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1 The Adsorption, Desorption, and Exchange Reactions of Oxygen, Hydrogen, and Water on Platinum Surfaces. 11. Hydrogen Adsorption, Exchange, and Equilibration Y. K. PENG AND P. T. DAWSON~ Chet?~istry Department atzd Instit~rte for Materials Research, McMaster University, Hamilton, Ontario L8S 4MI Received August 13, 1974 Can. J. Chem. Downloaded from by on 05/14/18 Y. K. PENG and P. T. DAWSON. Can. J. Chem. 53,298 (1975). The adsorption, desorption, exchange, and equilibration reactions of hydrogen and deuterium on a platinum filament have been investigated by thermal desorption mass spectrometry. A surface saturated with hydrogen at 120 "K has a coverage 4.2 x 1014 molecules ~ mand - ~ gives desorption spectra with four distinct peaks: P,(165 OK), P,(220 OK), m(280 OK), and P4(350 OK). Apparent activation energies and pre-exponential factors were determined for the P,-, A-, and P4-peaks. For both co-adsorption and sequential adsorption of H, and D, the mass 2, 3, and 4 desorption spectra have identical shapes and the gas desorbs at equilibrium throughout. It is concluded that hydrogen adsorbs dissociatively. Exchange andequilibration were studied at 120, 210, and 285 "K by determining the surface composition and isotope distribution after varying fractions of preadsorbed H had been replaced. Following exchange at 120 OK the desorption spectra show a higher Dcontent and alack of equilibrium in thedesorbinggas at low temperature. In most other experiments the mass 2,3, and4desorption spectra had identical shapesand thegas desorbed at equilibrium. The results are interpreted by a modelwhich requires that the polycrystalline platinum surface is intrinsically heterogeneous. It appears that different mechanisms are unnecessary to interpret the differences in kinetics observed for exchange and equilibration at low temperatures. Y. K. PENG et P. T. DAWSON. Can. J. Chem. 53, 298(1975). On a Ctudie, par spectrometric de masse impliquant la desorption, les reactions d'equilibre et d'echange de I'hydrogkne et du deuterium sur un filament de platine. Une surface saturee avec de I'hydrogkne B 120 "K supporte 4.2 x lo4 moldcules ~ met - conduit ~ B des spectres de desorption contenant 4 pics distincts: P,(165 OK), W(220 OK), m(280 OK) et P4(350 OK). On a determine les energies d'activation apparente et les facteurs pre-exponentiels pour les pics P,, P,. et P4. Pour la co-adsorption et I'adsorption en sequence de Hz et D2, les spectres de disorption des masses 2,3 et 4 ont des formes identiques et le gaz se desorbe B I'equilibre B tout moment. On en conclut que l'hydrogkne s'absorbe d'une fa~on dissociative. On a etudie I'echange et l'equilibre A 120, 210et 285 "K en determinant la composition la surface et la distribution isotopique aprks que diverses fractions d'hydrogkne prkadsorbe avaient Ctt remplacees. Apres &change a 120 OK, le spectre de desorption montre un contenu en deuterium qui est plus eleve et un manque d'equilibration dans les gaz desorbants B base temperature. Dans la plupart des experiences, les spectres de desorption des masses 2,3 et 4 ont des formes identiques et les gaz se desorbent a l'equilibre. On interprkte les resultats B I'aide d'un modkle qui demande que la surface polycristalline du platine soit intrinskquement hctcrogkne. I1 semble qu'il ne soit pas necessaire de faire appel Bdifferents mecanismes pour interpreter les differences dans les cidtiques observees pour I'echange et l'equilibration a base temperature. [Traduit par le journal] Introduction The adsorption and isotope interchange reactions of hydrogen and deuterium have been studied on many transition metal surfaces (1, 2). The isotope interchange reactions have been classified as either exchange, in which one isotope is preadsorbed I Dz(g) + H(a) -, HD(g) + D(a) or equilibration, for which both isotopes are 'To whom correspondence should be addressed. initially present in the gas phase PI Hz(g) + D2(g) * ~HD(Jz) Many basic types of mechanism have been proposed for the exchange and equilibration reactions, together with some subtle variants, but the experimental evidence for any mechanism is limited and our understanding of these reactions is still very poor. Of particular interest is the observation that under the same experimental conditions the kinetics of the exchange and equilibration reactions on platinum differ, from

2 PENG AND DAWSON: HYDROGEN INTERACTION WITH PLATINUM 299 which it is inferred that these reactions proceed via different mechanisms (3, 4). In an attempt to gain more information on these reactions, the adsorption, exchange, and equilibration processes have been investigated by thermal desorption mass spectrometry using a platinum filament which has been characterized in earlier studies of oxygen adsorption (5). In the experiments described in this paper the isotopic composition of the surface- has been determined at various stages during exchange with and equilibration of the gas phase. This is in contrast with previous studies in which these reactions have been followed by gas phase composition measurement alone. Experimental Methods The ultra-high vacuum system used in this work is the same as that described previously (5, 6). The sample filament, a 12 cm length of 0.01 in. diameter platinum (Johnson, Matthey and Mallory, Grade I), equipped with in. diameter sensing leads for temperature measurement and control, was spot welded to heavy tungsten leads. This sample was from the same spool and processed in the same way as that described in the first paper of this series (5). In low temperature experiments a filament temperature of 120 K was obtained by immersing the heavy tungsten leads of the press seal in liquid nitrogen. Hydrogen and deuterium were purified by diffusion through palladium. Hydrogen and deuterium pressures were monitored with a quadrupole mass spectrometer which was calibrated against an ion gauge attached directly to the reaction vessel. This ion gauge was removed from the system after calibration. The relative sensitivity for Hz and D2 was found to be 1.5. However, in contrast with flash experiments in a closed system, the areas under the mass 2 and mass 4 desorption spectra differed by only -5z since the measured pumping speed for Hz was greater than that for D2 by a factor of The desorption spectra were obtained with a slow linear heating rate of 5.6 deg s-i using an automated Kelvin double bridge. Results Adsorption of Hydrogen on Platinum Tl~ermal Desorption Spectra The thermal desorption spectra obtained after exposure of the clean platinum filament to various doses of hydrogen are shown in Fig. 1. The adsorbed phase obtained by saturating the surface at 120 O K with hydrogen at Torr is unstable at temperatures higher than 140 O K and produces a desorption spectrum, spectrum e, with four distinct peaks with peak maxima at 165, 220, 280, and 350 O K, designated the PI-, P2-, P3-, and P4-peaks, respectively. The saturation coverage obtained under these conditions is 4.2 x 1014.molecules cmp2; this coverage is TEMPERATURE ( O K 1 FIG. 1. Thermal desorption spectra obtained after hydrogen adsorption on platinum at 120 K for (a) 2 x lo-', (b) 6 x lo-', (c) 3 x (d) 2 x and (e) 6 x Torr s. unaffected by prolonged pumping at the adsorption temperature. Desorption spectra obtained with increasing exposure, spectra a-e reveal that the P-peaks develop successively as the coverage increases. This is most simply interpreted as the population of binding states of steadily decreasing binding energy with increasing coverage; however, account may have to be taken of possible induced heterogeneity in addition to the intrinsic heterogeneity expected for a polycrystalline surface. At 210 and 285 O K the adsorbed phases produced spectra similar to spectra c and a respectively of Fig. 1. At Torr the saturation coverages were 2 x 1014 molecules cmp2 at 210 O K and 8 x lol3 molecules cm-2 at 285 O K. Tsuchiya et al. (7) also observed four peaks (a, p, y, and 6) in hydrogen desorption spectra from platinum black. The u and y peak maximum temperatures correspond closely to those observed in the present work for the PI- and p,- peaks respectively and their P falls exactly between the p2- and P3-peaks, however, no features analogous to their 6 were observed. Desorption Kinetics Extensive overlap between the peaks makes it difficult to determine the order for the desorption kinetics. The kinetic parameters for the desorption process have been determined from the variation of the peak maximum temperature, Tm, with heating rate, b, assuming first order kinetics. Plots of log (Tm2/b) vs. l/tm for values

3 300 CAN. J. CHEM. VOL. 53, 1975 TABLE 1. Activation energies, E *, and pre-exponential factors, v, for hydrogen desorption from platinum Desorption peaks Eq(kcal mol-l) v (s-') Can. J. Chem. Downloaded from by on 05/14/18 of b varying from 1.2 to 28 deg s-' were linear. From the slopes of these lines (5, 8) the kinetic parameters for desorption shown in Table 1 were calculated. Tsuchiya et al. only report kinetic parameters for their y hydrogen for which E* = 12 kcal mol-' and v = 3.8 x lo5 (7); these values compare well with those shown in Table 1 for the P,-peak which occurs at the same temperature. Sticking Probability The initial sticking probability, determined by the flash desorption method, was 0.29 at 120 OK and 0.15 at 285 OK. This temperature dependence suggests a weakly-bound precursor to adsorption in the p-state (9-1 1) and is in good agreement with that reported by Norton and Richards (4) although the magnitudes they report are slightly smaller. Procop and Volter report a much lower initial sticking probability of -4 x (12) and Tsuchiya et al. (7) observed the opposite temperature dependence, indicating an activation energy to adsorption. Adsorption of Hydrogen and Deuterium Co-adsorption At 120 OK, the sample was exposed for 10 min to a flowing mixture of H, and D, in approximately equal amounts at a, total pressure of lo-' Torr. The mass 2, 3, and 4 desorption spectra obtained after this co-adsorption are shown in Fig. 2. It should be noted that all the spectra have identical shapes. Furthermore, if fh,, fd,, and fhd are the fractions of H,, D,, and HD in the desorbing gas, we can define the reaction quotient, Q, for the desorbing gas as It can be seen that the reaction quotient has a magnitude identical with the equilibrium constant, K, for the entire desorption spectrum. Sequential Adsorption At 120 OK, the surface was first subjected to a limited 2 min exposure to H, gas at 2 x lo-* TEMPERATURE C O K 1 FIG. 2. Mass 2, 3, and 4 desorption spectra obtained after the platinum filament had been exposed at 120 "K to a flowing equimolar mixture of H2 and D2 for a total exposure of 5 x Torrs. The inset shows the reaction quotient for the desorbing gas, Q = ( fhdz/( f H2)(fp2), as data points compared with its values at equll~br~um (curve); K = 4.24 exp (- 157/RT) (25). Torr and then after pumping the H, gas from the system the surface was saturated using D, gas at Torr for a further 3 min. In the time required to saturate the surface very little exchange would occur at 120 OK. The mass 2, 3, and 4 desor~tion s~ectra obtained after this sequential adsorptioa are shown in Fig. 3. As was the case following co-adsorption, the spectra have identical sha~es and the values determined for Q are fairly close to the equilibrium values. These results show that the adsorbed isotopes desorb at equilibrium, i.e. no steps leading to equilibrium, such as adatom migation, are slow. Furthermore, they strongly suggest that hydrogen is dissociatively adsorbed in the B-state on platinum. The argument is somewhat tautological in that we are defining adsorption in the atomic state to be adsorption for which complete isotope mixing occurs on desorption. It is perhaps more correct to conclude that hydrogen adsorbs in the P-state in a manner such that the residual interaction between the hydrogen atoms is small and the interaction with the original partner is not appreciably different from that with other hydrogen atoms. Exchange and Equilibration Reactions These reactions have been studied at three different temperatures by determining the surface composition and isotope distribution in the

4 PENG AND DAWSON: HYDROGEN INTERACTION WITH PLATINUM 301 Can. J. Chem. Downloaded from by on 05/14/18 FIG. 3. Mass 2, 3; and 4 desorption spectra obtained after the platinum filament had been exposed at 120 OK, first to H, gas up to approximately one-half monolayer coverage and subsequently to D, gas up to saturation coverage. The reaction quotient is shown as an inset. desorption spectra after varying fractions of preadsorbed hydrogen had been replaced. Exchange at 120 O K The hydrogen saturated surface (Fig. 1 e) was exposed to D, gas at Torr in a closed system. After a fraction of the preadsorbed hydrogen had been replaced the isotopic composition of the adlayer was determined from the mass 2, 3, and 4 desorption spectra. Spectra obtained after 28,48, and 64% of the preadsorbed hydrogen had been replaced are shown in Figs. 4, 5, and 6 respectively; the reaction quotient, Q, FIG. 4. Mass 2, 3, and 4 desorption spectra obtained after exposing a saturated hydrogen adlayer (Fig. le) at 120 OK to gas phase deuterium at Torr for 10 min. At this stage, 28% of the preadsorbed hydrogen has been exchanged FIG. 5. Mass 2, 3, and 4 desorption spectra obtained after exposing a saturated hydrogen adlayer at 120 OK to gas phase deuterium at Torr for 50 min. 48% of the preadsorbed hydrogen has been exchanged FIG. 6. Mass 2, 3, and 4 desorption spectra obtained after exposing a saturated hydrogen adlayer at 120 OK to gas phase deuterium at Torr for 120 min. 64% of the preadsorbed hydrogen has been exchanged. is also recorded. Note that, especially in Figs. 5 and 6, the desorption spectra obtained for the three isotopic species are quite different and the reaction quotient does not correspond to its equilibrium value, particularly below 250 OK where the PI- and P,-peaks occur. The exchange reaction is a slow process at 120 OK but 80% of the preadsorbed hydrogen was replaced after a five-hour exposure. This contrasts with the much more limited exchange observed by Tsuchiya et al. (3).

5 302 CAN. J. CHEM. VOL. 53, 1975 Equilibration at 120 OK Similar experiments have been carried out to see how the isotopic composition of the surface varies during equilibration of an equimolar gas phase mixture of Hz and D, at 2 x Torr on a platinum surface which had been saturated with hydrogen. After only a few minutes the gas phase had equilibrated whereas only a small fraction (-20%) of the preadsorbed hydrogen had been replaced. In a blank experiment, with the platinum filament removed, the equilibration was very slow confirming that the rapid equilibration in the gas phase was taking place on the platinum surface. The observation that exchange is slower than equilibration at low temperatures has been reported previously (3, 4). The desorption spectra obtained after 20 min, when 28% of the preadsorbed hydrogen had been replaced, are shown in Fig. 7. These spectra are very similar to those shown in Fig. 4 after 28% replacement by exchange. However, after 50 min, when 48% of the hydrogen had been replaced the spectra (Fig. 8) - differ from those obtained during exchange (Fig. 5) in that the shapes are identical and Q K throughout the desorption process. This is to be expected if the whole surface is in equilibrium with an equilibrated gas mixture. Exchange and Equilibration at 210 and 285 OK The exchange reaction proceeds rapidly at these temperatures and preadsorbed hydrogen can be completely replaced. Similar desorption FIG. 7. Mass 2, 3, and 4 desorption spectra obtained after exposing a saturated hydrogen adlayer at 120 "K to an equimolar gas phase mixture of Hz and D2 at 2 x Torr for 20 min. At this stage 28z of the hydrogen has been replaced TEMPERATURE C OK 1 FIG. 8. Mass 2, 3, and 4 desorption spectra obtained after exposing a hydrogen saturated adlayer at 120 "K to an equimolar gas phase mixture of Hz and Dz at 2 x Torr for 50 min. 48% of the preadsorbed hydrogen has been replaced TIME (MIN ) FIG. 9. Variation of the fraction of preadsorbed hydrogen remaining on the surface with time of exposure to gas phased, at 3 x lo-' Torr. The filament temperature was 210 "K throughout. spectra were obtained when the hydrogen saturated surface was exposed either to D, gas or a mixture of Hz and D,; mass 2, 3, and 4 spectra have identical shapes with Q equal to K throughout. Figure 9 shows the variation in the fraction of hydrogen remaining on the surface after it had been saturated at 210 O K and then exposed at 210 OK to D, gas at 3 x Torr for varying times. The straight line obtained in this semilog plot shows that the rate of exchange is first order with respect to the coverage of adsorbed hydrogen, i.e. In (nln,) = kt. This has been reported in studies where the variation in gas phase composition was measured (4) rather than the surface composition. The rate constant, k, determined from the slope of this plot is 5 x 10"

6 PENG AND DAWSON: HYDROGEN INTERACTION WITH PLATINUM 303 molecules cm-2 s-' which is about 20 times smaller than the value reported by Norton and Richards (4). However, the gas pressure used in the present work was 400 times smaller, so the difference between the values for k could be attributed to a gas pressure dependence for a pseudo rate constant. The results would agree if the reaction order in D, was 0.50 which is the upper limit for the order determined during equilibration at 208 OK (4). Considering the different methods used to follow the reaction, the agreement is satisfactory. Discussion The Adsorbed States of Hydrogen on Platinum The adsorption of hydrogen on platinum surfaces has been studied extensively but the nature of the adsorbed states is not well understood. It is generally accepted that hydrogen is adsorbed in at least two states (13-16). Stronglybound hydrogen is associated with a negative surface potential whereas weakly-bound hydrogen, present on the surface at higher coverage, has a positive surface potential (13, 15, 16). Strongly and weakly adsorbed hydrogen have infrared absorption bands at 4.86 p and 4.74 p respectively (14). Hydrogen in the stronglybound state is believed to be atomic but there is little agreement concerning the nature of the weakly-bound state, whether it is atomic (14, 17, 18) or molecular (13, 16). Any reversibly adsorbed species, i.e. any species which can be removed by pumping at the adsorption temperature, could not be detected in the present thermal desorption experiments. However, the weakly adsorbed, electropositive, species desorbs into vacuum between 200 and 250 OK (15, 16) and must therefore have been detected in the present work. The complete isotope mixing observed in both co-adsorption and sequential adsorption supports the view that this species is adsorbed atomically, in the sense discussed earlier. The reversibly adsorbed species, probably the precursor to adsorption in the P-state, present when the surface is in equilibrium with gas phase hydrogen is probably moleculr?r. While this even more weakly bound hydrogen undoubtedly plays an important role in the equilibration and exchange reactions, it seems unlikely that these molecules are sufficiently perturbed to effect isotope exchange between molecules within this adlayer. Desorption Kinetics The kinetic parameters for desorption of P- hydrogen (Table I) are characterized by low pre-exponential factors when compared with the value expected for simple first order processes of s-'. This implies a statistically improbable transition state in the desorption mechanism. The calorimetric heat of adsorption of hydrogen extrapolated to zero coverage has been reported as 28 kcal mol-' on supported platinum (19) and 25 kcal mol-' on a platinum filament (20). Thus the measured activation energies (Table 1) are much lower than the heats of adsorption and must be apparent activation energies for desorption, again suggesting that the mechanism is complex. One possibility is a two-step process in which formation of an adsorbed molecular complex represents the first, rate-limiting, step. However, one should not ignore the possibility that the observed kinetic parameters are an artefact of the experimental method. Thus if we assume a value of v = 1013 s-', then the calculated E* for the P, desorption would be 21 kcal mol-' in better agreement with the heats of adsorption. The nature and origin of such "slow" desorption processes (21) is worth further investigation. Heterogeneity of the Surface The role of intrinsic, as opposed to induced, heterogeneity of platinum surfaces in the binding of hydrogen is uncertain. It has been argued that the surface is homogeneous in one case because the various forms of adsorbed hydrogen seem to be interrelated (7) and in another, the argument is based on the linearity of the first order exchange plots and the completeness of exchange (4). On the other hand the small amount of hydrogen which is exchangeable at low temperature has been taken to indicate a heterogeneous surface (22). However, contamination could reduce the extent of exchange (23). In sequential adsorption experiments the observation of a memory effect is a strong indication that the surface is heterogeneous. In the sequential adsorption experiment shown in Fig. 3 there is no evidence of such a memory effect but this does not necessarily indicate that the surface is homogeneous as will be discussed later. The desorption spectra observed after exchange of 48 and 64% of the preadsorbed hydrogen, Figs. 5 and 6, have widely differing shapes for the desorbing isotopes. This is very

7 304 CAN. J. CHEM. VOL. 53, 1975 difficult to understand if the surface were homogeneous. In these experiments the center of the filament was at. a higher temperature than the ends which were in good thermal contact with the coolant. Thus it could be claimed that the exchange is faster in the center of the filament and that during desorption the gas desorbs first from this same portion of the filament. The experiments were therefore repeated with the whole reaction vessel and sample filament press seal completely immersed in an isopentane slush at 113 OK. The results were unchanged and therefore the differences in the shapes of the mass 2, 3, and 4 desorption spectra cannot arise from a nonuniform temperature of the sample during exchange. The surface must be heterogeneous. Modelfor the Exchange and Equilibration Results Assuming that P-hydrogen is adsorbed dissociatively on a heterogeneous platinum surface we can interpret the present observations with the following model. For simplicity, consider that only two types of crystal plane, A and B, are present on the surface. The binding energy of hydrogen adatoms on plane A is assumed to be slightly lower than that on plane B. In the exchange reaction, prior to the introduction of D, gas both plane A and B are saturated with P-hydrogen adatoms and every adatom on a given plane is assumed to have the same binding energy. In the presence of D, gas exchange will occur more rapidly on plane A where the hydrogen is more weakly bound. After 50x of the preadsorbed hydrogen has been replaced the surface isotope composition will be predominantly D on plane A but still mainly H on plane B, represented schematically in Fig. 10a. Isotope mixing on each plane can be expected below 140 OK (1 6), the temperature where desorption starts, and therefore the isotope composition of the gas desorbing from each plane should be determined by the equilibrium constant for the isotope exchange reaction, K. However, desorption from a surface containing planes of type A and B in macroscopic patches and with a surface isotope distribution as shown in Fig. 10a would give rise to a desorbing gas with a higher deuterium content at low temperature compared with that desorbing at higher temperatures (Figs. 5 and 6). We assume that the PI-, Pz-, P3-, and P4-peaks arise, at least in part, from the intrinsic heterogeneity of the DHDDHDDD Plane A H D H D H D H D Plane A HDHHHDHH Plane B HDHDHDHD Plane B FIG. 10. Schematic diagram showing the distribution of isotopes between type A and B planes after 50% of the preadsorbed hydrogen had been replaced by exposure to (a) D, gas and (b) an equimolar mixture of H, and D,. polycrystalline surface. These desorption features overlap considerably so if, for example, the PI- and P,-peaks were attributed to-type A planes (two peaks could arise from adatom interactions, i.e. induced heterogeneity) whereas the p2- and P4-peaks were from type B planes then we would expect an apparent reaction quotient, Q, deviating considerably from its equilibrium value, as observed at lower desorption temperatures. At higher temperatures adatom mobility will increase and mixing between different macroscopic planes will produce a statistical distribution of adatoms accounting for the observation that Q approaches its equilibrium value at higher desorption temperatures (Figs. 5 and 6). In this way the model successfully interprets the two salient features of the exchange results, the higher mass 4 component at low desorption temperatures and the deviation of Q from K. In contrast, during equilibration of an equimolar gas phase mixture of Hz and D, the isotope composition of each plane will successively reach the equilibrium value. Figure lob schematically represents the isotope distribution on the surface after 50% of the preadsorbed hydrogen has been replaced. The mass 2, 3, and 4 desorption spectra obtained from the surface represented in Fig. lob should have identical shapes and a reaction quotient; Q, close to the equilibrium value, K, as observed in Fig. 8. This model for the exchange and equilibration results makes - it ~ unnecessarv to invoke different mechanisms to explain the difference in kinetics of exchange and equilibration(3, 4). The equilibration reaction will occur predominantly on plane A, where hydrogen is more weakly bound, until the gas phase is equilibrated whereas in the

8 PENG AND DAWSON: HYDROGEN INTERACTION WITH PLATINUM 305 exchange reaction, after hydrogen on plane A has been replaced further exchange occurs on plane B with a slower rate. Thus the model would predict that both reactions would occur rapidly in the initial stages but the exchange reaction will slow down as the preadsorbed hydrogen concentration decreases. Actually during the initial stage of exchange some of the preadsorbed isotope appears rapidly in the gas phase and this is subtracted before constructing the kinetic plots (4). Though the origin of this desorbing gas is unspecified, it could represent the initial fast exchange reaction on type A planes. However the reason why this appears initially as homonuclear molecules is not clear. At low temperature a difference in the activation energy for reaction on type A and B planes of as little as 1 kcal mol-' would result in rates differing by a considerable factor, i.e. -- lo2 at 100 O K. At higher temperatures the difference becomes less significant, accounting for the observation that the rates of equilibration and exchange become comparable at higher temperatures (3, 4). Sequential Adsorption In the sequential adsorption experiment, Fig. 3, there is no evidence of a memory effect, complete isotope mixing occurs on desorption. Complete mixing does not necessarily indicate that the surface is homogeneous since mixing can occur during desorption or at the adsorption temperature if the adatoms are sufficiently mobile. However since after the exchange reaction (Figs. 5 and 6) the mass 2, 3, and 4 desorption spectra are quite different in shape, the adlayer cannot have sufficient mobility to produce the complete mixing observed following sequential adsorption (Fig. 3). Interpretation of the exchange results requires that the surface be heterogeneous and so some alternative explanation for the complete mixing observed after sequential adsorption must be found. The total sticking probability for hydrogen on polycrystalline platinum is initially high and remains more or less unchanged up to half-coverage and then drops very rapidly (4). Adsorption of gases on single crystal planes shows a similar variation of sticking probability with coverage and, moreover, there is no simple correlation between initial sticking probability and binding energy (24). Thus a similar variation of sticking probability for hydrogen adsorption on type A and B planes of polycrystalline platinum seems likely. In this case, preadsorption of hydrogen up to half-coverage would populate both type A and type B planes and in the subsequent adsorption of deuterium the remaining empty sites on both planes will be occupied. On completion of the sequential adsorption the isotope distribution on the surface would resemble that shown in Fig. lob. Isotope mixing on each plane prior to desorption would result in complete mixing. Since Q does not exactly equal K following sequential adsorption (Fig. 3) we conclude that the distribution of isotopes on different planes is not identical as is suggested in the simple model of Fig. lob. Exchange and Equilibration Mechanisms The exchange and equilibration reactions occur rapidly at temperatures below those required for desorption of the binding state investigated in this work, the p-state. Clearly additional adsorbed states which are unstable in the absence of gaseous hydrogen are important in achieving exchange and equilibration. An increase in surface coverage on introducing gas phase hydrogen presumably either reduces the binding energy of the whole adlayer, through adsorbate-adsorbate interactions, permitting exchange with the gas phase, or provides a distinct binding state of intermediate energy between the p-state and the gas phase and therefore a pathway to exchange. The nature of these additional states has not been examined in this work and therefore we can provide no direct evidence for any of the detailed mechanisms proposed for the exchange and equilibration reactions. Conclusions The results described in the present paper show that, for polycrystalline platinum surfaces, heterogeneity plays an important role in the exchange and equilibration reactions of hydrogen. It is shown that different mechanisms are not required to interpret differences in the kinetics observed for these reactions. However it is not established that different mechanisms do not exist. Particularly we would stress that different mechanisms may exist at different temperatures; thus, at low temperature exchange may occur between the atomically adsorbed p-state and a (molecularly) adsorbed precursor state whereas at high temperature and low pres-

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