Quick viewing(Text Mode)

Correlation Between Catalytic Activity and Bonding and Coordination

Correlation Between Catalytic Activity and Bonding and Coordination

Proc. NatI. Acad. Sci. USA Vol. 82, pp. 2207-2211, April 1985

Correlation between catalytic activity and bonding and coordination number of and on transition surfaces: Theory and experimental evidence (catalysis/transition /surface structure/electronic structure) L. M. FALICOV*t AND G. A. SOMORJAItt Departments of *Physics and fChemistry and tCenter for Advanced Materials, Lawrence Berkeley Laboratory, University of California, Berkeley, CA 94720 Contributed by L. M. Falicov, Nov'ember 13, 1984

ABSTRACT Correlation between catalytic activity and body-centered cubic (bcc) crystal as opposed to the (110) low-energy local electronic fluctuations in transition metals is face, or the (1120) surface of a hexagonal-close-packed (hcp) proposed. A theory and calculations are presented which crystal as opposed to the closed packed (0001) face], or have indicate that maximum electronic fluctuations take place at a high concentration of step edges and kink vertices. The high-coordination metal sites. Either (i) atomically rough structures of some of these surfaces are shown schematically surfaces that expose to the reactant molecules atoms with large in Figs. 1-5. numbers of nonmagnetic or weakly magnetic neighbors in the One of the key questions in that first or second layer at the surface or (ii) stepped and kinked emerges from modern studies concerns the surfaces are the most active in carrying out structure-sensitive determination of those metal sites where catalytic processes catalytic reactions. The synthesis of from N2 and H2 occur (3-6); these must be distinctly different from the sites over and surfaces, 'H2/2H2 exchange over where strong bonding of atoms and molecules takes place stepped crystal surfaces at low pressures, and the accompanied by a low and slow turnover. In this (C-C bond breaking) of isobutane at kinked paper a theory is presented that argues that catalysis is platinum crystal surfaces are presented as experimental evi- predominant at high-coordination sites: the more bulk-like dence in support of the theory. the site, the greater the probability of electron-state fluctua- tions, which participate catalytically in the breaking and Metal catalysts are used to carry out a multitude of selective formation of chemical bonds and yield high turnover. This chemical reactions that range from the synthesis of ammonia model points out the catalytic importance of metal atoms from and , to hydrogen-deuterium located in the second layer of open surface structures-the exchange, to selective conversion reactions iron (111) surface for example-which are highly coordi- (dehydrocyclization, isomerization, dehydrogenation, and nated but are nonetheless exposed to the incoming reactant hydrogenolysis to mention a few). These catalysts perform molecules. Similarly, the increased activity at stepped sur- selective bond breaking and rearrangements for millions of faces is explained by the exposure of high-coordination turnovers and produce molecules continuously atomic sites at the bottom of the steps. without deactivation (poisoning). Yet modern surface sci- Adsorbates tend to form strong stable bonds at lower- ence studies have revealed by isotope labeling and other coordination sites, where the atoms have few electron-state appropriate experiments that most of the surface is covered fluctuations, where s- and p-bonding predominates, and with strongly adsorbed species whose turnover is much where charge-transfer processes are more common; all these longer than the times required to produce the desired prod- effects lead to the formation of more stable surface com- ucts for the catalytic reaction (1). It is therefore obvious that pounds. These low-coordination sites have only secondary those reaction steps that need metal sites must occur at those few locations on the metal surface that remain uncovered. Thus, the questions arise: How does catalysis occur in the presence of these strongly chemisorbed stagnant layers of almost monolayer coverage? What is the nature of the active metal sites that remain uncovered? Surface science experiments have also shown that the chemical bonding of adsorbed atoms and molecules is struc- ture sensitive. The heats of adsprption of atoms and mol- ecules could vary as much as 10-30% from site to adsorption site on the same crystal surface (2). Top, bridge, threefold, and fourfold sites are available on flat, low Miller index surfaces. Surfaces that are rough on the atomic scale are often found, by or by catalytic reaction d studies, to bind atoms and molecules more strongly and to dissociate strong chemical bonds (NN, C-H, C-C, C=O) more readily (2). Rough surfaces are those with higher Miller index, in which the outermost layer is more sparsely populated, thereby exposing atoms in the second FIG. 1. Geometry for a stepped (111) surface in the face-centered- and other inner layers [for example the (111) face of a cubic (fcc) structure. Open circles represent atoms in the second layer, which is complete. circles represent atoms in the first layer, in which every third row is removed to create steps. This The publication costs of this article were defrayed in part by page charge arrangement can also describe a stepped (0001) surface of a hcp payment. This article must therefore be hereby marked "advertisement" structure (the difference between the two appears in the third, in accordance with 18 U.S.C. §1734 solely to indicate this fact. unseen, layer). 2207 Downloaded by guest on October 1, 2021 2208 Chemistry: Falicov and Somorjai Proc. Natl. Acad. Sci. USA 82 (1985)

fcc (977) fcc (755) fcc (533) fcc (14,11,10) fcc (10,8,7) fcc (13,11,9) FIG. 3. Various kinked surfaces in the close-packed fcc lattice.

four outer electrons fluctuating among the 4f, 5d, and 6s shells. (iii) Spin fluctuations, in which the magnitude and the orientation of the local magnetic moment fluctuates about its fcc (443) fcc (332) fcc (331) mean vectorial value. Spin fluctuations are common in the rare-early and in the 3d-transition-metal compounds. FIG. 2. Various stepped surfaces in the close-packed fcc lattice. (iv) Term and multiplet fluctuations, present in incom- plete shells of complex structure, and common in transition- importance in catalytic reactions that require specific metal metal, rare-earth, and elemental and compound sites, as they may sterically block catalytically active high- . coordination atomic sites, participate in or All these fluctuations require the presence of low-energy transfer processes, or change the local work function. electronic excitations which must have a large spectral weight in the catalytically active site. THEORY The most important criterion in determining the presence The chemisorption of an atom or B at a given or absence of fluctuations is the location of a given element location in an A transition-metal surface is essentially a in the . There are several tabulations of the consequence of the local electronic properties both of the heat of adsorption of small molecules (CO, H2, 02, N2) on atom or molecule and of the metal site or sites in the transition metals that exhibit decreasing values with increas- immediate neighborhood. ing d-shell occupancy (7). There are also correlations re- A given electronic configuration of, say, an isolated ported for the variations of catalytic activities for hydro- transition-metal atom A and atom B either leads to the genolysis, , and other chemical reactions that formation of a stable AB compound or yields an unreactive show trends dependent on d-shell occupancy (8, 9). From no-compound-formation situation. This typical either-or re- this evidence it is clear that the catalytic activity of the action situation is the opposite to what is needed in an transition-metal atoms and is directly related to their effective catalyst, where in principle A should attract B to unfilled d shells. Both s and p shells tend to form deep-lying filled (bonding) form a weakly bound AB intermediate compound but be orbitals, highly energetic unoccupied antibonding orbitals, readily available to release B in the presence of C so that the or wide metallic conduction bands with a relatively low final product BC be formed and A be ready to start the density of states at the Fermi level. None of these situations process all over again. leads to the sought-for low-energy fluctuations. This traditional view of the intermediate-compound theory The highly degenerate f shells are dominated by the very of catalysis can be viewed from a different point of view: the strong electron-electron intrasite coulomb repulsion. There- particular transition-metal atom A under consideration fore f levels are essentially atomic in character: most local should be susceptible to stable low-energy electronic fluc- charge and configuration fluctuations are suppressed. The tuations in its original state. In that case the intermediate levels are discrete and only occasional configuration or term AB compound-intrinsically related to the fluctuations-can fluctuations, as is the case of cerium, survive. be easily formed. Since low-energy fluctuations are also The d shells provide the ideal happy medium: the 10-fold present after a weak AB is formed, the compound can also degeneracy and relative narrowness of the bands provide be easily broken and the catalytic reaction may thus take metallic character with a high density of states.- But the place. The electronic fluctuations of importance may be of bands are not dominated by coulomb repulsion effects with many kinds. In particular: the consequent "'discretization" of the spectrum, appear- (i) Charge fluctuations, in which the electronic charge in ance of energy gaps, formation of magnetic moments, and the of A about its mean value. Atoms of a loss of "metallic" character of the d orbitals-all conditions vicinity fluctuates that lead to the suppression of local electronic fluctuations. metal are the traditional examples of charge fluctuators. The elec- band model of alkali metals consists The effect changes across the periodic table. The classical noninteracting tronic fluctuations increase in going from the 3d to the 4d to of a bulk atom neutral on the average, but with a 1/4 the 5d series: the d bandwidth increases in that direction, the probability each of being positively or negatively singly coulomb repulsion effects are less dominant, magnetism charged. Electron-electron coulomb repulsion tends to de- disappears, and ordinary configuration fluctuations increase. crease or suppress large charge fluctuations. The effect increases from left to right in the periodic table: (ii) Configuration fluctuations, in which the occupation the group VIII elements are the most favorable. numbers of single-electron orbitals are no longer good quan- Because of computational considerations we have ex- tum numbers. Transition-metal and rare-earth compounds plored theoretically in detail the local electronic structure of and structures are typical examples of configuration fluctua- various surfaces of the late 3d transition metals: Ni, Co, Fe, tors. The configuration alternates, for instance, be- and Cr. We have also explored heterogenous structures tween (3d)10, (3d)9(4s)1, and (3d)8(4s)2. The cerium atom, a involving either two of these elements or one of the elements commonly known valence or configuration fluctuator, has its and their noble-metal neighbor, Cu. Downloaded by guest on October 1, 2021 Chemistry: Falicov and Somorjai Proc. Natl. Acad. Sci. USA 82 (1985) 2209 Table 1. Number n of d holes for nickel atoms in reactants should be, according to this argument, catalytically various configurations active. Configuration Z(Ni) Z(Cu) n Another measure of the fluctuations in transition metals is the local spin imbalance at each site. When the coulomb Bulk nickel 12 0.56 repulsion between electrons becomes strong enough, the (111) Surface nickel atom 9 0.38 system tends to lessen or minimize its influence by suppress- Outstep nickel atom a (Fig. 1) 7 0.25 ing metallic fluctuations. This suppression results in a ten- Outstep nickel atom b (Fig. 1) 7 0.24 dency either towards an insulating state or to the formation Instep nickel atom c (Fig. 1) 11 0.52 of magnetic states (11, 12) with local spin imbalance (local Instep nickel atom d (Fig. 1) 10 0.45 magnetic moment) and, at low temperatures, long-range Ni under a (111) Cu monolayer 9 3 0.52 order fer- Ni monolayer on Cu (111) 3 6 0.46 magnetic (ferromagnetic, antiferromagnetic, rimagnetic, spiral arrangements, etc.). Those conditions that lead to the formation or enhancement of spin imbalances do not favor-i.e., they suppress or decrease-catalytic activ- Nickel is particularly interesting, since it is the last of the ity. 3d group VIII transition metals. When nickel is in its metallic We have calculated the spin imbalance S (twice the local form, the ground state is a mixture of three configurations magnetic moment divided by the g-factor) for a variety of (10) that, on the average, result in the d shell being occupied structures, sites, and circumstances (13-19). Our results are by 9.44 electrons. The presence of the n = 0.56 d holes, shown in Table 2. which are a measure of the configuration fluctuations, is Since imbalances are one of the manifestations of responsible for the catalytic activity of Ni. The larger n, the spin larger the configuration fluctuations of a particular Ni atom, reduced fluctuations, we expect, for a given element, cata- and the greater the catalytic activity of that particular site. lytic activity to be anticorrelated to its spin imbalance. This Our calculations for nickel in its paramagnetic state-which anticorrelation and our calculations (Table 2) point once should also be applicable to and to platinum again to catalytically active sites in locations of high co- yield a close correlation between n and the coordination ordination, and preferentially to atoms surrounded by a number Z of a particular site (10). Our results are shown in large number of nonmagnetic or weakly magnetic metallic Table 1, where Z(A) denotes the number of nearest neigh- neighbors. bors of chemical species A. An analysis of these results yields the following simple EXPERIMENTAL EVIDENCE conclusion: configuration fluctuations are larger for those We present here experimental evidence for the high catalytic atoms that have a large number of metallic neighbors. activity of atomically rough surfaces, where high-coor- Therefore transition-metal atoms in sites that are bulk-like, dination sites in the first or second layer at the surface are have a high metallic coordination, and are accessible to the exposed. We also show how stepped surfaces that expose

Table 2. Spin imbalance S of the magnetic transition metals in various configurations Configuration Z(Ni) Z(Cu) Z(Fe) Z(Co) Z(Cr) S Ferromagnetic bulk fcc nickel 12 0.56 Nickel in Ni3Cu quenched alloy 8 4 0.42 Nickel in NiCu quenched alloy 4 8 0.08 Nickel in NiCu3 quenched alloy 0 12 0 Nickel at the (100) free surface 8 0.74 Nickel at the (111) free surface 9 0.65 Nickel under a (100) Cu monolayer 8 4 0.37 Nickel under a (111) Cu monolayer 9 3 0.38 Nickel monolayer on top of Cu (100) 4 4 0.48 Nickel monolayer on top of Cu (111) 6 3 0.08

Ferromagnetic bulk hcp 12 1.56 Cobalt in the FeCo ordered alloy 8 0 1.78 Cobalt at the (100) surface of the FeCo alloy 4 0 2.03 Cobalt at the (110) surface of the FeCo alloy 6 0 1.86 Cobalt monolayer on top of Cu (111) 3 6 1.63

Ferromagnetic bulk bcc iron 8 2.12 Iron in the FeCo ordered alloy 0 8 2.66 Iron at the (100) free surface 4 2.90 Iron at the (110) free surface 6 2.55 Iron at the (100) free surface of the FeCo alloy 0 4 2.95 Iron at the (110) free surface of the FeCo alloy 0 6 2.75 Iron under a (100) Cr monolayer 4 4 1.95 Antiferromagnetic bulk (incommensurable ) 8 0.59 Antiferromagnetic bulk chromium (commensurable phase) 8 0.67 Chromium at the (100) free surface 4 3.00 Chromium at the (110) free surface 6 2.31 Chromium monolayer on top of Fe (100) 4 0 3.63 Chromium monolayer on top of Fe (110) 2 4 2.25 Downloaded by guest on October 1, 2021 2210 Chemistry: Falicov and Somorjai Proc. Natl. Acad. Sci. USA 82 (1985)

(111) (100) (110) (1120) (1010) (0001)

C4 (solid) C4 (solid) C6 (solid) C7 (dashed) C8 (dashed) C8 (dashed) C7 (solid) C6 (solid) C9 (solid) 1000 C11 (dashed) C10 (dashed) 500 (111) lO,000 5,000 (1120) ;,% 100 la *0 1,000 *t5) 50 (100) 500 5)t

m 10 z ;100 (1010) 50 (0001) (110) 1 Nonactive

FIG. 4. Surface structure sensitivity of iron-catalyzed ammonia synthesis. The symbols C,, indicate n-coordinated sites. Yield is FIG. 5. Strong surface structure sensitivity of ammonia synthe- expressed in arbitrary units. sis on rhenium single crystals. Symbols as in Fig. 4.

high-coordination sites at the step inner edges and at kinks highest coordination, is the most active for breaking the large exhibit high turnover rates for various reactions. binding energy (103 kcal/mol) of the H-H bond. Greatly Enhanced Rates of Ammonia Synthesis on Open Enhanced Hydrogenolysis Activity at Kink Sites on Plati- Crystal Faces of Iron and Rhenium That Expose the High- num Crystal Surfaces. Platinum is widely used as a catalyst Coordination Atoms in the First and Second Layers. The for the rearrangement of . It is particularly rate-determining step in the synthesis of NH3 from N2 and useful for the conversion of straight chain to aro- H2 is the of N2 (20). Chemisorption studies matic molecules and branched , to produce gasoline indicate that on the open (111) crystal face, where the with high octane number. The conversion of n-hexane or high-coordination iron atoms in the second layer are exposed n-heptane is often used as a test reaction that probes the to the incoming nitrogen molecules, the dissociation prob- selectivity of various platinum crystal surfaces to yield the ability of N2 is much higher than on the close-packed (110) face. studies on iron single crystal sur- faces, shown in Fig. 4, revealed that the rate of ammonia 1.0 I formation is 500 times greater on the Fe (111) than the Fe (110) face, while the Fe (100) face exhibits catalytic activity 0.9-H (a) _9W)Y EW0t= 900 that is intermediate between the other two crystal faces (3). Pt (332) The closed-packed hexagonal (0001) face of rhenium is 0.8 - virtually inactive for the synthesis of ammonia, while the open (1010) and (1120) crystal faces have high reactivities, many 0.7 orders of magnitude higher than the (0001) face (4). These active crystal surfaces exhibit an open structure with many 0 high-coordination sites exposed to the incoming molecules both in the first and in the second layers at the surface. In Fig. + 05aPt (332) 5 we show the relative reaction rates on the different single crystal surfaces along with the surface structure, displaying 0.4- atoms of different nearest-neighbor configurations. Reactive Molecular Beam Surface Scattering Studies of 0.3- 'H2/2H2 Exchange on Stepped Platinum Single Crystal Sur- faces. The reaction of hydrogen and deuterium molecules to 0.2 form 1H2H is one of the simplest reactions that take place readily on transition-metal surfaces, even at temperatures as 0.1 - Pt (111) low as 100 K. This reaction was studied by using mixed molecular beams of 1H2 and 2H2 on the (111) and stepped -80 -60 40 -20 0 20 40 60 80 surfaces of platinum (5). The reaction probability was 0.35 Angle of incidence (6), degrees on the stepped surfaces, much higher than the 0.07 value on the (111) surface under identical experimental conditions. FIG. 6. Reaction probability for 'H2/2H2 exchange over a plati- The reaction probability also depended markedly on the direc- num single crystal surface as a function of angle of incidence of the tion of approach of the reactant molecules to the stepped molecular beam, normalized to the incident 2H2 intensity. Curve a, with the to the in 6. The of 1H2H was the platinum (332) surface step edges perpendicular surface. This is shown Fig. production incident beam. Curve b, the platinum (332) surface where the side of the the highest when the reactants struck the open step projection of the beam on the surface is parallel to the step edges. a of 2 when the structure, and it decreased by about factor Curve c, the smooth, close-packed platinum (111) surface. 4, Angle inner edge of the step was shadowed. These results indicate of azimuth; I, beam intensity. Frequency of chopping was 10 Hz and that the inner corner atom, which is also the one with the surface temperature was 800°C. Downloaded by guest on October 1, 2021 Chemistry: Falicov and Somorjai Proc. Natl. Acad. Sci. USA 82 (1985) 2211

dominant in atoms of high coordination 0.03V.V. _ ) - C, - C, gases number that are surrounded by a large number of other metallic nonmagnetic or weakly magnetic atoms. Isobutane . Ethane, This proposition immediately points out the suitability, for this purpose, of rough, stepped, or kinked surfaces of metals t.) 0.012-_ and properly chosen alloys, which make accessible to the

ct reactants atomic sites of high coordination. 0 It is also important to remark that not all intermediate ._ steps of a catalytic reaction are structure sensitive, and that 0.0 structure-insensitive processes (which in general are not the rate-determining ones) may be in fact mediated by tena- ciously held overlayers, which insulate the metal from direct contact with some of the reactants. 1I Our proposition has obviously profound implications in 0 Pt (100) Pt (111) Pt (13,1,1) Pt (10,8,7) the manufacturing of catalysts, their chemical composition and morphology, as well as in the understanding of the FIG. 7. Reaction rates for the hydrogenolysis of isobutane on processes that lead both to their effectiveness and to their various single crystal surfaces of platinum. Rates are given in units demise (poisoning). of molecules per site per second. The kinked (10,8,7) crystal face is the most active for this reaction. This work was supported at the Lawrence Berkeley Laboratory by the Director, Office of Energy Research, Office of Basic Energy Sciences, Division of the Department of Energy desired products (21). The hydrogenolysis of alkanes to pro- under Contract DE-AC03-76SF00098. duce low molecular weight gases-methane through propane- by C-C bond breaking is an undesirable but important re- 1. Davis, S. M., Zaera, F. & Somorai, G. A. (1982) J. Catal. 77, action path. It was found that platinum crystal surfaces that 439-459. kink sites are the most active ones 2. Somojai, G. A. (1981) Chemistry in Two Dimensions: Sur- have high concentrations of faces (Cornell Univ. Press, Ithaca, NY). in catalyzing this unwanted reaction (22, 23). This is shown in 3. Spencer, N. D., Schoonmaker, R. C. & Somorjai, G. A. Fig. 7. Hydrogenolysis can be inhibited by the introduction of (1982) J. Catal. 74, 129-135. a small concentration of , which strongly binds to these 4. Ascher, M. & Somorjai, G. A. (1984) Surf. Sci. 143, L389- L392. sites and blocks the C-C bond-breaking process. 5. Salmeron, M., Gale, R. J. & Somorjai, G. A. (1977) J. Chem. Phys. 67, 5324-5334. STRUCTURE SENSITIVITY OF CATALYTIC 6. Davis, S. M., Zaera, F. & Somorjai, G. A. (1984) J. Catal. 85, REACTIONS 206-223. 7. Toyoshima, I. & Somorjai, G. A. (1979) Catal. Rev. Sci. Eng. The examples of catalytic reactions described above (am- 19, 105-159. monia synthesis, 1H2/2H2 exchange at low pressures, and 8. Sinfelt, J. H. (1973) Adv. Catal. 23, 91-119. hydrogenolysis) all indicate pronounced structure sensitiv- 9. Sinfelt, J. H. (1974) Catal. Rev. 9, 147-168. ity. This structure sensitivity of catalytic reactions can be 10. Tersoff, J. & Falicov, L. M. (1981) Phys. Rev. B 24, 754-764. detected either through systematic variation of metal particle 11. Thalmeier, P. & Falicov, L. M. (1979) Phys. Rev. B 20, size (24, 25) or by reaction studies using metal single crystal 4637-4644. surfaces of different surface structure (26). For structure- 12. Thalmeier, P. & Falicov, L. M. (1980) Phys. Rev. B 22, sensitive catalysis processes the concentration of the most 2456-2465. 13. Tersoff, J. & Falicov, L. M. (1982) Phys. Rev. B 25, active metal sites determines the turnover rates. In these 2959-2961. circumstances the availability of atoms with high coordina- 14. Tersoff, J. & Falicov, L. M. (1982) Phys. Rev. B 26, 459-461. tion and correspondingly large electronic fluctuations is of 15. Tersoff, J. & Falicov, L. M. (1982) Phys. Rev. B 26, key importance. There are also, however, structure-insen- 6186-6200. sitive reactions, such as many hydrogenation processes (27, 16. Victora, R. H. & Falicov, L. M. (1983) Phys. Rev. B 28, 28), in which the structure of the metal sites is not as 5232-5242. important. Reactions of this type are likely to occur on the 17. Victora, R. H. & Falicov, L. M. (1984) Phys. Rev. B 30, strongly adsorbed overlayer (29) that essentially insulates 259-262. the metal from most of the reactants and reaction products 18. Victora, R. H., Falicov, L. M. & Ishida, S. (1984) Phys. Rev. B and may 30, 3896-3902. reduce the metal role to that of atomizing some of 19. Victora, R. H. & Falicov, L. M. (1985) Phys. Rev., in press. the smaller reactant molecules (H2 or 02, for instance). 20. Ertl, G. (1983) in Robert A. Welch Conference Proceedings on As an example, during hydrogenation a stable Heterogeneous Catalysis, ed. Hackerman, N. (Robert A. Welch ethylidyne monolayer forms on top of the metal surface, and Foundation, Houston, TX), Vol. 25, pp. 179-207. the hydrogenation occurs via hydrogen transfer through this 21. Zaera, F. & Somorjai, G. A. (1982) J. Phys. Chem. 86, layer to the ethylene molecules that are weakly adsorbed on 3070-3078. top of it. The rates of structure-sensitive reactions (e.g., the 22. Gillespie, W. D., Herz, R. K., Petersen, E. E. & Somorjai, breaking of the H2 molecule) are controlled by the surface G. A. (1981) J. Catal. 70, 147-159. roughness, whereas the structure-insensitive processes 23. Davis, S. M., Zaera, F. & Somorjai, G. A. (1982) J. Am. (e.g., the Chem. Soc. 104, 7453-7461. hydrogenation of ethylene by the atomic hydrogen) 24. Boudart, M. (1969) Adv. Catal. 20, 153-166. are mediated by the tenaciously held overlayer. It is impor- 25. Dominguez, J. M. & Yacaman, M. J. (1980) in Growth and tant to realize that most catalytic reactions may require both Properties ofMetal Clusters, ed. Bourdon, J. (Elsevier, Amster- ingredients: the proper atomic surface structure and a dam), pp. 493-503. strongly adsorbed overlayer. 26. Somorai, G. A. (1983) in Robert A. Welch Conference Proceed- ings on Heterogeneous Catalysis, ed. Hackerman, N. (Robert A. CONCLUSION Welch Foundation, Houston, TX), Vol. 25, pp. 166-178. 27. Blakely, D. K. & Somorjai, G. A. (1976) J. Catal. 42, 181-196. The catalytic activity in structure-sensitive processes is 28. Manogue, W. H. & Katzer, J. R. (1974) J. Catal. 32, 166-169. directly related to the existence of sites with low-energy 29. Zaera, F. & Somorjai, G. A. (1984) J. Am. Chem. Soc. 106, electronic fluctuations. These fluctuations are most pre- 2288-2293. Downloaded by guest on October 1, 2021

Top View