Role of Chemisorption in Simple Catalytic ReactionsBY V. PONEC, Z. KNOR AND S . &RN*Institute of Physical Chemistry, Czechoslovak Academy of Sciences,Mgchova 7, Prague 2Received 13th January, 1966Hydrogen and oxygen sorption and their interaction have been studied on evaporated films ofPt, Pd, Rh, Ni, Fey Mo, Cu and Mn. Hydrogenation of cyclopropane has been followed on films ofNi, and sorption and interaction of hydrogen and nitrogen have been measured on films of Fe.Oxygen adsorbed on nickel does not react at 78°K with hydrogen adsorbed subsequently, but restrictsthe hydrogen adsorption roughly proportional to the degree of the surface coverage by oxygen. Atroom temperature there is extensive interaction of the preadsorbed oxygen with subsequently admittedhydrogen only if part of the surface had been left uncovered by oxygen, otherwise atomization of H2in the gas phase is necessary.Platinum behaves in the same way even at 73°K. With molybdenumthere is no interaction between the preadsorbed oxygen and admitted hydrogen at 273°K even ifhydrogen is present in the adsorbed state, atomization is again necessary.Products of the interaction at 273°K differ with various metals in a characteristic way. With Niand Mo the reaction is stopped after all oxygen had already reacted and the reaction cannot berepeated on the same surface at this temperature. With Pt, Pd and Rh, however, the sequence ofoxygen and hydrogen sorption can be repeated without limitation. Cu behaves differently. Thereis no hydrogen adsorption and no interaction with oxygen even on surfaces not fully precovered byoxygen.We conclude that hydrogen enters the reaction with oxygen always in the adsorbed state.There seems no connection between the efficiency of the metal in this reaction and the presence orabsence of hydrogen causing a " positive " effect in the change of the film conductivity. Twoconditions are required if a metal is to be active in the 024-H~ reaction : (i) all reactants must beadsorbed, (ii) their heat of adsorption has to be low.In hydrogenation of cyclopropane on nickel a fast reaction takes place only on a very smallportion of the surface, while on the remaining part the dissociated and dehydrogenated species areremoved slowly. Thus, various adsorbed particles play a different role in the catalytic reaction.Further results show that adsorption is a necessary prerequisite for a catalytic reaction, but itselfdoes not represent a sufficient condition.Tn all systems studied, adsorption of both components islargely competitive.We have been studying processes on clean surfaces of metals in the form ofvacuum evaporated films. The possible existence of several kinds of adsorbedspecies or of different types of chemisorption bonds in the adsorption of one andthe same molecule has been indicated previously.1-5 Consequently, we directedour study to the following questions, which form the borderline between the problemsof adsorption and of catalysis: (i) which kinds of bonds or of adsorbed speciescan be identified by measuring adsorption isotherms, isobars, kinetics of adsorp-tion and, particularly, of electric quantities, such as resistance and work function ?(ii) what is the role of various kinds of bonds or of the strength of bond (the valueof which can be estimated by calorimetric measurements) in simple reactions ?(iii) which molecules, participating in the reaction, enter into it in the adsorbedstate? In this paper we survey results which might be helpful in answering thesequestions.EXPERIMENTALThe following metals were studied: Ni, Pd, Mo, Rh, Cu, Fe, Mn.The films wereevaporated from filaments of the respective metals, except for Cu and Mn which were14150 ROLE OF CHEMISORPTIONevaporated from electroplated layers of W and Mo. We studied, first, the adsorption andinteraction of hydrogen and oxygen, but results were obtained also with nitrogen andcyclopropane.All experiments were carried out in high-vacuum systems the descriptionand function of which, together with the procedure of evaluation of the data have beengiven.6-8 After 3-4 days of evacuation, degassing of the filament and baking out of theapparatus, the evaporation of the film took place at a vacuum of 10-9 to 10-8 torr ; withCu and Mn it was 10-8 to 10-7 torr.The film resistance changes were measured in a vessel in which the sorption and re-actions of atomized hydrogens could also be followed. The resistance was measuredby a d.c. Wheatstone bridge. The work function changes were measured by the diodemethod, the current-voltage curves being ascertained before and after the adsorption ofgas.7 The vessel used was constructed in principle according to the design described inref.(9). Gas was admitted always to a cold cathode that was cleaned before the measure-ment of each current-voltage curve by repeated flashing at ca. 2300°K. Heats of ad-sorption were measured 10 in a Beeck-type calorimeter 11 (heat capacity 1.4 calldeg., sensi-tivity of 1 x 10-3 cal corresponding to 1 crn of deflection of the measuring device 10) usingan a.c. method which permitted the automatic recording of the heat effects.12 The surfacearea was measured using Kr or Xe.13 Catalytic activity in the hydrogenation of cyclo-propane was followed manometrically in a closed apparatus 14 at low pressures.RESULTS AND DISCUSSIONADSORPTIONComparison of the extent of chemisorption of gas with that of physical adsorptionof an inert gas allows estimation of the degree of surface coverage by the chemisorbate.With further assumptions, and with a suitable choice of chemisorbate, this comparisonmay also give an estimate of the " valency " of the metal surface atoms.As shown in table 1, hydrogen covers the surface of most metals to a high degree.In palladium, hydrogen dissolves 193 20 (at 78°K almost up to the ratio of Pd/H = 1)TABLE SURFACE COVERAGE OF VARIOUS METALS AT COMPLETE ADSORPTION OFmetalsFeNoNiRhPtlatticebody-centredcubecentredcubeface-centredcubeface-centredcubeface-centredcubebody-HYDROGEN 15-18no.of sites on1 cm2 of thesurfacex 1015(110) 1-72(100) 1-21*(211) 1.98(110) 1-44(loo) 1-02'(211) 1.72(111) 1-86(100) 1-61(110) 1.14(111) 1-62(100) 1.41(110) 0.99(111) 1.50(100) 1.31(1 10) 0.92mean no. of sites on1 cm2 at equal dis-tribution of 511planes x 10131-431 *371.541.331 924no. of H atomsadsorbed on 1 cm*x 10150-761.181.20-991 -04* According to Brennan, Hayward and Trapriel175 the number of atoms actually exposed bythis plane (and accessible to gases) is double that which corresponds to the number of atoms in asingle surface layerv. PONEC, z. KNOR AND s. E E R N ~ 151and only with Mn is the adsorption at 78°K very small and increases 21 substantiallyonly with increasing temperature.On copper, gaseous hydrogen is adsorbed atT<30OoK only when atomized 21 in the gas phase.A rapid chemisorption of hydrogen at 78°K is followed, as a rule, by a slowchemisorption of small extent (5-10 % of the total amount adsorbed.5~ 8, 16, 179 22, 23)The kinetics of the slow chemisorption is described by the logarithmic equation:be = 2.3 log/(t+to)+C, where C, to and b are constants and to is usually close tozero. The kinetics are related to the film porosity: on more dispersed films, theconstant b is higher.8, 23 The adsorption kinetics at 78°K are similar for metalsof a different electronic structure (see below) such as Ni,59 23 Fe,17* 22 on the onehand, and Mo8 and Rh16 on the other. At around 273"K, the adsorption ofhydrogen is practically instantaneous on all transition metals.The hydrogen isobars 16 on these metals differ to a greater extent.Accordingto the type of isobars, schematically shown in fig. 1, metals can be divided into twoloo 200 300 'K )[30 200 300TFIG. 1 .-Schematic illustration of two types of isobars ; type I, for N i , 5 , 2 3 Fe,17*26, Co 24 ; type 11,for Mo,* Rh.16,26 E, ratio of the amount adsorbed at given temperature to that directly measuredat 78°K ; Aemax, maximum divergence of isobar branches.groups. All metals without exception show a " divergence " (AE) of the twobranches of the isobars, the upper branch being always nearly reversible. Thedivergence suggests that a part of the film surface with sites of high heats of adsorptioncannot be covered at 78°K and that this activated process can only take place athigher temperatures.This process competes with a decrease in the extent of ad-sorption with increasing temperature, caused by desorption of hydrogen fromsites with a lower heat of adsorption. As a result, a maximum appears on thelower branch of the isobars. The film structure (i.e. probably the difficult accessi-bility of some adsorption sites) is responsible for at least a part of the activatedadsorption. The value of of the hydrogen isobars on nickel changed inparallel with the film dispersity and was greatest with the most dispersed films.23The most characteristic difference of transition metals for hydrogen adsorptionis revealed by changes of their electric properties.Changes of resistance of filmswere studied in the greatest detail. The types of dependences obtained can bedivided into three groups schematically shown in fig. 2 : (i) those with a maximum(hydrogen on Fe,191 279 28 C0,27 Ni,lS 239 279 289 297 Pd19- 20), followed by aminimum on some metals (Ni) at 78°K. (ii) Those with decreasing slope, and afte152 ROLE OF CHEMISORPTIONthe minimum-at 78°K-also with an ascending branch (Pt).179 30 (iii) Those withpermanent increase (d(AR)/dO)>O) showing at 8+1 at T = 78°K a greater resistanceincrease (hydrogen on M o , ~ Rh,16 Mn 173 27 ; also Ti 28, Cr 27). The work functionchanges, accompanying the hydrogen adsorption,39 319 329 29 are similar.I ' 3aeFIG. 2.-Schematic illustration of types of resistance changes AR/R with the extent of adsorption8 for various metals (see text).Oxygen, as contrast hydrogen, effects an increase of film resistance with Ni,17Pt,17 Rh,16 No 8 and Fe 17 of the type shown in fig.3a and with Mn of the typegiven in fig. 3b. Nitrogen is chemisorbed on iron to a small extent only, but itcauses a relatively large change of resistance.17 The dependence of resistance changeon the extent of adsorption is here practically a linear one (at 273°K). Cyclo-propane 14 behaves similarly to oxygen. Also other strongly adsorbed gases be-have in an analogous way.33-35 Therefore the positive effect, ie., the decrease ofthe film resistance with increasing adsorption on clean or on already partially coveredsurfaces is exhibited almost exclusively by hydrogen which is distinguished by itssmall dimensions.The rare existence of the positive effect occurs with other gases only if the ad-sorption tends to become physical in nature.Thus, according to Hansen andLittmann,36 xenon exhibits the positive effect on extremely thin films composed ofisolated islands. (With these films it is possible to accept the explanation suggestedby Broeder et aZ.37) From other gases described, signs of the positive effect werefound with CQ 389 33 at 78°K and higher pressures.38~ 33 However, a weak ad-sorption (which for CQ and Xe is most likely of the molecular type) is not alwaysaccompanied with a decrease of film resistance. We found 7 9 17 with nitrogenthat a weak reversible adsorption at 78°K on a surface already covered by chemi-sorption at 273"K, caused an increase of the resistance, although the change of thework function points to a polarization of the type Me-Ni.Displacement of hydro-gen from an iron surface by carbon monoxide proved that a part of the hydrogev. PONEC, z. KNOR AND s. C E R N ~ ‘ 153desorbed at 273°K first passes into a weakly bonded state giving a positive effect,and only then desorbs.38According to this mechanism, “ positive ” hydrogen may be weakly bound bymeans of d-orbitals of atoms on which possibly CO is already adsorbed. Thismakes it possible also to understand that at 78°K simultaneous adsorption of COand H2 is possible on the same surface and that the extent of adsorption for bothgases is practically the same as on the clean surface.The increased concentration04 08 0 1 2 3 40 a02FIG. 3.-Dependence of the relative resistance change (a, 6) on the extent of hydrogen sorptionat 298°K. For clean surface of Ni-film (a) and a surface partially covered by oxygen pre-adsorptionDependence of the hydrogen sorption (@HJ on the extent of oxygen preadsorption (@02) at 78°K(c) and 298°K (d). @ express the extent of sorption in multiples of hydrogen “monolayers”(complete hydrogen adsorption at 78°K).(b). 8 = 1 corresponds to the maximum hydrogen uptake in each case.of “ positive ” hydrogen is shown in the results of Siddiqi and Tompkins 39 (theirfig. 2). The time course of resistance changes on admission of hydrogen in theregion of the maximum of the curves on nickel, etc.17~ 29 and, furthermore, the factthat hydrogen, during desorption, first of all passes through the state in which itis bound by a bond causing the positive effect, suggests that such a bond constitutesan intermediate stage not only for desorption but also for adsorption.This wouldimply, according to the interpretation proposed below, a confirmation of the modelsuggested by Gundry and Tompkins 5 to explain the kinetics of adsorption.The positive effect and its occurrence together with the negative effect, at variousdegrees of coverage of the same transition metal have not as yet been explainedsatisfactorily. The positive effect of hydrogen cannot be explained by interactionwith impurities (especially with oxygen).On iron 17 and nickel,409 41 hydrogendoes not react with oxygen at low temperatures and in the platinum+hydrogensystem 17 the time course of the resistance changes in the region of the positiveeffect has a distinctly different character in the presence or absence of oxygen onthe platinum surface. Nor does it seem correct either to relate the positive effectto hydrogen dissolution and to the formation of H+ species as has been suggested,4154 ROLE OF CHEMISORPTIONbecause with palladium-in which hydrogen dissolves and the electron structureof which is similar to that of nickel-the dissolved hydrogen causes an increase ofresistance.Igs 20 Finally, neither the existence of species of the H f , H i type or ofmore complex species of various charges,ls 43 etc., is a convincing explanation.A simultaneous co-existence of two differently charged species-ions-on the surfaceof a conducting metal does not seem possible because adsorbed particles are mobileat temperatures above 150OK.4 The explanation of the positive effect by speciesof the H+ type meets with other difficulties.For example, nickel is a metal of pre-dominating electron-conductivity, whilst iron or molybdenum are metals of pre-dominating hole-conductivity.45~ 46 Thus, the similarity of the curves showing thedependence of the resistance changes on the extent of adsorption for Ni and Fewould signify that hydrogen is adsorbed at low coverages on nickel as a negativeion and on iron as a positive ion. But, at the same time, hydrogen at low coveragesalways increases the work function of the above metals and with iron and nickelthe strength and polarity of bond are very similar (see table 2).With molybdenum,formation of the positive hydrogen ion, which would have to be assumed over thewhole extent of adsorption, is still less probable, because with molybdenum the bondis more polarized in the sense of Mo+H-. (Placing of hydrogen under the surfaceatoms is unlikely, because of the great strength of the molybdenum lattice; thus,chemisorbed gases would penetrate to a far smaller depth than with iron and nickel.)It seems, therefore, one must look for an explanation of the positive effect in termsof the electron structure of the transition metal.19, 23TABLE 2.4OMPARISON OF STRENGTH AND POLARITY OF HYDROGEN ON SOME hlETALSNi Fe M Oheat of adsorption at 840,work function change A$maroom temperature (kcal mole-1) 26-30,47 26 10 32 47 40 10at 8- 1, [V] - 0.35 31 - 0.26 7 - 0.43 31 -0.28 7 - 0.20 7+ 3- +work function change A&=;the atomized hydrogen sorption ;78°K; compared with clean sur-face, PI -0.06 * 7 -0.15 * 7film resistance change (AR/A8) ;atomized hydrogen sorption + + +* after 30 min sorption the value depends on the amount of added atomized hydrogen.Transition metals are distinguished by a characteristic electronic sfructure.45p 47-50Electron orbitals of ns and (n-l)d atoms with f.c.c.metals and of as, np and(n- 1)d atoms with b.c.c. metals hybridize and give rise to delocalized orbitals andto corresponding bands of energetic levels, as for metals of a simpler electronstructure.51 It appears, however, that it is not a single broad band of levels thatis formed, but that there are several more or less distinguishable and mutually over-lapping sub-bands,4s the energy levels of which predominantly belong to orbitalsof one kind, e.g., with f.c.c.metals, to orbitals of practically only s or only d char-acter.52 Thus, one often speaks approximately of separate s- or d-bands of transi-tion metals. With b.c.c. metals there are also orbitals localized on individualatoms in addition to the spd band which-as indicated by the positive sign of thv. PONEC, z. KNOR AND s. EERNY 155Hall constant-clearly divides further into sub-bands.45 Atomic localized orbitals areformed from dz2 and d3c2-yz orbitals of isolated atoms. Electrons of not fully occupiedd-bands or electrons on localized d-levels are responsible for the main part of themagnetic moment of metals.52 The following two specific effects have been sug-gested to explain the high resistance of transition metals : (i) the scattering of con-ducting electrons by jumping into holes in the d-band, which-due to their higheffective mass-do not themselves participate in the conduction process.52 (ii) Thechange of the electron mobility by interactions with unpaired spins of electronsin the d-bands or in localized d-levels.49~ 53 These effects can assert themselvesalso in the adsorption of gases.On the basis of the semi-empirical theory of transi-tion metals the following model is suggested.On all metals, excepting platinum, hydrogen is adsorbed at low surface coverageby means of s-orbitals (f.c.c.metals) or spd-orbitals (b.c.c. metals) by a covalentbond. Thus, the participation of these orbitals in electrical conduction is restrictedand the metal resistance increases. If the metal has at its disposal for bondingalso electrons on not fully occupied d-levels, irrespective of whether the electronsbelong to localized or delocalized orbitals, then hydrogen is bound at a highersurface coverage with these orbitals by a weaker bond which reduces the film re-sistance by means of the effects (i) and (ii). The comparison of the electronegativitiesof metals 47 and hydrogen suggests that the covalent bond is, most probably, weaklypolarized in the Me+H- sense.It can be inferred, therefore, that hydrogen, whichcauses,31, 37 in most cases, an increase of the work function, is above the metalsurface. The hydrogen bond in the region where the film resistance is decreased,e.g., on Ni, also causes a decrease of the work function.29~ 31 This can mean eitherthe bond polarization is in the reverse direction or, more probably, that orbitalsof d-character extend to a lesser distance than s-orbitals of higher quantum number,which leads to the placing of hydrogen, polarized as Me+H-, under the surface level.With platinum, which has few reactive s-orbitals and a high work function, andoxidizes with great difficulty, hydrogen is bound with Me-Hf polarization first withd-orbitals and only at higher surface coverage also with s-orbitals which are, onplatinum, less advantageous for the chemisorption bond.This is the reason whythe change of film resistance in hydrogen adsorption on platinum is reversed ascompared with other transition metals.The adsorption of atomized hydrogen on a surface already covered by the ad-sorption of non-atomized hydrogen, leads to a decrease of the value of work function(table 2) and, eventually, even to a decrease of the work function below the valueat a clean surface.29 The explanation can be sought in the hydrogen bond (whichis the site of slight negative charge) with the metal atoms under the outermost metallayer already occupied by the preceding adsorption.Since these atoms in the lowerlayers of metal have a greater number of neighbouring atoms than the surface atoms,the hydrogen bond with them is weaker and therefore the adsorption of atomizedhydrogen, at lower temperatures, is reversible.21 The assumption of bonding withatoms at different distances from the surface of metal explains why if adsorptionof oxygen is followed by uptake of atomized hydrogen, interaction takes place,whilst with admission of gases in the reverse order the sorbed atomized hydrogendoes not enter into interaction.21Oxygen (and also most likely other strongly adsorbed gases) bonds with metalatoms so strongly that it tears them off the metal, thus the film resistance is moremarkedly increased than by hydrogen adsorption.54~ 36 The bond with d-orbitalsof these atoms is either impossible or it does not reveal itself in the film resistancevalues156 ROLE OF CHEMISORPTIONINTERACTION I N THE ADSORBED LAYERThe competitive character of adsorption of gases, which can react together athigher temperatures, has been found by us (in addition to Hz+CO+Fe system38)also in other systems.Hydrogen is displaced by oxygen from molybdenum surface ; 8adsorption of hydrogen on iron precludes the slow chemisorption of nitrogen,l7 etc.On the basis of our own experiments 409 4 1 9 21 on the interaction of pre-adsorbedoxygen or hydrogen with the other gas in the gaseous phase, we concluded thathydrogen interacts with oxygen in the adsorbed state. The same condition probablyholds also for oxygen when it interacts with adsorbed hydrogen.Conditions forinteraction were examined in the following experiments.Oxygen was pre-adsorbed, e.g., on nickel 40941 ; hydrogen subsequently admittedat 78°K did not react with the pre-adsorbed oxygen ; it merely restricted the subsequentadsorption of hydrogen to an extent roughly proportional to the part of the surfacewhich had been left unoccupied by oxygen (fig. 3c). At room temperature an ex-tensive interaction took place with the pre-adsorbed oxygen,403 41 but only if somepart of the surface had been left uncovered by oxygen adsorption. Interactionat 298"K, and its absence at 78"K, is evidenced by two facts : (a') change of the filmresistance and of the work function at 298°K is, after admission of hydrogen tosurface with pre-adsorbed oxygen, the reverse of the normal change with clean sur-face (fig. 3a).(b) The extent of hydrogen consumption was several times greater onsurfaces with pre-adsorbed oxygen (if the extent of oxygen pre-adsorption was suf-ficiently great) than on clean surfaces of the same area (fig. 34. If no part of thesurface is left free, consumption of hydrogen decreases almost to zero and no inter-action can be detected by measuring the film resistance. However, under analogousconditions, Quinn and Roberts 55 found a certain interaction by the work functionmeasurement. If the surface of nickel is completely covered by oxygen, reactionof hydrogen with pre-adsorbed oxygen can take place at 273°K if hydrogen is atom-ized in the gaseous phase.21 Platinum behaves in the same way, the only differ-ence being that the reaction can be detected but at 78°K.With M o , ~ Mn 17 andFe 17 no signs of interaction are found at 273"K, even if hydrogen is present on thesurface in the adsorbed state. On these metals, hydrogen can react with the layerof pre-adsorbed oxygen 179 21 only if it had been atomized in the gaseous phase andit again participated in the reaction after it had been sorbed by film.8, 41 Forsorption of oxygen by the film containing irreversibly bound pre-adsorbed hydrogen,the film resistance change is shown in fig. 4, and indicates that oxygen can react inthe adsorbed state. As the adsorption is undoubtedly faster than interaction itseems probable that oxygen also enters the reaction in the adsorbed state.The extent of interaction of hydrogen with pre-adsorbed oxygen at 273°K differsfor various metals.On Fe 17 and Mn 17 the extent is small ; on nickel 40, 41 andmolybdenum,8 reaction stops after the pre-adsorbed oxygen had reacted to a degreecorresponding to a ratio H/O not much greater than unity. On platinum,l7palladium19 and rhodium,16 oxygen and hydrogen can mutually react at 273°Kwithout limitation and the product of reaction desorbs from their surface. Oncopper,21 no adsorption of gaseous hydrogen takes place and therefore hydrogendoes not interact with pre-adsorbed oxygen. However, after hydrogen had beenatomized in the gaseous phase, it reacts easily with pre-adsorbed oxygen. Thisreaction can be repeated without restriction which means that, with Pt, Rh, Cu and Pd,the product desorbs at 273°K from the surface and makes the continuation ofreaction possible.Where reaction Metals clearly differ in the products of interaction at 273°Kv.PONEC, z. KNOR AND s. E E R N ~ 157can be repeated without limitation, i.e., on Pd, Pt, Rh and Cu, the most probableproduct is desorbable water. With other metals, the reaction proceeds probablyonly to the stage of formation of the OH group. This is indicated by the indirectindications just mentioned, and also by other results.56~ 57 The formation of theOH group at 273°K also agrees well with the fact that water on the surface of nickeland iron decomposes at 273°K with formation of hydrogen which is liberated intothe gaseous phase.s*$ 59 The fact that even with atomized hydrogen a desorbableproduct does not form on Ni, Fe, Mo and Mn at 273"K, shows that the additionof further hydrogen to the OH group, which is the most probable product of inter-action, is a difficult process and that water, which can be produced under suitableconditions (e.g., nickel catalysts are used in industry for removal of traces of oxygenat low temperatures), is probably formed by a mechanism other than that of themere addition of H+OH.5 10 15t, minFIG.4.-Time course of film resistance change and of oxygen sorption on Pt film covered by pre-adsorption of irreversibly bound hydrogen.The overall order of the activity of metals : Pt, Pd, Rh; Cu (H atoms) > Ni > Mo,Fe, Mn, Cu; as derived from the temperature and other conditions under whichhydrogen + oxygen interaction takes place, is generally in good agreement withthe catalytic experience.There is no direct connection between the catalytic activityand the presence or absence of the positive effect on the film resistance againstcoverage plots, i.e., between catalytic activity and the various kinds of chemisorptionbonds of hydrogen. The same conclusion holds also for other catalytic reactionsof hydrogen.605 71 Nor can other parameters, such as the shape of the isobars, thekinetics of adsorption, number of oxygen layers, extent of adsorption, etc., bedirectly correlated with catalytic activity. (This is not surprising if the above ex-planation of the adsorption kinetics and isobars is accepted.) The most activemetals are evidently those which adsorb oxygen and hydrogen most weakly.How-ever, as e.g., with copper, and similar non-transition metals, the low heat of ad-sorption of reacting gases is not a sufficient condition for the reaction to procee158 ROLE OF CHEMISORPTIONeasily at a low temperature. Nor is the adsorption of reactants a sufficient con-dition for a catalyst to be active for the reaction, as it is shown by results obtainedwith Mo, Fe and Mn. However, we arrive at the above order of metal activitiesin oxidation (and other reactions) of hydrogen if the two requirements are com-bined: (i) adsorption under reaction conditions of the reactants is the necessaryprerequisite; (ii) the high activity requires a low heat of adsorption of participantsentering reaction.The high reactivity of more weakly bound species has been postulated.7~ 61962In our experiments with cyclopropane we also found that only the part of the nickelsurface not covered by strongly bound products of cyclopropane chemisorptionis active in fast hydrogenation.According to results obtained by Thomson andWislilade 63 and Stephens 64 the same conclusion probably holds for the hydro-genation of ethylene.Sachtler and Fahrenfort 65 first proved directly from the hydrogen-deuteriumexchange on gold that the adsorption of both reactants is a necessary prerequisitefor a catalytic reaction; this is also probably the interpretation of the hydrogen-deuterium exchange on other non-transition elements.67 Sachtler and de Boer,66and also Kazansky and Voevodsky,68 further proved experimentally that hydrogenenters oxidation in the adsorbed state on gold66 and palladium.68 A similarconclusion results from our experiments for Ni,40* 41 Mo 8 and Pt.17 Since on thesemetals the adsorption of oxygen during the reaction is possible (this also holds forgold according to the latest results 69, the mechanism requiring all participants toenter the reaction in the adsorbed state again seems to be the most probable.Thegeneral character of this conclusion suggests the reason why the Rideal type ofmechanism, where some of the reaction participants react by an impact from thegaseous phase or from the physically adsorbed state, has-contrary to the preceding(Bonhoeffer-Farkas) mechanism-never been directly proved experimentally butwas based on kinetic arguments only.Beeck70 first suggested and proved a correlation between the high heat of ad-sorption and low catalytic activity.A similar correlation follows from resultsobtained by other authors in spite of their own different interpretations of theirresults.609 71 There are also several theories of the catalytic activity of solids, basedon a correlation with thermodynamic parameters (for review, see ref. (71)). How-ever, these correlations do not apply, e.g., to metals which do not adsorb one or moreof the reactants, such as, e.g., Au, Ag, Cu, Pb, Sn and other non-transition elementsin hydrogenation or oxidation reactions, or Pt, Pd and Ni in ammonia synthesis,etc.This clearly shows that energetic factors alone cannot serve as a guide for arational choice of catalysts. Nevertheless, Pt and Rh are generally, for a numberof reactions among the most active metals and adsorb gases comparatively weakly(see the general rule of Tanaka and Tamaru 72), whilst Mo, W, Ta, Ti, etc., belongto the least active group of metals and adsorb all gases very strongly. Nickel andcobalt, etc., occupying a position between these extreme groups, are closer to thegroup of active metals which is in accordance with the values of heats of adsorption.NOW, one may ask what is the basis of the low heat/high activity correlation.The reasons might be as follows : (i) high heat of adsorption means a low potentialenergy of adsorbed species and if the exothermic effect of the overall reaction isnot high, the surface reaction among adsorbed particles is highly endothermic.Since the endothermicity represents the minimum possible activation energy, thenalso the surface reaction is accompanied by a high activation energy.(ii) Re-actants form a product with an exothermic or a small endothermic effect, so thatthe final step-the desorption of products-must be strongly endothermicv. PONEC, z. KNOR AND s. EERNS 159According to the general rule of Tanaka and Tamaru72 the higher is the heat ofdesorption of the products, the higher is the heat of adsorption of the reactantsand thus again the correlation of the activity with the heat of adsorption of theinitial reactants is obtained.(iii) On metals with a higher heat of adsorption ofmore complex molecules such as hydrocarbons, the degree of surface coverage bya layer of non-reactive products of dissociative chemisorption is doubtlessly greaterthan on metals with a lower heat of adsorption.70~ 64 The metals then differ in thearea of the working surface and its rate of the reaction on this surface where againeffects (i) and (ii) are operative.From the possible operation of mechanisms (i)-(iii) there result definite con-clusions for the relation between the absolute value of the heat of adsorptionand the activation energy. The greatest differences in energies of activation on thevarious metals is expected with mechanism (i) ; according to mechanism (iii) metalsdiffer but in the value of the frequency factor.However, the activity of some metals cannot be explained along the lines above.Among these is the recombination of H-atoms60 and other gases, and the decom-position of formic acid.73 Hydrogen atoms are weakly adsorbed by a number ofnon-transition elements, nevertheless the activity of these elements in recombinationis low.Therefore, the correlation of catalytic activity with heat of adsorption showsa maximum at the optimum heat of adsorption, i.e., for the lower heats of adsorption(or of bond strengths or other similar parameters), the activity of metals increaseswith the heat of adsorption, and decreases for the region of higher heats of ad-sorption.This can be explained as follows. On transition metals where the lowheat/high activity correlation holds, hydrogen recombines and desorbs in a singlestep with an activation energy close to the heat of adsorption of hydrogen with zero“ intrinsic ” activation energy (fig. 5 4 . On non-transition metals it is not the de-sorption itself but the recombination of atoms in the adsorbed state that is difficult(see scheme 5b) and is connected with a high intrinsic (non-endothermic) activationenergy. The different rate-determining steps for transition and non-transitionU(4 (b)FIG. 5.-Schematic illustration of the course of potential curves for hydrogen adsorption on transi-tion (a) and non-transition (b) metals. 1, Physical adsorption ; 2, weak adsorption of atoms boundwith d-orbitals of metals ; 3, strong chemisorption of atoms.Chemisorption on non-transitionmetals endothermic (2’) and exothermic (3’) ; E&s, activation energy of desorption. Curve 2 doesnot represent the “equilibrium ” d-orbital-H bond of p. 155.elements explains the existence of two branches in the correlation of catalyticactivity with the heat of adsorption. In formic acid decomposition, again a goodvolcano-shaped correlation with the heat of formation of formates was obtained,In order to explain the maximum, we must again suppose a different rate-determinin160 ROLE OF CHEMISORPTIONstep for metals on the right and left side of the correlation. It seems improbablethat in the formation of the activated complex adsorption only should play therate-determining role with metals of the left side of the correlation,73 because, e.g.,on silver an adsorbed layer is easily formed by adsorption of formic acid.74 But theassumption of a different rate-determining step for metals on the left and rightside of the heat-activity correlation itself, is acceptable.Thus we conclude that, in addition to the necessity of adsorption of reactants,there exists another prerequisite limiting the operation of the correlation of highactivity with low heats of adsorption, viz., that such correlation can hold goodbut for a group of metals with the same rate-determining step in the given reaction.The low heat/high activity correlation does not hold where Bronsted empirical rulerelating the activation energy with the heat of adsorption operates as is so with non-transition elements on the left side of correlations in hydrogen recombination, formicacid decomposition and possibly in further reactions.In conclusion, an electronic factor may operate when the above rules hold.Asuitable electronic structure makes possible a rapid adsorption of reactants withouta noticeable activation energy, though even strong bonds must dissociate duringadsorption (fig. 5a). The electronic structure of metals is, furthermore, decisivefor the strength of the adsorption bond and consequently operates through rule(ii) concerning the magnitude of the heat of adsorption, and also renders it possiblethat the surface reaction proceeds without a high intrinsic activation energy.Acombination of all effects of the electronic factor lead to complications in correlatingactivities with electronic structures. It seems, therefore, that any correlation of thecatalytic activity with parameters characterizing merely the electronic structure ofmetals, will be found and understood only by analyzing step by step the mechanismof the overall catalytic reaction as indicated above.1 Sachtler and Dorgelo, Bull. SOC. chim. Belg., 1958, 67, 465.2 Mignolet, Bull. SOC. chim. Belg., 1958, 67, 358.3 Mignolet, J. Chim. Physique, 1957, 54, 19.4 Dowden in Chemisorption, ed. Garner (Butterworths, London, 1957), p. 3.5 Gundry and Tompkins, Trans. Faradizy SOC., 1956, 52, 1609.6 Knor and Ponec, Coll. Czech. Chem. Comm., 1961,26, 529.7 Knor and Ponec, Coll.Czech. Chem. Comm., 1966, 31, in press.8 Ponec, Knor and &rnS;, Coll. Czech. Chem. Comrn., 1965, 29, 3031.9 Pritchard, Trans. Faraduy SOC., 1963, 59,437.10 CemS;, Ponec and HlBdek, J. Catalysis, 1966, 5, 27.11 Beeck, Cole and Wheeler, Disc. Faraday Soc., 1950, 8, 314.12 Hlhdek, J. Sci. Instr., 1965, 42, 198.13 Ponec and Knor, Coll. Czech. Chem. Comm., 1962, 27, 1091.14 Knor, Ponec, Herman, DolejBek and cernf, J. Catalysis, 1963, 2, 299.15 Knor and Ponec, COIL Czech. Chem. Comm., 1961, 26, 961.16 Ponec, Knor and CernS;, Coll. Czech. Chem. Comm., 1965,30, 208.17 Ponec, Knor and cerny, unpublished.18 Boreskov and Vasilevitch, Kinet. katuliz, 1960, 1, 69.19 Knor, Ponec and eernS;, Kinet. kataliz, 1963, 4, 437.20 Suhrmann, Wedler and Schumicki in Structure andProperties of Thin Films (Willey, New York,21 Ponec, Knor and tern$, J.Catalysis, 1965, 4, 485.22 Porter and Tompkins, Proc. Roy. Soc. A , 1953, 227, 529.23 Ponec and Knor, Actes 2nd Congr. Int. Catalyse, (Technip, Paris, 1961) p. 195.24 Ponec, unpublished.25 Gundry and Tompkins, Trans. Faraday Soc., 1957, 53, 218.26 Lanyon and Trapnell, Proc. Roy. SOC. A , 1955,227, 387.27 Suhrmann, Hermann and Wedler, 2. physik. Chem., 1962,35, 155.28 Zwietering, Koks and van Heerden, J. Phys. Chem. Solids, 1959, 11, 18.1959), p. 278v. PONEC, z. KNOR AND s. ~ E R N J 16129 Tretyakov and Balobnev in Mechanizm vzaimodeystviya metullov s gazarni (Izd. Nauka, MOSCOW,30 Suhrmann, Wedler and Gentsch, 2.physik. Chem., 1958, 17, 350.31 Culver and Tompkins, Adv. Catalysis, 1959, 11, 67.32 Crossland and Pritchard, Surface Sci., 1964, 2, 217.33 Wedler and Fouad, 2. physik. Chem., 1964,40, 12.34 Suhrmann, Schwandt and Wedler, 2. physik. Chem., 1962, 35, 47.35 Gryamov, Shimulis, Yagodovsky, Dokl. Akad. nauk U.S.S.R., 1960, 160, 1132.36 Hansen and Littmann, 2. Elektrochem., 1963, 67, 970.37 Broeder, van Reijen, Sachtler and Schuit, 2. Elektrochem., 1956, 60, 838.38 Cukr, Merta, Addmek and Ponec, Coll. Czech. Chem. Comm., 1965,30,2682.39 Sidiqqi and Tompkins, Proc. Roy. SOC. A , 1962,268,452.40 Ponec and Knor, Qll. Czech. Chem. Comm., 1962,27, 1443.41 Ponec, Knor and Cern3, Proc. 3rd Int. Congr. Catalysis, Amsterdam (North Holland Co.,42 Sachtler and Dorgelo, 2. physik. Chem., 1960, 25, 69.43 Suhrmann, Adv. Catalysis, 1955, 7, 303.44 Gomer, Disc. Faraday SOC., 1959, 28, 23.45 Goodenough, Physic. Rev., 1960, 120, 67.46 Frank, Appl. Sci. Res. B, 1957, 6, 379 ; 1958, 7, 41.47 Hayward and Trapnell, Chemisorption (Butterworths, London, 1965).48 Mott and Stevens, Phil. Mag., 1957, 2, 1304.49 Mott, Adv. Physics, 1964, 13, 325.50 Vonsovsky and Izyumov, Usp. Fiz. Nauk U.S.S.R., 1962, 77, 379 ; 1962, 78, 4.51 Reitz, Adv. Solid State Physics, 1955, 1, 1.52 Mott and Jones, The Theory of the Properties of Metals and Alloys (Oxford, 1936).53 Coles, Adv. Physics, 1958, 7, 40.54 Sachtler and van Reijen, J. Res. Inst. Cat., Hokkaido Univ., 1962, 10, 87.55 Roberts, Proc. 3rd Int. Congr. Catalysis, Amsterdam (North Holland Co., Amsterdam, 1965),56 Farnsworth, Schlier and Tuul, J . Physics Chem. Solids, 1958, 9, 57.57 Campbell and Thompson, Trans. Faraday SOC., 1961, 57, 279.58 Suhrmann, Heras, Heras and Wedler, Ber. Bunsenges. physik. Chem., 1964, 68, 511.59 Lazarov, lecture held at the Faculty of Sciences, Charles University, Prague, 1964.60 Bond, Catalysis by Metals (Academic Press, New York, London, 1962).61 Trapnell and Rideal, Disc. Faraday Soc., 1950, 8, 114.62 Dowden, Bull. SOC. chim. Belg., 1958, 67, 439.63 Thomson and Wishlade, J. Chem. Soc., 1963,4278.6'1 Stephens, J. Physic. Chem., 1958, 62, 714.65 Sachtler and Fahrenfort, Actes 2nd Congr. Int. Cutalyse, Paris (Technip, Paris, 1961), p. 831.66 Sachtler and de Boer, J. Physic. Chem., 1960,64, 1579.67 Fensham, Tamaru, Boudart and Taylor, J. Physic. Chem., 1955, 59, 806.68 Kazansky and Voevodsky, Dokl. Akad. nauk U.S.S.R., 1957,116, 633.69 Kulkova and Levchenko, Kinet. kataliz, 1965, 6, 765.70 Beeck, Disc,Faraday SOC., 19S,O, 8, 118.71 Ponec and Cerny, Rozpravy Cs. akad. v6d (Disc. of Czechoslovak Acad. of Sci.), 1965, 75,72 Tanaka and Tamaru, J. Catalysis, 1963, 2, 366.73 Fahrenfort, van Reijen and Sachtler, 2. Elektrochem., 1960, 64, 216.74 Tamaru, Trans. Faraday Soc., 1959,5§, 824.75 Brennan, Hayward and Trapnell, Proc. Roy. SOC. A , 1960,256,81.1964), p. 50.Amsterdam, 1965), p. 353.p. 365.no. 5 (in English)