4196
J . Phys. Chem. 1991, 95,4196-4206
FEATURE ARTICLE Surface Sclence Studies of the Electronic and Chemlcai Properties of Bimetallic Systems Jod A. Rodriguez and D. Wayne Goodman* Department of Chemistry, Texas A&M University, College Station, Texas 77843-3255 (Received: December 10, 1990)
Recent studies dealing with the electronic, chemical, and catalytic properties of well-defined bimetallic surfaces (prepared by vapor-depositing one metal onto a crystal face of a second metal) are discussed. The results show that a metal atom supported on a dissimilar metal can be electronically perturbed and that this perturbation can dramatically alter the chemical and catalytic properties of both constituents of the bimetallic system. In many cases, the metal adatoms exhibit properties toward the chemisorption of H2, 02,CO, and C02and reactivitiestoward small hydrocarbons that are significantly different from those seen for the pure metal. For supported monolayers of Ni, Cu, and Pd a correlation is observed between shifts in surface core-level binding energies and changes in the desorption temperature of CO from the metal adlayers. The shifts in core-level binding energies and CO desorption temperatures are a consequence of (1) electronic interactions between the metal overlayer and metal substrate and (2) variations that occur in the admetal-admetal interactions when the admetal adopts the lattice parameters of the substrate. Examples are provided which demonstrate the relevance of single-crystal studies for modeling the behavior of high surface area supported bimetallic catalysts. The coupling of an apparatus for the measurement of reaction kinetics at elevated pressures with an ultrahigh-vacuum system for surface analysis allows detailed studies of structure sensitivity, the effects of surface composition on catalytic activity, and, in certain cases, identification of reaction intermediates by postreaction analysis. The roles of 'ensemble" and 'ligand" effects in mixed-metal catalysts are discussed in the light of data obtained by using well-defined bimetallic surfaces.
I. Introduction In recent years there has been a substantial effort in the surface science community to investigate the structural, electronic, and chemical properties of bimetallic systems. Bimetallic surfaces are extensively used in many important industrial processes in the areas of catalysis, electrochemistry, and microelectronics fabrication. Mixed-metal systems are superior over their single-metal counterparts in terms of catalytic activity and/or selectivity.' Many fundamental studies have been focused on trying to understand the roles of "ensemble" and "ligand" effects in bimetallic catalysts.'+ Ensemble effects are defined in terms of the number of surface atoms needed for a catalytic process to occur. Ligand effects refer to t h w modifications in catalytic activity or selectivity that are the result of electronic interactions between the components of the bimetallic system. In gathering information to address these questions, it has been advantageous to simplify the problem by utilizing models of bimetallic catalysts such as the deposition of metals on single-crystal substrates in the clean environment familiar to surface science. These well-defined bimetallic surfaces have offered a unique possibility to correlate surface chemical reactivity with atomic-level surface structure. Experiments performed using modern techniques of surface sciences-' have given exciting insights into phenomena that accompany the formation of surface metal-metal bonds, improving our understanding of the chemical properties of bimetallic surfaces. These types of studies reveal that the metal overlayer-metal substrate interaction can modify to a large extent the electronic properties of both constituents of the bimetallic system. Correlations have been found between variations in the chemical and electronic properties of supported metal films. The coupling of an apparatus for the measurement of reaction kinetics at elevated pressures with ultrahigh-vacuum (UHV)systems for surface analysis*-IOhas allowed detailed studies of the effects of surface structure on the catalytic activity and selectivity of bimetallic *To whom correspondence should be addressed. 0022-3654/91/2095-4196$02.50/0
systems for CO methanation, ethane hydrogenolysis, n-hexane isomerization, and cyclohexane dehydrogenation. In many cases, the results obtained by using well-defined bimetallic surfaces have revolutionized the way in which "ensemble" and 'ligand" effects are viewed. In this article, we review recent studies that deal with the electronic, chemical, and catalytic properties of well-defined bimetallic surfaces. We will focus our attention on systems formed by the combination of transition and/or noble metals. We begin with a discussion of studies on the structure (section 11) and electronic properties (section 111) of bimetallic systems generated by deposition of one metal onto a singlecrystal surface of a second metal. Next, the results of works dealing with the adsorption of CO, COz, H2,02,and small hydrocarbons on these bimetallic surfaces are presented (section IV). Finally, we show studies concerned with catalytic reactions on well-defined bimetallic surfaces (section V). 11. Atomic Structure of Metal Overlayers
A large number of crystalline bimetallic systems have been prepared by vapor-depositing one metal onto the surface of a second metal. Low-energy electron diffraction (LEED)?." Auger (1) Sinfelt, J. H. Bimetallic Catalysrs; Wiley: New York, 1983. (2) Sachtler, W. M. H. Faraday Discuss. Chem. Soc. 1981, 72, 7. (3) Campbell, C. T. Annu. Rev. Phys. Chem. 1990, 41, 775. (4) Rodriguez, J. A.; Goodman, D. W. In New Trends in CO Activation; G u m , L., Ed.; Elsevier: Amsterdam, 1991; Chapter 3. (5) Ertl, G.; Kilppers, J. Low Energy Elecrrons and Surfuce Chemistry; VCH Verlagsgesellschaft: Weinheim, 1985. (6) Woodruff, D. P.; Delchar, T. A. Modern Techniques ofSurfacr Science; Cambridge University heas: New York, 1986. ( 7 ) Somorjai, G.A. Chemistry in Two Dimensions: Surfaces; Cornell University Press: Ithaca, 1981. ( 8 ) Campbell, C. T. Adu. Carol. 1989, 36, 1 . (9) Sexton, B. A.; Somorjai, G. A. J. Carol. 1977, 46, 167. (10) Goodman, D. W.; Kelley, R. D.; Madey, T. E.; Yates, J. T. J . Caral. 1980, 63, 226.
0 1991 American Chemical Society
Feature Article electron spectroscopy (AES),IzJ3field ion microscopy (FIM)," reflection high-energy electron diffraction (RHEED),I9 ion scattering spectroscopy (ISS),15116X-ray photoelectron diffraction (XPD),17J8and Auger electron forward ~ c a t t e r i n g ~have ~ J * been employed to examine the atomic structure of metal overlayers. The combined use of several of these techniques provides complementary information about surface composition, adsorption sites, surface structure (long-range two-dimensional ordering), and growth mode (three-dimensional ordering) of the deposited metal film. Experimental evidence indicates that films with submonolayer coverages exhibit a large diversity of spatial arrangements, varying from randomly adsorbed atoms to densely packed islands with approximately bulk structure.12*20*21 This variety of structures is a consequence of different types of lateral interactions between the metal adatoms. The adatom-adatom interaction energies along a surface can be probed directly by using thermal desorption mass spectroscopy (TDS). Overlayer systems that show a TDS peak shifted to higher temperature with increasing coverage usually have net attractive interactions between the adatoms, while those characterized by a shift to lower temperatures have adatomadatom repulsions. Noble-metal films are typical examples of systems with net attractive lateral interactions, forming two-dimensional islands at submonolayer coverages and presenting an increase in the desorption temperature with increasing coverage.'3,22-25 This is in sharp contrast to the behavior found for alkali metals adsorbed on transition-metal surface^.^^.^^ The lateral interactions among the atoms in a metal overlayer with submonolayer coverage are mainly the consequence of a competition between two terms in the adatom-adatom interaction energy:12~23*28 (1) an attractive term due to the formation of chemical bonds between the adatoms and (2) a repulsive term caused by the fact that metal adatoms always have an associated partial charge and repel each other. Transition- and noble-metal overlayers have only small charge-induced repulsions which are easily overcome by the cohesive energy gained in making adsorbate-adsorbate bonds, so that net attractive lateral interactions are usually observed for these systems. LEED studies of crystal growth show quite clearly that there can be preferred orientational relationships when two metals with dissimilar crystal lattices are forced into intimate contact.2031The case of fcc( 1 1 1) films growing on bcc( 1 10) substrates has been extensively investigated, with certain preferred rotational alignments (Nishiyama-Wassermann or Kurdjumov-Sachs orientations) observed, depending on the bulk ratio of adatom/substrate nearest-neighbor distance.2031The term "pseudomorphic growth" ( I 1) MacLaren, J. M.; Pendry, J. B.; Rous, P. J.; Saldin, D. K.;Somorjai, G. A.; Van Hove, M. A.; Vvcdensky, D. D. Surface Crystallographic Information Seruice; Reidel: Dordrecht, 1987. (12) Bauer, E. In The Chemical Physics of Solid Surfaces and Heterogeneous Catalysts; King, D. A., Woodruff, D. P., Eds.; Elsevier: Amsterdam, 1984; Vol. 3. (13) He, J. W.; Shea, W. L.; Jiang, X.;Goodman, D. W. J . Vac. Sei. Technol. A. 1990,8, 2435 and references therein. (14) Bassett. D. W. Thin Solid Films 1978,48,237 and references therein. ( I 5) Schmitz, P. J.; b u n g , W. Y.; Graham, G. W.; Thiel, P. A. Phys. RN. B 1989, 40, 11477. (16) (a) Attard, G. A.; King, D. A. Surf. Sci. 1987,188,589. (b) Horrell, B. A,; Cocke, D. L. Catal. Reo.-Sci. Eng. 1987, 29, 447. (17) Egelhoff, W. F. Phys. Reu. Lett. 1987, 59, 559. (18) Egelhoff, W. F. Crit. Reo. Solid State Mater. Sci. 1990, 16, 213. ( 19) Reflection High Energy Electron Diffraction and Reflection Electron Imaging at Surfaces; Larsen, P. K.,Dobson, P. J., Eds.; Plenum: New York, 1988. (20) Bauer. E. Appl. Sur/. Sci. 1982, 11112,479. (21) Bauer, E.; van der Merwe, J. H. Phys. Reo. B 1986, 33, 3657. (22) Niemandtsverdriet, J. W.; Dolle, P.; Markert, K.;Wandelt, K. J . Yac. Sci. Technol. A 1987, 5. 875. (23) Kolaczkiewicz, J.; Bauer, E.Surf.Sci. 1986, 175, 508. (24) Houston, J. E.;Pcden, C. H. F.; Blair, D. S.;Goodman. D. W. Surf. Sci. 1986, 167, 427. (25) Jiang. X.;Goodman, D. W. Surf. Sci., in press. (26) Rodriguez, J. A.; Clendening, W. D.; Campbell, C. T. J. Phys. Chem. 1989, 93. 5238. (27) Bonzel, H. P. Surf.Sci. Rep. 1988, 8, 43. (28) Bauer, E.; Kolaczkiewicz, J. Proc. IYInt. Yac. Congr. Y l n t . Conf. Solid Surf. Madrid 1983, 363.
The Journal of Physical Chemistry, Vol. 95, No. 11, 1991 4197 FRANK-VAN DER MERWE
SUBSTRATE STRANSKI-KRASTANOV
SUBSTRATE VOLMER-WEBER
SUBSTRATE
Figure 1. Schematic diagram of the three growth modes usually observed for metal overlayers. Ni/Ru(0001)
1 @!
700 K ANNEAL
1100KANNEAL
2ML
4ML
6ML
NiiW(100)
L1 1ML
2ML
3ML
TPD AREA (ARB. UNITS)
Figure 2. (a) Adsorption of Ni on Ru(0001) plotted as the Ni(848 eV):Ru(273 eV) AES ratio versus Ni desorbed in TPD (from ref 30). (b) Adsorption of Ni on W(100)plotted as the Ni(848 eV):W(179 eV) AES ratio versus Ni desorbed in TPD (from ref 31).
refers to a situation where the metal overlayer adopts a lattice constant which differs from its bulk crystal structure but which match coherently the lattice of the underlying substrate. Typical examples of this type of growth are the systems Cu/Ru(OOO1),U Cu/W(l Ni/Ru(OOOl),w Ni/W(I 10),DJ1932 Ni/W(100),"J2 Pd/W(I 10),33J4and Pd/Ta(l In general, only the first monolayer grows pseudomorphically, while subsequent layers tend to present lattice constants that are closer to the bulk crystal structure of the admetal. As we will see below, the strain induced by the substrate on the adatoms in a pseudomorphic monolayer (29) Lilienkamp, G.; Koziol, C.; Bauer, E. Surf.Sci. 1990, 226, 358. (30) Berlowitz, P. J.; Houston, J. E.; White, J. M.; Goodman, D. W. Surf. Sci. 1988, 205, 1. (31) Berlowitz, P. J.; Goodman, D. W. Surf.Sci. 1987, 187, 463. (32) Kolaczkiewicz, J.; Bauer, E. Surf.Sci. 1984, 144, 495. (33) Schlenk, W.; Bauer, E . Surf. Sci. 1980, 93, 9. (34) Berlowitz, P. J.; Goodman, D. W. Lungmuir 1988, 4, 1901. (35) Ruckman, M. W.; Murgai, V.; Strongin, M. Phys. Reo. B 1986,34. 6759.
4198 The Journal of Physical Chemistry, Vol. 95, No. 11, 1991
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can perturb to a large extent their electronic and chemical properties. Extensive experimental results point to the existence of three distinct growth modes for metal films with coverage beyond a m o n ~ l a y e r . ~ JEach ~ , ~ ~mode is named after investigators associated with their initial description: Frank-wn der Merwe growth (FV), Stranski-Krastanov growth (SK), and Volmer-Weber growth (VW). Figure 1 shows a schematic view of the topology of these modes. The growth mode adopted by a metal film depends largely upon the relative surface energies of the pure admetal ( T A ) , the pure substrate ( T S ) , and the interface (7A_S).21'37 The ideal layer by layer of FV mode can only be expected when AT = T A '. TA-S - T~ < 0. The surface energy mismatch parameter, r A S I O 8 ( 1 4 2 0 ( 1 4 2 0 = 21(rA- T S ) / ( T A , + T S ) ~ is , generally less than for systems BINDING ENERGY (ev) BINDING ENERGY (ev) where this mode IS observed.21 The VW mode, whereby threeFigure 3. (a) Ultraviolet photoelectron spectra for monolayer (dashed dimensional islands (cluster crystallites) of the admetal grow from curve) and greater-than-monolayer coverages of Pd on Nb( 110) (from the lowest coverages (without a uniform monolayer ever being refs 37 and 43). (b) UPS spectra of various coverages (6) of Pd on formed), is usually found when AT = T A + TA-S - T~ > 0. W(100)(from ref 44). The SK growth mode is frequently seen for transition- and noble-metal overlayers. This situation appears when the heat of in the admetal-admetal interactions when the admetal adopts the adsorption of the vapor-deposited metal onto the first adsorbed lattice parameters of the substrate and (2) electronic interactions monolayer (or bilayer) is smaller than the bulk cohesive energy between the admetal and substrate (Le., formation of chemical of the adsorbate, a phenomenon that can be a consequence of bonds, transfer of charge, polarization effects, etc.). electronic interactions between the substrate atoms and the first III.1. Pd Ouerlayers. Figure 3 shows UPS results for Pd films adlayer(s) or caused by the strain induced by the substrate lattice supported on Nb(110)37*43 and W(100).44 At BW = 1, LEED on the geometry of the first monolayer. Figure 2 shows AES and shows a commensurate 1 X 1 ordered overlayer for Pd/Nb(l TPD results for Ni overlayers on Ru(OOO1)Mand W( lo^).^' On At slightly higher Pd coverages (- 1.4 monolayers (ML)) the these substrates, Ni grows layer by layer at 100 K. Layers in diffraction pattern changes to that of Pd(l1 l).43 For Pd/Wexcess of the first pseudomorphic monolayer are metastable, ( LEED patterns below 1 ML indicate that the Pd adlayer forming three-dimensional islands (cluster crystallites) at temis predominantly pseudomorphic. For certain annealing conditions, peratures above 1000 K.30*31This type of behavior (FV growth several pseudomorphic Pd layers can be formed, leading to a thin at 100 K, SK growth at much higher temperatures) has been Pd( 100) bcc film.3g In Figure 3 the Pd monolayers are devoid observed for many bimetallic systems: C ~ / R u ( 0 0 0 1 ) ,Cu/ ~~ of density of states (DOS) at the Fermi level (EF).In contrast, Rh(100),25Cu/W(l Cu/Re(0001)?8 Cu Mo(l lo)," Pd/ Pd multilayers and bulk Pd45exhibit emission spectra characterized W(110),'3*34Pd/W(100),~*39 Ni/M0(llO),~~~~Ni/W(110)~~~ 2 by a large DOS at EF.Thus, adsorption of a single layer of Pd and Fe/Mo(l 10).13+40 on Nb( 1 10) and W( 100) produces adatoms with a valence band In the last paragraphs we have discussed growth modes that structure similar to that observed in noble metals!5 An identical are usually observed when the admetal and the substrate have phenomenon has been reported for the Pd/Ta(l and Pd/Wa low miscibility. More complicated situations can arise when (1 systems. the components of the bimetallic system present a large misciSelf-consistent tight-binding calculations for Pd,,/Nb( 1lo)& b i l i t ~ . ~ . ~In' those cases substrate atoms can migrate into the indicate a chemisorption bond mainly covalent. Theoretical results film, forming alloys of variable composition and crystal structure for an unsupported layer of Pd show a substantial density of states as a function of annealing t e m p e r a t ~ r e . ~An , ~ ~interesting pheat EF.& Thus, the Pd-Nb interaction is essential in order to nomenon has been observed for Rh/Ag(100)I5 and Fe/Au(100).42 produce the noble-metal behavior observed for a monolayer of The components of these systems have very limited miscibilities. Pd on Nb( 110). The 4d levels of an unsupported monolayer of However, their equilibrium configurations are not predicted by Pd and of the Nb(ll0) substrate are very close to the Fermi level, any of the three traditional growth modes described above. In with the centroid of the Pd(4d) band at lower energy. Upon these systems the equilibrium structure is that of a Ag-Rh-Ag adsorption of Pd, the Pd(4d) and Nb(4d) levels interact to yield or Au-Fe-Au sandwich. The formation of the "sandwicheswis bonding and antibonding bands that appear below and above the thermodynamically driven by the difference in surface free energies Fermi level.& The bonding bands are mostly Pd(4d)-like, showing between Ag and Rh or Fe and Au.15 only a small contribution from the Nb(4d) levels. A net result of the Pd-Nb interaction is a shift of the 4d levels of the Pd 111. Electronic Structure of Metal Overlayers monolayer toward higher binding Pd atoms adsorbed X-ray and ultraviolet photoelectron spectroscopies (XPS and on top of the first Pd overlayer do not interact directly with the UPS) have been extensively used to study the core and valence Nb(l10) substrate, and the position of the Pd(4d) levels prolevels of metal overlayers. In this section we discuss recent results gressively varies toward the bulk value. obtained for Ni, Cu, and Pd films supported on several metal Figure 4 illustrates the effects of adsorbate coverage (film substrates. These results show clearly that metal atoms in a thickness) on the Pd(3d5,,) XPS peak positions of the Pd/Wsupported monolayer can have electronic properties very different (1 Pd/Re(0001)," and Pd/Mo(l lo)* systems. The peak from those observed for the bulk metal. Particularly interesting positions reported for Pd coverages in excess of 1 ML represent are the cases of strained pseudomorphic overlayers, in which the a product of electrons emitted from surface and subsurface atoms. electronic perturbations are a result of (1) variations that occur For the case of Pd( loo), theoretical calculation^^^ suggest that
+
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(36) Kern, R.; Le Lay, 0.;Metois, J. J. In Current Topics in Materials Science; Kaldis, E., Ed.; North-Holland: Amsterdam, 1979; Vol. 3, Chapter 3. (37) Zangwill, A. Physics at Surfaces; Cambridge University Press: New York, 1988. (38) He, J. W.; Goodman, D.W. J . Phys. Chem. 1990, 94. 1496. (39) Prigge, S.; Roux, H.; Bauer, E.Surj. Sci. 1981, 107, 101. (40) Tikhov, M.; Bauer, E. Sur/. Sci. 1990, 232, 73. (41) Barnard, J. A.; Ehrhardt, J. J.; Azzouzi, H.; Alnot, M.Surf. Sci. 1989, 211/212. 740. (42) Bader, S.D.; Moog,E. R. J. Appl. Phys. 1987, 61, 3729.
(43) ECBatanouny, M.;Strongin, M.;Williams, G. P.; Colbert, J. Phys. Rev. Lett. 1981, 46, 269. (44) Graham, G. W. J. Vac. Sei. Technol. A 1986, 4, 760. (45) Baer, Y.; Heden, P. F.; Hcdman, J.; Klasson, M.; Nordling, C.; Siegbahn, K. Solid Stare Commun. 1970, 8, 517. (46) (a) Kumar, V.; Bennemann, K. H. Phys. Reu. B 1982,26,7004. (b) Kumar, V.; Bennemann, K. H. Phys. Reo. B 1983, 28, 3138. (47) Campbell, R. A.; Rodriguez, J. A.; Goodman, D. W. Surf. Sci. 1990, 240, 7 1. (48) Rodriguez, J. A.; Campbell, R. A,; Goodman, D. W. J . Vac. Sci. Technol. A., in press.
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The Journal of Physical Chemistry, Vol. 95, No. 11, 1991 4199
Pd(3d5/2) XPS
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BINDING ENERGY Pd COVERAGE, ML Figure 4. Pd(3dy2)peak position as a function of Pd coverage: (a) Pd/W(l lo), (b) Pd/Re(0001), and (c) Pd/Mo(l IO). (d) Relative Pd(3d512)XPS binding energies of bulk Pd, surface atoms of Pd( lOO), and Pd monolayers on W(1 IO), Re(0001), and Mo( 110) (from refs 47 and 48).
the Pd(3d5 ), XPS binding energy of the surface atoms is -0.4 eV lower than that of bulk Pd. A similar difference has been observed experimentally for Nim and Pt51surfaces. These shifts in binding energy are a consequence of variations in the coordination number of the surface atoms compared to bulk atom^.^^"^ If we reference the combined peak of bulk and surface atoms in 40 ML of Pd on W(110) to that of Pd(100) (see Figure 4d). a difference of -0.8 eV is obtained between the Pd(3ds 2) binding energy of a pseudomorphic monolayer of Pd on W( 116) and that of the surface atoms of Pd( 100). The corresponding shifts for Pdl.o/Re(OOO1) and Pdl.o/Mo(l 10) are 0.65 and 0.90 eV, respectively. The influence of adsorbate coverage found in Figure 4 for the Pd(3d5 2) peak is similar to that seen in Figure 3 for the Pd(4d) band! the core and valence levels of supported Pd monolayers appear at higher binding energy than those of bulk Pd atoms. The Pd monolayer densities on W(110) (1.42 X lOI5 atoms/ cm2), Mo( 110) (1.43 X 10l5atoms/cm2), and Re(0001) (1.52 X 10" atoms/cm2) are somewhat higher than the surface atomic density of Pd(100) (1.32 X 1OI5 atoms/cm2). Thus, the effective coordination of the surface atoms in Pd( 100) is smaller than in Pdl,o/W(lIO), Pdl,o/Mo(l lo), and Pdl.o/Re(OOO1). This l e a d s 4 7 9 4 to an increase in the Pd(3d) XPS binding energy of the supported Pd monolayers with respect to the corresponding values for the surface atoms of Pd(100). However, the fact that the Pd(3dJi2) level of bulk Pd atoms is at lower binding energy than those of supported Pd monolayers suggests that the shifts observed in Figure 4d are a consequence of not only an increase in the coordination number of the Pd adatoms but also the product of strong electronic interactions between the Pd adlayers and subs t r a t e ~ . ' ~The , ~ data of Figure 4 are consistent with a model in which there is transfer of electrons from occupied orbitals of the Pd adlayers into empty electronic states of W, Mo, or 111.2. Cu Overluyers. For submonolayer depositions at 100 K, Cu grows in a dispersed mode on Ru(0001), forming twodimensional (2D) islands pseudomorphic to the substrate upon (49) Arlinghaus, F. J.; Gay, J. G.; Smith, J. R. fhys. Reu. B 1981, 23,
5152.
(50) Egelhoff, W. F. fhys. Reu. k i t . 1983, 50, 587. (51) Egelhoff, W.F. Sur/. Sci. Rep. 1987, 6. 253. (52) Eastman, D. E.; Himpsel, F. J.; van der Veen, J. F. J . Vac. Sci. Technol. A 1982, 20, 609. (53) Purcell, K. G.; Jupille, J.; King, D. A. Sur/. Sci. 1989, 208, 245.
annealing to 300 K.24 This behavior is seen to continue up to a coverage near 1 ML. Additional Cu deposition to 2 ML shows an epitaxial Cu( 111) structure. Past the second monolayer, Cu films are metastable and coalesce into three-dimensional (3D) islands upon annealing to 900 K. Figure 5 displays UPS difference spectra for Cu overlayers on R~(O00l).~' At submonolayer coverages (top panel), the spectra present features at binding energies of approximately 3.6, 2.8, and 1.5 eV. Theoretical calculations utilizing the linearized augmented plane-wave (LAPW) method" indicate that the structures at 3.6 and 1.5 eV correspond to two bonding and antibonding interface states, respectively. These interface states are mainly a Consequence of the interaction between the Cu(3d) and the Ru(4d) levels. It is clear from the spectra of Figure 5 that the electronic properties of Cu films with submonolayer coverage are very different from those of Cu multilayers. In Figure Sb, the strong feature at just below 2 eV is associated with the sharp 3d band of bulk Cu. The interface state at 1.5 eV observed for Cqo/Ru(OOO1) has not been found in other bimetallic systems involving Cu overlayers: Cu/Pt(ll 1),55Cu/Nb(110),56 and Cu/W(l XPS studies for Cu/Pt( 11l)?5 Cu/Rh( Cu/Re(0001)," and C u / R ~ ( 0 0 0 1 indicate )~~ that the Cu(2p3l2) binding energy increases with film thickness. Figure 6a-c shows typical results. Coverages of less than 1 ML show little shift in peak position. This phenomenon can be attributed to the formation of 2D islands of Cu on the substrates. For coverages larger than 1 ML, there is a monotonic increase in the Cu(2py2) peak position up to a value of 932.9 eV at coverages greater than 5 ML. In Figure 6a-c the total shifts in Cu(Zp,/,) binding energies are 0.6 eV for Cu/ Rh(100), 0.3 eV for Cu/Ru(0001), and 0.15 eV for Cu/Re(0001). Figure 6d shows the relative Cu(2p3/,) binding energies of the supported Cu monolayers with respect to those of the surface and bulk atoms of Cu(100). The changes in C U ( ~ P , /peak ~ ) position seen in Figure 6c can be simply attributed to an increase in the (54) Houston, J. E.; Peden, C. H.F.; Feibelman, P. J.; Hamann, D. R. Sur/. Sei. 1987, 192, 457. (55) Shek, M.L.; Stefan, P. M.;Lindau, 1.; Spicer. W . E. Phys. Rm. B 1983, 27, 7277, 7301. (56) El-Batanouny, M.;Strongin, M . fhys. Rev. B 1985, 31, 4798. (57) Rodriguez, J. A.; Campbell, R. A.; Goodman, D. W .J. fhys. Chem. 1991. 95, 2477.
(58) Rodriguez, J. A.; Campbell.
244, 211.
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4200 The Journal of Physical Chemistry, Vol. 95, No. 11, 1991
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Rodriguez and Goodman
< 1 ML) to 853.05 eV (dNi > 10 ML).
CLEAN Ru(0001)
The shift in Ni(2p3p) binding energy is likely a consequence of the variation in the coordination number of the Ni atoms. A very similar Ni(2p3 2) binding energy is obtained for a monolayer of Ni on Ru(O0 1) and the surface atoms of Ni( 100). (The surface atomic densities of Nil~o/Ru(OOO1)and Ni( 100) are almost identicaL6I) In contrast, Ni monolayers supported on W(110)47 and M0(110)~'show Ni(2p3,& binding energies close to that of bulk Ni and -0.3 eV higher than the corresponding value for the surface layer of Ni(100). This phenomenon suggests that the electron density of the Ni atoms in Nil,o/W(llO) and Nil,o/Mo(llO) is lower than that of the surface atoms of Ni(100)!7+61 111.4. Summary. A supported metal monolayer can have electronic properties that are very different from those observed for the bulk metal. The strongest electronic perturbations are seen for systems that involve a combination of a metal with an almost fully occupied valence band and a metal with a valence band more than half empty. It appears that bonding of a pair of metals generally involves a gain in electron density by the element initially having the larger fraction of empty states in its valence band.61 The direction of electron transfer can be understood in terms of orbital mixing: hybridization of the occupied states of an electron-rich metal A with the unoccupied levels of an electron-poor metal B leads to a loss of A character in the occupied states and hence a reduction in the electron density on metal A.61 For supported monolayers of Ni and Pd (electron-rich admetals), the magnitude of the perturbations induced by the loss of electron density increases as the transition-metal substrate ymovesn from right to left in the periodic table.61 In contrast, for supported Cu monolayers (electron-poor admetal) the strongest admetal-substrate interactions are observed for transition-metal substrates that are on the right side of the periodic table.
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BINDINQ ENERGV O V
Figure 5. Normal incidence angle-resolved UPS spectra taken near the R point in the surface Brillouin zone for the Cu/Ru(0001) surface as a
function of Cu coverage. B-ISrefers to the two bonding interface stata, Cu-SS to the pair of Cu surface states, and AB-IS to the antibonding pair of interface states (from ref 54). effective coordination of the Cu atoms. (The Cu monolayer density on Re(0001) is almost identical with the surface atomic density of C U ( ~ O O ) .On ~ ~ )the other hand, for Cu overlayers on Rh(100) and Ru(0001) the XPS data suggest" strong electronic interactions between the adlayers and the substrates. For CuIp/Ru(0001) and Cu,,/Rh( 100) the XPS results are consistent with a model in which the electron density of the Cu adatoms is larger than that of the surface atoms of C U ( ~ O O ) . ~ ~ 111.3. Ni Ooerlayers. Ni forms a pseudomorphic monolayer on R ~ ( 0 0 0 1 ) Results .~ of UPS and theoretical calculati~ns~~ show the presence of interface and surface states. The principal interface feature occurs at the Fermi level and is only -606 filled.sg For Ni/W(l IO), UPS studies indicate a structural transition from a pseudomorphic to a slightly distorted f a ( 111) monolayer.@' The ultrathin films of Ni on W(110) have very narrow electronic bands, which broaden with coverage above 2 ML. The smallest bandwidths are observed for the less densely packed bcc layer (eNj C 1 ML).@ At point xz- and yz-like bands are predominant. XPS studies for Ni/Ru(0001) as a function of Ni coverage6I show an increase in the Ni(2p3/,) peak position from 852.75 (eNi
r,
(59) (a) Houston, J. E.; Berlowitz, P. J.; White, J. M.; Goodman, D. W. J . Vuc. Sci. Technol. A 1988, 6, 887. (b) Houston, J. E.; White, J. M.; Feibelman, P.J.; Hamann. D. R. Phys. Rev. B 1988, 38, 12164. (60) Koziol, C.; Lilienkamp, G.; Bauer, E. Phys. Rev. E. 1990, 41. 3364. (61) (a) CampFll. R. A.; Rodriguez, J. A.; Goodman, D. W. Sur/. Sci., in pras. (b) Rodnguez, J. A.; Campbell, R. A.; Goodman, D. W. Submitted
for publication.
IV. Chemical Properties of Metal Overlayers The studies discussed in the previous section show that a metal atom supported on a matrix of a dissimilar metal can be significantly perturbed and that this perturbation can dramatically alter the electronic properties of both constituents of the bimetallic system. As a consequence, it can be expected that these electronic perturbations will modify the chemical properties of the components in the mixed-metal system. The results of many studies dealing with the chemisorption of simple molecules (CO, H2, 02, etc.) on well-defined bimetallic surfaces indicate that indeed this is the case. In the present section we discuss studies focused on the adsorption properties of mixed-metal systems, placing emphasis on the differences with respect to the chemistry of the isolated metals and on the behavior of strained metal overlayers. 1V.I. Adsorption of CO. CO is an ideal molecule to investigate the chemisorption properties of bimetallic surfaces. There is extensive information about the surface chemistry of this molecule on many monometallic substrates, and the bonding mechanism is much better known for CO than for any other simple molecule. In addition, CO is involved in many catalytic processes of industrial importance. On transition metals, the adsorbed CO molecule desorbs at temperatures in the range between 300 and 600 K. In contrast, molecular desorption of C O from noble metals is at much lower temperatures: Cu, -200 K; Ag and Au, 300 K) and an increase in the features corresponding to desorption from Cu (peaks at T C (62) Foord, J. S.;Jones, P.D.Surf Sci. 1985, 152/153,487. (63) Peebles, H. C.; Beck,D. D.; White, J. M.; Campbell, C. T. Sur/. Sci. 1985, 150, 120.
The Journal of Physical Chemistry, Vol. 95, No. 11, 1991 4201
Feature Article
-/-
I
I
~ C U ( l O 0 (rurfrcr+bulk) )
Cu/R h(lO0)
032.70 0
1
I
,
2
3
,
4
,
,
6
,
6
,
7
,
a
,
0
+
J
10'20
Cu COVERAGE, ML Figure 6. C U ( ~ P ,peak ~ ~ )position as a function of Cu coverage: (a) Cu/Rh(100), (b) Cu/Ru(0001), and (c) Cu/Re(0001). (d) Relative Cu(2p,/l) XPS binding energies of bulk Cu, surface atoms of Cu(100), and Cu monolayers on Rh(100), Ru(0001), and Re(0001) (from ref 48).
1
i@
2095
I A
ecu= 0.59 ML
pjy,, ,
-,
,
r,L
ANNEALT = 100 K
Figure 7. TPD results correspondingto CO adsorbed to saturation levels on clean Rh(100) and on surfaces containing submonolayer (a) and multilayer (b) coverages of Cu (from ref 25).
2200
2100
2000
1900
1800
WAVENUMBER (cm-')
Flpre 8. IR spectra of CO on Cb,s9/Rh(100). Cu was deposited onto Rh(100) at 100 K and flashed to the indicated temperature. Ten langmuirs of CO was dosed after each anneal, and the IR spectrum was collected at 85 K (from ref 64).
300 K). The peak at -290 K can be assigned to CO desorbing from the edges of Cu islands. The trends observed in Figure 7 for CO/Cu/Rh( 100) are similar to those found in the CO TPD spectra of C O / C ~ / R u ( 0 0 0 1 )and ~ ~CO/C~/Re(0001).'~ Data of Fourier-transform infrared reflection-absorption spectroscopy (IRAS) for the CO/Cu/Rh(100) system show adsorption of CO on a-top and bridge sites of Rh and on a-top sites of C U . The ~ morphology of the Cu,o.6/Rh(100) surfaces is very temperature dependent." At 100 K, the results of Figure 8 indicate adsorption of C O on disordered 2D domains of Cu (peak at -2120 cm-I). Flashing the surface to higher temperatures produces a progressive formation of large 2D islands of Cu pseudomorphic to the substrate (peak at -2095 cm-1).64 An IRAS study of the mobility of CO on C u , , / R h ( 100) indicates that at -80 K on nonsaturated surfaces CO is adsorbed on Cu and Rh.65 An increase of the temperature to 250 K induces migration of CO from Cu (weak m e t a l 4 0 bond) to Rh (strong metal-CO bond). The IRAS results discussed here for CO/ Cu/Rh( 100) are very similar in many aspects to those seen for CO/Cu/Ru(OOOI).~
The data of Figure 7 for CO/Cu,,o/Rh(lOO) show a CO TPD maximum increased by approximately 70 K with respect to the peak maximum on C U ( ~ O O ) . * ~For ~ ~ 'CO adsorption on Pdl.o/ W(l the CO TPD spectra exhibit a desorption temperature that is 180 K lower than the corresponding value of Pd( 100). The variations in the CO TPD maxima suggest that the electronic properties of a metal atom bonded to a dissimilar metal can be significantly perturbed. Figure 9 displays a qualitative correlation between the increase or decrease in CO desorption temperature and relative shifts in surface core-level binding energies (Pd(3ds 2) Ni(2p3& or Cu(2p3/,); all measured before adsorbing CO)h*s+ In general, a reduction in binding energy of a core level is accompanied by an enhancement in the strength of the bond between C O and the supported metal monolayer. Likewise, an opposite relationship is observed for an increase in core-level binding energy. The correlation observed in Figure 9 can be explained in terms of a model based on initial-state effects!' The chemisorption bond
(64) He, J. W.;Kuhn, W. K.; Leung, L. W.; Goodman, D. W. J . G e m . Phys. 1990, 93, 1463. (65) Kuhn, W. K.; He, J. W.; Goodman, D. W. Chem. Phys. Leu. 1990, 172, 331.
(66)Hoffmann, F. M.;Paul, J. J . Chem. Phys. 1987,86, 2990. (67)Rodriguez, J. A.; Campbell, R. A.; Goodman, D. W. J . Phys. Chem. 1990,94, 6936.
-
-
~~
Rodriguez and Goodman Pdpa(110) T
~~~;~,~
PdMI(110) -t + Pd/Ru(0001)
Cu/Ru(0001)
~
;
~
~
v
~
-c .,50K+&
-31:;'
1.02 ML Cu
0.63 ML
939 937 935 933 931 929
cu
x 7.6
939 937 935 933 931 929
BINDING ENERGY, eV
Fwre 10. Effects of H adsorption on the Cu(2p3p)spectra of Cu/Re(OOO1) surfaces. The Cu overlayer was vapor-deposited at -350 K and annealed to 5 0 0 K before dosing 3 0 langmuirs of H2at -150 K (from ref 58).
(68) (a) Bagus, P. S.;Hermann, K.;Bauschlicher, C. W.J . Chem. Phys. 1984.81, 1966. (b) Hermann, K.;Bagus, P. S.;Nelin, C. J. Phys. Reu. B 1987. -. - . 35. - - , 9467. . .- . . (69) Rogozik, J.; Dose, V. Sur/. Sci. 1986,176,L847. (70) Egelhoff, W.F. Phys. Rev. B 1984, 29, 4769. (71) Citrin, P. H.; Wertheim. G. K.;Baer. Y. Phys. Rev. B 1983,27,3160. (72) Egelhoff, W. F. J . VUC.Sci. Technol. 1982. 20. 668. (73)Salmeron, M.;Ferrer, S.;Jazzar, M.;Somorjai, G. A. Phys. Reu. B
.
1983, 28, 1158.
(74) Rodriguez, J. A.; Campbell, C. T. J . Phys. Chem. 1987, 91,2161. (75) (a) Lindholm, E.; Li, J. J . Phys. Chem. 1988,92,1731. (b) Tronc, M.;Azria, R.; Le Coat, Y. J . Phys. B 1980, 13. 2327. (76)Shustorovich, E.; Baetzold. R. C. Science 1985, 227,876.
direction of the CO-induced shifts is consistent with a reduction in the electron density of metal adatoms upon CO adsorption. This decrease in electron density is probably a consequence 0P7vS8(1) transfer of electrons from the metal overlayer into the CO(2r) orbitals (7-back-bonding) and (2) charge transfer from the metal adlayer to the substrate (induced by a repulsive interaction between the admetal a-charge and electrons in the CO(5u) orbital). IV.2. Adsorption of H2. The dissociative adsorption of H2 on bimetallic surfaces is an important step in many catalytic reactions. On transition metals, dissociation of H2occurs readily, producing hydrogen adatoms that recombine and desorb as H2 at temperatures between 300 and 900 K. On the other hand, a substantial activation barrier for H-H bond cleavage makes difficult the dissociation of H2on noble-metal surfaces. Studies for adsorption ) ~
ea a
b
1
0.4 YL N l / W (100) 0.1 YL 111
YL N I / W (100) +A 0.7 0.8 YL N I / W (100) 1.1 YL N I / W (100)
' 0
0 0.4 YL Nl
1.2 YL NI
1d4 1 1.5
I
I
I
1.7
1.9
2.1
1.6
1.7
1.8
2.0
1.9
2.1
1000/T (K-')
1 0 0 0 / T (K-')
Figure 14. (a) Arrhenius plots of the rates of ethane hydrogenolysisvs Ni coverageon W(l lo), at a total pressure of 100 Torr (H2/C2H6= 99) (from ref 86). (b) Comparison of the methane production from ethane over scveral Ni coverages on W( loo), Ni on W(1 IO), (in the limit of zero Ni coverage), and a Ni(100) catalyst (from ref 86). The total reactant pressure was 100 Torr (H2/CZH6= 99).
Y 3.5
::h,, 1 3.5 r
v)
Re/Pt(lII)
701
I
0
0
1.o
\
0.5
\
EPITAXIAL ALLOYED
T
" I 7
E
air
RHENIUM COVERAGE (ML)
L
\
1.o
2.0
3.0
PLATINUM COVERAGE (ML)
Figure 15. Ethane hydrogenolysis activity of Re/Pt( 11 1) (a) and Pt/ Re(0001) (b). Reaction conditions: Pahm = 10 Torr, PH2= 100 Torr, and T = 573 K (from ref 92).
hydrogenolysis on Ni/W(100) surfaces are shown in Figure 14b.@ In contrast to the data on W( 1lo), the turnover frequencies on Ni/W( 100) are independent of Ni coverage. Also shown in Figure 14b are data from a Ni(100) catalyst9' and the rates from Ni/ W(110) in the limit of zero Ni coverage. For all these systems, the activation energies and turnover frequencies are essentially identical. Ethane hydrogenolysis is a structure-sensitive reaction?' Kinetic studies show a much higher hydrogenolysis activity for Ni( 100) than for Ni( 111)?l This difference was attributed to the spacing of the high-coordination sites on the crystal^.^^ Ni( 100) has a spacing of 2.5 A between 4-fold hollow sites, while Ni( 111) has a spacing of 1.4 A between 3-fold hollow sites. The greater spacing on Ni( 100) facilitates breaking of the ethane C-C bond.91 This suggests that if the spacing between Ni atoms is sufficiently large, the rate of ethane hydrogenolysis will be large and independent of Ni structure. This is the case for Ni/W(100), where the distance between high-coordination Ni sites is -2.7 A and the rates are independent of Ni coverage. The strained Ni overlayer on W( 100) is more "open* than Ni on W( 110). A study of ethane hydrogenolysis over Cu/Ru(0001)80 shows a decrease in the catalytic activity of the surface with Cu coverage. The effect of Cu at low coverages is to simply block active Ru ~~~~
~~
~~~
~
~
~
I
40L
0 \O
0.0
3.0
+ 2 CH4
m 50
0.0
2.0
I C2Hs + Hz
0.5
1.0
1.5
2.0
2.5
3.0
'NI
Figure 16. Rate of ethane hydrogenolysis as a function of surface Ni coverage (in monolayers) on Pt( 111) at T = 580 K,H2/C2H6= 100, and PT =
101 Torr. The rate was normalized by using the atom density of the Pt(l11) substrate (1.505 X atoms an-*), assuming that each Ni adatom blocks a Pt surface atom (from ref 8 5 ) .
sites on a one-to-one basis, due to the formation of 2D Cu islands.s0 Specific rates of ethane hydrogenolysis on Re/Pt( 111) and Pt/ Re(0001) catalysts are shown in Figure 15.92 The addition of 0.6-1 ML of Re to Pt(ll1) results in surfaces with a hydrogenolysis activity much larger than that of Pt( 111) or Re(0001). High catalytic activity is also observed for Pt
