, 218.3 Knor and Ponec, CON. Czech. Chem. Comm., 1961, 26, 579.4 Becker, Ado. Catalysis, 1955, 7, 135.5 Wheeler, Structure and Properties of SoZid Surfaces, (ed. Gomer and Smith) (Univ. of Chicago6 Rootsaert, van Reijen and Sachtler, J. Catalysis, 1962, 1, 416.7 Wortman, Gomer and Lundy, J. Chem. Physics, 1957, 27, 1099.Press, 1953), p. 439104 GENERAL DISCUSSIONWedler for nickel 1 and for iron 2 are lower than those obtained by other authors.3-5Apart from the problem of the calorimeter calibration I would ask whether it can beexcluded that too low values of heat are systematically obtained because of (a) theindirect method of evaluation requiring one to ascertain the kinetics of the heatevolution, and (b) the transfer of some part of the evolved heat from the interior of thecalorimeter through the gas phase to the surrounding ice-bath without being registeredby the thermometer winding.Dr.M. W. Roberts (Queen's University, Belfast) said : I do not consider the extentof hydrogen uptake on nickel to be any guide as to the cleanliness of the film surface.For example (see table 1 of our paper), nickel surfaces which have adsorbed a mono-layer of oxygen at - 195" after heating to 22 or 150" in vacuo can then adsorb hydrogenextensively.The heat of hydrogen adsorption may, however, be very different fromthat observed with a clean metal ; the work function change is certainly different.Prof. G. Wedler (Technische Hochschule, Hannover) said : tern? uses the amountof hydrogen adsorbed on 1 cm2 to compare the cleanliness of his films and our films.This method is not valid since the films have different thicknesses and differentroughness factors which cannot be determined exactly enough to compare thecoverages. Furthermore, contaminated or partially precovered films sometimesadsorb even more gas than clean films.6-8Dr. Cernf 9 evaporates his films in a vacuum of 10-7 to 10-8 torr and keeps themin this vacuum up to 18 h before the first doses of gas are admitted.The constantgas pressure represents an equilibrium pressure or the equilibrium between the rate ofdesorption of gas in the apparatus and the rate of pumping. Since the evaporatedfilms act as good getters, they will chemisorb all the active gas. The composition ofthe gas in an ultra-high vacuum system before and after the evaporation of a nickelfilm has been studied by Gentsch10 in this laboratory by means of an omegatron.Before the evaporation the residual gas contained about 40 % CO. After theevaporation>f the film all the CO was gettered. Under these conditions mentionedabove Dr. Cernys films must be contaminated to a noticeable extent even if theresidual gas only contains some small percentage of active gases.The great influence of the surface contamination on chemisorption effects, especi-ally in the system Ni+H2, is clearly demonstrated by the observed changes of theelectric resistance.When ordinary high vacuum conditions (10-6 to 10-7 torr) wereused, only a decrease in the resistance of the nickel films could be observed with thehydrogen adsorption.11 With the improvement of the vacuum conditions the initialincrease of the resistance became more and more determining (ref. (3), (8), (9) and (10)of our paper). Similar observations have been made with the chemisorption of formicacid 12 and water vapour.13Fig. 1 demonstrates that even a residual pressure of 5 x 10-9 torr has a greatinfluence on the adsorption effects in the Ni+H2 system at 273°K.The fully drawn1 Wedler, Z. physik. Chem., 1960, 24, 73.3 Brennan, Hayward and Trapnell, Proc. Roy. SOC. A, 1960, 256, 81.4 Klemperer and Stone, Proc. Roy. Sac. A, 1957, 243, 375.5 Beeck, Adv. Catalysis, 1950, 2, 151.6 Ponec and Knor, Coll. Czech. Chem. Comm., 1961, 26,29.7 Siddiqi and Tompkins, Proc. Roy. SOC. A, 1962, 268,452.8 uinn and Roberts, Trans. Faraday Sac., 1962, 58, 569.9 8 ern?, Ponec and Hlhdek, J. CataZysis, 1966, 5, 27.10 Gentsch, 2. physik. Chem., 1960, 29, 55.11 Suhrmann and Schulz, Z. ph.ysik. Chem., 1954, 1, 69.32 Suhrmann, Kern and Wedler, 2. physik. Chem., 1963, 36, 165.13 Suhrmann, Heras, Heras, Wedler, Ber. Bunsenges.physik. Chem., 1964, 68, 511, 990.2 Wedler, 2. physik.Chem., 1961, 27,388GENERAL DISCUSSION 105lines refer to the nickel film discussed in detail in our paper. It had been evaporatedunder a vacuum of < 2 x 10-10 torr. The exact pressure will have been about 10-11tsrr as can be seen from measurements with an omegatron.1 The broken lines referto the adsorption of hydrogen on a nickel film evaporated under a residual gaspressure of 5 x 10-9 torr. Both the films were treated in exactly the same manner.The rise in resistance, however, is smaller and the heat of adsorption is higher in thesecond case. If these curves (broken lines) are shifted so that the maxima of theresistance coincide (dotted curves), no difference between the behaviour of the twofilms is to be observed at higher coverages.This may indicate that at high coveragesthe same processes will occur. At smaller coverages, however, the heat of adsorptionis clearly higher for the contaminated film.C I1.51 -alo’ - 1 . , . . . . , , , , , ; 10 0.5 I a 0 coverage (lo15 molecules/cm2)F~G. I .-Variation of the resistance of nickel films and of the heat of chemisorption with coverage ofhydrogen at 273°K.residual gas pressure 2 x 10-10 torr ;- - - - - residual gas pressure 5 x 10-9 torr ; . . . . . . curve - shifted by 0.4 x 1015 molecules/cm*.The increase of the heat of adsorption by a partial precoverage of the films hasalso been reported by other authors, e.g., by Dr. Cernf himself in the system Mo + H2after preadsorption of oxygen,2 by Klemperer and Stone (ref. (13) in our paper) in thesystem Ni+02 after preadsorption of hydrogen, by Bagg and Tompkins 3 in thesystem Fe+H2 after preadsorption of CO and in the system Fe+02 after H2 pre-adsorption.Concerning the structure of the films, the crystallographic planes exposed to thegas phase may be different in the films used by the other authors and in the films used1 Gentsch, private communication.2 Knor and Ponec, Cull.Czech. Chem. Comm., 1961, 26, 579.3 Bagg and Tompkins, Trans. Faraday Suc., 1955, 51, 1071106 GENERAL DISCUSSIONby us. tern9 does not consider that we used very thin films, which were rather smooth(roughness factor 1-5-2) whereas tern9 and the other authors used films 10-20 timesas thick with roughness factor of 5-10. These rather porous films may have a greaterperceEtage of high indexed planes in their internal surface and therefore a greaterheterogeneity.The statement of Cerng that all the heats of adsorption reported by us were smallerthan those reported by other authors is not correct.For oxygen adsorption on ironfilms at room temperature Bagg and Tompkins 1 found 71 kcal/mole, we 2 observed100-120 kcal/mole, and Brennan reported a value of 133 kcal/mole (ref. (19) in ourpaper). The dependence of the heat of adsorption of oxygen on nickel on surfacecoverage has never been reported by us. Preliminary measurements of 0 2 adsorptionon a Ni film have only been used to study the properties of the new type of the calori-meter.Concerning the last question of Cernf, the kinetics of the adsorption must beconsidered in the evaluation of the heats of adsorption.An extrapolation of thecooling curve without any correction leads to wrong (too high) values of the heat ofadsorption. A transfer of part of the heat evolved in the calorimeter through the gasphase without registration by the thermometer is impossible, since in our experimentsthe outer jacket of the calorimeter is pamped to a vacuum of < 10-8 torr.DP. D. A. King (ZnzperiaZ CoZZege) (communicated) : Two sets of anomalous resultsobtained by Brocker and Wedler could both be attributed to an underestimation of theamount of gas pumped out of the cell during desorption. First, they report that asthe nickel films approach saturation and an equilibrium pressure is established, highercoverages could apparently be attained by simply pumping gas out of the cell, and thenreadsorbing further doses.Secondly, adsorption in this system proceeds veryrapidly, with an activation energy for adsorption which must be very close to zero, sothat heats of adsorption and desorption should bc equivalent : however, they evaluatecalorimetrically a desorption heat at high coverages as 19 kcal/mole compared withheats of adsorption of - 15 kcal/mole at similar coverages. The accuracy with whichthe amount of gas pumped out of the cell can be determined is limited by difficultiesin the absolute calibration of pressure gauges at low pressures, and the estimation ofvolumes in a system containing pumps. Thus, if this amount had been consistentlyunderestimated by -20 % due to calibration error, the subsequent AR/& values andchemisorption heats determined on readsorption (their fig.3) would be coincident.Similarly, using the same correction factor, the desorption heat would be reducedfrom 19 to 15 kcal/rnole, consistent with the heats of adsorption.Dr. D. Brennan (University of Liverpool) said : It is useful to make some generalcomments about the shapes of film calorimeters and the determination of their heatcapacities. The use of a spherical shape for a calorimeter gives rise to a number ofcalorimetric dificulties, without a compensatory advantage of simplifying the problemof the distribution of the adsorbate within the film. Unless a central diffuser is used,as in the work reported earlier by Hayward and coworkers, to achieve a uniformdistribution of adsorbate prior to the adsorption, the geometry of an inlet tubeconnected to a spherical vessel could be more complex than that of a cylindrical tube.In the absence of uniform distribution of the adsorbate, a cardinal requirement ofany calorimeter is that a given quantity of heat liberated in any part of the calorimetershould give the same response.This is readily achieved in a cylindrical vessel since,with care, uniform wall thickness can be obtained and the sensing wire can also beuniformly wound. Thus, unit length of sensing wire has the same amount of glass1 Bagg and Tompkins, Trans. FaradGy Soc., 1955,51, 1071.2 Wedler, Z.physik. Chern., 1961, 27, 388GENERAL DISCUSSION 107associated with it and, no matter if the film is thicker in one place than in another orthe adsorbate is not uniformly distributed, the calorimetric response will be the samefor a given heat of adsorption no matter where in the calorimeter it is liberated.Now, in a spherical vessel, the amount of glass to be associated with unit length ofwinding for 2 given pitch will depend on the location; the turns near the poles areespecially exceptional. The importance of this effect will depend on how closelywound the sphere is; clearly, the smaller the pitch, the less serious the effect.Thepoint to be made, however, is that even with a wall of uniform thickness, the calori-metric sensitivity is not everywhere the same. This intrinsic non-uniformity is ofimportance when there is non-uniform distribution of adsorbate, but it could also beof importance in the determination of the heat capacity.The quantitative solution of the problem of the distribution of the heat liberatedby current fiowing from small electrodes attached to a large conductor is oftendifficult.For a spherical shell conductor, as for the spherical calorimeter, qualita-tively the best that can be achieved is a symmetrical, as opposed to a uniform, dissipa-tion of heat, and this requires diametrically opposed circular electrodes and uniformconductor. Here, it is virtually impossible to prepare a uniform evaporated metalfilm under these circumstances. If the intrinsic non-uniformity of the calorimeter issignificant, then the measured heat capacity will be a complex function of the diameterof the calorimeter, the number of turns of the resistance thermometer and the distri-bution of the current in the film.Should the electrode assembly not be completelysymmetrical, then it might be, for example, that more heat would be dissipatedequatorially and the derived heat capacity would then be even more unrepresentativeof the calorimeter as a whole.Additionally, it is very difficult to make a sphere with uniform wall thickness andto check that the wall is indeed uniform without first breaking the calorimeter; sinceuniformity of wall is so important, this is a serious deficiency. Again, it is verydifficult to wind the resistance wire uniformly on to such a surface.Regarding thesealing of the resistance wire into the wall, there is some risk that this could disturb theuniformity of the wall; at the same time, the need for such a measure is doubtful,since there is no evidence that the rate of heat transfer through the wall to the wirewound on it is in any way inadequate to the needs of this kind of experiment. If thewire were used as a heater, as it might conceivably be in a calibration run, thenimproved thermal contact could be some advantage, but the limiting factor is thethermal conductance of the glass between turns. Clearly, the details of design ofthese calorimeters are very important.In contrast to these difficulties, the procedures for calibration of cylindrical calori-meters are relatively reliable, if not entirely free of troubles.Two main methodshave been adopted, viz., to dissipate electrical energy in a film deposited in thecalorimeter, or to use the resistance thermometer or additional external winding as aheater. We believe that dissipation of heat in a film is the most satisfactory method inthat it most closely simulates an adsorption measurement. This nethod has beencriticized on the grounds that it is difficult to get electrica! contact with the film but itis our experience that this can be achieved satisfactorily. We have used a number ofdifferent types of contacts, and always the results have been independent of the natureof the contact and the film in cse. The method also has the powerful advantage thatdeterminations can be made for different parts of the tube. There is no validity inthe criticism 1 that, since the boundaries of the film are indistinct, the area of heatliberation is imprecise and the derived values of heat capacity could be a function of1 tern$, Ponec and HIBdek, J. Catalysis, 1966, 5, 27108 GENERAL DISCUSSIONthe current in the film.The only consideration is that the film should be entirelywithin the uniform region of the tube constituting the calorimeter.We are not entirely happy about heat capacity determinations involving externalheaters, particularly the method in which a stationary state is established and then thecurrent switched off. Our attempts to use the resistance thermometer as a heater 1have never given good results because the temperature of the wire was always appreci-ably above that of the wall.The use of a second winding as heater seems to be moresuccessful, but we remain concerned about temperature gradients ; temperatureequilibm ation longitudinally is relatively slow. Finally, the stationary state procedureis so different to that used in an adsorption determination, that, other methods beingavailable, its use in our opinion is unnecessary.There are three points that I would like to make specifically in relation to the paperby Brocker and Wedler. (i) While it is true that our ultimate vacua fall no lower thanthe pressure ranges 10-8 to 10-7 torr, the area of our films is such that there is not thecontamination available to dirty the available surface to any significant extent.Further, where comparision can be made between results obtained on evaporatedmetal films and on small area surfaces maintained in ultra-high vacua, agreement issatisfactory.(ii) The films of Brocker and Wedler are very thin in comparison withthe films used by other investigators. Their roughness factor is only 2, whereas oursis about 10 ; indeed their films are not sufficiently thick to show a smooth dependenceof area on weight. The properties of such very thin films are likely to be perturbed bythe substrate and to be different from those of thicker films. (iii) If the heat of adsorp-tion of hydrogen on nickel were as low as reported by Brocker and Wedler, then it isdifficult to understand why it is not possible to remove the entire adsorbed layer bypumping for about an hour.On the other hand, it is also difficult to understand whya small part of the adsorbed layer can be removed so quickly by pumping, even thoughthe associated heat is reported to be 19 kcal mole-1.Prof. G. Wedler (Technische Hochschule, Hannover) said : The difficulties men-tioned by Brennan are well known to us. Therefore we have critically studied possibleerrors in the determination of the heats of adsorption. After considerable experiencehad been gained in this laboratory, reliable spherical calorimeters can be built.The deviation from the average thickness of the calorimeter bulb has beenmeasured and was less than 3- 5 % for more than 80 % of the bulb. Only the bottomof the vessel and the part near the inlet tube were slightly thicker.This has been takeninto account, when the thermometer wire was sealed on the bulb. The averagedistance between the windings was 3 mm. Care was taken, however, that the amountof glass per cm of the thermometer wire was the same all over the bulb, i.e., a smallerdistance between the windings was used at the bottom and at the top of the bulb.The diameter of the contact foils was 1 cm, the diameter of the bulb 5 cm. There-fore more heat is liberated near the foils than at the “ equator ” of the bulb. Sincethe ratio ‘‘ amount of glasslcm of thermometer wire ” is the same all over the bulb,the non-uniformity of the heat liberation cannot influence the determination of theheat capacity. Furthemore, we found experimentally that using the stationarycalibration method both the warming curve as well as the cooling curve are representedby an exponential law with exactly the same value of r/C.This result indicates thatthe initial differences in the distribution of temperatures are very quickly balanced,presumably due to heat radiation within the bulb. In this respect the sphericalgeometry of our calorimeter is more advantageous than a cylindrical one.Furthermore, (a) the values of r/C obtained by use of the stationary method andthe pulse method differ for the calorimeter used in this work by 6 %. The values of1 Whaba and Kernball, Trans. Faraday Soc., 1953,49, 1351GENERAL DISCUSSION 109r/C obtained from the cooling curves when measuring the heats of adsorption liealways between the values found by the two calibration methods.(b) If the outerjacket is pumped to high vacuum (< 10-8 torr) the experimental value of the coolingconstant r (29 mW/deg.) agrees with the theoretical value (32 mW/deg.), which isobtained with the assumption that all the loss of heat is due to radiation from theuniformly heated bulb. (c) The value of the heat capacity C does not depend on theenergy used in the calibration even if it is altered by a factor of 10. It is also independentof the value of Y, if this is increased by admission of gas into the outer jacket of thecalorimeter, so that heat radiation and heat transfer by the gas phase are responsiblefor the loss of heat. ( d ) The value of C obtained by calibration using the pulsemethod ( e g , 0-63 cal/deg.) agrees well with the value calculated from the weight andthe specific heat of the material of the bulb (0.62 cal/deg. with the same calorimeter).All these results clearly show that the objections made by Brennan with respectto the calorimeter design are not relevant for the spherical calorimeters used in thiswork.The differences in the heat of adsorption of hydrogen on nickel reported byBrennan and the other authors and those found by us are at least 45 %. It is certainlynot possible to explain such a large difference by uncertainties in the determination ofthe heat capacities of the calorimeters.Prof. W. M. H. Sachtler (Amsterdam) said: Wedler mentions that the latticeparameter is slightly shorter for nickel films than for bulk nickel.We have made thesame observation for nickel and copper. With some copper films the lattice parameterwas 6 x 10-3 A shorter than in bulk copper. I would ask whether Wedler attributesthis contraction to the effect of surface tension in small crystals or whether he proposesa different interpretation.Prof. G. Wedler (Technische Hochschule, Hannover) (communicated): In reply toSachtler, we suppose that the contraction of the lattice we have observed with thin filmsof nickel and copper (ref. (23) and (24) of our paper) is due to the influence of the surfacetension. There is, however, no direct proof for this assumption. The effect couldalso be attributed to lattice rearrangements at the surface due to chemisorptionprocesses.However, we observed the lattice contraction for high-vacuum evaporatedfilms irrespective of the vacuum conditions applied during the X-ray diffractionmeasurement. Concerning Dr. King’s question, the possible error in the determina-tion of the amount pumped out of the cell certainly is far smaller than 20 %, since theeffective volumes have been measured by different methods and the calibration of thepressure gauge is reliable in the range of about 10-4 torr. Furthermore, an additionalirreversible sorption of hydrogen by nickel films at high coverages has also beenobserved in previous work using different ultra-high-vacuum apparatus and otherpressure gauges (ref. (3) of our paper).Dr. D. A. King (Imperial College) (communicated) : Observations made during thedeposition and subsequent sintering of nickel films in an ultra-high vacuum systemindicate that the reading on a pressure gauge attached to the cell during deposition isnot necessarily an indication of the degree of contamination of the film, and that thedegree of contamination is reduced by depositing heavy films.The apparatus andprocedure was similar to that used by Hayward, Taylor and Tompkins (this Discussion),background pressures of 1-2 x 10-10 torr being attained. Nickel filaments (JohnsonMatthey “spec pure ”) were outgassed for 20 h at a temperature just below thatrequired for evaporation of metal. When deposition was carried out, at a rate of - 1017 atoms sec-1, with the glass substrate uncooled (i.e., at N 328°K) the cell gaugepressure rose to 10-8 torr and thereafter fell continuously until the gauge was indicat-ing 6 x 10-10 torr after 40 mg had been deposited ; a fall to the “ background ” valueof 2 x 10-10 torr occurred when deposition was terminated.In contrast, when the cel110 GENERAL DISCUSSIONwalls were cooled to 78 or 195"K, no increase in the gauge reading above " back-ground " was observed during film deposition, in agreenent with the observation ofBrocker and Wedler. However, on subsequently warming the films to room tempera-ture after the deposition, with the cell open to the pumps, desorption spectra wereobtained (fig. 1). The dashed curve is the pressure burst obtained with a 27 mg filmdeposited at 78"K, while curves 1-4 are successive spectra obtained by depositing - 15mg at 78"K, warming to room temperature, recooling to 78°K and repeating theprocedure (total film weight 60 mg).Contaminant desorbing over this temperaturerange was considerably reduced by the latter procedure ; however, on finally heatingthe film to 460°K the pressure rose to a maximum of 3 x 10-7 torr.TFIG. 1.An estimate of the degree of surface contamination is difficult to make. From thearea under the desorption curve 1, and the conductance of the exit port, it was calcul-ated that 1014 molecules were desorbed during the warm-up to 290°K. As the filmarea was - l o 3 cm2, this represents a negligible degree of contamination ; however, itis clear from the effect of heating to 460°K that contaminants with higher desorptionheats were present-although it is possible that the contaminants are not concentratedat the metal surface but diffuse from the bulk during the heating procedure.The pressure reading in the gauge during film deposition does not sample the fluxof contaminant at the cell walls, as contaminant atoms or molecules from the filamentmust make at least one collision with the metal film before entering the gauge tubula-tion.In particular, if the sticking coefficient s for contaminant on the metal surfaceis unity (as it is for CO on W, see Gomer's paper and my discussion comment on it)the guage pressure will not rise above the " background " value during deposition,despite the presence of contaminant. Thus, the observation of a pressure rise duringdeposition, as when the substrate is uncooled, does not indicate that the film is morecontaminated but rather that at the higher temperature s < 1 for the contaminants.Thedegree of contamination is, in fact, not reduced by cooling the substrate duringdepositionGENERAL DISCUSSION 11 1Prof. S. Z. Roginskii (Moscow) said : Direct measurements of the differential heatsof chemisorption and of reactions involving chemisorbed molecules are of considerablesignificance. In connection with the conclusion of Brocker and Wedler I show a plot(fig. 1) from our experimental data (Tretyakov et al.) on the effect of H2 chemisorptionon the changes of the work function A$ andon the electroconductivity AK of nickel filmsobtained under very pure conditions atp - 10-10 torr (hydrogen was purified bydiffusion through palladium).The curve forvariations in the work function with coverageshows a maximum and a bend; two extremepoints and a bend majj be seen as well on thecurve for electroconductivity. This is evidencefcr the occurrence of three chemisorptionforms. These facts as well as the differentstrength of adsorption on various metal faces,and the very sharp drop in the rate of ex-change between adsorbed deuterium andordinary gaseous hydrogen, makes it difficultto understand the origin of the observedconstancy in differential heats of adsorption.This is also not consistent with the results ofcertain earlier thermochemical measurements.It would be of interest to find out the reasonfor these discrepancies.Dr.A. A. Hslscher (Amsterdam) said: Inthe adsorption of oxygen on cobalt andnickel Brennan et nl. found a smaller heatof adsorption at low coverage than at highcoverage. Might this be due to an increasein surface energy in the initial stages of-10 - 4P=I*10 id=2.I07 7 -----cIcI0 0.5 1.00FIG. 1.adsorption? Can a difference in surface energy, i.e. in ordering in the surface layersafter adsorption at 78 and 300"K, also be responsible for the fact that at the samecoverage the heat of adsorption at 78 is smaller than at 300"K? Brennan andGraham assume that it is difficult for the cobalt and nickel atoms to move upon adsorp-tion at 78°K. We think, however, that our field ion microscope experiments haveshowlz that the displacement of tungsten atoms upon adsorption of carbon monoxideand nitrogen at 78°K is possible.Would not one expect a.fortiori rearrangements tooccur upon yoxygen adsorption on cobalt and nickel?Dr. S . Cernf (Inst. Physic. Chem., Prague) said: The low value of the initialpoints on the heat-coverage curves of oxygen of Brennan and Graham for tungsten,cobalt and nickel in fig. 2 and 3 of their paper, has been observed also with othersystems. Wedler 1 found the same phenomenon with oxygen on iron, as I have withoxygen on molybdenum and in a more pronounced form with oxygen on nickel.2 Itis important to elucidate whether these low values are merely some experimentalartefacts of ininor importance, or due to some deeper and more substantial cause.The marked effect with oxygen, and its absence with carbon monoxide,3 seem to excludean explanation involving a partial adsorption of the first doses in the cold traps before1 Wedler, Z.physik.Chem., 1961, 27, 388.2 Cernf, Diss. (Institute of Physical Chemistry, Czechoslovak Academy of Sciences, Prague, 1963).3 Brennan and Hayes, Phil. Trans. A, 1965, 258, 347112 GENERAL DISCUSSIONthe calorimeter vessel (the extent of CO adsorption in the traps would be expected tobe the same as that of 0 2 , if not greater). Brennan et al. used cylindrical etchedcalorimeters, Wedler used a spherical calorimeter, and I used cylindrical drawncalorimeters with controlled wall homogeneity.1 This suggests that the observedphenomenon cannot be ascribed to some specific effect of the calorimeter used.Thefact, that both Brennan and Graham, and myself have found this phenomenon in amore pronounced form with low-melting metals (Ni, Go) in comparison with high-melting metals (W, Mo) seems to point to some deeper cause. Mechanisms whichcould be responsible are sinteriiig or rearrangement of the films by serious local risesin temperature 2 which has been mentioned by Holscher, or eventually splitting ofmetal-metal bonds 3 by initial doses. More heat-coverage curves carefully measuredin the initial region for gases with different bond energy and for various metals seemdesirable in order to find the mechanism responsible for this phenomenon.Dr. Gert Ehrlich (Schenectady, New Yurk) said: The behaviour of oxygen at ametal surface is markedly affected by the atomic arrangement of the interface.This isclearly revealed when rhodium is allowed to interact with 0 2 in a field emission micro-scope. At 79"K, this interaction lowers electron emission monotonically ; the majoreffects are concentrated along the zone running from the (100) towards the (1 10) plane.During the initial stages (c), the (110)'s themselves are excluded from change and thedarkening stops at the (430). When after prolonged exposure at 79°K (as typified bye) the oxygen is pumped out and the surface is warmed to 300"K, electron emissionincreases and drastically changes its distribution over the surface. After warming, thepreviously dark zone from (100) to the (1 10) becomes brightly emitting, and insteadthe zones from (100) to (111) recede into darkness.The same type of pattern isobtained by continuous exposure to oxygen at room temperature (h). Adsorption ofadditional oxygen at 79°K (i), after exposure of the surface at room temperature,lowers the overall electron emission ; the (1 10)'s are darkened, and planes in the [ 1 101zone adjacent to the (100) become emitting. The pattern characteristic of adsorptionentirely at low temperatures is not restored.These observations are consistent with the view that at 300°K oxygen brings aboutsome reorganization of the surface, but without any particular specificity for a givensurface feature or plane. At low temperatures, however, a chemisorbed layer isformed that in some way favours planes with kinked steps, such as the (310), at theexpense of less jagged planes like the (221).Part of this chemisorbed material istransformed into the high temperature state on warming ; the remainder is labile andis evolved into the gas phase. Most interesting are the differences in the roomtemperature behaviour of Ni and Rh. On the former,4 the oxygen appears to interactspecifically with the (1 10) planes. On Rh, there are as yet no indications for any suchprefereme.Mr. B. R. Wells (Queen's University, Belfast) said: With reference to fig. 4 ofBrennan's paper, the sequence from 4a to 4b shows an ordered rearrangement ofadsorbed oxygen to a configuration which does not occur in nickel oxides, all withinone atomic layer.This contrasts with what is known about the nickel+oxygensystem in that non-stoichiometey plays an important role. If we are to accept theresults of low-energy electron diffraction studies, we know that in the initial interactionsat - 300°K such phases as Ni70 and Ni30 exist near the gas/metal surface,s and. Quinn1 Cerny, Ponec and HlBdek, J. Catalysis, 1966, 5, 27.2 Brennan and Hayes, Phil. Trans. A , 1965, 5, 27.3 Sachtler and van Reijen, J. Res. Inst. Catalysis, Hokkaido Univ., 1962, 10, 87.4 EhrIich, Ann. N. Y. Acad. Sci., 1963, 101, 722.5 Park and Farnsworth, J. Appl. Physics, 1964, 35, 2220GENERAL DISCUSSION 113and Roberts 1 , 3 have shown that variations in the surface potentials obtained on theoxidation of nickel reflect differences in stoichiometry.Some recent work in thislaboratory 2 indicates that an oxygen deficient phase similar to the Ni30 of Park andFarnsworth 1 may be formed by heating a nickel surface, saturated at 77°K withoxygen, in vacuo to -360°K ; this implies a penetration of - 10 A by the oxygenspecies. Furthermore, this oxygen-deficient phase requires a higher activation energyof formation than the simple 6 : 6 coordination nickel oxide structure. Therefore, ifthis process can take place at -360°K it is questionable to dismiss the so-called" oxide state " on warming from 77 to 273°K. Because the surface does not react asa bulk nickel oxide surface is not a categorical argument against the 6 : 6 coordinationconfiguration in the surface, only that the subsurface metallic nickel is still influencingthe surface characteristics.If an oxygen-saturated nickel surface at 77°K is warmed in UdCUO to 273°K andrecsoled, when further oxygen is added at 77"K, although the work function increaseis only 1.0 eV this increase is caused by only +th to kth of the gas adsorbed initially,and which gave rise to a change of 1.4 eV on the clean metal.2 If we treat this asadsorption on an essentially metallic system, a monolayer of this adsorbed oxygenwould give rise to a work function change of between 5 and 6 eV.A more likelyexplanation is that this is adsorption on a semiconducting oxide, and therefore somesort of " oxide state " must have been formed.Dr. J. W. Geus (Staatsmijnen, Geleen, Netherlands) said: Measurements of theeffects of chemisorption of oxygen on the electrical conductance of evaporatedtungsten films confirm the interpretation given by Brennan et al.for this metal.4 Whenoxygen is admitted at 77°K to a tungsten film, the conductances measured at both 77and 273°K decrease equally until about one oxygen atom per metal surface atom istaken up. Since the decrease in conductance measured at 77°K is the same bothbefore and after heating to 273"K, no major rearrangement of the adsorbed layer iscaused by heating to 273°K. After adsorption of about one oxygen atom per metalsurface atom more oxygen is adsorbed at 77°K without this having an effect on theconductance. On heating to 273"K, the pressure first rises and then slowly decreases,while the conductance decreases symbatically.On recooling, the same decrease inconductance is observed at 77°K. This behaviour is in accordance with the observa-tion of Brennan and Graham, viz., that an activation energy is needed to chemisorbstrongly more oxygen on a tungsten surface covered with a monolayer. In thissecond stage of the adsorption process the decrease in conductance per oxygenmolecule sorbed is clearly smaller. When oxygen is admitted to a tungsten film at273"K, a small fraction of each dose is sorbed beyond a monolayer during migrationof oxygen over the regions of the surface already covered to the less accessible parts ofthe porous tungsten films. Since the effect of the oxygen sorbed beyond a monolayeron the conductance is smaller, the total effect per oxygen molecule adsorbed on theconductance is slightly weaker, when the oxygen is admitted at 273°K.A second remark has to be made about the character of the chemisorptive bondof oxygen on nickel.First, chemical bonds generally have to be interpreted by takingboth covalent and ionic contributions into account. Only when either the covalentor the ionic contribution strongly dominates, a description as covalent or ionic,respectively, is justified.At present the effect of oxygen sorption on the ferromagnetism of nickel no longer1 Quinn and Roberts, Nature, 1963, 200, 648.2 Roberts and Wells, Trans. Faraday SOC., 1966, 62, 1608.3 Quinn and Roberts, Trans. Farday SOC., 1964, 60, 899.4 Geus, Koks and Zwietering, J. Catalysis, 1963, 2, 274114 GENERAL DISCUSSIONyields conflicting information.1 Experimental evidence is available demonstratingthat the magnetic moments of the chemisorbing nickel atoms are decoupled from theferromagnetism of the metal on oxygen sorption. The magnetic moments of thedecoupled nickel surface atoms are oriented in an external magnetic field to a degreemuch smaller than those of the ferromagnetic nickel atoms. Therefore, the niagnetiza-tion decreases on sorption of oxygen. However, when the magnetization is measuredunder conditions where no thermodynamic equilibrium is attained in the time of themeasurement, sorption of oxygen not only decouples the moments of the chemisorb-ing nickel atoms, but also causes the thermodynaniic equilibrium nagnetization to bemore closely approximated.When the latter effect dominates, the apparent rnagneti-zation increases on oxygen sorption, although the ferromagnetism of nickel is de-creased. A decoupling of the moments of the adsorbing nickel atoms from theferromagnetism of the metal by sorption of oxygen points to a donation of electronsfrom the nickel atoms to the oxygen atoms. Consequently, the chemisorption bondhas an appreciable ionic character.This can be concluded also from the results of Lewis,Z who investigated the effectof oxygen sorption of the K X-ray absorption edge of alumina-supported nickel.These results point to a bond between oxygen and a nickel surface analogous to thatexisting in bulk nickel oxide. However, the possibility that a fraction of the nickelparticles is completely oxidized, whereas the remaining part of the nickel has still notreacted with oxygen, is not fully excluded in Lewis’ experiments.1The fact that Park and Farnsworth 3 observed that the photoemission from nickelpartially covered with oxygen follows a Fowler curve, does not necessarily indicatethat bonding of oxygen to a nickel surface is effected by mainly covalent forces.Asstated by MacRae,4 both nickel atoms and nickel-oxygen complexes are present in thenickel surface at low coverages. The emission from the still metallic part of the surfaceobeys the Fowler curve, whilst the pre-exponential factor is strongly decreased. Athigh coverages, Park and Farnsworth no longer observe obedience to the Fowler curve.Dr.A. Frennet (Ecole Roy. Miiitaire, Brussels) said : We have studied the adsorp-tion 5 of krypton and xenon over a wide range of relative pressure ( z 10-5 <p/po < z 0,8) on films of Ni, Cu, Ag, W, Mo, Ti, Ta, Re, Rh, Pt and Pd. We apply two methodsto analyze the experimental isotherms : the B.E.T. and the Dubinin method. TheDubinin equation,6 was recently demonstrated as valid in the low pressure range byHobson.7 For example, if we analyse the adsorption isotherm of krypton on amolybdenum film at 77°K (fig. 1) using the B.E.T. method (fig. 2), three straigh’ L 1’ inesare necessary, over the pressure range. With each of these curves is associated avalue of the number nm of molecules contained in the monolayer and a value of theB.E.T.constant C. The problem is to get a criterion to chose which of these valueshas physical meaning. For this purpose, all the experimental points are plottedfollowing the Dubinin equztion (fig. 3). Two types of experimental points may bedistinguished, those that lie on a straight line, corresponding to the low pressurepart of the isotherm, as predicted by the equation, and those that lie above thestraight line on the left of the figure, and corresponding to the high relative pressurerange part of the isotherm and at the same time at coverages greater than the mono-layer.1 Geus and Nobel, J . Catalysis, accepted for publication.2 Lewis, J. Physic. Chem., 1960, 64, 1103.3 Park and Farnsworth, Surface Sci., 1965, 3, 287.4 MacRae, Surfcce Sci., 1964, I, 319.5 Delaunois, Frennet and Lienard, J.Chim. Physique, 1966, 63, 906.6Dubinin and Radushkevich, Proc. Acad. Sci. U.S.S.R., 1947, 55, 331.7 Hobson, Can. J. Physics, 1965,43, 1934115 GENERAL DISCUSSIONAs Gaines and Cannon 1 noted, the only criterion of B.E.T. applicability is thesaturation of the monolayer, i.e., the relation ( p / p ~ ) " = ~ , , , = - derived from theB.E.T. equation, must be fullfilled. If we now apply the B.E.T. method to the pointscorresponding to a coverage equal or greater than the monolayer as determined in the1+JcI I04 0.2 0-3 0% 04 a6 M 04)PiPo (x lo2)FIG. 1 .-Adsorption isotherm of krypton, at 77°K on a molybdenum film previously saturated withH2 at 273°K.19-0 i-' 121 *120(log Pipd2FXG.2.-The krypton isotherm represented by the Dubinin equation.Dubinin graph, we obtain curve 1 in fig. 2. This curve gives a value of n, = 6-45 x 1018molecules in good agreement with the value of nrn = 6.55 x 1018 molecules given by theDubinin equation.The good agreement between the number of krypton and xenon moleculescontained in the monolayer does not depend upon the nature of the metal film.1 Gaines and Cannon, J. Physic. Chern., 1960, 64,997116 GENERAL DISCUSSIONThus, the quantities of adsorbed krypton and xenon satisfy the relationshowing that, when the monolayer is filled, the adsorbed krypton and xenon do notknow the surface structure of the film. The B.E.T. constant C has small values onall the metals so that on Ti and Ta, where Cis -200, the monolayer is filled at relativepressure of ~5 % ; on Mo, Re, Rh and Pd, where C is 25-50, it is necessary to workat p/po> 10-15 %, and on some metals such as Pt the monolayer is not filled at p/povalues as high as 50 %.(nm)Kr/(%Jxt? = %/ai<r = 1,2070.1 0.7 09 0.4 0.5 0.6 0-7 04PJPOhave to be muItiplied by 10 and 100 respectively.C = 43.6 C = 297 C = 1670FIG.3.-The krypton isotherm represented by the B.E.T. equation. For curve 2 and 3 the coordinates(1) N , = 6.55 X 1018 ; (2) N , = 5.61018 ; (3) N, = 5 . 0 ~ 1018It is remarkable that two methods established from completely different assump-tions, give values of the number of molecules contained in the monolayer in goodagreement. We also believe that many of the isotherms obtained atp/po < few percenton metal films and analyzed using the B.E.T.method give suspicious values of thearea.Dr. D. Brennan (University of Liverpool) said: With regard to the remarks ofFrennet, in comparing the adsorptive capacities of various metals for krypton andxenon1 we were careful not to identify strictly any one parameter with the monolayercoverage. Rather, like parameter was compared with like for each of the adsorbates,with the conclusion that both species have equivalent covering capability. Thisresult was interpreted with good self consistency to mean site adsorption.1 Brennan and Graham, Phil. Trans. A, 1965, 258, 325GENERAL DISCUSSION 117We too have found that the observed isotherm at low pressure could be representedby the equation of Dubinin and Radushkevich.However, the gradient of the graphof log S against [log (P/P0)]2 was always greater for krypton than for xenon and therelative gradients varied from metal to metal so that the values of the intercepts on thelog S axis and the derived Sm values showed no consistency. For example, a largediscrepancy was found for titanium, viz., Sm (Kr) = 29-9 x 1017, Sm (Xe) = 21.2 x 1017atoms (cf. BET. monolayer values of 21.2 x 1017 and 19.8 x 1017 atoms, respectively),whereas close agreement was found for nickel, viz., Sm (Kr) = 12-7 x 1017, Sm (Xe) =12.3 x 1017 atoms (cf. B.E.T. monolayer values of 12.5 x 1017 and 13.2 x 1017 atoms,respectively). Possibly the origin of the difference between our findings and thosereported by Dalaunois, Erennet and Lienard is the fact that these authors used evapor-ated films which had been pre-treated with hydrogen or methane.Concerning the problem of selecting a parameter which can be closely identifiedwith the monolayer coverage, there is no evidence that Sm of the Dubinin-Radush-kevich equation has any utility.On the contrary, Hobson and Armstrong,1 find fornitrogen, argon and helium that obedience to the Dubinin-Radushkevich does notimply values of Snt which can be directly equated to the monolayer coverages (seealso Hobson 2). We believe the existence of a very marked point B means that thecorresponding coverage is close to the monolayer coverage. The B.E.T. equation,applied in the region of the point B, is merely a convenient algebraical device forobtaining a parameter which, under these circumstances must be very close to, if notequal to, the monolayer value.Monolayer values derived from the isotherm atpressures other than corresponding to point B must be regarded with diminishingconfidence as the pressures differ more and more from that of the point B and as thepoint B itself becomes less and less well defined.Dr. V. Ponec and Dr. S. Cernf (Inst. Physic. Chem., Prague) said: The results ofFrennet and his collaborators 3 have again proved the validity of an isotherm of thetype :Experimental data can be compared either with a model of volume filling of the micro-pores of the adsorbent (model I),495 or with a model of a plane monolayer on theheterogeneous surface (model II).6 We introduce the following quantities : E = 2.3RT log (ps/p), wherep, is the saturation pressure,p the equilibrium pressure, E representsthe free molar enthalpy, i.e., the thermodynamic adsorption potential (not thepotential of adsorption forces) ; W is the adsorption volume filled by the adsorbateat the pressure p , is., the volume of filled micropores, as generally accepted ; WQ isthe total adsorption volume (total volume of micropores); na is the number ofadsorbed molecules ; is the number of molecules in the complete monolayer ; urepresents the volume of the adsorbed molecule andco the area of the molecule adsorbedon the surface; k and k’ are constants.MODEL I is characterized by the following conditions :temperature ;log n, = c - B(1og p)’.(a) (aElaT)w=.a, = 0, i.e., the characteristic curve E = f ( W ) is invariant with(b) W = WO exp (- kez), [ Wo # f(T)], i.e., the Dubinin-Radushkevitch equation.1 Hobson and Armstrong, J .Physic. Chem., 1963, 67, 2000.2 Hobson, Can. J. Physics, 1965,43, 1934, 1941.3 Delaunois, Frennet and Lienard, J. Chim. Physique, 1966, in press.4 Dubinin, Chem. Rev., 1960,60, 235.5 Dubinin, in Chemistry and Physics of Carbon, vol. 3, ed. Walker (Dekker, Inc., N.Y.), 1966,in press. 6 Hobson and Armstrong, J. Physic. Chem., 1963, 67,2000118 GENERAL DISCUSSIONMODEL 11 is characterized by the following conditions :(c) (a~/dT),,~ = 0, or (c’) (dE/dT),,, = 0 ;( d ) nu = tl?n exp (- W),(i) The validity of the postulate (a) by itself permits thermodynamics calculationsand also the evaluation of the isotherms of other similar adsorbates from a singleisotherm of a standard adsorbate.(ii) The validity of postulate (a) does not dependon the validity of eqn. (b) and vice versa. Analogous statement holds for model 11.(iii) The condition (c) has substantially different thermodynamic implications com-pared with conditions (a) and (c’): validity of (c) means that the entropy changeconnected with the transfer of molecules from the liquid to the adsorbed state is zero,whereas validity of (a) (and most probably of (c‘) as well) means that by this transferentropy decreases or is zero.For microporous active charcoal and zeolitic molecular sieves a wide validity ofthe postulate (a) has been ascertained and independent information on the structureof these materials suggests that the validity of model I.I9;2 For active charcoal also,eqn.(b) holds well. For macro-porous or non-porous carbons eqn. (e) holds insteadof (b) :This relation after the substitution for E gives the Freundlich equation. Recently itwas ascertained that with materials exhibiting little or no porosity (Ti@, EaS04,glass, etc.) an equation holds which has the form either of (d) or of (e). Thepapers 3-5 seem to show that with non-porous substances the condition (c) is fulfilledand that, most likely, eqn. ( d ) and the validity of model I1 are involved.A final decision concerning the properties of the underlying physical model and theeventual decision of which condition (a), (c) and (c’) actually applies would requiredata for a greater number of adsorbents and a wider range of temperatures.How-ever the Dubinin-Radushkevitch equation is based on the temperature invariancy ofthe characteristic curve E = .f( W), whilst in ref. (4)-(6) the temperature invariancy ofthe characteristic curve E = f(7iU) is postulated. Therefore eqn. (d) describes an essenti-ally different physical situation than the Dubinin-Radushkevitch eqn. (6).The applicability of eqn. (d) for the surface area estimation and the comparison ofresults with those of the BET. equation form a separate problem. Experience hasshown that the B.E.T. equation usually does not hold well for microporous materialsand, moreover, gives non-realistic high values of their surface area.The structure ofthese materials justifies interpretation of experimental data in terms of model I, i.e.,on basis of volume filling of pores. For inaterials having no microporosity thesurface is gradually covered by layers of the adsorbate and thus it is justified tointerpret experimental data on the basis of model I1 or by means of eqn. (d), respec-tively. However, it is misleading to call eqn. (d) the Dubinin-Radushkevitch equationand thus to invoke properties inherent to model I, particularly the idea of volumefilling of the adsorption space. At a single temperature eqn. (b) and ( d ) simultaneouslyhold and lacking additional information it is difficult to decide which of these twocases actually applies.= f(T) ?I.W = WO exp (- k ‘ ~ ) , [ WO # f(T>].1 Dubinin, Chem.Rev., 1960, 60, 235.2 Dubinin, in Chemistry and Physics of Carbon, vol. 3, ed. Walker (Dekker, Inc., N.Y.), 1966,3 Hobson add Armstrong, J. Physic. Chem. 1963, 67, 2000.4 Endow and Pasternak, J. Vac. Sci. Techn., 1966, in press.5 Ricca, Bellardo and Medona, in preparation.in pressGENERAL DISCUSSION 119As can be expected from the physical model forming the basis of the B.E.T.theory, the BET. equation starts to hold from the coverage where the influence ofsurface heterogeneity is weakened and where higher adsorption layers are being formed,i.e., usually with na/nm about 0.8-1 -5. Eqn. ( d ) has been derived for monolayer adsorp-tion on heterogeneous surfaces and, accordingly, holds for the lowest pressures.Thepoint B, the value nm according to the B.E.T. equation and the value nm according tothe eqn. ( d ) are for theoretical isotherms always in the same region, and the agreementof surface areas evaluated in these three ways is therefore not surprising.There cannot be excluded the possibility that both equations, the B.E.T. as well as(d), are but appropriate empirical modes of finding a certain chosen point on isothermswithout any physical meaning. This approach, however, does not call for furtherdiscussion.Dr. J. Miipler, (Inst. hiorgan. Chem. Czechoslovak Acad. Sci., Prague) said;Frennet's finding that o x e / o ~ is constant and does not depend on the nature of theadsorbent has no general validity. In ref.(1) all the measurements of Frennet hasbeen made on the metal films previously saturated with hydrogen or methane, so thatthe variation with the measurements on the clean metal surfaces (see e.g., ref. (2), (3))is not surprising.Prof. R. A. W. Maul (Technischen Hochsclztale, Hannouer) said: Dr. Pone5 hasargued that the Dubinin-Radushkevich (D-R) equation cannot be applied to adsorp-tion phenomena on plane surfaces, since it had been derived in connection withsorption in highly porous carbons. As has been shown by Hobson4 the equationcan be derived by means of the Polanyi potential in connection with a certaindistribution for the adsorption energies. Thus the D-R equation can be effectivelyapplied also to non-porous matsrials as has been shown by a number of recent studies.In a paper by Haul and Gottwald 5 on residence times of rare gas atom adsorbedon Pyrex glass at low surface coverages it has, however, been shown, that, e.g., forxenon at temperatures between 130 and 158°K the intercept of the D-R plot issmaller by a factor of 2 than the B.E.T.monolayer capacity. A similar behaviour hasbeen found by Hobson 4 for argon between 63 and 77"K, whereas in our experimentsbetween 83 and 100°K both values are in good agreement. It is suggested that thiseffect might be explained by surface condensation phenomena occuring at tempera-tures below the two-dimensional critical temperature, the ideal value of which maybe taken as half the three-dimensional critical temperature according to De Boer.6In this case the intercept of the D-R plot can no longer be interpreted as a moiio-layer capacity in the sense of Kaganer.7 On the other hand, as has been mentionedin this Discussion by Dr.Frennet, the Belgian authors 8 have carried out adsorptionexperiments with Mr, Xe and CH4 on a large variety of metal films and found that evenat 78 and 90°K respectively the D-R and B.E.T. values of the monolayer capacity arein good agrcerncnt. The present authors 5 wish to emphasize that this is not incontrast to their findings since the two-dimensional critical temperature on metalsurfaces will presumably be considerably less than the ideal value due to polarizationeffects. Furthemiore with metal surfaces the arrangement of the adsorbed rare gasatoms may be strongly influenced by the geometry of the underlying crystal face.1 Delaunois, Frennet and Lienard, J. Chim. Physique, 1966, in press.2 Brennan, Graham and Hayes, Nature, 1963, 199, 1152.3 Brennan and Graham, Phil. Trans. A, 1965, 258, 325.4 Hobson and Armstrong, J. Physic. Clzern., 1963, 67, 2000.5 Haul 2nd Gottwald, Surface Sci., 4, 1966, in press.6 de Boer, The Dynamical Charccter of Adsorption (Clarendon Press, Oxford, 1953).7 Kaganer, Proc. Acad. Sci. U.S.S.R., 1957, 116, 603.8 Delaunois, Frennet and Lienard, J . Chim. Physique, 1966, 63, 906120 GENERAL DISCUSSIONProf. I(. S. W. Sing (Brunel University) (communicated) : It is generally acceptedthat the application of B.E.T. equation should be restricted to that part of the isothermwhich includes point B. For nitrogen adsorption at - 196" on non-porous hydroxyl-ated silica or alumina, point B corresponds unambiguously to the beginning of themiddle linear region of the isotherm (in accordance with the original designation ofEmmett and Brunauer I), being located 2 at a pressure close to 0-1 PO. The positionwith krypton, argon (and probably also xenon) is complicated and far from clear. Onhydroxylated silica 3 it would appear that the monolayer and multilayer adsorptionprocesses overlap, and that the statistical monolayer is not directly associated with anindistinct point B. On the other hand, on metal surfaces, although the heat of adsorp-tion is comparatively high and likely to lead to the formation of a well-definedmonolayer, the full significance of a characteristic point B is uncertain. According tosome workers,4~ 5 the krypton monolayer is completed on certain clean metals (Ni andCu) at pressures -0.1 PO, and is identified as such by a good point B ; others,6-*however, report a sharp point B with these adsorption systems at much lower pressures( < 0.01 PO). In seeking an explanation for these apparently conflicting results, thereis the possibility of a localized monolayer being formed on certain sites at low pressure,becoming more highly compressed with increase in pressure.9 Also, on a nearlyuniform surface of graphitized carbon,lo or sintered nicke1,ll krypton gives a stepwiseisotherm and the step-height (or point of inflexion) rather than point B appears tocorrespond to the monolayer capacity.121 Emmett and Brunauer, J. Amer. Chem. SOC., 1937, 59, 1553.2 Sing, Chem. Ind., 1964, 321.3 Sing and Swallow, Proc. Brit. Ceram. SOC., 1965, 39.4 Klemperer and Stone, Proc. Roy. SOC. A, 1957, 243, 375.5 Kington and Holmes, Trans. Furaday SOC., 1953, 49, 417.6 Roberts, Trans. Faraday SOC., 1960, 56, 128.7 Anderson and Baker, J. Physic. Chem., 1962, 66, 482.8 Brennan, Graham and Hayes, Nature, 1963, 199, 1152.9 Pierce and Ewing, J. Amer. Chem. SOC., 1962, 84, 4070.10 Amberg, Spencer and Beebe, Can. J. Chem., 1955, 33, 305.11 Fox and Katz, J. Physic. Chem., 1961, 65, 1045.12 Prenzlow and Halsey, J. Physic. Chem., 1957, 61, 1158

ISSN:0366-9033    

DOI:10.1039/DF9664100102

出版商:RSC    

年代:1966

数据来源: RSC