J . Phys. Chem. 1994,98, 5962-5965
5962
Interaction of Carbon Monoxide with Bimetallic Overlayers A. K. Santra and C. N. R. Rao' Solid State and Structural Chemistry Unit, Indian Institute of Science, Bangalore 560 01 2, India Received: January 4, 1994; In Final Form: April 7, 1994'
Interaction of carbon monoxide with a few chosen bimetallic overlayers has been investigated along with the core-level binding energies of the deposited metals by employing X-rays as well as UV photoelectron spectroscopies. Core-level binding energies of the deposited metals around monolayer coverages (0 1) are significantly different than those at high coverages or of the pure metals. Bimetallic overlayers such as Ni/Au and Cu/Pt showing large negative shifts in the surface core-level binding energy of the deposited metal interact strongly with carbon monoxide. In the case of Ni/Au (0Ni 0.85), CO dissociates around 280 K. In contrast to this behavior, the interaction of CO with P d / M o or W, showing large positive shifts in the surface core-level binding energy, is very weak, and the CO desorption temperature is much lower than that from the clean Pd metal surface. The CO desorption temperature generally increases as the surface core-level shift of the deposited metal becomes more negative; the separation between the (5a 1 ~ and ) 4 a levels of CO also increases in this direction. These results suggest that the variation in the strength of interaction of CO with bimetallic overlayers is a chemical manifestation of the shift in the surface core-level binding energies of the deposited metals a t monolayer coverages.
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Introduction Surfaces of bimetallic systems have attracted much attention because of their interesting electronic and catalytic properties. In this context, investigations of well-characterized bimetallic surfaces prepared by depositing one metal onto the surface of another assumes considerable significance. Recent electron spectroscopic studies of Goodman and co-workers1s2on the surfaces of bimetallic systems have shown that a metal supported on a dissimilar metal is electronically purturbed and that charge transfer plays an important role in such surface metal-metal bonds. According to these workers, the formation of metalmetal bonds between two different elements causes a flow of electron density to the element initially having a larger fraction of empty states in its valence band, contrary to the behavior encountered in the bulk alloys. That the shift in core-level binding energies of the deposited metal found in such bimetallic systems is not due to simple relaxation and related effects of the photoemission process is borne out by the observation of changes in the desorption temperature of carbon monoxide adsorbed on these systems.1.2 Wertheim and Rowe3 have suggested that besides charge transfer, other factors such as reference level, hybridization, and final-state screening should also be considered in explaining the core-level shifts in bimetallic surfaces. While these factors have to be taken into account, it seems likely that chemical consequences of the electronic purturbation between the two metals in bimetallic overlayers, such as the desorption temperature of CO and C-0 stretching frequency,*,4would result mainly from charge transfer. Such modification in the chemical reactivity of a metal when deposited on a dissimilar metal is an important surface phenomenon of possible technological significance as well. Since the magnitude of interaction of C O with bimetallic surfaces is reflected in the shift of the core-level binding energy of the deposited metal, AE,, we considered it most interesting to investigate bimetallic systems where the deposited metals exhibit large negative shifts of the core-level binding energy. We would expect such surfaces to interact strongly with CO. We have investigated a number of bimetallic overlayers where a wide
* To whom correspondenceshould be addressed. @
Abstract published in Advance ACS Abstracts, May 1, 1994.
variation in the magnitude of electronic purturbation between the two dissimilar metals was expected. The systems studied are Pd/Mo, Pd/W, Ni/W, Pd/Ag, Cu/Au, Ni/Cu, Cu/Pt, Ni/Au, and Ag/Pd, of which the last four are reported to exhibit large negative AE, values of the deposited metals.5" In contrast, the Pd/Mo or W system' shows a large positive shift of AE,. We have employed X-ray and UV photoelectron spectroscopies (XPS and UPS) to study the interaction CO with these surfaces. It may be recalled that the UV photoelectron spectrum of molecularly chemisorbed C O on surfaces of Ni and Pd shows characteristic bands due to the (Sa + la) and 4a molecular levels with a separation of -3.1 eV between the two states. The energy separation between the two states, A[4a - (5a lr)], is known tovary with strength of interaction between CO and the metal.9-11 The C(1s) spectrum in the XPS of molecularly adsorbed CO shows a feature around 285 eV, distinctly different from the dissociatively adsorbed (carbidic) species. The present study clearly demonstrates that the AE, values of the deposited metals can be directly related to the strength of interaction of C O with the surfaces, large negative A& values being associated with bimetallic overlayers showing strong interaction.
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Experimental Section Electron spectroscopic measurements were carried out with a VG ESCA 3 M K I1 spectrometer fitted with a sample preparation chamber at a base pressure of 2 X 10-lOTorr. A1 Ka (1486.6 eV) and H e I1 (40.8 eV) radiations were employed for the XPS and UPS measurements, respectively. High-purity (>99.9%) polycrystalline metal foils from Aldrich Chemical Co. were used for these studies. Polycrystalline foils were cleaned in-situ in the preparation chamber of the instrument by Ar+ ion etching and annealing until clear surfaces devoid of carbon and oxygen were obtained. High-purity metals weredeposited at room temperature under ultrahigh-vacuum (UHV) conditions on these freshly cleaned metal surfaces in the preparation chamber by means of resistive evaporation of high-purity metals wound around a thoroughly degassed tungsten filament. In order to quantify the surface coverage, 8, of the metal deposited, we have made use of method of Seah and Dench,12 which estimates from the internal
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0022-3654/94/2098-5962%04.50/0 0 1994 American Chemical Society
The Journal of Physical Chemistry, Vol. 98, No. 23, 1994 5963
Interaction of CO with Bimetallic Overlayers
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Figure 2. UP difference spectra of CO adsorbed on (a) Pd/W and (b)
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Pd/Mo at different Pd coverages (Ow).Insets show (a) C(1s) signals at different temperatureswhen Ow 1.2 and (b) variationin the normalized C(1s) intensities with temperature for different 6pd values. 0.0
TABLE 1: Core-Level Binding Energy Shifts of the Deposited Metals around Monolayer Coverages system AE:, eV AE*c,eV system AE:, eV Pd/Mo 0.60 0.90 Cu/Au -0.25 0.85 Ni/Au Pd/W 0.55 -0.70 0.35 Ni/W 0.00 Cu/Pt -0.65 AgJPd -0.70 0.00 PdJAg -0.30 Ni/Cu -0.35 0.00
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Ni/Cu
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Figure 1. Variation in the core-level binding energy of the adsorbed metal (relative to the bulk metal), AEcrin the various bimetallicsystems studied against the metal coverage, 0 (ML).
intensities of core-level photoemission peaks as given by
where I t and I", are the intensities from the pure bulk standard for the adsorbate and substrate metals, respectively, rA(ES)and A:(&) are respectively backscattering contributions and the mean free path of substrate metal@) electrons in the matrix of adsorbed metal (A), and for XPS r's are taken equal to zero. 9 is the angle of collection of the photoelectrons, taken to be 45O here. Themean free path, X,values are taken from the1iterat~re.l~ The surface coverage 0 is given in the number of monolayers. It is to be noted that calculations based on XPS intensities lead to errors in 0 values if the bulk standards are not taken under the same operating conditions and surface texture. In the present experiment, we have taken bulk standards from the respective polycrystaline foils under the same operating conditions. Bimetallic overlayer surfaces at different metal coverages (0) were exposed to purified CO in the preparation chamber (1 langmuir = 10-6 T0rr.s). The adsorbed species were examined by C(1s) spectra as well as ultraviolet photoelectron spectra. Results and Discussion
In Figure 1 we have plotted shift in the binding energy of the adsorbed metal (relative to the bulk metal), AE,, in the various bimetallic systems studied against the metal coverage, 8. We see that the AE,value varies markedly with B in every case, reaching a constant value around B 1.0, except in Pd/Ag. The AE, value at monolayer coverages of the deposited metal (0 l), AE',, varies from a highly positive value to a highly negative value in the different bimetallic overlayers studied by us. For
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example, the Pd/Mo and Pd/W systems show shifts of -0.60 eV while Ag/Pd, Ni/Au, and Cu/Pt systems show shifts of around -0.70 eV. In the case of Pd/Ag, there is an increase in AE, for 0 < 1, possibly due to the alloy formation at the interface. In Table 1 we have listed the AE, values of the deposited metal around monolayer coverages, AEL, in the different bimetallic overlayers studied by us. In Table 1 we have also listed the shifts in surface core-level binding energies, AE*,,obtained after applying the surface core-level shift correction.14JS In Figure 2 we show the difference spectra of CO adsorbed on Pd/ W and Pd/Mo, where the AE*,(Pd) values are highly positive (Table 1). The spectra in the two cases are quite similar. The separation between the (5u l n ) and 4 a levels remains nearly the same (3.1 e v ) with the change in 6pd. In the inset of Figure 2a, we show the C(1s) feature of the molecularly chemisorbed CO on Pd/W at 80 K. This feature disappears around 310 K when Bpd 1.2. The desorption temperature of CO on clean Pd metal is around 450 K. In the inset of Figure 2b, we have shown the variation of the desorption temperature of CO in the Pd/Mo systems for different epd values. From these results we conclude that the magnitude of interaction between Pd and CO in Pd/Mo or w decreases as 6pd approaches unity. In Figure 3 we have shown the difference U P spectra of CO adsorbed on Pd/Ag a t different Pd coverages. In the Pd/Ag system, AE*,is zero; however, the separation between (56 In) and 4u levels of CO increases with the decrease in Ow. For approximately monolayer coverage (0pd 1.2), where we observe a AEc of -0.3 eV, the A [4u - (50 + ln)] is 3.7 eV (compared to 3.1 eV on the surface of Pd metal or at large epd in the Pd/Ag system). This observation suggests that the CO-Pd interaction strength increases with decrease in 6pd. Accordingly, the CO desorption temperature is higher on Pd/Ag (8, 1.2) compared to clean Pd metal by -50 K (see inset of Figure 3). Notice the presence of the C(1s) signal due to molecularly adsorbed CO at 450 K on this Pd/Ag surface. The Ni/W overlayer system show a AE*,value equal to 0.35 eV. However, we do not find any change in energy separation betweenthe4uand(5u+ In)levelswith&i(Figure4a). Neither do we see any change in the desorption temperature compared
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The Journal of Physical Chemistry, Vol. 98, No. 23, 1994 'Pd'l
Santra and Rao
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Figure 5. (a) UP difference spectraof CO adsorbed on Ni/Au at different spectrum with temperature is shown (b) for 2.5.
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e N i values. Change in C( 1s) 6Ni 0.85 and (C) for eNi
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Figure 3. UP difference spectra of CO adsorbed on Pd/Ag at different
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Owvalues. InsetshowsthechangeintheC(1s) spectrumwithtemperature, for epd 1.2. eNl=i 2
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Figure 6. UP difference spectra of CO adsorbed on (a) Cu/Pt and (b) Cu/Au at different values. Insets show the changes in the C(1s) spectra with temperature.
Figure 4. UP difference spectra of CO adsorbed on (a) Ni/W and (b) Ni/Cu at different Ni coverages (eNi). Insets show the change in C(1s)
(Figure 5c) where the 285-eV feature due to molecular CO persists even at 300 K. The observation of dissociation of CO on the Ni/Au at eNi 1 is indeed a graphicdemonstration of thesurface modification brought about in bimetallic overlayers. Furthermore, this observation also gives a chemical meaning to the shifts in surface core-level binding energies in the metal overlayers. Interaction of C O with the Cu/Au and Cu/Pt surfaces appears to be similar to that found in the Pd/Ag or Ni/Cu systems. In both the cases, the intramolecular shake-up satellite of CO in the U P spectrum disappears with decrease in Cu coverage (Figure 6). It may be recalled that the U P spectrum of C O adsorbed on a clean Cu metal surfaceI6shows a shake-up satellite (- 13.6 eV) due to excitation to the 27r* orbital accompanying 4 a ionization. Disappearance of the shake-up satellite indicates that the interaction between Cu and CO is stronger in the metallic overlayer. Accordingly, the C O desorption temperature is considerably higher in both the Cu/Pt and Cu/Au systems. The Cu/Pt system, with a larger negative value of AE*, (-0.5 eV), shows a higher C O desorption temperature (-320 K) than the Cu/Au system (-290 K) with a smaller AE*,(-0.10 eV). The absence of dissociation of CO on the Cu/Pt (Ocu 1) surface, although the AE*,is comparable to that in the Ni/Au system, may be because the interaction of CO with clean Cu is initially so weak that the surface modification can only make it strong enough to increase the desorption temperature. Rodriguez et ~ 1 . l have . ~ used the peakdesorption temperatures to determine Td values since such values are directly related to
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spectra with temperature. to the case of the clean Ni metal (see inset of Figure 4a). In the case of the Ni/Cu system, where AE*,is 0.0 eV, however, there is an increase in 4 a 4 5 a l7r) separation with the decrease in ONi(Figure4b). Theseparationis 3.5 eVwhenBNi 1.1 compared to 3.3 eV when e N i 2.1. The C o desorption temperature is also higher than that of the clean Ni metal surface. From the inset of Figure 4b we see that the molecularly adsorbed CO, at low Ni coverage (&i l.l), does not desorb fully even at 450 K. The Ni/Au overlayer system is specially interesting. This system shows a large negative value of U * ,(-0.35 eV) and also major changes in the nature of the adsorbed CO species. From the difference UP spectra of CO adsorbed on the Ni/Au surfaces (Figure 5a), we see that the separation between the 4a and (5u + IT) levels increases with the decrease in e N i at 80 K. The separation is 3.7 eV when ONi is 0.85. On warming this surface ON^ 0.85) to 280 K, we only see a feature around 7.0 eV in the UP spectrum, suggesting dissociation of CO. The C(1s) spectra also confirm the occurrence of dissociation of C O on the ON^ 0.85 surface. The C(1s) spectrum at 80 K (Figure 5b) shows the 285-eV feature due to molecularly adsorbed CO. On warming the surface to 280 K, we see a feature at 283 eV due to carbidic carbon arising from the dissociation of CO. This behavior is to be compared with that of the BNi 2.5 surface
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The Journal of Phpical Chemistry, Vol. 98, No. 23, 1994 5965
Interaction of CO with Bimetallic Overlayers
metal. In Figure 7a we have plotted the energy separation between the 4a and (5u l?r) levels against the change in core-level binding energy, AE,,in the Pd/Ag, Ni/Cu, and Ni/Au systems. The plots clearly show that the A[4a - (5a l r ) ] energy separation increases with decrease in AE, accompanying the decrease in metal coverage, 0 (see Figure 1 and Table 1). In Figure 7b we have plotted the change in the CO desorption temperature, ATd, relative to the clean metal surface, against the shift in the surface core-level binding energy of the deposited metal at the monolayer coverage, M*,.The AT, value becomes increasingly positive as AI?*, becomes more negative. These results show that AE*,has a chemical meaning. The plots in Figure 7 and the observation of dissociative adsorption of CO on the Ni/Au surface appear to be chemical manifestations of the changes in core-level binding energy of the deposited metal in the bimetallic overlayers.
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References and Notes
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0.0 0.4 0.8 A E c (ev) Figure 7. (a) Variation in the separation between 4u and (5u 1 ~ ) levels, A[4u - (5u IT)], with the change in core-level binding energy, AE,,in Pd/Ag, Ni/Cu, and Ni/Au systems. (b) Variation in the CO desorption temperature, ATd, relative to the clean metal surface, against the shift in the surface core-level binding energy, AE*c. -0.4
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the adsorption energies. The XPS detection technique employed here slightly overestimates Td, but the changes in Td are however properly reflected. Thus, we are able to relate the changes in the desorption temperature of CO as well as in the separation between the4aand (Sa+ lr) levelsofCOwithA&valuesofthedeposited
(1) Rodriguez, J. A.; Goodman, D. W. J. Phys. Chem. 1991,95,4196 and references cited therein. (2) Rodriguez, J. A.; Goodman, D. W. Science 1992, 257, 897 and references cited therein. (3) Wertheim, G.K.; Rowe, J. E. Science 1993, 260, 1527. (4) Rodriguez, J. A.; Goodman, D. W. Science 1993, 260, 1527. (5) Motoyoshi, Y.; Kishi, K.;Ikoda, S. J . Catal. 1982, 77, 200. (6) Shek, M. L.; Stefan, P. M.; Lindau, I.; Spicer, W. E. Phys. Rev. B 1983, 27, 7301, 7277. (7) Stiener, P.; Hiifner, S . Solid State Commun. 1981, 37, 279. (8) Pervan, P.; Milun, M. Surf.Sci. 1992, 264, 135. (9) Rhodin, T. N.; Brucker, C. F. Solid State Commun. 1977,23,275. (10) Rao, C. N. R.; Vijayakrishnan, V.; Santra, A. K.; Prins, M. W. J. Angew. Chem., In?. Ed. Eng1:7992, 31, 1064. (11) Rajumon, M. K.; Hedgde, M. S.; Rao, C. N. R. Catal. Lett. 1988, 1, 351. (12) Seah, M. P.; Dench, W. A. Surf.Interface Anal. 1979, I , 2. (13) Penn, R. D. J. Electron Spectrosc. Relat. Phenom. 1976, 9, 29. (14) Egelhoff, Jr., W. F. Surf. Sci. Rep. 1987,6,253 and references cited therein. (15) Citrin,P.H.; Wertheim,G. K.;Baer,Y. Phys. Rev.B1983,27,3160. (16) Rao, C. N. R.; Kamath, P. V.; Prabhakaran, K.; Hegde, M. S . Can. J. Chem. 1985,63, 1780.
