Nature of Metal-Metal Bonding in Mixed Metal Catalysts

Guni, L u al. (Editors),New Frontiers in Caalysis Proceedings of the 10th International Congress on Catalysis, 19-24 July, 1992,Budapest, Hungary 0 1993 Elsevier Science Publishers B.V. All righa Ileserved

NATURE OF METAL-METAL BONDING IN MIXED METAL CATALYSTS R. A. Campbella,J . A. Rodrigueg and D.W. Goodmana u

aDepartment of Chemistry, Texas A&M University, College Station, TX 77843-3255, USA bChemistry Department, Brookhaven National Laboratory, Upton, NY 11973, USA

Abstract

The nature of the electronic and physical properties of ultrathin Cu and Pd films on Ta(ll0) have been studied using X-ray photoelectron spectroscopy (XPS) and temperature programmed desorption (TPD). The results indicate that monolayers of Cu and Pd on Ta(ll0) have lower electron densities than that measured for the surface atoms of Cu(100) and Pd(100), respectively. By comparison of the present results with those for other Cu, Ni and Pd films, correlations are found among the electronic perturbations of the monolayer films, the metal-substrate cohesive bond strength and the chemisorption properties of the monolayer films toward CO. A simple model is described to explain the direction and magnitude of the electronic perturbations. Surface electronegativity is defined and is found to be quite different from electronegativities in bulk alloys. The electronic state of the metal overlayer is determined to be an excellent indicator of the ability to chemisorb CO. 1. INTRODUCTION

The catalytic stability, selectivity and/or activity of a bimetallic catalyst are known in many cases to be superior to the anaolgous properties of a monometallic catalyst [ 11. There has been much emphasis on understanding the changes that occur in the structural, electronic and chemical properties of catalysts upon the addition of the second metal through studies employing model catalysts [2-41. The model catalysts are prepared in an ultrahigh vacuum (UHV) environment by vapor depositing a second metal onto a transition metal single-crystal substrate. These bimetallic systems then can be probed by a variety of surface science techniques or the UHV system can be coupled to an elevated pressure reactor for studying reaction kinetics [3]. As part of an effort to understand the physical and electronic properties responsible for the catalytic advantages of bimetallic systems, our laboratory has been involved in a systematic study of the properties of Pd, Ni and Cu films supported on transition metal substrates. In this paper we report the results for Pd and Cu ultrathin films on Ta(ll0). The physical and electronic properties of the metal films were studied

334 by X-ray photoelectron spectroscopy (XPS) and temperature programmed desorption (TPD). By comparing the present results with those previously reported for Pd, Ni and Cu ultrathin films, we provide correlations for the chemical and electronic properties of bimetallic systems. The studies show correlations exist among the metal-substrate bond strengths, charge transfer between the overlayer and substrate, and the chemisorption properties of these systems. The results indicate that the cohesive metal-substrate bond strength is directly affected by charge transfer where there is a gain of electron density by the element initially having the greater fraction of empty states in its valence band. We present a qualitative determination of electronegativity at the surface which is found to differ greatly from that found for bulk electronegativities. It will also be shown that the electronic state of the bimetallic system is a very good indicator of its ability to chemisorb CO. 2. EXPERIMENTAL

A conventional UHV chamber with a base pressure of 1 4 x Torr was used for this work and is described in detail elsewhere (51. The system was equipped with Auger electron spectroscopy (AES), XPS, low energy electron diffraction (LEED) and TPD capabilities. Manipulator capabilities allowed for resistive heating to 1600K and liquid nitrogen cooling to 115K. An electron beam assembly allowed for flashing the sample to 2300K. The Ta(ll0) crystal was mounted by spotwelding to Ta support leads and the surface temperature was monitored by a W/5%Re-W/26%Re thermocouple spotwelded to the sample edge. The surface was cleaned by successive annealing cycles to 2500K. The cleanliness and long-range order were verified with AES, XPS and LEED. Pd and Cu were evaporated onto the crystal surface at a substrate temperature of -350K (unless stated otherwise) by resistively heating a W wire wrapped with high purity Pd or Cu wire. Following evaporation the surfaces were annealed to 5OOK to remove any contaminants, with no impurities detectable by AES or XPS. All adsorbate coverages are reported with respect to the number of Ta(ll0) surface atoms, 1.30 x 10” atoms/cm2, with one Cu (Pd) atom corresponding to B~.(~,,)= 1.0 ML. A linear heating rate of 10K/s was used in all the desorption experiments. The Pd(3d), Cu(2p) and Ta(4f) XPS spectra in section 3 were obtained with an Al Ka X-ray source. The Pd(3d5,,) and Cu(2p3/,) binding energies were referenced against the Ta(4f) peak with an experimental error of k0.03 eV. Detection was normal to the surface in XPS and AES. 3. RESULTS

The desorption spectra of Cu (m/e=63) from the Ta(ll0) surface are shown in Fig. 1. The spectra are similar to other desorption spectra of Cu from single crystal substrates [6-lo]. The feature at 1225-1325K is attributed to desorption of the first Cu monolayer. The low temperature feature at 1050-1225K, which displays a common leading edge and continues growing with increasing Cu coverage, is due to multilayer Cu

335 The results for the desorption of Pd monolayers from the Ta(ll0) surface have been previously reported [15]. These results indicate that the first monolayer of Pd on Ta(ll0) has an activation energy for desorption of 97 kcal/mol and a desorption temperature of 1540K. Pd films of greater than one monolayer exhibit TPD spectra with Pd desorption features both above and below the tempeiature observed for the Pd monolayer. These results are indicative of Pd dissolution into the Ta(ll0) substrate and alloy formation during TPD analysis. It was concluded that Pd multilayers, with the exception of the first layer, when heated to temperatures above 600K, form an alloy with the substrate. In Fig. 2b the Pd(3d5/,) binding energy is plotted as a function of Pd coverage (monolayers). The binding energies of the Pd(3d5/,) peaks are measured after the Pd was deposited at a substrate temperature of -350K with a subsequent anneal to 500K. The results are similar to those observed for the Cu/Ta(llO) system. A constant Pd(3d5,,) binding energy of 336.25 eV is observed up to -1 ML. Between 1 and 1.3 ML a slight increase in the binding energy is observed which is attributed to the relaxation of the first Pd monolayer [16]. At Pd coverages >1.3 ML, the binding energy decreases until a value of 335.60 eV is reached at -20 ML. Pd coverages of <2.5 ML were calibrated using TPD area analysis and Pd(3d)/Ta(4f) XPS intensity ratios. The 20 ML Pd coverage was approximated by the attenuation of the Ta(4f) peak. Using the results described for the Pd monolayers [16,17] we find that a supported monolayer of Pd atoms on Ta(ll0) is perturbed +0.90 eV with respect to the surface atoms of Pd(100). These results are consistent with charge transfer from the Pd overlayer to the Ta(ll0) substrate. 4. DISCUSSION

The present XPS results as well as those previously reported [7,14,16-191 have shown that monolayers of Cu and Pd are electronically perturbed with respect to the surface atoms in the Cu and Pd (100) surfaces. In Fig. 3a the Cu(2p3/,) binding energy shift is shown, with respect to the Cu(lO0) surface atoms, for the Cu,,o/Ta(llO) system and for the Cu,,,/Mo( 110) [ 181, Cu,,o/Re(OOO1) [7], Cu,,o/Ru(OOO1) [ 141 and Cu,,o/Rh(lOO) [14] systems. The results indicate that charge is transferred from the Cu overlayer to the substrate when the substrate has a valence d-band that is less than half occupied (Ta, Mo) with the perturbation increasing the less occupied the valence band. For substrates with a valence d-band more than half occupied (Ru, Rh) charge transfer from the substrate to the Cu overlayer is implied, with the perturbation increasing the more occupied the valence band. Little perturbation is observed for a valence band that is half occupied (Re). A simple model has evolved to explain the direction and magnitude of the charge transfer in Cu overlayers. This model is explained taking into consideration orbital mixing: hybridization of the occupied states of metal A (electron rich) with the unoccupied levels of metal B (electron poor), leading to a loss of “ A character in the occupied states and thus a reduction in the electron density of metal A. In this model it is considered that Cu has a 4s valence band that is half occupied. Thus, when Cu is placed on a substrate that has a less than half occupied valence band (electron poor with respect the Cu 4s valence level) there is charge transfer from Cu overlayer to the substrate. Similarly, when the Cu monolayer is adsorbed onto a substrate with a valence

336 desorption. The Cu coverages in this section are calculated using TPD area analysis with saturation of the high temperature feature being defined as oCu= 1.0 ML. The Cu(2pS,,) peak position is plotted as a function of Cu coverage in monolayers in Fig. 2a. The Cu films were prepared by vapor depositing Cu onto the Ta(ll0) substrate at -350K and then annealing to 500K. The C u ( 2 ~ XPS ~ ~ ~spectra ) were then acquired with the resulting peak positions plotted in Fig. 2a. For submonolayer Cu coverages the Cu(2p3/,) peak position remains at a constant binding energy of 933.03 f 0.03 eV. The binding energy decreases to 932.90 eV at -2 ML and remains constant with increasing Cu coverage.

7 CuTTa(ll0)

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900

1000

1100

1200

1300

,

,

1

2

,

3

::

-! 20

OVERLAYER COVERAGE, ML

1400

TEMPERATURE K

Figure 1. Temperature programmed desorption of Cu from Ta(ll0).

Figure 2. a) C u ( 2 ~ ~binding ~ ~ ) energy as a function of Cu coverage on Ta(ll0). b) Pd(3d,,,) binding energy as a function of Pd coverage on Ta( 110).

The results from Fig. 2a indicate that atoms in a Cu monolayer supported on Ta(ll0) have a higher binding energy than bulk Cu atoms. This is quite different from the relative value of surface atoms compared to bulk atoms in a Cu(100) single crystal. Measurements indicate that the surface atoms have a Cu(2p3/,) binding energy that is 0.22 eV below that found for bulk Cu atoms [ll]. The difference in binding energy between bulk and surface atoms is a consequence of the surface atoms having a lower coordination number [ 12-13]..Using the method described in reference [ 141 the Cu(2p3/,) peak position is shifted t0.30 eV with respect to the surface atoms of Cu(100). This result is consistent with charge transfer from the Cu overlayer to the Ta(ll0) substrate.

337

!mn

band that is more than half occupied (electron rich), Cu accepts charge from substrate. The magnitude of the perturbation increases (decreases) as the valence level of the substrate becomes less (more) occupied. When the substrate has a half occupied valence band there is no core-level perturbation predicted. This theory correctly explains the results in Fig. 3a. 0.8 0.6

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1280 1260 1240 1220 1200

1440

1180 1160

1420

1140

1400

Ta(ll(1) W(110) Re(0001) Ru(0001)

Figure 3. a) The Cu(2p3,J binding energy Pd(3d3/,) shift relative to Cu( 100) surface atoms for a Cu monolayer on several single crystal surfaces. b) Thermal desorption temperatures of a Cu monolayer from transition metal surfaces.

1120

Ta(ll0) Mo(ll0) Re(0001) Ru(0001) Rh(l00)

Figure 4. Shifts in the binding energy for monolayers of Pd on several substrates relative to the surface atoms of Pd( 100). b) Thermal desorption temperature of Pd monolayers from several single crystal surfaces.

The results in Fig. 4a for the perturbation in the Pd(3d,,,) binding energy, with respect to the surface atoms of Pd( loo), for Pd monolayers [ 17,191 on several substrates indicate that in all cases there is charge transfer from the Pd overlayer to the substrate. The data also indicates that the magnitude of the perturbation decreases as the substrate “moves” to the right in the transition series. This result can be explained using our electron rich/electron poor model employed for the Cu overlayer systems. In this case Pd is always considered the electron rich metal since its valence d-band is more occupied than any of the substrates. Thus, as the electron density of the valence band increases (left to right in the transition series) the magnitude of the perturbation, from a loss of Pd electron density, decreases due to a reduced fraction of empty valence level in the substrate.

338 The desorption temperature of a Cu monolayer is a good qualitative indicator of its cohesive bond strength with the substrate. In Fig 3b. the desorption temperature for a monolayer of Cu from the Ta(llO), Mo(ll0) [6], Re(0001) [7], Ru(0001) [8,9] and Rh( 100) [ 101 substrates is presented. The strongest Cu-substrate bond strengths occur when the substrate is found at the extreme left or right in the transition series. The highest desorption temperatures are found for Cu from the Ta(ll0) and Rh(100) substrates, whereas the lowest desorption temperature occur for the Re(0001) substrate. Desorption temperatures of Pd monolayers [ 1519-211 from several substrates are presented in Fig 4b. These data show a general trend of a stronger Pd-substrate bond strength for substrates with the least occupied valence band. The Pd,,o/Ta( 110) systems has a desorption temperature of 1540K, the highest observed, and the Pd,,o/Ru(OOO1) shows the lowest desorption temperature (1440K). In Figs. 3 and 4 the largest perturbation in the core-level binding energies correlates with the largest shift in the Cu and Pd desorption temperatures. The Cu overlayer systems show the largest perturbation in the Cu(2p3/,) binding energy for the Ta( 110) and Rh( 100) substrates, the same substrates that show the highest desorption temperature for a Cu monolayer. It is important to notice that in one case the Cu substrate bond is influenced by charge transferto the substrate and in the other by charge transfer from the substrate. The Pd monolayers also show a correlation between the shifts in the Pd(3d3,,) binding energy and Pd desorption temperature with the largest shifts observed for the Ta(ll0) substrate. Thus by comparing the TPD results with the XPS core-level shifts it is evident that charge transfer between the metal overlayer and substrate is an important aspect of the cohesive bond strength. The degree and direction of charge transfer in metal overlayer systems is also an indicator of the relative ability of the overlayer to withdraw or donate electrons to the substrate, that is, its electronegativity [22]. The electronegativities for transition metals, in general, are found to increase from left to right in the series, falling sharply for the noble metals [23]. Experimental and theoretical results [24-271 indicate that for three dimensional alloys, involving Ta, W, Au and Pt, charge is expected to flow from the element on the left to the element on the right of the Periodic table. The charge transfer measured with XPS allows for the determination of the electronegativities of Cu, Pd and Ni monolayers with respect to the substrate. Qualitatively the electronegativities at the surface have been determined to be as follows: Ta(ll0) > W(110) > Mo(ll0) > Re(0001) = Cu > Ru(0001) > Rh(100) > Ni > Pd, decreasing from left to right in the transition series. This general trend is exactly the opposite from that observed for bulk alloys. The origin of these modified electronegativities is not clear; however, these results indicate that charge transfer at the surface cannot be accurately predicted using bulk electronegativities. In addition to the perturbations observed for the electronic properties of model bimetallic systems, the chemical properties of the overlayer have also been found to be perturbed. For example, the desorption temperature of CO from the Pd,,o/Ta(llO) system [15] is found to be 235K below that observed for Pd(100). In Fig. 5 the shift in the CO desorption temperature for Cu, Ni and Pd monolayers is shown relative to the temperature measured for CO desorption from the corresponding (100) metal surfaces along with the shift in the core-level binding energies of the clean overlayers. It is noticed that for these systems there is a good qualitative correlation between the two sets

339 of data. This correlation is explained through the use of the Blyholder bonding model for CO to transition metals [28,29]. In this model we assume that the CO-metal bond is dominated by the donation of electron density from the occupied electronic states of the metal into the unoccupied CO 2n' molecular orbitals ( A backdonation). Recent theoretical [30-321 and inverse photoemmission [33] studies have shown that n backdonation is more important than bonding between the CO 5a molecular orbitals and the unoccupied metal electronic states. In the systems in whic'h there is charge transfer from the overlayer (Pd, Ni) to the substrate, there is an increase in the separation between the occupied valence levels of the overlayer, E* below the vacuum, and the empty 2n' orbitals of CO, E, above the vacuum level. Recent studies have shown that shifts in core-level binding energies are parallelled by similar shifts in the levels measured by ultraviolet photoemmission (UPS) [34,35] and work function measurements [36].

Pd/W(l 10) 4 Pd/Re(0001)-

Ni/W(110)

+0.80 eVI -180 K

Figure 5. Correlation between the shift in surface core-level binding energy and the shift in CO TPD maximum. The properties of the Pd, N i and Cu monolayers are compared with the corresponding values for the (100) face of the pure metals.

Ka

+ 0.80 eV -l,o

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L

Ni/Mo(l10)

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-30

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Cu/Ru(0001)

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0 Shift in XPS Surface Core Binding Energy Shift in CO TPD maximum

340 According to first order perturbation theory an increase in the E,,-E, separation should lead to a decrease in the CO-metal bond strength. The data for Ni and Pd overlayers indicate that the largest perturbations in the core-level binding energies also show the largest decrease in the CO desorption temperatures. For the Cu overlayer cases in which the substrate donates charge to the Cu, the E,,-E, separation is decreased, resulting in a stronger CO-Cu bond strength. These results indicate that the density of states near the Fermi level of the metal overlayer is, in general an excellent indicator of the ability to chemisorb CO. In Fig. 5 the Ni,,o/Ru(OOO1) and Cu,,,JRe(0001) systems do not conform to the general trends. A reduction in the core-level binding energy is not accompanied by a reduction in the CO desorption temperature. Instead an increase in the CO desorption temperature is observed. These systems have approximately the same occupation levels in the valence band and thus the admetal-substrate interaction can affect the relative position and symmetry of the admetal valence levels in the absence of charge transfer and without significantly altering the core-level binding energies. For systems involving similar valence band occupancies, the CO chemisorption properties cannot be predicted simply from the observed core-level shifts. 5. CONCLUSIONS

1) The electronic and chemical properties of Cu, Ni and Pd monolayers supported on transition metal substrates are significantly different from those found for Pd( loo), Ni(100) and Cu(100). 2) Charge transfer is an important component of the factors that contribute to the ability of the metal overlayers to form a bond with the substrate, thus affecting the metalsubstrate bond strength. 3) Surface electronegativities have been determined and are found to be different from the results observed for bulk electronegativities. 4) The electronic state of the metal overlayer is an excellent indicator of its ability to chemisorb CO. 6. REFERENCES

1 2

3

4 5

6 7

J. H. Sinfelt, Bimetallic Catalysts, Wiley, New York, 1983. J. A. Rodriguez and D. W Goodman, J. Phys. Chem., 95 (1991) 4196; and references therein. J. A. Rodriguez and D. W. Goodman, Surf. Sci. Rept., 14 (1991) 1; and references therein. C. T. Campbell, Ann. Rev. Phys. Chem., 41 (1990) 775; and references therein. R. A. Campbell and D. W. Goodman, Rev. Sci. Instrum., in press. R. A. Campbell and D. W. Goodman, to be published. J. A. Rodriguez, R. A. Campbell, and D. W. Goodman, J. Vac. Sci. Technol. A, in press.

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8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36

K. Christman, G. Ertl, and H. Shimuzu, J. Catal., 61 (1980) 397. J. T. Yates, C. H. F. Peden, and D. W. Goodman, J. Catal., 94 (1985) 576. X.Jiang and D. W. Goodman, Surf. Sci., 255 (1991) 1. W. F. Egelhoff, Phys. Rev. B, 29 (1984) 4769. D. E. Eastman, F. J. Himpsel, and J. F. van der Veen., J. Vac. Sci. Technol., 20 (1982) 609. W. F. Egelhoff, Surf. Sci. Rept., 6 (1987) 253. J. A. Rodriguez, R. A. Campbell, and D. W. Goodman, J. Phys. Chem., 95 (1991) 2477. B. E. Koel, R. J. Smith and P. J. Berlowitz, Surf. Sci., 231 (1990) 325. R. A. Campbell, W. K. Kuhn, and D. W. Goodman, to be published. R. A. Campbell, J. A. Rodriguez, and D. W. Goodman, Surf. Sci., 240 (1990) 71. J. A. Rodriguez, R. A. Campbell, and D. W. Goodman, J. Phys. Chem., 95 (1991) 5716, R. A. Campbell, J. A. Rodriguez, and D. W. Goodman, J. Chem. Phys., submitted for publication. P. J. Berlowitz and D. W. Goodman, Langmuir, 4 (1988) 1091. W. Schlenk and E. Bauer, Surf. Sci., 93 (1980) 9. L. Pauling, The Nature of the Chemical Bond, Cornell University Press, Ithica, NY, 1960. R. E. Watson and L. H. Bennett, Phys. Rev. B, 18 (1978) 6439. R. E. Watson, L. J. Swartzendruber, and L. H. Bennett, Phys. Rev. B, 24 (1981) 6211. R. E. Watson and L. H. Bennett, Phys. Rev. Lett., 43 (1979) 1130. R. E. Watson, J. W. Davenport, and M. Weinert, Phys. Rev. B, 35 (1987) 508. R. E. Watson, J. W. Davenport, and M. Weinert, Phys. Rev. B, 36 (1987) 6396. G. Blyholder, J. Phys. Chem., 68 (1964) 2722. G. Blyholder, J. Phys. Chem., 79 (1975) 756. P. S. Bagus, K. Herman, and C. W. Bauschlicher, J. Chem. Phys., 81 (1984) 1966. W. Muller and P. S. Bagus, J. Vac. Sci. Technol. A, 3 (1985) 1623. K. Herman, P. S. Bagus, and C. J. Nelin, Phys, Rev. B, 35 (1987) 9467. G. Rangelov, N. Memmel, E. Bertel, and V. Dose, Surf. Sci., 251 (1991) 965. M. W. Ruckman, V. Murgai, and M. Strongin, Phys. Rev. B, 34 (1986) 6759. G. W. Graham, J. Vac. Sci. Technol. A, 4 (1986) 760. D. R. Baer, C. W. Hubbard, and R. L. Gordon, J. Vac. Sci. Technol. A, in press.

342

DISCUSSION Q: V. Poncc (The Netherlands) Scvcral very good thcorctical papers calculated elcctron density contours for several monolayers, like Fe on W (or other similar combination). These maps do not indicate any charge transfer of importance. Even monolayers of alkali metals (and that is an extreme case) do not show an extended charge transfcr. Why do you insist that you B.E. shifts (XPS data) prove an clectron transfer'! The shifts can be explained alternatively, for example, by other initial state effects, than charge transfer.

A: D. W. Goodman Our theory is based on general [rends observed for Cu, Ni and Pd films supported on several metal substratcs. It explains in a simple and clcar way all the existing experimental data. Reliable quantum-mechanical calculations that deal in a systematic way with the bimetallic surfaces in Figures 3, 4 and 5 have not been published. A detailed explanation for the correlations in Figures 3 and 4 will require the use of a theoretical method that is able to predict cohesive energies and charge distributions at a quantilative level. It is wcll known that shift5 in core-level binding energies can depend not only on chargetransfer processes but also on other phenomena. However, for the bimetallic systems investigated in this study, the magnitude and direction of the core-level shift arc dominated by charge transfer effects. For example, in the case of Pd/Ta(llO) the direction of charge transfer predictcd by XPS agrecs with the results of work function measurements, UPS, CO-FTIR, CO-HREELS and CO-TPD. In a similar way, XPS and all the other techniques indicate that the bond of Ni on W(110) is less ionic than that of Pd [ 11, and that Pd transfers more charge to W(110) than to Ru(0001) [2]. J. A. Rodriguez and D. W. Goodman, Surf: Sci., 257,897 (1992) [I] R. A. Campbell et al., fhys. Rev. B, in press (1992) [2] 0:D. A. King (United Kingdom) I am going to attempt to come between Vladiniir and Wayne. To Vladimir, I would point out that recent intcrprctations of theoretical calculations for Cs on W surfaces at low coverages, suggesting that there is no charge transfer, are incorrect. The charge is transferred to the image plane between the ion and its image, according to the classical model: this happen to Iic between Cs and W atoms, but this does not indicate covalency, as pointed out recently [3]. And to Wayne, I would point out that in the same paper we showed that you cannot interpret core levcl shift data in terms of charge transfer alone: final state (relaxation) and environmcntal contributions can be at least as big, and in some cases these various efforts are additive, but in others they can cancel each other out. Benesh and King, Chem. Phys. L e f f .(1992) [3]

A: D. W. Goodman We cannot rule out that final-state effects contribute to the core-level shifts of the bimetallic surfaces. Ncverthelcss, the correlations in Figures 1, 3, 4 and 5 indicate that the core-lcvel shifts are dominated by initial-state effccts. Particularly interesting are the data for Cu/Rh(100) and CuDa(ll0) in Figure 3a. The change in the direction of the core-level shift cannot bc explained on the basis of final-state effects.

Q: H. Niemantsverdrict and R. van Santen (Thc Netherlands)

1) Binding cncrgy shifts caused by alloying are relatively small, therefore we wonder if strain effects in the overlaycr have an effect as well. If we take a surface layer of Pd on bulk Pd, as referenced, and attribute, for simplicity, the surface core level shift to the narrowed d-band and thc resulting negative chargc on the surface atoms, then stretching of the Pd surface laycr narrows the d-band furthcr and increases the negative charge on the surface atoms. Compression goes the other way. Do you have an idea about the magnitude of this "horizontal" contribution to the binding energy shifts of your metal layers.

343 2) Although we do not disagree that charge transfer is involved, to some extent, we are not sure that it is the only reason for the altered chemisorption of CO. It is well documented that interaction energies of adsorbed molecules with surfaces change with the geometry and the coordination of metal atoms in the surface. Strain effects alter the local geometry of the overlayer atoms from substrate to substrate. It seems to us that your interpretation in terms of changes in electron donation properties is too simple, as structural and electronic effects are inseparable. A: D. W. Goodman 1) According to your hypothesis stretching of the Pd surface layer should increase the negative charge on the surface atoms, moving their core-levels to lower binding energy. The experimental results show a different trend. A pseudomorphic monolayer of Pd on Ta(ll0) has a lower atomic density and a higher Pd 3dq2 binding energy than the surface layer of Pd(ll1) or Pd(100). The "strain effect" also cannot explain the data in Figure 3a. for Cu/Rh(100) and Cuna(l10). The surface atomic densities of Cu/Rh(100) and Cuna(l10) are both much smaller than that of Cu(lOO), but the direction of the core-level shift is different in each case. The "strain effect" plays a role in the properties of a metal overlayer, but in many bimetallic systems, the importance of this effect is secondary when compared to that of charge transfer [ 11. 2) The "strain effect" cannot explain the CO chemisorption properties of Pd and Ni overlayers [4]. For monometallic surfaces, it is well established that the strength of the metalCO bond increases when the atomic density of the surface decreases. Thus, strained overlayers should show an increase in the strength of the metal-CO bond. However, a monolayer of Pd on Ta(ll0) or W(110) has a smaller surface-atomic density and co desorption temperature than those of Pd(ll1). The effects of stretching the Pd atoms are overcome by those of charge transfer from Pd to the Ta or W substrate. The Pd-Pd separation in Pd/Re(0001) is almost identical to that of Pd(lll), nevertheless, the supported Pd monolayer shows a decrease of -100 K in the CO desorption temperature. This change in the chemical properties cannot be attributed to structural differences. R. A. Campbell et al., Surf: Sci., 240, 71 (1992) [2] Q: W. Griinert (Germany) 1) The electron transfer reported by you should be also reflected in the structure of the conductive bands of your systems. Have you performed UPS with your systems ? 2) What is the reason for the intermediate increase of the Pd FWHM when you build up a system with many monolayers of Pd on you system ?

A: D. W. Goodman 1) UPS results indicate that the shifts in the core-levels track the shifts in the valence levels [ 11 2) For systems with medium Pd coverages, the convolution of electron emissions from the metal-metal interface and top layers is responsible for the increase in the FWMH. Q: D. Chadwick (United Kingdom) Many of the metals used in your study have loss features associated with their photoelectron peaks in a metal-metal system with significant metal-metal bonding, one might expect the loss processes associated with on component to be weakly excited in the photoelectron spectrum of the other component. Have you attempted to study the loss features in the photoelectron spectra ? A: D. W. Goodman This is an interesting point and indeed may well be the case; however, we have not carried out these kinds of experiments.

344

Q: J. C. Bertolini (France) You show a nice correlation between the upward shift of Pd core levels (for Pd deposited on metals) and the CO bond strength. But, with respect to particle size effect it is exactly the contrary (i.e. for very small sizes the Pd 3d levels shift upwards while the CO bond strength is increased). Can you comment on that '!

A: D. W. Goodman We are not familiar with the details of the experiments to which you refer for very small Pd particles. The comparison of photoemission results for a solid and a small particle is not a trivial task d u e to changes in the screening of the core-hole. In a solid metal, the electron screening of the core-hole is much more effective than in a small metal particle, and as a consequence the core levels of a solid should appear at a lower binding energy than those of a particle. Thus, it is not surprising that uncoordinated atoms of very small Pd particles have a higher Pd 3d binding energy and a stronger metal-CO bond than the atoms of solid Pd.

0:D. Wang (China) What is the "degree of applicability" of your developed model of metal-metal interaction to the case of supported bimetallics of very small particle size? In particular, please comment with reference to the difference between your models of mixed metals where the individual components are in separate phases and bimetallics in the form of small particles where the individual components are well-mixed [ 5 ] . [S] J. H. Sinfelt, Bimetallic Catalysts, Wiley, New York, (1983) A: D. W. Goodman After comparing our results for bimetallic surfaces with those reported in the literature for bulk three-dimensional alloys, we have found important differences in the direction of the charge transfer between metals [I]. The nature of a heteronuclear metal-metal bond depends strongly on the geometry of the system. Bimetallic systems involve species with similar electron donor-eleclron acceptor properties, and the subtle balance that determines the flow of charge transfer between elements can be easily affected by changes in the coordination number or in the geometrical arrangement of the atoms. In principle, data for bimetallic surfaces or bulk alloys should be extrapolated with caution when predicting the behavior of small particles.