Hydrogen Sorption by Palladium-Gold Wires B Y DANIEL D. ELEY * AND EDWARD J. PEARSON Chemistry Department, University Park, Nottingham, NG7 2RD Received 30th September, 1974 Hydrogen sorption was studied on PdAu wires carefully cleaned in ultra high vacuum, at a hydrogen pressure of 6.25 pPa and wire temperatures of 100, 150,200 and 304 K. For Pd at 100 K the initial sticking coefficient was 0.16, falling to 0.006 at a fractional monolayer coverage 0 = 0.8. Sorption against time curves at 100 K for hydrogen continued smoothly to coverages 8 > 3.0. Deuterium for the same collision number ( P D ~ = 2/!&) gave a similar rate up to fl - 1.0 when the uptake became slower. Temperature programmed desorption gave one peak and desorption activation energies, which extrapolated to a 0 = 0 value of 100 kJ mol-1 for Pd, 90PdlOAu and 70Pd30Au and 45 kJ mol-1 for 55.2Pd44.8Au.These activation energies decreased sharply with coverage, to a value for the three Pd rich alloys of -20 kJ mol-I, comparable with the heat of solution. Hydrogen sorptions for a given exposure increased slightly from Pd to 90PdlOAu, decreasing sharply towards zero at 40Pd60Au, where the holes in the d band have disappeared. The results are discussed critically in relation to earlier data, and a tentative model advanced involving (mainly) /3 chemisorbed H (on Pd atoms, immobile at loOK), c( chemisorbed H2 (weakly held on octahedral holes in 100 planes) and dissolved H atoms under the surface. Although it is concluded that holes in the d band are necessary for strongly chemisorbed /3-H, a role for PdnAu, surface ensembles cannot be rulzd out.Starting with the ortho-para hydrogen conversion, we have now published work on eight reactions on our original set of 42 SWG polycrystalline PdAu wires,2 with three papers in preparation on further reactions, now carried out under ultrahigh vacuum condition^.^ In this paper we study the temperature programmed desorption of hydrogen from these wires, to obtain information on the bond strength and general behaviour relevant to our work on catalytic mechanisms. Since Pd and Au form a homogeneous set of solid solutions for all compositions, they are currently of great interest in academic studies, as witness recent reviews.4* In addition, Auger analysis has established that PdAu alloys have identical bulk and surface compositions' in vacuo,? whereas PdAg shows Ag enrichment in the surface with respect to the bulk.6 At the outset it should be realized that the main problem with Pd alloys is to distinguish adsorbed hydrogen on the metal surface from absorbed hydrogen in the bulk.In ordinary high vacuum conditions it was found necessary to add finely divided transfer catalysts, such as Pd black or UH3 7* * to secure a rapid surface reaction leading to equilibrium concentrations of dissolved H in reasonably short times, in Pd wires or foils at temperatures <120°C. More recently it has been found that equilibrium dissolved H in massive Pd may be rapidly achieved at temperatures as low as 177 K, by using ultrahigh vacuum equipment which excludes mercury vapour and tap grea~e.~*l These are the conditions adopted in the present research.Wagner 12- l3 discussed the solution of hydrogen in Pd terms of one or other of two successive steps. being rate determining. However, the surface kinetics involved still merit investi- gation. It appears that it is not the first strongly chemisorbed hydrogen but a H2(g) + 2H(ads), H(ads) $ H(disso1ved) 'r Preliminary work by Dr. B. Moore has confirmed this result on the present wires. 1-8 223224 H ADSORPTION BY Pd-Au WIRES successively weakly cheinisorbed hydrogen lo (either atoms lo or molecule^)^^ which is the precursor to dissolved M atoms. This is the background to the present investigation. A previous flash-desorption study of hydrogen from PdAu wires was published by Tardy and Teichner.'' EXPERIMENTAL A conventional ultrahigh vacuum equipment l6 was constructed from 3 cm-diameter glass tubing.This comprised a cylindrical vessel, 3 cm-diameter, holding axially a 15 cm length of 42 SWG PdAu wire, with an Alpert ion gauge and omegatron mass spectrometer closely adjacent. Hydrogen gas could enter this system at one end via a V.G. variable leak metal valve, leaving at the other end via a pinhole leak connected to the pumping system. The equilibrium or steady-state pressure pes = 6.25 pPa for hydrogen was fixed for each experiment by the setting of this metal valve. The Alpert gauge, a Mullard 106-17, containing a lanthanum boride coated filament, was run at a low enough temperature to avoid dissociation of the hydrogen into atoms.It was used to measure the desorption spectra. During bakeout the omegatron was used to monitor the background impurities, which on cooling usually yielded a vacuum below 53 nPa (4x 10-lo Torr). The 42 SWG alloy wires were from the original spools provided for Couper's work by Johnson Matthey. British Oxygen Grade X hydrogen, stated purity 99.999 %, was used without further purification. The filaments were heated electrically with an electronic control circuit that kept the average filament temperature to _+ 1°C in the steady state, and also allowed us to impose a linear heating regime In our experiments we used p = 2 K s-l, a relatively slow rate compared with N 100 K s-l used in much flash desorption work. Resistance against temperature calibration plots for the filaments were determined using a series of thermostat baths and furnaces, and agreed well with published data.15 The PdAu filament was initially cleaned by the following treatment: (1) outgassed (50 nPa) at 1000 K for 6 h, (2) heated in 6 mPa oxygen at 1000 K for 20 h, (3) pump, then 6 mPa hydrogen at 1000 K for 6 h, (4) pump, then 6 pPa oxygen for 15 min, (5) pump, then 6 pPa hydrogen for 15 niin, (6) pump to 50 nPa at 1000 K for 10 min.Between runs the wire was given 5 min reduction as for (5), followed by outgassing for 2 min as for (6), and every 2-3 days the wire was subjected to (4) followed by (9, repetition of this treatment always yielding the same final state. H atom bombardment followed by outgassing of the wire between runs gave virtually the same result as the oxidation-reduction technique. Sticking probability determinations by Ehrlich's method l6 involved equilibrating the wire at 1050 K and hydrogen pressure pes, then suddenly cooling the wire to the desired temperature and recording the p against t curve, adsorption coverages being calculated from the integrated p against t curve.Temperature-programmed desorption (TPD) data were analysed by one or other of three methods in terms of order of reaction a, activation energy E, and frequency factor v($, in the equation for the isothermal rate of desorption dnldt, where there are n adsorbed molecules per m2 dnldt = n'vg) exp (- E/RT). The methods were : (a), from Tp, the temperature at which the desorption rate is a maximum, e.g. for a = 2 from a plot of log,, noT; against l/Tp, no being the initial adsorbed concentra- tion.and (from Redhead)l7 (b), from a plot of log,, (radn/dt) against 1/T, which gives a line of slope E/2.303R, if E is independent of coverage and the correct value of a is used (see Degras) ;l (c), methods (a) and (b) fail to yield straight lines if E depends on coverage n, but the E against n relation may be derived from plots of log (- dn/dt)given against 1 /T.D . D . ELEY AND E. J . PEARSON 225 RESULTS SITE DENSITY To predict the uptake of hydrogen molecules in a chemisorbed monolayer, it is necessary to assume a density of adsorption sites on Pd and the PdAu alloys. We shall assume as in earlier work l * that the Pd wire has a roughness factor of 1.0 and exposes equal area of (1 10) and (100) places, giving 1.16 x lo1 Pd atoms or single sites per m2.Therefore assuming dissociative atomic adsorption of the hydrogen we predict a monolayer uptake of 5.8 x 10l8 molecules m-2, which we also apply to the PdAu alloys and denote by M where appropriate in the figures. There are clearly several uncertainties in this assumption. For example, Pd being f.c.c. it might have been more appropriate to assume equal areas of the lowest energy planes (1 11) and (loo), which would give a monolayer uptake of 7.2 x lo1 molecules m-'. The first assumption, however, appears to fit in better with the present results, besides being consistent with our earlier work. STICKING COEFFICIENT The cleaned Pd wire at 100 K was exposed to a pressure of 6.25 pPa hydrogen, and the result is shown in fig.1. 10 2 0 30 40 5 0 M 6 0 coverage/lOl' molecule m-2 monolayer coverage (see text). FIG. 1.-The sticking probability of hydrogen at 6.25 pPa on the Pd wire at 100 K. M denotes TEMPERATURE-PROGRAMMED DESORPTION The cleaned and outgassed wire was heated to 1000 K and exposed to 6.25 pPa hydrogen, conditions resulting in negligible sorption. It was then cooled to the required initial temperature, namely 100, 150,200 or 304 K, a suitable exposure time allowed for adsorption and absorption to occur, after which the linear temperature programmer was switched on and the desorption " spectrum " recorded. Exposures are recorded in Langmuir units, 1L = Fig. 2 then shows three TPD curves from pure Pd following exposures of 1.41,4.23 and 9.87 L at 100 K and 6.25 pPa hydrogen.By integration surface coverages were calculated for the given exposures, and coverage against time plots drawn. An estimated error for Torr s = 133 pPa s.226 22 0 20c 18C & 160 ‘3 140 8 ei a .-( E 120 \ E! 100 8 J E 8 0 8 8 r- r( H ADSORPTION BY Pd’-Au WIRES #A I00 2 0 0 3 0 0 t emperature/K FIG. 2.-Desorption spectra of hydrogen from a Pd wire initially exposed at 100 K and 6.25 pPa, for the number of Langmuirs (L) indicated. Each experiment starts at pes = 6.25 pPa.D. D. ELEY A N D E . J . PEARSON 227 these coverages is 5-7 %. Three such curves are shown for Pd, 90PdlOAu and 70Pd30Au at 100 K and 6.25 pPa in fig. 3. Fig. 4(a) shows the effect of exposure and alloy composition on surface coverages of the wire held at 100 K, and fig.4(6) N I E 100 9 0 8 0 70 6 0 5 0 40 100 9 0 8 0 7 0 6 0 5 0 4 0 (4 at %Pd (b) at % Pd FIG. 4.-(u) Effect of alloy composition on hydrogen uptake for various exposures (L) at 100K and 6.25 pPa. U,56 L ; V, 28 L ; 0,14 L ; x ,5.6 L. (b) Effect of alloy composition on hydrogen uptakes for 28 L exposure, at four temperatures and 6.25 pPa. The dotted line is the result due to Tardy and Teichner (see text). 0, 150; 0, 100; V, 200; x , 304 K. *0° t t F1 I E 0 5 10 15 2 0 2 5 3 0 35 4 0 adsorption timelmin FIG. 5.-Comparison of H2 uptake (0) at pHz = 6.25 pPa with Dz uptake ( A ) at P D ~ = 2/2, = 8.75 pPa, by a 70Pd30Au wire at 150 K.228 H ADSORPTION BY Pd-Au WIRES shows the effect of the temperature and composition of the wires on coverages follow- ing an exposure of 28 L (the apparently anomalous position of the 150 K curve being noted).In fig. 5 the coverage against time relationship for deuterium is compared with that for hydrogen on 70Pd30Au, the deuterium pressure pD2 being adjusted to 8.75 pPa, i.e. $pH2, so as to give the same impingement rate on the wire. It is apparent that the rate of sorption of deuterium is identical to that of hydrogen up to - 1 monolayer coverage, when it becomes noticeably slower. 15.5 16.0 16.5 1 0 5 1 ~ FIG. 6.-First order (A) and second order (0) Degras plots for desorption of hydrogen from a Pd wire. The initial sorption temperature was 200 K. 3 0 0 3 5 0 4 00 450 5 0 0 1 0 5 1 ~ FIG. 7.-Desorption activation energy plots for a Pd wire corresponding to the following coverages (left to right) x ,4.8 x lo1' ; 0, 15.0 x 1017 ; U, 24.5 x 10'' ; 0,45.0 x lox7 ; all in molecule m-2.D .D. ELEY AND E . J . PEARSON 229 Activation energy for desorption plots were made according to all 3 methods, methods (a) and (b) (the latter for both first and second order reactions) giving curved plots indicative of a dependence of activation energy on coverage, and fig. 6 shows an example of method (6) applied to pure Pd. It was therefore decided to analyse our data by method (c), which involves deriving (dnldt) for the same coverage from a number of runs at different temperatures. Here we had to be satisfied with straight lines based on 3 points, as in the examples in fig. 7, the slopes of which gave E12.303 R. Fig. 8 shows that the E values so derived for Pd, 90PdlOAu, and 70Pd30Au lie virtually on the same E against coverage curve, which extrapolates back to zero coverage to give an initial activation energy (" initial heat of adsorption ") of 100 kJ mol-1 with a visually estimated error of 10 kJ mol-'.In the fig. 8 inset we have plotted the initial (zero coverage) E values as a function of alloy composition. 2 0 40 8 0 100 120 140 ~overage/lO'~ molecule m-2 FIG. 8.-Desorption activation energy as a function of coverage for: CI, Pd ; 0, 90PdlOAu ; A, 70Pd30Au and x , 55.2Pd44.8Au wires. The inset plot gives the initial activation energies at zero coverage as a function of composition (estimated error bars have been inserted). DISCUSSION SOLUTION OF HYDROGEN We first enquire whether our conditions of pH2 = 6.25 pPa and TK are likely to give rise to the ap phase transition? in the palladium hydride, assuming an equilibrium concentration of dissolved hydrogen is reached.gives a linear plot of loglops, against 1/T(K) which we assume may be extrapolated below 194.5 K to our lowest temperature of 100 K. As a result we derive values of pslx as 300 K, 1333 Pa; 200 K, 0.43 Pa; 150 K, 133 pPa and 100 K, 0.013 nPa. From this it may be seen that at equilibrium only the a phase may be formed in our experiment at 300, 200 and 150 K although if a steady pressure of 6.25 pPa were maintained it should give the P-hydride phase at equilibrium at 100 K. However, the time duration of our experiments is such that only the a phase can be formed at 100 K. are used both to label the two palladium hydride phases, and to distinguish the weakly and strongly adsorbed hydrogen states in desorption spectra.Lewis t cc and230 H ADSORPTION BY Pd-Au WIRES give a formula relating n = [H]/[Pd] for solution equilibrium in the a hydrogen phase, as a function of T and p over the range + 75 to - 78°C. Assuming this formula is applicable to our temperatures, we calculated for the working pressure p = 6.25 pPa at 100 K, the value rz = 2 x and at 200 K, n = 3.7 x If the wire for simplicity exposed only the (100) face, and if r, wire radius = 0.1016 mm and Ypd the radius of the Pd atom = 0.137 nm then Wicke and co-workers7* rw 2 . 8 3 ~ ~ ~ = 2.6 x 105. N- total number of Pd atoms in wire number of Pd atom in wire surface - So assuming H = Pd for the surface, then at 100 K total dissolved H/surface H - 520 and at 200K the ratio is 0.96.Given therefore a sufficient initial exposure of the wire to hydrogen for equilibrium hydride formation, and sufficiently rapid de-solution kinetics, we should expect the flash desorption peak due to dissolved H to be dominant over that for adsorbed H following an initial exposure of the wire at 100 K, and the two peaks to be comparable following initial exposure at 200 K. There may, of course, be several adsorption peaks if there are several adsorbed species with different binding strengths. When only one peak is observed overall this may be due to overlap of the solution and adsorption peaks, or perhaps because diffusion is too slow to yield a solution peak within the time of the temperature scan.STICKING COEFFICIENT ON Pd Aldag and Schmidt 2 o found for hydrogen on a polycrystalline Pd wire an initial sticking coefficient of S = 0.1 3 falling to 0.001 at higher coverages, which later they associated with the weakly bound a state detected in TPD, which they regarded as a precursor to solution. Fig. 1 shows our very similar results, S = 0.16 falling to S = 0.006 at a coverage of 4.5 x 10'' molecules m-2, i.e. a fractional coverage 0 of 0.78. These values support the view that our initial Pd surface is clean. Tardy and Teichner on the other hand reported much lower initial sticking coefficients for Pd, -10-4.15 COVERAGE AGAINST TIME RELATION It may be seen from fig. 3 that the rate of uptake at 100 K and 6.25 pPa is closely similar for Pd, 90PdIOAu and 70Pd30Au wires, and that a monolayer coverage is reached at -8 min or 22.5 L exposure, with no indication of any knee at this point.The rate after the monolayer shows some scatter but it appears highest on 90PdlOAu, a result brought out in several experiments in fig. 4. A similar run on a 70Pd30Au wire at 150 M in fig. 5 compares deuterium (8.75 pPa) and hydrogen (6.25 pPa) rates of uptake, showing that the deuterium rate becomes the slower after -1 monolayer uptake. This suggests that the initial process is formation of a strongly bound chemisorbed monolayer, the p state of Aldag and Schmidt,20 followed by onset of the solution process, where we expect the rate of solution of deuterium to be determined by its permeability constant, which is less than that for The permeability constant itself is a product of solubility (where deuterium < hydrogen) 8 * 9* 23 and diffusion constant (where apparently conflicting ratios have been reported, as &ID, = 1.31 at 302.5"C 24 and &ID,, = 0.62 at 25"C, extrapolating to 1.0 at 300°C).The concept of an initial strongly chemisorbed p state, a precursor to dissolved hydrogen was suggested by Lynch and Flanagan lo* l1 and fits in with the deuterium effect as noted above. However, the continuity of the hydrogen sorption curve, and the fact that already at a coverage of 0.75 monolayer the activation energy for desorption has decreased to a value approximating to the 22D. D. ELEY A N D E . J . PEARSON 23 1 heat of solution, suggests that adsorption and solution cannot be separated, even during uptake of the first monolayer.Auer and Grabke I4 observed that the solution rate is first order in hydrogen and suggested that the solution precursor was a hydrogen molecule, adsorbed with its axis perpendicular to the surface with one atom embedded in an octahedral interstitial site in the surface, following a process H,(ads) + [H(ads) . . . H(sub-surface)] + 2H(dissolved). They found kinetic evidence for a blocking action of H atoms (or protons) adsorbed in these sub-surface interstices. On this view their H,(ads) may be equated with the a-state of Aldag and Schmidt,20 although these authors report that at 100 K the a-state was not saturated even at 10-4Torr, containing much more hydrogen than the P-state, leading them to identify it with dissolved H atoms remaining near the surface. Our fig.3 for 100 K and fig. 5 for 150 K show that uptake is still continuing after 3 monolayer equivalents of H, e.g. 1 monolayer H and 2 ‘‘ mono- layers ” of dissolved H, and one is led to wonder whether at such surface concentra- tions something analogous to a surface a -+ p phase transition needs to be considered in the top 2 or 3 layers of metal atoms. TPD PEAKS Most of our TPD peaks, e.g. those in fig. 2, refer to an initial coverage of <1 monolayer of hydrogen, so we may assume that the single maximum observed in the range 220-3OOK refers to adsorbed hydrogen. Conrad et aZ.26 working with H,-Pd (110) also found single desorption peaks but understandably at the higher temperature of 360 K, because of a higher heating rate of 20 IS s-l.Aldag and Schmidt 2o who adsorbed hydrogen on polycrystalline Pd at 200 K and applied an even greater heating rate - 100-500 K s-I (L. Schmidt, personal communication) found their main /3 peak (including 3 sub-peaks PI, p2, p3) centred around 450 K. The peaks observed by all three groups clearly refer to the same adsorbed species, which we all attribute to atomic hydrogen, Pd-H. Aldag and Schmidt,20 by adsorbing hydrogen at 100 K, were able to detect a weakly bound a state, regarded as a precursor to solution, around 250 K. This peak did not appear in Conrad’s or our desorption spectra, presumably because of our much lower heating rates. On the other hand, by using very long exposures of 600L at room temperature Conrad et al.found a second desorption peak setting in at 500-700 K which they associated with dissolved hydrogen. Presumably the high activation energy associated with a peak at this temperature, means the dissolved H is being “ desorbed ” as atoms. ACTIVATION ENERGIES OF DESORPTION Since chemisorption on transition metals usually has a negligible activation energy, it is usual to equate activation energies of single-step desorptions with the heats (negative enthalpies) of adsorption of the species concerned, here labelled P. On the other hand, where bulk diffusion is the rate limiting reaction, the temperature maximum may be related to the activation energy for diffusion by an equation analogous to Redhead’s equation.27 The activation energy against coverage graphs for Pd, 90PdlOAu and 70Pd30Au all extrapolate to an initial heat of 100 kJ mol-1 at 8 = 0, which may be compared with Aldag and Schmidt’s 2o P peak value for 202 kJ mol-l, Conrad’s 26 87 kJ mol-I for Pd (1 11) and 102 kJ mol-I for Pd (1 lo), Beeck’s 2 8 105 kJ mol-l for Pd films, and Vert’s 29 electrochemical value of 1 1 3 kJ mol-I.Our values fall sharply with increasing coverage, to values of 15 kJ mol-I232 H ADSORPTION BY Pd-Au WIRES for Pd, 17.5 kJ mol-1 for 90PdlOAu and 19 kJ mol-l for 70Pd30Au. These values are lower than the a state desorption activation energy of 54-58 kJ mol-l of Aldag and Schmidt,20 and the isosteric heat of Lynch and Flanagan’s lo precursor state to solution which was 36.4-44.8 kJ mol-I. They are in fact much closer to values of the isosteric heats of evaporation of dissolved hydrogen, given as 19.3 kJ mol-l for Pd, 29.0 kJ mol-1 for 84.7Pdl5.3Au, and 37.8 kJ mol-l for 73.5Pd26.5A~.~’ Other values for this latter quantity for pure Pd are 23.3 and 27.4 kJ m ~ l - l , ~ ~ all “ mols ” referring to HZ.The fall of adsorption energy with coverage observed here may denote the effect of a repulsive interaction between neighbouring H atoms in an immobile film, i.e. immobile at the adsorption temperature of 100 K.31 However, it may also arise from (a) chemisorption on different planes, (b) increased adsorption into the a state, and (c) increased solution contributions at the higher coverages. In contrast, Conrad et a1.26 report for H2-Pd (1 10) that the value of 102 kJ mol-’ remains constant from 8 = 0 to -8 = 0.5, thereafter falling, and similar behaviour appears to have been found b ~ V e r t .~ ~ This latter behaviour was shown by Roberts 31 to be expected for dissociative chemisorption on a uniform set of sites with nearest neighbour repulsion energy, when the film of atoms is mobile, as seems likely when prepared at room temperature as in these two cases. In the inset of fig. 8 we see that the zero coverage E shows a strong decrease after 70Pd30Au to 45 kJ mol-1 at 55.2Pd44.8Au, which if continued would reach 0 kJ mol-l at 45Pd55Au, a composi- tion approaching that where the 0.6 holes in the 4d band of Pd are filled by 5 s electrons from the Au atoms. We clearly cannot detect hydrogen uptake beyond this point (fig. 4) in agreement with Tardy and Teichner.15 CHEMISORPTION A N D ALLOY COMPOSITION In fig.4(a) and (b) we see that the main result is a profound decrease in the coverage of the Pd rich alloys which occurs around 55Pd45Au’ to a virtually zero value at 40Pd60Au, where the holes in the Pd d-band are completely filled by s-electrons from the Au. The dotted curve in fig. 4(b) is Teichner and Tardy’s result l5 for very different conditions, namely 6900 L exposure to hydrogen at an unspecified tempera- ture, which appears to have been 278 K. Their results show a similar absorption for Pd but a much more marked maximum at intermediate Pd-rich compositions, and also the same profound decrease in adsorption of 55Pd45Au. These results, together with the activation energies of desorption, support the view that holes in the d band play a dominant role in the bonding energy of the strongly chemisorbed ( p ) hydrogen, although the relation is clearly not a simple linear one.A flat peak over say Pd to 70Pd30Au has been shown by Dowden to result from adsorption on Pd,Au, Kobosev ensembles.32 On the other hand the maximum adsorption found in this region compels us to draw an analogy with the parallel finding for the a-phase in dissolved hydrogen.33 Although under some conditions this maximum is not found for Pd-rich PdAu alloys,34 nevertheless, it seems always present so long as p < pas, the critical pressure for the ccp hydride phase transition, which occurs in both Pd and PdAg and presumably PdAu, alloys.35 It appears to be due to the effect of added Ag(Au) in lowering the energy required to expand the octahedral interstitial sites which hold the dissolved protons in bulk Pd.35 It also seems possible that the a- adsorption of H2 molecules in surface octahedral interstitial sites will be similarly favoured.Clearly, the maximum uptake at 90PdlOAu is to be associated with subsurface dissolved protons, with a possible contribution at low exposure times from aH,, the precursor to dissolved hydrogen. This explanation seems more likely than that of a ‘‘ second-order geometric effect ” on the adsorption of the stronglyD. D. ELEY AND E . J . PEARSON 233 chemisorbed PH offered ear lie^,^ both for the adsorption of hydrogen, and the catalysis of the poH2 and H2 + D2 reactions. We shall explore the relevance of D2 weakly adsorbed on octahedral sites in a later paper ; it might be the species respon- sible for the D2 exchange reaction observed with strongly chemisorbed PH by A.and L. F a r k a ~ , ~ ~ in which case the (111) Pd surface, which has no octahedra1 interstices, should be relatively less active in this reaction. Referring to fig. 4(b) the decrease in coverages over the series 100, 200, 300 K (150 K being anomalous), is in line with expectation for " pseudo-equilibrium " values of QH at the very low ambient pressure used. Our coverages for pure Pd may be compared with the saturation coverages of Aldag and Schmidt,20 the latter at 0.66 pPa and 10 L exposure, in brackets, uiz. 100 K, 70 x 1017 molecules m-2 ; 200 K, 34 x 1017 (60 x 1017) ; and 304K, l o x 1017 (300K, 25x 1017).In view of the differences in assumptions, approximations and experimental conditions between the two laboratories, we may regard the agreement as satisfactory. The anomalously high coverage in our experi- ments at 150 K if confirmed by further work would suggest the operation of a small activation energy for adsorption of PH in reducing the coverage at 100 K below that for 150 K. A SURFACE MODEL In fig.9 we show a picture of the(100)plane of f.c.c. Pd. The strongly chemisorbed H atom is regarded as forming an electron pair bond to a single Pd atom, which is consistent with the agreement between the observed heat value for QH and Stevenson's calculated value 37 for the covalent bond H2 +2Pd + 2PdH AHads = -94.1 kJ mol-1 (calc) - 100 kJ mol-1 (expt). H,vW I FIG.9.-The (100) plane of Pd, with the various postulated chemisorbed PI, P2 and oc hydrogen species indicated by white spheres. The partial electronic charges are noted, H2 VW is van der Waals adsorbed hydrogen held over 2Pd sites. This is the hydrogen with a negative surface potential (Q;) according to Dus.~* When this film is virtually complete, we can have weak chemisorption of atoms and molecules, pz and a, both with positive surface potential^.^^ The existence of a234 H ADSORPTION BY Pd-Au WIRES strongly bound negative species followed by a weakly bound positive species was originally inferred from electrical conduction and thermoelectric 40 studies of hydrogen on palladium films. Whether it is sensible to postulate that the fli and a forms can also be chemisorbed on the interstices of the (1 11) plane is a matter for further study.So far as the bulk is concerned, the (1 11) interstices form the transition state for a dissolved proton diffusing from one octahedral hole to its neighbour in the lattice.41 Therefore it is likely that fl; and a hydrogen will be less stable on this (1 11) plane. Denoting 0, as a surface octahedral interstitial site, and nb by a similar site immediately under the surface, we summarise the available data for the processes in this discussion below. H2 OsOb = H * C]sH nb H2+Os + H 2 . 0 , AH = -44.8 kJ mo1-1 lo AH = -33.9 kJ mol-1 Eact = 28.4 kJ mol-1 l4 AGO - -21.3 kJmol-l 42 AHo - -23 kJmol-l l1 Eac,(diffusion) = 24.0 kJ m ~ l - ~ . ~ ~ H nsob * UsH ob H2$2C3b + 2H. Ob H a b a b +- nbH ob In fig.10 we have roughly sketched some potential-energy curves for these various species on the surface and sub-surface of a (100) Pd lattice plane, in the absence of lateral surface interactions. It seems clear from these curves that if HB is to be in equilibrium with dissolved H, it must be thermally activated into a surface mobility (perpendicular to the plane of the paper) so that it may freely diffuse into the metal surface FIG. 10.-A rough potential energy scheme for surface and sub-surface hydrogen on Pd (100). The section is taken through the surface octahedral holes (full lines), and through the neighbouring Pd atom (dotted lines). octahedral sites, and hence into the bulk metal. This mobility may just be setting in around 25"C, but the process is unlikely to be competitive with the much lower activation energy solution process involving a adsorbed H2 until much higher temperatures, when a p* rate of solution should set in.The effect of alloying Pd with Au is to lower slightly the bulk H potential energy, and after -70Pd30Au to raise that for chemisorbed HB. The important matter of surface mobility betweenD. D . ELEY AND E . J . PEARSON 235 weakly and strongly chemisorbed H atoms on adsorption and solution of hydrogen in Pd has been discussed recently by Bucur, Mecea and 111drea.~~ It seems quite likely that for Au rich alloys (beyond 40Pd60Au) that HB becomes more weakly held than Ha, and all the chemisorbed hydrogen H2 or H is weakly held on the octahedral hole sites. Thus although Au will catalyse pH2 and H, + D2 reactions indicating the presence of chemisorbed hydrogen, there is no measurable H2 uptake by ordinary volumetric methods.45 This model is, of course, extremely tentative, and does not involve any steps or other defect states which may be needed in a more complete consideration of the problem.A recent review 46 has concluded that “ the dynamical conditions of formation and decomposition of hydride phases ” are important in catalysis and the present work gives strong support to this statement. The authors wish to thank the S.R.C. for a studentship awarded to E. J. P., and I.C.I. for a grant to purchase the omegatron mass spectrometer. ’ A. Couper and D. D. Eley, Disc. Faraduy SOC., 1950, 8, 172. D. D. Eley, J. Res. Inst. Catalysis, Hokkaido University, Sapporo, 1968, 16, 101.D. D. Eley, Chem. and Znd., 1976, 12. R. L. Moss and L. Whalley, Ado. Catalysis, 1972, 22, 115. E. 6. Allison and G. C. Bond, Catalysis Rev., 1972, 7, 233. B. J. Wood and H. Wise, Surface Sci., 1975, 52, 151. E. Wicke and G. Bohmholdt, Z. Phys. Chem. N.F., 1964,42,115. E. Wicke and G. H. Nernst, Ber. Bunsenges Phys. Chem., 1964,68,224. J. D. Clewley, T. Curran, T. B. Flanagan and W. A. Oates, J.C.S. Faraday Z, 1973, 69, 449. lo J. F. Lynch and T. B. Flanagan, J. Phys. Chem., 1973,77,2628. ’’ J. F. Lynch and T. B. Flanagan, J.C.S. Faraday Z, 1974, 70, 814. l2 C. Wagner, 2. Phys. Chem. A, 1932,159,459. l 3 C. Wagner, Z. Elektrochem., 1938,44, 507. l4 W. Auer and H. J. Grabke, Ber. Bunsengw Phys. Chem., 1974, 78, 58. l5 B.Tardy and S. J. Teichner, J. Chim. Phys., 1970,67, 1962,1968. l6 G. Ehrlich, Adv. Catalysis, 1963, 14, 265. l 7 P. A. Redhead, Vacuum, 1962,12,203. l 8 D. A. Degas, 27iesis (Faculty of Science, Paris, 1966). l9 F. A. Lewis, The Palladium Hydrogen System (Academic Press, New York, 1967), p. 25. 2o A. W. Aldag and L. D. Schmidt, J. Catalysis, 1971, 22,260. ’ R. M. Barer, Dzj5usion in and Through Solids (Cambridge University Press, Cambridge, 1941), p. 184. 22 L. A. Rubin, Engelhard Ind. Inc. Tech. Bull., 1961,7, No. 122, 55. 23 M. J. B. Evans and D. H. Everett, Hydrogen in Metals (School of Metallurgy, Birmingham 24 W. Jost and A. Widmann, 2. Phys. Chem. B, 1935,29, 17; 1940,45,285. 2 5 G. Bohmholdt and E. Wicke, Z. Phys. Chem. N.F., 1967,56,133. 26 H. Conrad, G. Ertl and E. F. Latta, Surface Sci., 1974,41,435. 27 G. Farrell and G. Carter, Vacuum, 1967, 17, 15. 28 0. Beeck, Disc. Faraday SOC., 1950, 8, 118. 29 Z. L. Vert, I. A. Mosevitch and I. P. Tverdovskii, Doklady Akad. Nauk, S.S.S.R., 1961, 30 K. Allard, A. Maeland, J. W. Simons and T. B. Flanagan, J. Phys. Chem., 1968,72,136. 31 J. K. Roberts, Some Problems in Adsorption (Cambridge University Press, 1939). 32 D. A. Dowden, Catalysis, Proc. Fifth Znt. Congress on Catalysis (Miami Beach), 1972, 1, 621 3 3 A. Sieverts, E. Jurisch and A. Metz, 2. anorg. Chem., 1915,92, 329. 34 A. Maeland and T. B. Flanagan, J. Phys. Chem., 1965, 69, 3575. 35 A. Brodowsky and E. Poeschl, Z. Phys. Chem. N.F., 1965,44,143. 36 A. Farkas and L. Farkas, J. Amer. Chern. SOC., 1942,64, 1594. 37 D. P. Stevenson, J. Chem. Phys., 1955, 23,203. 38 R. Dug, Surface Sci., 1973, 42, 324. ” R. Suhrmann, G. Wedler and G. Schumiki, in Structure and Properties of Thin Films, ed. C. A. Neugebauer, J. B. Newkirk and D. A. Vermilyea (Wiley, New York, 1959), p. 268. University, Jan. 5 and 6, 1976), preprints. 140, 149. (North Holland Press, Amsterdam, 1973).236 H ADSORPTION BY Pd-Au WIRES 40 D. D. Eley and J. Petro, Natuw, 1966, 209, 501. 41 H. Buchold, E. Wicke and G. Sicking, Hydrogen in Metals (School of Metallurgy, Birmingham University, Jan. 5 and 6, 1976), preprints. 42 V. Breger and E. Gileadi, Electrochim. Acta, 1971, 16, 177. 43 H. Zuchner, 2. Naturforsch, 1970, 25a, 1490. 44 R. V. BUCW, V. Mecea and E. Indrea, Hydrogen in Metals (School of Metallurgy, Birmingham University, Jan. 5 and 6, 1976), preprints. 45 M. J. Chappell and D. D. Eley, in preparation. 46 W. Palczewska, Adv. Catalysis, 1975, 24, 245. (PAPER 4/2011)