Probing ensemble effects in surface reactions. 1. Site-size

806

J. Phys. Chem. 1989, 93, 806-814

Probing Ensemble Effects in Surface Reactions. 1. Site-Size Requirements for the Dehydrogenation of Cycllc Hydrocarbons on Pt(ll1) Revealed by Bismuth Site Blocking C. T. Campbell,* J. M. Campbell, P. J. Dalton, F. C. Henn, 1. A. Rodriguez, and S. G. Seimanides Chemistry Department, Indiana University, Bloomington, Indiana 47405 (Received: December 17, 1987; In Final Form: August 4, 1988)

Catalytic reactions on bimetallic surfaces are often thought to be controlled by ensemble effects, whereby a side reaction requiring a large ensemble of active sites can be selectively suppressed by diluting the active metal with a second, inert metal. Unfortunately, the lack of knowledge of surface structure and the complications due to coexisting electronic effects have, until now, precluded accurate determinations of ensemble requirements for surface reactions. We analyze here applications of a new method for determining ensemble sizes that partially overcomes these obstacles and allows for semiquantitative assessment of ensemble effects. The method involves the controlled blocking of sites on a well-defined transition-metal surface with a dispersed overlayer of inert bismuth adatoms. The interactions of five cyclic hydrocarbons (cyclopentane, cyclohexane, cyclopentene, cyclohexene, and benzene) with Pt( 1 1 1) have been studied in this way in an accompanying series of papers. In particular, the influence of Bi upon the competing dehydrogenation and desorption kinetics of these adsorbed molecules has been qualitatively measured. This present paper correlates the results for those five molecules and fits them with a simple kinetic model to extract the absolute ensemble requirements for the surface dehydrogenation reactions. The method and model may have applicability to a broad range of surface reactions. In addition, an "effective ensemble requirement" is defined, whose value is useful in predicting ensemble effects in catalysis. Trends in the value of the kinetic parameters and the ensemble requirements with hydrocarbon character are discussed. The absolute ensemble requirements for the dehydrogenation of these adsorbed hydrocarbons are surprisingly large and indicate in some cases that at least six additional free Pt atoms are necessary for dehydrogenation (beyond those required for adsorption). Mechanistic implications of these results are discussed.

I. Introduction The selectivity of chemical reactions occurring at solid surfaces can often be controlled by ensemble effects.'-" As introduced by Sachtler to describe bimetallic catalysts, the term "ensemble requirement" refers to the number of active sites on the surface that are required for a particular reaction.' Thus, reactions that require a large ensemble or group of active sites can be selectively suppressed at the expense of small-ensemble reactions by removing or poisoning a fraction of the active sites. For example, the selectivity of Ni for hydrogenolysis versus dehydrogenation can be greatly increased by adding an inactive metal (Cu) to form an a l l ~ y . ~ - "This % ~ has been attributed to the larger ensemble requirement for C-C bond-breaking reactions compared to C-H bond c l e a ~ a g e . ~In, ~the bimetallic catalysts, the active (Ni) sites are diluted by the substitutional presence of the inert metal (Cu), and therefore the probability of finding a large ensemble of free Ni sites is very small. Although the ensemble concept is already widely used to advantage in industrial applications, there still remains a gap in our ability to fundamentally understand ensemble effects or even to quantitatively measure the ensemble requirement for a given reaction. Many efforts to measure ensemble sizes have used bimetallic catalysts that combine an active metal with an inert net tal.'^'^-'^-'^ By observing how the reaction rate decreases with the concentration of the inert metal and modeling this with a suitable kinetic expression that involves the number of active sites ( A ) necessary for the reaction, a quantitative estimste of the ensemble requirement A can, in be obtained. Unfortunately, the surface concentration of bimetallic catalysts rarely equals that in the bulk2s3and, therefore, such estimates can be in substantial error unless supported by surface analysis of the topmost atomic layer. More recently, surface scientists have attempted to address this issue by vapor-depositing an inert transition metal (such as Ag, Au, or Cu) onto the surface of an active metal (such as Pt,Ru, or Ni) and again observing the decay in the reaction rate as the concentration of the inert overlayer increase^.^*^^^^ Even this method has been plagued with difficulties since these metal overlayers tend to coalesce into twodimensional islands,22-24,26-28 leaving large regions on the active *Alfred P. Sloan Research Fellow. To whom correspondence should be addressed.

surface free of the masking species. Thus, a knowledge of the lateral distribution of the two metals in the topmost atomic layer is also necessary. Therefore, until now, no experiment has accurately measured the ensemble size requirement for any surface region except, perhaps, for simple chemisorption,18 where the ensemble size can also be inferred from a knowledge of the structure of the adsorbate. Most recently, we have demonstrated that Bi may be an excellent choice as an inert site blocker since (1) it spreads uniformly over the surface of active transition metals, (2) it is very inert in chemisorption reactions, and (3) it has the same electronegativity as the active transition metals and should therefore only have (1) Sachtler, W. M. H. LeVide 1973, 163, 19. (2) Sachtler, W. M. H.; van Santen, R. A. Ado. Cutul. 1977, 26, 69. (3) Ponec, V. Adu. Curd. 1983,32, 149. (4) Sinfelt, J. H. Sci. Am. 1985, 253(3), 90. (5) Sachtler, W. M. H. CHEMTECH 1983, July, 434. (6) Ponec, V. C a r d Rev.-Sci. Eng. 1975, 11, 41. (7) Sinfelt, J. H.; Cusumans, J. A. Aduunced Muterials in Cutulysis; Academic: New York, 1977; p 1. (8) Clarke, J. K. A. Chem. Rev. 1975, 75,291. (9) Sachtler, J. W. A,; Somorjai, G. A. J. Carol. 1983, 81, 77. (10) Dalmon, J. A,; Martin, G. A. J. Cural. 1980, 66, 214. (1 1) Sachtler, W. M. H. J . Mol. Curd. 1984, 25, 1. (12) Iglesia, E.; Boudart, M. J . Curul. 1983, 81, 224. (13) Burton, J. J.; Pugel, T. M. J . Cutul. 1977, 47, 280. (14) Eley, D. D.; Moore, P. B. Surf. Sci. 1981, 111, 325. (15) Yu, K. Y.; Ling, D. T.; Spicer, W. E. J . Curd. 1976, 44, 373. (16) Chehab, F.; Kirstein, W.; Thieme, F. Surfuc. Sci. 1981, 108, L419. (17) Herrington, E. F. G.;Rideal, E. K. Trans. Furuduy SOC.1944, 40, 505.

(18) Campbell, C. T.; Paffett, M. T.; Voter, A. F. J . VUC.Sci. Technol. 1986, A4, 1342. (19) Alstrup, I.; Anderson, N. T. J . Cutul. 1987, 104, 466. (20) Vickerman, J. C.; Christmann, K.;Ertl, G.J. Carol. 1981, 71, 175. (21) Shimizu, H.; Christmann, K.; Ertl, G. J . Cutul. 1980, 61, 412. (22) Yates, J. T.; Peden, C. H. F.; Goodman, D. W. J. Curd. 1985, 94, 576. (23) Peeblas, H. C.; Beck, D. D.; White, J. M.; Campbell, C. T. Surf. Sci. 1985, 150, 120. (24) Foord, J. S.; Jones, P. D. Surf. Sci. 1985, 152, 487. (25) Sachtler, J. W. A.; Biberian, J. P.; Somorjai, G. A. Surf. Sci. 1981, 110, 43. (26) Paffett, M. T.; Campbell, C. T.; Taylor, T. N. Langmuir 1985, I , 741. (27) Paffett, M. T.; Campbell, C. T.; Taylor, T. N.; Srinivasan, S. Surf. Sci. 1985, 154, 284. (28) Bauer, E.; Kolaczkiewicz, J. Proc. IX In?. Vacuum Congr. V Int. Congr. Solid Surf., Madrid, Spain, 1983, 363.

0022-3654/89/2093-0806$01 .SO10 0 1989 American Chemical Society

Probing Ensemble Effects in Surface Reactions minimal electronic influence^.'^,^'^' (See ref 30 for details of the electron transfer between Bi and a Pt surface.) We have made preliminary tests of the utility of Bi as a site-blocking agent by addressing the ensemble requirements for the chemisorption of CO, H2, and 0 2 1 8 on Pt( 11 1). Those results were encouraging in that Bi appeared to block sites effectively but had no or only minimal electronic influences on the coadsorbates. For example, dissociative H2 adsorption was poisoned twice as fast as molecular C O adsorption, yet the activation energy for desorption of neither molecule changed by more than 8% as Bi was added.18~31Recent results by Windham et al.52for ethylene chemisorption and decomposition on Bi-dosed Pt(1 l l ) are also very encouraging in that decomposition of ethylene was suppressed by Bi without substantial electronic influences of Bi upon chemisorbed ethylene as probed by thermal desorption spectra or vibrational spectra. However, it was obvious that the study and detailed kinetic modeling of more reactions requiring larger ensembles would be necessary before the utility of Bi overlayers in quantitatively assessing ensemble requirements could be proven. The present work correlates results from a series of papers (three of which accompany this where the interactions of simple hydrocarbons (cyclopentane, cyclopentene, cyclohexane, cyclohexene, and benzene) with a Bi-dosed R(111) surface have been studied by using thermal desorption mass spectroscopy (TDS) and X-ray photoelectron spectroscopy (XPS). The studies show that Bi blocking is a suitable method for measuring the site-size requirement for chemisorption of these hydrocarbons but that quantitative XPS combined with TDS is better suited to this task. The results of these studies also suggest that, indeed, Bi blocking may be an effective method for measuring the ensemble requirements for the dissociation of adsorbed molecules and for other reactions occurring entirely on the surface. Specifically, very small coverages of Bi can almost completely suppress the dehydrogenation of adsorbed hydrocarbons, thereby enhancing their desorption. This occurs over a Bi coverage range so small that the activation energies for desorption and dehydrogenation are only very weakly affected (as evidenced by invariant peak temperatures and line shapes in TDS). Thus, the suppression of dehydrogenation appears to be dominated by simple steric effects, where Bi adatoms block free Pt sites which are required for dehydrogenation to proceed. Clearly, such sites would be necessary if the mechanism for dehydrogenation involved a concerted reaction where one or more C-H bonds were being broken as the corresponding H-Pt bonds were formed. A major goal of the present paper is to develop a relatively simple kinetic model involving ensemble effects that can be used to quantitatively explain the dissociation of adsorbed species on surfaces where the active sites have been partially diluted (for example, by Bi blocking). This model encompasses the major physical factors that should be considered in modeling such experiments and that became apparent in our accompanying studies32-36of the coadsorption of hydrocarbons with Bi on Pt(ll1). It successfully and quantitatively accounts for the bismuth poisoning of the dehydrogenation of all these adsorbed hydrocarbons on Pt( 11 1). In cases where the chemistry for the hydrocarbon on Pt is favorable, the model even allows for semiquantitative determination of the number of free Pt sites required for elementary dehydrogenation steps. This, in turn, provides unique insight to the reaction mechanisms.

The Journal of Physical Chemistry, Vol. 93, No. 2, 1989 807 11. Experimental Detail and Results In the experiments we hope to model here, which are described in more detail in the original paper^,'^-^^ a certain Bi coverage was first deposited on the smooth Pt( 111) surface. At the temperatures and low coverages of most interest here (esi* < 0.4), the bismuth overlayer is disordered and thought to be nearly randomly dispersed across the surface30 in 3-fold hollow sites.5E Then a small exposure of some hydrocarbon (C,H,) was coadsorbed with this bismuth a t -100 K,where the adsorbed hydrocarbon is still completely molecular. (The adsorbed hydrocarbon first fills vacant Pt sites on the surface.) The surface was then rapidly heated while monitoring the thermal desorption mass spectra (TDS). Near the desorption temperature, there is a competition between dehydrogenation of the hydrocarbon and its intact molecular desorption. The probability of dehydrogenation was measured by the area of the resulting H2evolution peaks in TDS, and that for desorption was measured by the area under the parent hydrocarbon peaks. In general, these intensities were easily self-normalized to probabilities since there was usually almost 100% dehydrogenation with no Bi present and always 100% desorption at high Bi coverage. In the case of the alkanes, the dehydrogenation probability was somewhat below unity at zero Bi coverage. Its absolute value was determined by quantitative XPS, and it was then used to convert the TDS intensities into probabilities. This experiment was repeated as a function of bismuth precoverage (e,,*). In general, dehydrogenation is poisoned with increasing Bi coverage due to the blocking of free Pt sites, which are required to abstract hydrogen atoms from the adsorbed molecule in initiating dehydrogenation. As the dehydrogenation is killed, molecular desorption correspondingly increases. It is important to note that the experiments to be presented and modeled here were all performed in the limit of low hydrocarbon (and hydrogen) exposure, such that the functional dependence of decay in the dehydrogenation probability with was experimentally proven to be independent of hydrocarbon (and hydrogen) coverage. Thus, there was no self-blocking of dehydrogenation by the adsorbed hydrocarbon or its fragments. In addition, for the coverage ranges studied here, the sticking probabilities of the hydrocarbons were independent of BBi* so that a series of fixed exposure corresponds to a constant hydrocarbon coverage. We have obtained such results for the following hydrocarbons on Pt( 11 1): cyclopentane,36 ~yclopentene,3~ cy~lohexane,3~ cy~ l o h e x e n e benzene,32 ,~~ and perdeuterated b e n ~ e n e . ' ~The raw TDS data are presented and discussed in the original papers. The analyzed results are summarized as the data points in Figures 1-5, respectively, where the probability for dehydrogenation during the TDS is plotted as a function of Bi precoverage. The scatter in the data points is mainly due to error in measuring the dehydrogenation probabilities, since the error bars on BE,* are very small (0.59) this is certainly not the case, since the Bi adatoms exert mutual repulsive interactions that force the Bi into ordered overlayer structures where Bi-Bi distances are maximized.30 However, at low OBi* ( 0.2. Rigorous simulations of that type would certainly be valuable to further our understanding of these systems and to put our ensemble size measurements on more quantitative grounds (particularly for those systems where the major poisoning occurs for Osi* > 0.15). Another assumption implicit in eq 1 is that each Bi adatom blocks three Pt surface atoms. This is based on the size of the Bi atoms (-34% larger than Pt49) and the fact that they are thought to sit in 3-fold hollow sites when OB{* I 0.59.30v58 Structural determination of the geometric location of Bi adatoms at low coverages (OBi* < 0.3) would be very useful to confirm this latter point. According to Mitchell,50such high-symmetry hollow sites are generally preferred by atomic adsorbates. A model whereby each Bi atom poisons a larger ensemble of Pt atoms by long-range electronic effects would manifest itself in energetic and kinetic effects observable in TDS. Such effects were not seen,32-36 and we have thus proceeded in this work by assuming that the dominant role of Bi is simply as a steric site blocker. The value of the 3 used here can be taken as an upper limit on the number of Pt atoms sterically blocked by each Bi adatom, since at high coverages each Bi atom only occupies the space of 1.8 Pt atoms.% Therefore, the absolute ensemble requirements determined here can be taken as a lower limit on the true values. A final consideration here relates to the limits we have placed on the extent of electronic influences of the Bi adatoms on the kinetic parameters. We have folded all such effects into a single parameter (AB). We have chosen limits on AB based upon observations of the minor temperature shifts induced by low Bi coverages in the molecular desorption TDS peaks and in the dehydrogenation-rate-limited H2 TDS peaks (see accompanying papers). In this analysis we have assumed that these desorption and dehydrogenation TDS peaks can be treated independently using a first-order Redhead kinetic treatment. However, desorption and dehydrogenation are in competition here, and they can have small influences upon the TDS peak positions and line shapes of each other. What is really needed to completely prove this point are separate kinetic measurements of desorption and dehydrogenation where the other pathway is not complicating the analysis. This could, for example, be accomplished at low Bi coverages for the dehydrogenation of cyclopentene (OBi* < 0.2) or cyclohexene (eBi*< 0.3), since here desorption is not a competitive process. An ideal experiment would be to directly monitor the kinetics of dehydrogenation as a function of Bi coverage using a quantitative surface analytical method that could give the concentrations of adsorbed reactants and adsorbed products versus time and temperature. With accurate measurements, one could directly probe the influence of Bi upon the activation energy and preexponential factor for the surface dehydrogenation reaction. Similarly, lighter alkanes ( kl), then the surface coverage of I, (e,) should be very low and determined by the rapid equilibrium of eq 12: 61

= [kleC,Hy(l - 3eBi)"l/(k-leH)

(14)

The rate of overall dehydrogenation to stable product Pa is then given by rate = k24( 1 - 38Bl)" = (k2kl/k-l)(e,xH,/eH)( 1 - 30Bi)"+m (15)

Therefore the overall ensemble requirement is given by the sum of those for the individual intermediate steps: A = n + m. (In the TDS experiments of the type discussed here, the hydrogen coverage (0,) will be largely determined by how far the reaction has proceeded to product Pa, since is very small in this model.) A similar sequence of equilibrated hydrogenation/dehydrogenation steps has been proposed to explain the kinetics of benzene hydrogenation on palladium catalyst^.^^ The above argument could be extended to include any number of intermediate dehydrogenation steps that are in rapid equilibria, with the result that the overall ensemble requirement is just the sum of those for each individual step. Notice that this model does not require that the concentration of the partially dehydrogenated intermediates be large. These intermediates should, in fact, be quite unstable and at much lower concentrations than the reactant hydrocarbon or product (P,). This would explain why these intermediate species have not been observed in surface spectroscopies, whereas species with loss of several hydrogens have been isolated and confirmed in most of the cases. If the above mechanism is correct and if surface diffusion of hydrogen is possible, then the mechanism requires considerable hydrogen scrambling in the molecularly desorbed reactant. Indeed, our TDS spectrum following a saturation exposure of an equimolar mixture of benzene (C6H6) and perdeuterated benzene (C6D6) to clean Pt( 1 11) shows very significant molecular benzene desorption peaks for all of the mixed isotopic benzenes (Le., 6-1 3% of c6H6).32 Tsai et aL41 report considerable isotopic exchange in the benzene desorbed from coadsorbed mixtures of c-C6HI2and c-C6DI2on stepped Pt( 1 1 1). They report a similar result for Similarly, Surnam et c-C6H,o/c-C6D,omixtures as saw rapid H-D exchange in the vibrational spectrum of adsorbed benzene (C,H,,,) on Pt(l10) when exposed to D2 below 350 K. From coadsorbed c-C6HI2/C-C6Dl2mixtures on Pt(l1 l ) , we saw no observable isotopic scrambling in the desorbing cyclohexane TDS peak, although there is very little C-D bond cleavage in that case due to a very strong kinetic isotope effect.33 From coadsorbed deuterium (from D,) and c-C6H12on P t ( l 1 I ) , we also saw no observable deuterated cyclohexane in TDS, although the resulting dehydrogenation product (adsorbed benzene) had incorporated (51) Chow, P.; Vannice, M. A. J . Cuiul. 1987, 107, 140. (52) Windham, R. G.; Kcel, B. E.; Paffett, M. T. Lmgmuir 1988,4,1113. (53) McMillan, D. F.; Golden, D. M. Ann. Rev. Phys. Chern. 1982, 33, 493. (54) This trend comes from comparing the activation energies for dehydrogenation of the following adsorbed s ecies on Pt(ll1): (a) cyclopentane (sp3) 13 kcaI/m01;~~ (b) cyclohexane (sp ) 13.4 k~al/rnol:~~ (c) ethylene (di-u bonded, therefore sp3) 13 k c a l / m ~ l(d) ; ~ ~cyclopentene (di-u bonded, therefore sp') 13.3 k~al/rnol;'~ (e) cyclohexene (di-u bonded, therefore sp3) 13 kcal/ mol;35(f) benzene (aromatic, parallel to surface, sp2) 28-33 k~al/mol;~* (g) c-C5HS (aromatic, parallel to surface, sp2) 28-32 kcal/m01.~~ (55)'Avery, N. R. Surf.Sci. 1985, 163, 357. (56) Demuth, J. E.; Ibach, H.; Lehwald, S.Phys. Rev. &ti. 1978, 40, 1044. (57) Avery, N. R. Surf. Sci. 1984, 146, 363. (58) According to Mitchell,5o the highest symmetry hollow sites are generally preferred by atomic adsorbates.

P

814 The Journal of Physical Chemistry, Vol. 93, No. 2, 1989

Campbell et al.

significant deuterium ( ~ 4 % ) In . ~TDS ~ from coadsorbed deuterium (from D2) and c-CsH8 on Pt( 11 l ) , we saw some incorporation of deuterium into the molecularly desorbing cyclopentene (-3%) and also into the adsorbed dehydrogenation products (Hydrogen isotope exchange in adsorbed alkylidynes occurs with an activation energy of only -7 kcal/mol on Pt(111).43 In any case, there is significant evidence for rapid hydrogen isotopic exchange in adsorbed hydrocarbons on Pt surfaces. The amount of scrambling is not as large as one might assume based on the rapid equilibrium proposed above for eq 12. However, there are two effects that could minimize isotopic scrambling but that are still consistent with the proposed mechanism. First, it is possible that the dehydrogenation/rehydrogenationequilibrium of eq 12 is rapid compared to the rate of hydrogen adatom diffusion on the surface, so that the same hydrogen atom largely remains associated with the same carbon atom of the molecule. Note that the same condition that favors molecular desorption over dissociation (i.e., high surface coverage) also would tend to inhibit hydrogen diffusion. Second, there could be a large kinetic isotope effect in the dehydrogenation and rehydrogenation, so that hydrogen elimination and addition is strongly favored. Evidence for a large kinetic isotope effect a t least in the dehydrogenation reaction has been reported for two of the molecules on Pt(l1 l).3z33 Both of these effects would be more important for molecules that have lower desorption/dehydrogenation temperatures, since hydrogen diffusion is slower at lower temperatures and since a lower dehydrogenation temperature implies lower activation energies for kl and k-I and therefore larger kinetic isotope effects. Indeed, the extent of hydrogen isotopic scrambling decreases with decreasing desorption/dehydrogenation temperature for the molecules mentioned above on Pt(ll1). Further evidence for the ease of hydrogenation of hydrocarbons on P t ( l l 1 ) is seen in the TDS spectrum of coadsorbed ethylene and hydrogen, where large amounts of ethane are produced at only 220 K.44 Similarly, c - C ~ Hcan ~ , ~self-hydrogenate (probably via a Ha intermediate) to produce cyclopentadiene gas at -500 K when coadsorbed with BL4’ The presence of Bi (for OBi* 5 0.4) markedly influences neither the heat of adsorption of hydrogen adatoms (