Alkane activation on transition-metal surfaces - ACS Publications

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Langmuir 1993, 9, 655-662

Feature Article Alkane Activation on Transition-Metal Surfaces: Beams, Bulbs, and New Insights W . Henry Weinberg Department of Chemical Engineering, University of California, Santa Barbara, California 93106-5850 Received May 13, 1992. In Final Form: November 19, 1992 A review is presented of some of our recent work in the area of alkane activation on transition-metal surfaces. In particular we consider the dissociative chemisorption of methane, ethane, propane, and isobutane on the ( l l O ) - ( l X 2 ) reconstructed surfaces of platinum and iridium. Both trapping-mediated and direct activation are discussed. Specific issues that are considered and quantified include the surface reactivity, the selectivity (in the case of propane and isobutane), and the vibrational assistance of direct diesociation.

I. Introduction

comdexes. while useful as models. inevitablv contain geometric constraints resulting from the intrakolecular Advances in the research field of alkane activation and nature of the interaction that make it difficult to infer conversion would be enormouslybeneficialto the American very much concerning the unconstrained, intermolecular economy, in general, and to the natural gas, petroleum, activation of interest. and chemical industries, in particular. As an example, One indication,a t least,of the ability of metallic surfaces the chemical conversion of methane, for instance by to activate C-H and C-C bonds in h e s is the rate of oxidative coupling to ethane or partial oxidation to alkane hydrogenolysis, and wide variations have been methanol, is of extreme scientific and technological observed among these rates for the group VI11transition importance.’ The inherent obstacle to alkane activation metals.5 For example, in the case of ethane hydrogenolysis can be viewed as arising from two opposing effects. Since ~ Torr and PH* by silica-supported catalysts ( P c ~=H22.8 alkanes are relatively inert, activation will generallyrequire = 152 Torr),the observed reaction probabilities at 457 K severe conditions such as high temperatures or very active are approximately 9 X 10-16 for platinum, 2 X 10-10 for substrates. However, controlled selectivetransformations iridium,and 3 X lP9for ruthenium with platinum showing will be disfavored, in general, by these conditions. The the lowest reactivity and ruthenium showing an activity key would appear to be in finding systems that are capable that is second only to osmium.”1° Furthermore, the of activating alkanes under increasingly more moderate hydrogenolysis of alkanesis a “structuresensitive”reaction conditions, as we have done in the case of the Ir(ll0)in the sense that the specific activity is a function of the (1x2) and Pt(llO)-(lxZ) surfaces (vide infra), and in extent of dispersion of the supported metal: and the quantifying the initial activation reaction as well as the selectivity can be a function of dispersion as ~ e l l . l l - ~ ~ subsequentsurface reactivity of the alkyls that are formed. Recent data substantiate the fact that ethane activation It is quite possible that the initial chemical interaction on platinum is sensitive to the geometrical structure. As between a reactive metal surface and an alkane involves discussed more fully in section 11, we have measured an the formation of a three-center, two-electronbond between activation energy (referenced to a gas-phase energy zero) a filled C-H bonding orbital and an unfilled metal orbital. of 2.8 kcal/mol for C2H6 dissociative chemisorption on the This postulate is motivated by results from organometallic “corrugated”Pt(llO)-(lX2) surface,15and Rodriguez and chemistry, where a large number of stable complexes Goodman16have measured a value of 8.9 kcal/mol on the containing intramolecular interactions of this type, the ‘smooth” Pt(ll1) surface. These results tend to verify so-called agostic interaction, have been found.2 Subsethe putative connection between alkane hydrogenolysis quently, kinetic and mechanistic studies of severalsystems, and alkane activation, and they serveto quantify the extant in which intramolecular alkane activation is achieved, of structure sensitivity for one specific example of the point clearly to the formation of a reactive intermediate latter. This observed structure sensitivity is different from prior to C-H bond cleavageand suggest strongly that this that of hydrogenation reactions, for example, where the intermediate is in fact the v2(C-H)-alkane c o m p l e ~ . ~ , ~ (5) Sinfelt, J. H. Adu. Catal. 1973,23, 91. However, since such an organometallic intermediate has (6) Sinfelt, J. H. Catal. Rev. 1969, 3, 175. not been observed spectroscopically, it has not been (7) Sinfelt, J. H.; Yates, D. J. C.; Taylor, W. F. J. Phys. Chem. 1965, possible to obtain information concerning the nature of 69, 1877. (8) Sinfelt, J. H.; Yaks, D. J. C. J. Catal. 1967,s. 82. the interaction. Similarly, stable agostic, organometallic (1) Meunier,B.;Chaudret,B. Perspectives in the Selectiue Activation C-H and C-C Bonds in Saturated Hydrocarbons;Reidel: Dordrecht, 1987; Vol. CXXX of the NATO AS1 Series. (2) Brookhart, M.; Green, M. L. H. J. Organomet. Chem. 1983,250, 395. (3) Periana, R. A.; Bergman, R. G.J. Am. Chem. SOC.1986,108,7332. (4) Jones, W. D.; Feher, F. J. J . Am. Chem. Soc. 1986, 108,4814.

of

(9) Sinfelt, J. H.; Yaks, D. J. C. J. Catal. 1968, 10, 362. (10) Yaks, D. J. C.; Sinfelt, J. H. J. Catal. 1969, 14, 182. (11) Leclercq, G.;Trochet, J.; Maurel, R. C. R.Acad. Sci. Ser. C 1973, 276, 1353. (12) Brunelle, J.-P.; Sugier, A.; Le Page, J.-F. J. Catal. 1976,43,273. (13) Yao, H. C.; Shelef, M. J. Catal. 1979,56, 12. (14) Foger, K.; Anderson, J. R. J. Catal. 1979, 59, 325. (15) Sun,Y. K.; Weinberg, W. H. J. Vac. Sci. Technol. 1940, AS, 2445. (16) Rodriguez, J. A.; Goodman, D. W. J. Phys. Chem. 199O,S#,5342.

0743-7463/93/2409-0655$04.00/00 1993 American Chemical Society

656 Langmuir, Vol. 9, No. 3, 1993

Weinberg

extent of metal dispersionhas little influenceon the specific by others have been invaluable to us in the planning of both our beam and our bulb studies, and the work we catalytic activity.17-'9 discussin section I1builds on this previous work. In what Much fine and relevant work has been carried out during follows we state only our own opinions concerning alkane the past 20 years concerning alkane adsorption and activation,and our own interpretations of our experimental activationby well-characterizedtransition-metal surfaces. These studies include both molecular beam s~attering~O-~~data. It is beyond the scope of this paper to attempt to defend or to criticizethe data and interpretations of others. and "bulb" r e a c t o w investigations. These contributions Our research on C-H bond activation in alkanes originated with our somewhatunexpected observation(at (17) Boudart, M. Adv. Catal. 1969,20,153. (18) Boudart,M.;Aldag,A. W.;Benson, J.E.;Dougharty,N.A.;Harkms, the time) that the highly corrugated, reconstructed IrC. G. J. Catul. 1966, 6, 92. (110)-(1X2)s u r f a ~ e ~ ~sufficientlyreactive 9~is to effect the (19) Boudart, M.; Aldag, A. W.; Ptak, L. D.; Benson, J. E. J. Catal. dissociative chemisorption of cyclopropane at 100 K, the 1968, 11, 35. (20) Stewart, C. N.; Ehrlich, G. Chem. Phys. Lett. 1972, 16, 203. lowest surface temperature in~estigated.9~The concen(21) Stewart, C. N.; Ehrlich, G. J. Chem. Phys. 1975, 62, 4672. tration of dissociativelyadsorbed cyclopropane molecules (22) Rettner, C. T.; Pfniir, H. E.; Auerbach, D. J. Phys. Rev. Lett. at 100 K is quite significant, approximately 2.1 X 1014 1985,54, 2716. cm-2;cf. a reconstructed unit-cell concentration of 4.7 X (23) Rettner, C. T.; Pfniir, H. E.;Stein, H.; Auerbach, D. J. J. Chem. Phys. 1986,84,4163. 1014 cm-2.89~w992993 Clearly, the observed reactivity is (24) Lee, M. B.; Yang, Q. Y.; Tang, S. L.; Ceyer, S. T. J. Chem. Phys. inherent to this particular surface and is not limited, for 1986,85, 1693. example, to a low concentration of defect sites. Pread(25) Steinrack, H.-P.; Hamza, A. V.; Madix, R. J. Surf. Sci. 1986,173, L571. sorbed hydrogen linearly poisons the dissociative adsorp(26) Hamza, A. V.; Steinrack, H.-P.;Madix,R. J. J.Chem.Phys. 1986, tion of cyclopropaneat lWK, with a fractionalprecoverage 85, 7494. of hydrogen (8H)equal to l / 3 being sufficient to inhibit (27) Hamza, A. V.; Steinrfick, H.-P.; Madix, R. J. J. Chem.Phys. 1987, 86, 6506. dissociation completely.1o1Indeed,this observedpoisoning (28) Beckerle, J. D.; Yang, Q.Y.; Johnson, A. D.; Ceyer, S. T. J. Chem. by preadsorbed hydrogen, but not by postadsorbed Phys. 1987,86,7236. hydrogen, allowed us to conclude that the cyclopropane (29) Ceyer, S.T.; Beckerle, J. D.; Lee, M. B.; Tang, S. L.; Yang, Q.Y.; Hines, M. A. J. Vac. Sci. Technol. 1987, A5, 501. is indeed dissociatively chemisorbed at 100 K.91J01 ~~

~~~

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(30) Rettner, C. T.; Auerbach, D. J. InKinetics ofZnterfaceReactions; Grunze, M., Kreuzer, H. J., Eds.; Springer-Verlag: Heidelberg, 1987; p 145. (31) Lee, M. B.; Yang, Q. Y.; Ceyer, S. T. J.Chem.Phys.1987,87,2724. (32) Hamza, A. V.; Madix, R. J. Surf. Sci. 1987, 179, 25. (33) Schoofs, G. R.; Arumainayagam, C. R.; Madix, R. J. J. Vac. Sci. Technol. 1988, A6, 882. (34) Rettner, C. T.; Pfniir, H. E.;Stein, H.;Auerbach, D. J. J. Vac. Sci. Technol. 1988, A6, 899. (35) Ceyer, S. T. Annu. Rev. Phys. Chem. 1988,39,479. (36) Schoofs, G. R.; Arumainayagam, C. R.; McMaster, M. C.; Madix, R. J. Surf. Sci. 1989, 215, 1. (37) Arumainayagam, C. R.; McMaster, M. C.; Schoofs, G. R.; Madix, R. J. Surf. Sci. 1989, 222, 213. (38) Luntz, A. C.; Bethune, D. S. J. Chem. Phys. 1989,90, 1274. (39) Beckerle, J. D.; Johnson, A. D.; Yang, Q.Y.; Ceyer, S. T. J. Chem. Phys. 1989, 91, 5756. (40) Beckerle, J. D.; Johnson, A. D.; Ceyer, S. T. Phys. Rev. Lett. 1989, 62, 685. (41) Yang, Q. Y.; Johnson, A. D.; Maynard, K. J.; Ceyer, S. T. J. Am. Chem. SOC.1989, 111,8748. (42) Ceyer, S. T. Langmuir 1990, 6, 82. (43) Kang, H. C.;Mullins, C. B.; Weinberg, W. H. J. Chem.Phys.1990, 92, 1397. (44)Mullins, C. B.; Weinberg, W. H. J. Chem. Phys. 1990,92,3986. (45) Mullins, C. B.; Weinberg, W. H. J. Chem. Phys. 1990,92,4508. (46) Beckerle, J. D.; Johnson, A. D.; Ceyer, S. T. J. Chem. Phys. 1990, 93, 4047. (47) Arumainayagam, C. R.; McMaster, M. C.; Madix, R. J. Surf. Sci. 1990,237, L424. (48) Mullins, C. B.; Weinberg, W. H. J. Vac. Sci. Technol. 1990, AB, 2458. (49) Kang, H. C.; Mullins, C. B.; Weinberg, W. H. J. Vac. Sci. Technol. 1990, AB, 2538. (50) Rettner, C. T.; Mullins, C. B.; Bethune, D. S.; Auerbach, D. J.; Schweizer. E.K.: Weinbere. W. H. J. Vac. Sci. Technol. 1990. AB. 2699. (51) Harris, J.'; Simon, j.iLuntz, A. C.; Mullins, C. B.; Rettner; C. T. Phys. Rev. Lett. 1991, 67, 652. (52) Arumainayagam, C. R.; McMaster, M. C.; Madix, R. J. J. Vac. Sci. Technol. 1991, A9, 1581. (53) Verhoef, R. W.; Kelly, D.; Mullins, C. B.; Weinberg, W. H., Surf. Sci.. in ureas. -... --r---I

(54) Yates, J. T., Jr.; Madey, T. E. Surf. Sci. 1971,28, 437. (55) Madey, T. E. Surf. Sci. 1972,29, 571. (56) Winters, H. F. J. Chem. Phys. 1975, 62, 2454. (57) Winters, H. F. J. Chem. Phys. 1976, 64, 3495. (58) Schouten, F. C.; Kaleveld, E. W.; Bootsma, G. A. Surf. Sci. 1977, 63, 460. (59) Schouten, F. C.; Gijzeman, 0. L. J.; Bootsma, G. A. Surf. Sci. 1979, 87, 1. (60) Yates, J. T., Jr.; Zinck, J. J.; Sheard, S.; Weinberg, W. H. J. Chem. Phys. 1979, 70, 2266. (61) Brass, S. G.: Reed, D. A.: Ehrlich, G., J . Chem. Phvs. 1979, 70. 5244. (62) Salmeron, M.; Somorjai, G. A. J. Phys. Chem. 1981,85, 3835. (63) Wittrig, T. S.; Szuromi, P. D.; Weinberg, W. H. J. Chem. Phys. 1982, 76,3305. (64) Weinberg, W. H. Surv. Prog. Chem. 1983, 10, 1.

(65) Szuromi, P. D.; Weinberg, W. H. Surf. Sci. 1985, 149, 226. (66) Szuromi, P. D.; Engstrom, J. R.; Weinberg, W. H. J. Phys. Chem. 1985,89, 2497. (67) Firment, L. E.; Somorjai, G. A. J. Chem. Phys. 1977, 66, 2901. (68) Sault, A. G.; Goodman, D. W. J. Chem. Phys. 1988,88, 7232. (69) Cheaters, M. A.; Gardner, P.; McCash, E. M. Surf. Sci. 1989,209, 89. (70) Engstrom, J. R.; Goodman, D. W.; Weinberg, W. H. J. Am. Chem. SOC.1986.108.4653. (71) Liu, A.'C.; Friend, C. M. J. Chem. Phys. 1987,87,4975. (72) Lo, T.-C.; Ehrlich, G. Surf. Sci. 1987, 179, L19. (73) Brass, S. G.; Ehrlich, G. Surf. Sci. 1987,187, 21. (74) Brass, S. G.; Ehrlich, G. Swf. Sci. 1987, 191, L819. (75) Beebe, T. P., Jr.; Goodman, D. W.; Kay, B. D.; Yates, J. T., Jr. J. Chem. Phys. 1987,87,2305. (76) Brass, S. G.; Ehrlich, G. J. Chem. Phys. 1987, 87, 4285. (77) Kay, B. D.; Coltrin, M. E. Surf. Sci. 1988, 198, L375. (78) Lo, T.-C.; Ehrlich, G. Surf. Sci. 1988, 198, L380. (79) Egawa, C.; Iwasawa, Y. Surf. Sci. 1988,198, L329. (80) Engstrom, J. R.; Goodman, D. W.; Weinberg, W. H. J. Am. Chem. SOC.1988,110, 8305. (81) Liu, A. C.; Friend, C. M. Surf. Sci. 1989, 216, 33. (82) Chorkendorff, I.; Alstrup, I.; Ullman, S. Surf. Sci. 1990,227,291. (83) Alstrup, I.; Chorkendorff, I.; Ullman, S. Surf. Sci. 1990,234,79. (84) Brand, J. L.; Arena, M. V.; Deckert, A. A.; George, S. M. J. Chem. Phys. 1990,92, 5136. (85) Arena, M. V.; Deckert, A. A.; Brand, J. L.; George, S. M. J. Phys. Chem. 1990,94,6792. (86)Weinberg, W. H.; Sun, Y.-K. Science 1991,253, 542. (87) Hanley, L.; Xu, 2.;Yates, J. T., Jr. Surf. Sci. 1991, 248, L265. (88) Johnson, D. F.; Weinberg, W. H. Unpublished results. (89) Chan, C.-M.; Van Hove, M. A.; Weinberg, W. H.; Williams, E. D. Solid State Commun. 1979,30,47. (90) Chan, C.-M.; Van Hove, M. A,; Weinberg, W. H.; Williams, E.D. Surf. Sci. 1980, 91, 440. (91) Wittrig, T. S.; Szuromi, P. D.; Weinberg, W. H. J. Chem. Phys. 1982, 76, 716. (92) Van Hove, M. A.; Weinberg, W. H.; Chan, C.-M. Low-Energy Electron Diffraction;Springer-Verlag: Heidelberg, 1986. (93) TheIr(llO)-(lX2)surface haeamisaingrowstructure,asexplained in detail e l s e ~ h e r e . ~With ~ ~ high-temperature ~ ~ ~ ~ - ~ ~ oxidation followed by reduction, it is also possible to prepare a clean, Ir(llO)-(lX3) surface98 or asurface with mixed (1x1) and (1x3) phases.sJ" We have in the past and shall continue in the future to study the (1x2) surface with a missingrow structure in order to compare the results with the Pt(llO)-(lx2) surface which has the same s t r u c t ~ r e . 9 ~ (94) Kellogg, G. L. J. Vac. Sci. Technol. 1987, A5, 747. (95) Miiller, K.; Witt, J.; Schutz, 0.J. Vac. Sci. Technol. 1987,A5,757. (96) Copel, M.; Fenter, P.; Gustafsson, T. J. Vac. Sci. Technol. 1987, A5, 742. (97) Chan, C.-M.; Van Hove, M. A. Surf. Sci. 1986,171, 226. (98) Hetterlich, W.; Heiland, W. Surf. Sci. 1989, 210, 129. (99) Bu, H.; Shi, M.; Masson, F.; Rablais, J. W. Surf. Sci. 1990, 230, L140. (100) Bu, H.; Shi,M.; Rablais, J. W. Surf. Sci. 1990,236, 135. (101) Wittrig, T. S.;Szuromi, P. D.; Weinberg, W. H. Surf. Sci. 1982, 116, 414.

Alkane Activation on Transition-Metal Surfaces Considerably more remarkable and exciting was our observation that the irreversible, dissociative adsorption of ethane and all higher molecular weight alkanes occurs at approximately 130 K on the Ir(llO)-(lx2) surface.63~~6 Molecular adsorption occurs at 100K (unlikecyclopropane, which suggests the expected ring opening of the cyclopropane), and for sufficiently low coverages only the dissociation reaction is observed to occur; i.e., there is no molecular desorption. Preadsorbed hydrogen linearly poisons the extent of the dissociation reaction, as was observed for cyclopropane, with OH = l/3 being sufficient for complete inhibition. Both reaction-limited and desorption-limited hydrogen extrusion from the surface were observed following the dissociative chemisorption of the alkanes. The former is due to the decomposition of stable hydrocarbon fragments on the surface, and the “molecularity” of the fragments corresponding to the reactionlimited hydrogen that desorbs above 450 K can be determined unambiguously. These molecularities are approximately C2Ho from ethane, C3H2 from both propane and cyclopropane, C4H4 from isobutane, C4H1 from n-butane, C&Lfrom both neopentane and n-pentane,C6H2 from n-hexane, and C7 Hs from n-heptane. Neither the mechanism of decomposition that leads to these molecularities nor the structure of the fragments is known however. On the other hand, we found the close-packed Ir(ll1) surface to be inactive for the dissociative adsorption of both propane and cyclopropane, Le., adsorption at 100K, followed by heating, resulted only in molecular desorption which was complete by 175 K.lo2 The very small extent of dissociative adsorption of cyclopropane which was observed, approximately3% of that observed on Ir(l10)(1X2), is probably limited to defect sites, the concentration of which was found to be approximately2.5% on this same Ir(ll1) surface via H2 adsorption and desorption measurements.lO3 Clearly, the heterogeneous activation by iridium of C-H bonds of alkanes is sensitive to the surface structure under these experimental conditions. We also found that the dissociative adsorption of alkanes on the reconstructed Pt(llO)-(lX2)surface is qualitatively similar to, but quantitatively different from that, on Ir(110)-(1X2).66 For example, ethane, propane, n-butane, and n-pentane are all adsorbed molecularly on Pt(ll0)(1x2) at 100 K. Whereas only reversible, molecular desorption of ethane and propane (complete below approximately 190 K) is observed upon heating, the irreversible, dissociative adsorption of n-butane and n-pentane occurs with a threshold temperature of approximately200 K [cf. approximately130K on Ir(llO)-(lX2)]. Neither of these surfaces was found to activate methane under these experimental conditions, since the desorptiontemperature is below the threshold temperature for reaction on both surfaces. Preadsorbed hydrogen linearly poisons the reactivity of Pt(llO)-(lX2) just as for Ir(llO)-(lX2),and the thermal desorption spectra of hydrogen from the irreversibly adsorbed n-butane and n-pentane implicate the presence of a number of stable, partially dehydrogenated surface intermediates,the identities of which remain UIlknOWn.

In collaborative work with Wayne Goodman,we studied the hydrogenolysis of ethane, propane, n-butane, and neopentaneon both the Ir(llO)-(lX2)and Ir(ll1) surfaces for alkane partial pressures between 0.2 and 5.0 Torr, hydrogen partial pressures between 20 and 500 Torr, and surface temperatures between 400 and 700 K.70>80The (102)Szuromi, P.D.;Engstrom, J. R.; Weinberg, W. H. J.Chem. Phys. 1984, BO, 508.

(103)Engstrom, J. R.;Tsai,W.; Weinberg, W. H. J.Chem. Phys. 1987, 87,3104.

Langmuir, Vol. 9, No. 3, 1993 657 major reaction channel for each of these alkanes except n-butane involves the cleavage of a single carbon-carbon bond (presumably following initial C-H bond cleavage and alkyl adsorption), resulting in “demethylization” of the parent alkane. The major reaction channels of n-butane on the two surfaces are the following: n-CrHlo + 2H2 2CH4 + CzH6 on Ir(lll), and n-C4Hlo + Hz 2C2H6 on Ir(llO)-(lX2). The latter was interpreted in terms of a bidentate, chelating metallacyclopentane intermediate, which is sterically forbidden on the closepacked (111)~urface.~’J This is yet another example of structure sensitivity in alkane activation and hydrogenolysis catalysis. Both this early work as well as that of our precursors, cited above, helped to shape the research in alkane activation that we have conducted more recently. The latter is discussed in section 11.

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11. Alkane Activation on Ir(llO)-(lX2) and Pt( 110)-(1x2) Surfaces A. Overview. Recently, we have carried out fundamental studies aimed at quantifying the dynamics and kinetics of alkane activation on group VI11 transitionmetal surfaces. In particular the initial probability of dissociativechemisorptionhas been measured as a function of the relevant independent variable (surfacetemperature or impact energy, as explained later) for methane, ethane, propane, and isobutane activation on well-characterized surfaces of platinum and iridium. Providing answers to four essential questions, which are relevant both from a scientific point of view as well as to practicing catalytic chemists and engineers,has guided our research in alkane activation. These four questions may be stated succinctly as follows. First, how does the geometrical structure of a surface affect both the reactivity and theselectivity of the alkane activation reaction? We have addressed this question by considering an extreme case, namely, the microscopically “rough” Pt(llO)-(lX2) surface, with a missing-row reconstruction, and the microscopically “smooth” Pt(ll1) surface [where we have made use of data from Wayne Goodman’s laboratory16for the flat Pt(l11) surface]. Second, how are the reactivity and selectivity of the alkane activation reaction influenced by the electronic structure of the surface for the same geometrical structure? We have addressed this question by considering alkane activation on the Ir(ll0)(1x2) and Pt(llO)-(lX2) surfaces. Third, what are the quantitative values of the rate coefficients of alkane activation, embodied by the probability of dissociative chemisorption, on surfaces of variable but well-characterized, geometrical and electronic structure? As mentioned above and discussed later, we have quantified the activation of methane, ethane, propane, and isobutane. Included in this research was the development of a methodology to evaluate the individual rate coefficients of primary and secondary C-H bond cleavage in propane, and primary and tertiary C-H bond cleavage in isobutane, an issue of great importance insofar as the selectivity of a catalyst is concerned. Fourth, under what conditions can catalytic reaction-rate data be analyzed using reaction-diffusion equations which assume mean-field behavior? We have addressed this issue by developing a dynamic Monte Carlo algorithm within the context of a lattice-gas model and have taken some preliminary steps toward answering this question. In so doing, a natural explanation of the compensation effect in catalysis was discovered. The compensation effect had been somewhat enigmatic to catalytic scientists for at least 70 years. B. Results and Discussion. In the recent past, our group has helped to clarify the existence of two different

Weinberg

658 Langmuir, Vol. 9, No. 3, 1993 1.o

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' R(a) + H(a) Figure 1. One-dimensionalpotential energy diagram (alongthe reaction coordinate),for the dissociative chemisorption of the alkane RH to an adsorbed alkyl (R) and a hydrogen adatom via a physically adsorbed alkane. This figure depicts the special case of unactivated chemisorption with respect to a gas-phase energy zero.

mechanisms of alkane activation, namely, direct dissociation, and trapping-mediated dissociative chemisorption. Direct dissociative chemisorption occurs on the time scale of a collision between the gas-phase molecule and the surface (510-l2s), and the rate of this reaction depends primarily on the translational and internal energies of the gas-phase molecule (and generally to a lesser extent on the temperature of the surface). In trapping-mediated dissociative chemisorption, the gas-phase molecule is trapped in the potential field of the surface (Le., it is adsorbed physically in the case of an alkane), and it accommodates to the temperature of the surface. The physicallyadsorbed moleculethen either may desorb with a rate coefficient kd

= kd") eXp(-Ed@)

(1)

where 4 l / k ~ T ,and , T, is the surface temperature, or may react (dissociate) with a rate coefficient

k, = k,(O)exp(-E,@)

(2)

Here, kda and kr(0)are the preexponential factors of the two rate coefficients and Ed and E, are the corresponding activation energies of the desorption and dissociation reactions. The rate of the trapping-mediated reaction is a function of the surface temperature. The gas temperature is important only insofar as it affects the probability of trapping into the physically adsorbed state. When there is no significantaccumulation of physically adsorbed molecular alkanes on the surface (i.e., at surface temperatures sufficiently high that the lifetime of the molecular alkane on the surface is much less than the reciprocalof the impingement rate), it is easy to show that the probability of the dissociation reaction (the ratio of the reaction rate to the impingement rate) is given by (3)

where f' is the probability of trapping into the physically adsorbed state. In this way we have separated the trapping-mediated alkane activation reaction into two "pieces": a dynamics factor, embodied in the trapping probability, f'; and a kinetics factor, given by kr/(kr + kd), which should be thought of as a rate coefficient that is properly normalized to become a probability. The value of the probability of trapping-mediated chemisorption, given by eq 3, is dependent on the value of the potential energy at which two potential energy curves cross (actually

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8 10 12 14 16 18

Ei~oso,58i( kcalimol) Figure 2. Trappingprobability as a function of Ei COS^,^ Bi (kcal/ mol) for ethane on Ir(llO)-(lx2)at T,= 77 K for Bi Oo (D), 22.6O (+I, and 45' (A).Neithertotal-energyscalingnornormal-energy scaling is observed, but the empirically observed scaling law is closer to the former.44

repel one another in this adiabatic case).'@ These two curves are that of the physically adsorbed alkane and that of the chemisorbed alkyl and hydrogen adatom. This discussion is clarified in Figure 1 for the special case in which the curves cross below the chosen zero of energy, which is the gas-phase alkane (RH)molecule far from the surface and at rest. Note that the activation energies of the two competing reactions are both referenced to the ground state of the physically adsorbed alkane, and note also that if E, < E d (as in Figure 11, then the reaction probabilitydecreasesas the surfacetemperature increases; cf. eq 3. Clearly, if E, > Ed, then the reaction probability increasesasthe surfacetemperature increases. Thissimple fact gives a trivial qualitative criterionfor decidingwhether the trapping-mediated dissociative chemisorption is activated with respect to the gas-phase energy zero. As discussed next, we have used these concepts to interpret our measured data for the trapping-mediated activation of a number of alkanes on the (110)-(1X2)surfaces of iridium and platinum. Employing supersonic molecular beam techniques, we have measured the initial probabilityof molecular trapping of C2Hs on the Ir(llO)-(lX2) surface at 77 K as a function of incident translational energy (or impact energy), Ei, and polar angle of incidence, Bi, with respect to the surface n ~ r m a l The . ~ ~impact ~ ~ energy was varied between 1.2 and 24 kcal/mol, and the angles of incidencewere Oo, 22.5O and 4 5 O . Under these conditions, the initial trapping probability was found to scale with Ei COSO.~ BiJ i.e., all trapping probabilities were found to lie on a "universal curve" when plotted as a function of Ei COSO.~ Bi. These results are shown explicitlyin Figure 2. Qualitatively,the scaling exponent, 0.5 in this case, can be thought of as follows: If the rate of interconversionbetween the parallel and perpendicular components of the linear momentum is rapid compared to the rate of exchange between the perpendicular component of momentum and the surface, then total-energy scaling with an exponent n = 0 would be expected. On the other hand, if the rate of intramolecular momentum exchange is slow compared to the rate of "intermolecular" momentum exchange, then normalenergy scaling with an exponent n = 2 would be expected. In the first case a sufficient fraction of all of the linear momentum must be transferred to the substrate, whereas in the second case only the perpendicular component of (104)In the case of alkane activation, it is possible that the measured barrier is really an effective barrier that is reduced somewhat with respect to the 'true" barrier due to atomic tunneling assistance near the top of the barrier.

Alkane Activation on Transition-Metal Surfaces

Langmuir, Vol. 9, No. 3, 1993 659 Table I. Measured Initial Rate Parameters for Methane and Ethane Activation on Pt(llO)-(lXZ) and Ir(llO)-(lx2)~

Pt(llO)-(lX2) CH4 Pt(ll0)-(1x2) CD4 Pt(llO)-(lX2) C & 3 Pt(llO)-(lX2) CzD6 Ir(llO)-(lX2) CzHa 0 See text for details.

t 0.11 0.001

I

'

'

1

'

1

'

1

I

'

'

I

0.002 0.003 0.004 0.005 0.006 0.007

-,1

K-1

T

Figure 3. Verification of the trapping-mediated mechanism proposed in eq 4 for the dissociative chemisorption of CzH6 on Ir(llO)-(lx2).

the momentum is relevant to the initial trapping (but the parallel component is relevant to the subsequent accommodation). These arguments should be viewed as both qualitative and tentative. More research needs to be done to describe trapping dynamics properly. We then made use of these measurements of the trapping probability in order to investigate, in a detailed way, the trapping-mediated dissociative chemisorption of C2H6 on the Ir(llO)-(lX2)surface.& Employing molecular beam techniques, the initial probability of dissociative chemisorption was measured as a function of impact energy, parametric in surface temperature. Employing these independently measured values of both P, and 5, the data were analyzed using eq 3 written in the form (4)

where

In order for this trapping-mediated mechanism to describe the measured data, a plot of In ((UP,)- 1)vs 1/T, should be linear; the slope would then be equal to -(Ed - E,)/kB and the intercept would be equal to In (kd(o)/k$O)).Figure 3 shows the results of 30 different measurements (each one of which is the average of a number of experiments, between 4 and 121, for surface temperatures between 150 and 500 K and impact energies between 1.2 and 10 kcal/ mol.lo5 The linearity observed in Figure 3 serves to verify the essential correctness of the trapping-mediated activation of ethane on Ir(llO)-(lX2). In particular, the data of Figure 3 imply that E d - E, = 2200 f 200 cal/mol, and kd(O)/k$O) r 390. This value of Ed - E, implies that the potential curves in Figure 1cross 2200 cal/mol below the gas-phase energy zero, and the fact that kd'o'/k$o' > 1is expected both on the basis of arguments involving phase space availability for desorption vs dissociation and ratios of partition functions.lm In order to determine the initial (low-coverage) value of k,, we measured independently the low-coverage value of kd which was found to be kd = 1013exp[-(7700 caVmol)Bls-l. Thus, the activation energy of the trapping-mediated dissociative chemisorption of (105)At an impact energy of 10 kcal/mol, there was a s m d component of direct dissociation, which was separately measured and subtracted from the total measured value of P, (vide infra). (106)Campbell, C. T.;Sun, Y.-K.; Weinberg, W. H. Chem. Phys. Lett. 1991, 179, 53.

14 400 15 600 2800

2 2 215

3500 -2 200

215 390

18900 20100 10500 11 200 5 500

5x10'2 5 x 10'2 5 X 10" 5 X 10" 3 X 10'0

ethane on Ir(llO)-(lX2) is 5500 cal/mol with respect to the proper reference energy, namely, the bottom of the potential well for physical adsorption. With this framework for analyzing experimental data, we next quantified the activation of CH4, CD4, C2H6, and CzD6 on the Pt(llO)-(lX2) surface.15 In addition to obtainingnumerical values of the reaction rate coefficienta (which can be used in engineering applications), the following two questions motivated this study: (1)What are the intrinsic differences in alkane activation that can be attributed to electronicstructure (platinum vs iridium) for two surfaces with the same geometrical structure? (2) What can be said concerning the observed kinetic isotope effect, e.g., is it mainly due to zero-point energy differences or to atomic tunneling? In this work we employed an ultrahigh vacuum microreactor and evaluated the probability of dissociative adsorption by posttitrating the surfacecarbon (depositedby the reactingalkanemolecules) by fully oxidizing it to C02 with flowing molecular oxygen. The carbon coverages were kept low to ensure a measurement of the initial activation reaction. Since the pressures of the alkane that were employed were never greater than 10-4 Torr, the gas-phase temperature in this 'bulb" reactor was always equal to the wall temperature, which was 300 K. We anticipated that a gas temperature which was this low would result in no detectable contribution from direct dissociation,and this indeed was found to be the case. The surface temperature was varied widely (between 335 and 875 K), and the measured probability of dissociation was found to be a function of surface temperature, implicating the anticipated trapping-mediated mechanism. From work carried out in our laboratory about ten years a g 0 , 6 ~ , ~ J Owe ~ anticipated that the Pt(llO)-(lX2)surface would be substantially less active than the Ir(llO)-(lX2) surface for the dissociative chemisorption of alkanes, and this expectation was confirmed. Indeed, since Pr 0 in all cases for the Pt(llO)-(lX2) surface, unlike the Ir(llO)-(lX2) surface. This implies that the potential curves of Figure 1cross above the gasphase energy zero for each of these alkanes on Pt(ll0)(1x2). This, of course, leads to much lower values of the probability of dissociation on Pt(llO)-(lX2) compared to

Weinberg

660 Langmuir, Vol. 9,No. 3, 1993

Ir(llO)-(lX2),as we observed qualitatively a decade ago. Second, note that the difference in activation energies of CH4 and C2H6 activation on Pt(llO)-(lX2) is approximately 8.4 kcal/mol, which, within experimental uncertainties, implies that all of the stabilization of the C-H bond in methane compared to ethane is mapped into the activation energyfor ita dissociationon this surface. Third, note that kdco)/krco) is much larger for ethane (and other higher alkanes, vide infra) than for methane. This is expected and can be understood easily by considering the ratio of partition functions for the transition state for desorption and the transition state for dissociation. Fourth, note that the activation energies for dissociation of the deuterated isotopes are about 1200 cal/mol (methane) and 700 cal/mol (ethane) greater than the perhydrido isotopes. This, tentatively, tends to implicate zero-point energydifferences rather than tunneling for this trappingmediated mechanism,sincewe have shownthe asymmetric C-H stretching mode to be the relevant one for direct methane and ethane activation (vide infra) and (h/2) (VCH - VCD) z 1100 cal/mol for this mode. This would be expectedto be the observed isotope effect if no vibrational motion is retained along the reaction coordinate in the transition state; otherwise a value slightly less than this would be expected. We cannot rule out on this basis, however,some tunneling assistance,e.g., at higher energies near the top of the barrier. Fifth, and finally, our results for C2H6 dissociation on Pt(llO)-(lX2) can be compared with those of Wayne Goodman's groupie for C2H6 dissociation on Pt(ll1). For trapping-mediated activation of CzHs on Pt(lll), they found that E r - E d 8900 cal/mol compared to our value of 2800 cal/mol for Pt(llO)-(lX2). This implies that the difference in activation energies for the smooth Pt(ll1) surface vs the corrugated Pt(ll0)(1x2) surface is about 6100 cal/mol. (Note the values of E d are essentially the same on the two surfaces.) This quantifies, in this case, the fact that ethane activation is a "structure sensitive reaction". This is especially interesting since our data in Table I allow us to evaluate electronic differences for the activation of C2H6 on Pt(110)-(1X2)vs Ir(llO)-(lx2),two surfaces with the same (or very similar) geometric structure. We conclude that the electronic difference is about 5000 cal/mol [2800 (-22OO)l. Since there are experimental uncertainties in each of these (four) measured activation energies, we do not wish to make too many claims at this point; but it is interesting that geometrical structural differences seem to be at least as important (if not more so) as electronic structural differences for ethane activation on platinum and iridium. Another issue of great scientificand practicalimportance concerns the extent of lo(primary) vs 2 O (secondary) vs 3O (tertiary) C-H bond cleavage in alkanes. The scientific interest revolves around the challenge of evaluating separate rate coefficients which have similar activation energies,and the practical importance concernsthe ability to probe the selectivity of a catalytic surface (since in many cases the initial bond that is cleaved in a reactant dictates the reaction product that will be formed). We have made measurements of the trapping-mediated dissociative adsorption of fully and selectively deuterated propane and developed a methodology whereby we have quantified loand 2 O C-H bond cleavage for this alkane on the P t ( l l O ) - ( l X 2 ) surface.@ The data measured in our ultrahigh vacuum (UHV) microreactor for three isotopes of propane, plotted as In P r vs l/T,, are shown in Figure 4. The fact that Princreases as T, increases means that the two potential curves of Figure 1cross above the gasphase energy zero, Le., E, - E d > 0 in this case (as was the case for methane and ethane on this surface). Since the

b 10.2

e a

Ppropane=1.7~10'6Torr

Figure 4. Initialprobabilities of trapping-mediateddissociative chemisorption of (a) C3H8, (b) CH3CD2CH3,and (c) C3Ds on Pt(110)-(1~2) as a function of reciprocal surface temperature.The of these three slopes- ( E , d k ~ )and the intercepts In (k,'o)/kdco)),pp lines are listed in Table 11. The error bar representsthe maximum variation in the measured rate at each temperature.

data for CH3CD2CH3 fall neither on the data for C3H8 nor on those for C3D8, we conclude there is both loand 2' C-H bond cleavage in propane under reaction conditions, not a surprising result. The challenge here is to evaluate two different activation energies and two different preexponential factors from data which show only one apparent activation energy and one apparent preexponential factor. We solved this problem by defining two conditional probabilities: ppis the conditional probability of formin a 1' Pt-propyl, given that the propane dissociates and is the conditional probability of forming a 2 O Pt-propyl, again given that the propane reacts. If Pr,j is the measured probability of dissociation of the ith isotope of propane, it follows that

$

(7)

and

Pp+PB=l

(8) Equation 7 is exact in the absence of a secondary kinetic isotope effect, and eq 8 is exact by definition. It is easy to solve eqs 7 and 8 simultaneously for ppand p, as a function of surface temperature, and the resulta are

pp= 0.61 exp[-(220 cal/mol)81 p, = 0.39 exp[+(205 cal/mo1)8]

(9)

(10) The elementary rate parameters (expressed,as before, as a ratio of elementary rate coefficients, kr/kd) for the formation of loand 2O Pt-pro yls on Pt(llO)-(lx2) are given by P r , C s H p p and Pr,C3H8 respectively, for perhydridopropane. For this isotope only, the result is exact because no approximations were used in the derivation. The elementary rate parameters for the other isotopes follow directly from the constraint that the activation energy for C-D bond cleavage is 870 cal/mol greater than that for C-H bond cleavage (independent of lo vs 2 O bonds). This value is the difference in the apparent activation energies for C3D8 and C3H8 for the measured data of Figure 4. The derived rate parametersthat describe the dissociativechemisorptionof propane on Pt(llO)-(lX2) are listed in Table 11. For example, for the perhydrido isotope of propane, we find that the initial probability of dissociative chemisorption may be written as

E,

Alkane Activation on Transition-Metal Surfaces

Langmuir, Vol. 9, No. 3, 1993 661

Table 11. Rate Parameters for the Dissociative Chemisorption of Propane on Pt(llO)-(lx2)4 reactant

Eapp = E, - Ed (cal/mol)

C3Hs CH3CD2CH3 C3Ds

1460 1800 2330

[kJ0)/kd(O)] app 6.1 x 10-3

(k,@)/kd(0))(l") (k,(0)/kd(0))(2')

6.1 x 10-3 6.1 x 10-3

2.4 x 10-3 2.4 x 10-3 2.4 x 10-3

3.7 X 3.7 X 3.7 X

-Ed (cal/moi)

E,(20)- Ed (cal/mol)

1680 1680 2550

1255 2125 2125

The measured (cf. Figure 4) quantity, Eapp= E, - Ed, is the apparent activation energy (with respect to a gas-phase energy zero) of dissociative chemisorption of the three isotopes of propane, whereas El(1o)- Ed and E,(20)- Ed are the true activation energies of lo and 2O Pt-propyl formation(withrespect to the same gas-phaseenergyzero). The apparent (measuredvia Figure 4) and the actualratiosof preexponential factors of the rate Coefficients of dissociative chemisorption (kJo)) and desorption of the physically adsorbed propane (kdc0)) are also given. kr P,s = (6.1 X

exp[-(1460 cal/mol)flI (11)

kd

exp[-(1680 cal/mol)fll + (2.4 X exp[-(1255 callmo1)flI (12) and since we independently measured k d to be approximately 1013exp[-(9700 cal/mo1)8], we can write eq 12 as

P,= (3.7 X

k, cz (3.7 X 10") exp[-(11380 cal/mol)fl] + (2.4 X 10") exp[-(10955 callmo1)flI (13) with units of s-l. The rate coefficientsfor C3H8 activation that are listed in Table I1(cf. eq 13)reproduce the data of Figure 4 exactly (by necessity). The rate coefficients for (CH3)zCDz and C3D8 listed in Table I1 reproduce the data of Figure 4 to within f 2 7%. This means that our constraint of ErCD ETCH= 870 cal/mol for both 1' and 2' C-H bonds is very good indeed; Le., under these conditions the secondary kinetic isotope effect can be neglected. To summarize, 2 O C-H bond cleavage is favored by 425 cal/mol compared to 1' C-H bond cleavage for propane activation on the Pt(llO)-(lX2) surface. The preexponential factor of the rate coefficients also favors 2 O C-H bond cleavage, by a factor of about 1.9, e.g., 2.4 X divided by 3.7 X 10-3 multiplied by 3 (the ratio of the number of primary and secondary C-H bonds in propane). This may also be seen by taking the ratio of eqs 9 and 10, namely

pp/p,= 1.56 exp[-(425 callmo1)fll

(14) We conclude immediately that 2' C-H bond cleavage is favored by 425 cal/mol energetically, and the preexponential factor favors 2 O C-H bond cleavage on this surface by a factor of 1.9 (i.e., 3/1.56). Inorder to quantify the selectivityof the P t ( l l O ) - ( l X P ) surface for 1' and 3' C-H bond cleavage, we measured, in our UHV microreactor, the initial probability of trapping-mediated dissociative chemisorption of three isotopes of isobutane, namely, (CH&CH, (CH3)3CDand (CD3)3CH.lo7 These measured data are shown in Figure 5. We employed the same methodology that we developed for propane (vide supra) to evaluate the conditional probabilities of formation of loand 3' Pt-isobutyls and, therefore, the elementary rate coefficients for 1' and 3O bond cleavage. We were also able to evaluate both the apparent and the elementary rate coefficientsthat should describe the isotope (CD&CD. A full delineation of the results may be found in Table 111. Only a few comments are in order here. First, the derived rate coefficients for (CH3)3CH describe the experimentally measured data exactly, of necessity. More importantly, the derived rate coefficients for (CH3)sCD and (CD&CH reproduce the measured data (over a temperature range between 325 and 650 K) to within f4%, again indicating both the absence of a significant secondary isotope effect and the reliabilityand uniqueness of the reported rate coefficients. (107) Weinberg, W. H.; Sun, Y.-K. Surf. Sci. Lett. 1992, 277, L39.

3

-' E

I , , , , , , , , , , , , , , ]

10-4 1.5

2.5

2.0

3.0

1000 K-1 T

-I

Figure 5. Initial probabilitiesof trapping-mediateddissociative chemisorption of (CH&CH, (CH&CD, and (CD3)sCH on Pt(110)-(1X2) as a function of reciprocal surfacetemperature.The and the intercepts (kio)/kd'o'),~pof these three slopes (-EaPp/k~) straight lines are listed in Table 111. The statistical uncertainty in the experimental data can be judged by the scatter in the data points that are reported in the figure.

As may be seen in Table 111, 3O C-H bond cleavage is favored in (CH3)sCH by 137 cal/mol over 1' C-H bond cleavage (i.e., 795-658), and the preexponential factor of the rate coefficients favors 3' C-H bond cleavage by a factor of about 3.2 (i.e., 8.4 X 10-4 divided by 2.4 X 10-3 multiplied by 9, the ratio of the number of loand 3O C-H bonds). How the energetic and entropic stabilization of 2' vs 1' and 3' vs 1' C-H bond cleavage depends on surface electronic and geometric structure is still unknown, since only the corrugatedPt(llO)-(lX2)surfacehas been studied to date in the necessary detail to address these issues. We have employed supersonic molecular beam techniques to study also the direct dissociativechemisorption of perhydrido- and perdeuteromethane and -ethane on the Ir(llO)-(lX2)surface.s3 Impact energies (Ei)between 1and 35 kcal/mol were used at angles of incidence (8i) of Oo, 22.5O, and 45O with respect to the surface normal. The surface temperature was varied between 550 and 1150 K in order to ensure there is no measurable contribution to the probability of dissociation (i.e., SO.01) from the trapping-mediated reaction channel. We found that the initial probability of direct dissociationwas not a function of the surfacetemperature, consistent with the traditional picture of direct chemisorption in the absence of phononassisted atomic tunneling. For both isotopes of both alkanes, normal-energy scaling was observed, i.e., the probability of chemisorption of each reactant fell on a "universal curve" when plotted as a function of Ei cos28i. By use of our version of the reflectivity technique, the threshold for an observable (20.01) probability of chemisorption varies between 6 and 11 kcd/mol for the four reactants. The observed offsetsin the translational energy for the thresholds for CH4 vs CDI and CzHs vs CZDG were approximately 4-5 kcal/mol,which tends to implicate the

662 Langmuir, Vol. 9, No. 3, 1993

Weinberg

Table 111. Rate Parameters for the Dissociative Chemisorption of Isobutane on P t ( 110)-(lX2)a

E, - Ed (cal/mol)

Espp

reactant (CHd3CH (CHMD (CD3)3CH (CD3)3CDb

755 925 1244 1645

(kJo)/kdco))wp 3.2 x 10-3

(~JO'/kdcO))(l')

3.2 x 10-3 3.2 x 10-3 3.2 x 10-3

2.4 x 2.4 x 2.4 x 2.4 x

10-3 1e3 10-3 10-3

('JO)/kd(0))(3')

8.4 X 8.4 X 8.4 X 8.4 X

lo4 l0-C lo-' loa

-Ed (cab")

-Ed (caVmol)

795 795 1685 1685

658 1548 658 1548

The measured (cf. Figure 5) quantity, Eapp= E, - Ed, is the apparent activation energy (with respect to a gas-phase energy zero) of dissociative chemisorption of (CH&CH, (CH&CD, and (CD&CH, and (k,co)/kdco)),pp is the measured (cf. Figure 5) ratio of the preexponential factors of the rate coefficients of dissociative chemisorption (kJo)) and desorption of the physically adsorbed isobutane (kd") for these three isotopes. The quoted values for (CD&CD were calculated on the basis of the measured values for the other three isotopes. The values of E,(10)-Ed and -Ed are the true activation energies of 1' and 3' Pt-isobutyl formation for the four isotopes of isobutane, and (kJo)/kdco))cl") and (k,'0'/kdc0))'30) are the actual ratios of the corresponding preexponential factors. Calculated values.

importance of atomic tunneling. (Asnoted earlier, zeropoint energy differences would give rise to a maximum offset of (h/2)(VCH- VCD), which is approximately 1.1kcal/ mol for C-H stretching modes.) Indeed, we were able to fit our data, for each isotope, to an Eckart potential to extract effective barrier heights and widths for atomic tunneling. The fact that the barrier heights are essentially identical for methane and ethane is further evidence that the mechanism of direct dissociation involves atomic tunneling in this case. We have also investigatedthe effect of vibrationalenergy on direct dissociative chemisorption of both isotopes of methane and ethane on Ir(llO)-(lX2) by using vibrationally hot molecular beams of variable translational energy." We found a significantvibrationalenhancement for both CD4 and c2D6 in the sense the thresholds for dissociation were much more precipitous and occurred at lower translationalenergiescomparedto direct dissociation from the vibrationalground state. In particular, we found sharp increases in the probability of chemisorption from below 0.01 to approximately 0.05 at thresholds that were downshifted by about 4-5 kcal/mol. The results for CH4 and C2H6 showed no measurable vibrational assistance (for the same nozzle temperature of 770 K), Le., the maximum enhancement in the probability of chemisorption was below 0.02 with no clearly lowered translational energy threshold. Considering the observed increase in the chemisorptionprobabilities and the downshifts in the threshold translational energies (and the lack thereof in the perhydrido isotopes),these results are most consistent with attributing the enhanced chemisorption probability to the u = 1 vibrational state of the asymmetric C-D stretching modes. Obviously, direct dissociationcan be made to dominate trapping-mediated dissociation when very high-translational energy molecular beams are employed (withnarrow, Gaussian velocity distributions). Occasionally, direct dissociation may be important in the more commonly employed bulb chemical reactors. For example, we have preliminary experimental data that suggest, for the activation of C& on Ir(ll1) in a bulb reactor with a gasphase Maxwell-Boltzmann energy distribution corresponding to 300 K, the initial probability of dissociation is independent of surface temperature from 500 to 1200 K.88This implicates the direct channel in this case, and we conclude that more research is needed to clarify when one should expect to observe either direct or trappingmediated chemisorption. We have also carried out a number of significant numerical simulations, within the context of lattice gas (108) Verhoef, R. W.; Kelly, D.; Mullins, C. B.; Weinberg, W. H. Submitted for publication in Surf. Sci. Lett.

models, with the aim of addressing the fourth question that was posed in section IIA. Since the compensation effect (the sympathetic variation of the preexponential factor and the activation energy of a reaction rate coefficient) had been an enigma to catalytic scientists and engineers since the beginning of this century (e.g., refs 109-112), we have made a concerted effort to study the origins of compensatory behavior in model LangmuirHinshelwood reactions employing Monte Carlo simulations. We have investigated in this way both transientll3 and steady-state114reactions, and we have thereby developed insight into the origin of the compensationeffect. In particular, the compensation effect in these model systems and analogousphysicalsystemscan be understood in terms of the temperature and coverage dependence of a distribution of actiuation energies for the surface reaction (arising when adsorbate interactions are extant among the reactants). In related work, we have performed Monte Carlo simulationsof a Langmuir-Hinshelwood reaction between two species adsorbed on a square lattice, with the goal of establishing how spatial correlations between the reactants influence the reaction rate.lls A novel feature of this investigation is that we explicitly simulated the diffusion of the reactants on the surface. The probability of adsorption was kept sufficiently low to maintain moderatelylow-surfacecoverages. Even for a reaction probability that is 100times smaller than the probability of migration, "islanding" of each of the reactants was found to occur (although the entire surface is never saturated). Depending on the reaction probability, the "islands" grow to finite steady-state sizes. By studying this simple model, we were able to show that quite large inhomogeneities can be reasonably expected to occur in catalytic systems, even when reaction rates are small compared to diffusion rates, and that these inhomogeneities affect total reaction rates. Acknowledgment. Principal support of this research was provided by the Department of Energy (Grant DEFG03-89ER14048). Additional funding was provided by the donors of the Petroleum Research Fund, administered by the American Chemical Society (Grant ACS-PRF23801-AC5-C),and by the UniversitywideEnergy Research Group of the University of California. The support of each of these organizations is gratefully acknowledged. (109) Constable, F. H. h o c . R. Soc. London 1925, A108, 355. (110) Cremer,E.;Schwab, G.-M. Z . Phys. Chem. (Leiprig)1929,A144, 243. (111) Kemball, C. h o c . R. Soc. London 1953, A217,376. (112) Cremer, E. Adu. Catal. 1965, 7, 75. (113) Kana, H. C.: Jachimowski. T. A.: Weinbera, W. H. J. Chem. Phys. 1990,93, 1418. (114) Fichthorn, K. A,; Weinberg, W. H. Langmuir 1991, 7, 2639. (115) Kang, H. C.; Weinberg, W. H.; Deem, M. W. J. Chem. Phys. 1990, 93, 6841.