J. Phys. Chem. 1987,91, 2337-2342
2337
Photoemission Studies of Physisorbed Xenon Atoms on Ruthenium-Copper Surfaces K. S . Kim,+ J. H. Sinfelt,* Corporate Research Science Laboratories, Exxon Research and Engineering Company, Annandale, New Jersey 08801
S. Eder, K. Markert, and K. Wandelt Institute of Physical Chemistry, University of Munich, 0-8000,Munich 2, West Germany (Received: November 10, 1986)
Studies of Ru-Cu surfaces were made by applying ultraviolet photoelectron spectroscopy (UPS)to xenon physisorbed on the surfaces. Experiments were conducted on samples prepared by depositing copper on the (001) plane, i.e., the basal plane, of a ruthenium single crystal and on the surface of a ruthenium powder as well. When the amount of copper present on the surface was approximately one-third to one-half of a monolayer, the xenon photoemission spectrum obtained with the ruthenium powder differed significantly from that obtained with a well-annealed, smooth (001) surface plane. The copper appears to form islands on the smooth (001) plane, but on the surface of the ruthenium powder it is not present in this form. Consequently,copper deposited on a smooth (001) plane of ruthenium would not seem to be a good model of a ruthenium-copper catalyst. Introduction of defects into the (001) plane prior to copper deposition yields a surface which corresponds more closely to that of typical catalysts. These findings provide a reasonable way of rationalizing differences observed when studies of the effect of copper on the ethane hydrogenolysis activity of a ruthenium single crystal are compared with studies of the influence of copper on hydrogenolysis activity in actual ruthenium-copper catalysts.
Introduction Bimetallic clusters or aggregates of ruthenium and copper are In these bimetallic entities, the copper of interest in is present on the surface of the ruthenium,% since the metals are virtually completely immiscible in the bulk.' The surface properties of ruthenium-copper catalysts have recently attracted the attention of a number of scientists who approach the problem by conducting experiments in which copper is deposited on the surface of a ruthenium single crystaL8-16 Data have been obtained showing that copper-ruthenium bonds at the interface are stronger than the bonds in metallic copper.8 These data are consistent with the earlier observations on catalysts,14 namely, that interaction of copper with ruthenium takes precedence over the formation of copper crystallites or clusters which are totally isolated from the ruthenium. In seeking to understand the interaction between copper and ruthenium in more detail, one can pose questions of the following type: How is the interaction of copper with a given crystal plane of ruthenium affected by the presence of defects in the plane, and how does information on single-crystal specimens relate to actual catalyst surfaces? To obtain information on questions of this type, we have conducted studies of ultraviolet (W)photoemission of electrons from xenon adsorbed on the surfaces of two types of specimens. One was prepared by depositing copper on the (001) plane, Le., the basal plane, of a ruthenium single crystal. The other was prepared by depositing copper on the surface of a ruthenium powder. The binding energies of electrons in physisorbed xenon atoms, as determined in photoemission experiments, are sensitive to the local properties of sites in the surface. Shifts in binding energy from one site to another reflect differences in crystal field potential, relaxation energy, and work function. Since noble gas atoms do not form chemical bonds with the substrate, the shifts do not include contributions which can be characterized as chemical shifts.l 7 For measurements of photoemission from physisorbed xenon, the temperature of the specimen is typically in the range of 60-90 K. The adsorption occurs layer by layer, so that the first layer is completed before the second layer begins to grow.18 On heterogeneous surfaces, different types of sites are frequently occupied by xenon atoms in a sequential manner, the order of Present address: National Science Foundation, Chemistry Division, Washington, DC 20550.
occupation being determined by the adsorption energies. Correspondingly, shifts in electron binding energies of xenon atoms are observed when adsorption occurs on the different sites. It has been shown that the measured shifts are large enough to distinguish between different crystal planes of many metals and between defect and regular sites on single-crystal metal surfaces. Such data also provide quantitative information on the relative abundances of different surface sites.lg Consequently, UPS studies of physisorbed xenon atoms are very useful for characterizing heterogeneities in surfaces of metals of interest in catalysis.
Experimental Section The experiments were carried out in an ultrahigh-vacuum system equipped with a helium resonance lamp (He I, 2 1.1 eV) and a hemispherical 150' electron energy analyzer for UPS. The system also contained a cylindrical mirror electron energy analyzer and a coaxial electron gun for Auger electron spectroscopy ( A B ) , which was used to monitor surface cleanliness and the deposition of copper on ruthenium surfaces. The system pressure was typTorr. A detailed description of the ically lower than 1 X sample stage with both cooling and heating capabilities is given elsewhere.*O The cooling was done with a two-stage closed cycle (1) Sjnfelt, J. H.; Barnett, A. E.; Carter, J. L. US. Patent 3617518, 1971. (2) Sinfelt, J. H. J. Catal. 1973, 29, 308. (3) Sinfelt, J. H.; Lam, Y. L.; Cusumano, J. A.; Barnett, A. E. J . Catal. 1976, 42, 227. (4) Sinfelt, J. H. Acc. Chem. Res. 1977, 10, 15. (5) Helms, C. R.; Sinfelt, J. H. Surf. Sci. 1978, 72, 229. (6) Sinfelt, J. H.; Via, G. H.; Lytle, F. W. J . Chem. Phys. 1980, 72,4832. ( 7 ) Hansen, M. Constitution of Binary Alloys, 2nd ed.; McGraw-Hill: New York, 1958; p 620. (8) Christmann, K.; Ertl, G.; Shimizu, H. J . Catal. 1980, 61, 397. (9) Shimizu, H.; Christmann, K.; Ertl, G. J . Catal. 1980, 61, 412. (10) Vickerman, J. C.; Christmann, K.; Ertl, G. J . Catal. 1981, 71, 175. (11) Shi, S.-K.; Lee, H.-I.; White, J. M. Surf. Sci. 1981, 102, 5674. (12) Vickerman, J. C.; Christmann, K. Surf. Sci. 1982, 120, 1. (13) Brown, A.; Vickerman, J. C. Surf. Sei. 1984, 140, 261. (14) Yates, J. T., Jr.; Peden, C. H. F.; Goodman, D. W. J . Catal. 1985, 94, 576. (15) Goodman, D. W.; Peden, C. H. F. J . Catal. 1985, 95, 321. (16) Peden, C. H. F.; Goodman, D. W. In Catalyst Characterization
Science: Surface and Solid State Chemistry; American Chemical Society: Washington, DC, 1985; ACS Symp. Ser. No. 288, pp 185-198. (17) Kim, K. S.; Winograd, N. Chem. Phys. Lett. 1975, 30, 91. (18) Wandelt, K. J . Vac. Sci. Technol. 1984, A2, 802. (19) Miranda, R.; Albano, E. V.; Daiser, S.; Wandelt, K.; Ertl, G. J. Chem. Phys. 1984, 80, 293 1. (20) Wandelt, K.; Daiser, S.; Miranda, R.; Furth, H. J. J . Phys. E 1984, 17, 22.
0022-365418712091-2337$01.50/00 1987 American Chemical Society
2338 The Journal of Physical Chemistry, Vol. 91, No. 9, 1987 helium refrigerator. For xenon adsorption, the sample was cooled to 60 f 2 K, unless otherwise noted. It was then flashed, usually to 5 0 0 K, to remove gases adsorbed during the initial cooling, after which it was recooled more rapidly to the same low temperature. Xenon (99.99% purity) was then admitted to the system via a variable leak valve to establish a pressure in the range of 10-8-10-7 Torr. After a desired exposure, the sample was closed off from the xenon, and the pressure in the system decreased rapidly to about Torr. A spectrum was then obtained. At 60 K, xenon atoms are adsorbed layer by layer on metal surfaces and a monolayer is formed after an exposure between 10 and 15 langmuirs. In some experiments at high xenon coverages, or in experiments using adsorption temperatures higher than 60 K, spectra were taken while the system was open to the xenon at a constant pressure between lod and Torr. Shorter cooling times were used for powder samples because their surfaces adsorbed residual gases more rapidly. Consequently, the adsorption temperatures were higher (75-90 K). Pressures were measured with an ionization gauge of the Bayard-Alpert type. Thermomolecular correction factors2* were not used. The Xe 5p spectra were obtained by subtracting the spectrum of the substrate from the total spectrum of the Xe covered substrate. The Xe 5p binding energies reported in this work have an uncertainty of approximately 0.1 eV. All Xe spectra were deconvoluted on the assumption that the lines were Lorentzian. Positions and widths of 5p3,* and 5p112lines for Xe atoms adsorbed on reference surfaces were determined first. The 5p3/2 lines of Xe atoms adsorbed on Ru surfaces could be fitted well with single Lorentzians, although they consist of two adjacent Lorentzian For Xe atoms on the Cu surfaces, however, it was necessary to use two Lorentzians for fitting 5p3/2 lines. The Ru(001) specimen was a disk 6 mm in diameter and 1 mm in thickness (Materials Research, 99.99% purity). It was cut and polished by using standard procedures. The preparation of the ruthenium powder has been described in a previous paper by one of the authors (J.H.S.),in which data on the chemisorption and catalytic properties are also p r e ~ e n t e d .The ~ surface area of the powder was approximately 5 m2/g. In the present work, the powder was heated to 900 K under ultrahigh vacuum and leached with ultrahigh-purity 3 N HC1 to remove small amounts of calcium. The treatment was repeated until calcium was no longer detectable by Auger electron spectroscopy. After the calcium was removed, the powder was burnished on a 1-mm Pt foil with a sapphire bead. The Ru disk and a Pt foil covered with the Ru powder were mounted with tungsten leads spot-welded to their backs. A chromel-alumel thermocouple was also spot-welded to the backs and was used for temperature measurement. The Ru disk was cleaned by a procedurezs involving successive sputtering and oxygen treatment at elevated temperatures, and it was annealed at 1500 K. The powder sample was cleaned by successive oxidation and reduction at 870 K at O2 and H2 pressures of 10“ and Torr, respectively, until no C, C1, and S were detected by Auger electron spectroscopy. It was then annealed at 870 K in situ for 10 min. Cu/Ru surfaces were prepared by vapor deposition of copper onto ruthenium maintained at a temperature of 100 K. The source of the vapor was a sample of copper in a resistively heated tungsten basket. The deposition time was controlled by a shutter in front of the copper source. Estimates of the extent of coverage of Ru(001) surfaces by copper were made from line shape analyses of xenon photoemission spectra obtained on the surfaces. Estimates were also obtained with the use of Auger electron spectroscopy. In this method, the coverages were determined from the peak-to-peak heights of the Cu (60 eV) and Ru (273 eV) (21) Fain, S. C., Jr.; Chinn, M. D.; Diehl, R. D. Phys. Reu. E : Condens. Matter 1980, 21, 4170. (22) Waclawski, B. J.; Herbst, J. F. Phys. Reu. Lett. 1975, 35, 1594. (23) Antoniewicz, R. P. Phys. Rev. Lett. 1977, 38, 374. (24) Mathew, J. A. D.; Devey, M. G. J. Phys. C 1976, 9, L413. (25) Jablonski, A,; Eder, S.; Wandelt, K., unpublished work.
Kim et al.
2-D O . 8 PhSW 7
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14.0 L
4.6
11.0 8.0 5.0
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6.6 8.8 Blnding Energy (elf)
Figure 1. Xe 5p3,2,1,2photoemission spectra for X e atoms adsorbed on a thick Cu(l11) overlayer taken as a function of X e exposure. The Cu overlayer was vapor deposited onto a Ru(001) surface at 100 K and epitaxially grown by annealing at 640 K. An exposure of 14 langmuirs corresponds to complete monolayer coverage (L = langmuirs).
Auger lines (in the derivative mode) according to the method of Holloway.26 This method uses relative Auger sensitivity factors determined in situ for pure Cu and Ru and inelastic scattering mean free paths for Cu and Ru Auger electrons in Cu. The latter values were taken from Seah and D e n ~ h .The ~ ~ Ru(001) disk was reused after evaporating deposited Cu at 1500 K in situ.
Results In this work, we studied Cu/Ru surfaces prepared by deposition of submonolayer amounts of Cu on “smooth” and “rough” Ru(001) and on the Ru powder. We also studied a thick Cu layer, deposited on the smooth Ru(001) surface, as a reference. The smooth Ru(001) surface was prepared by annealing the clean surface extensively at 1500 K in situ. The rough Ru(001) surface was prepared by bombarding the smooth surface with 2-keV argon ions. The ion dose was equivalent to about one-half of a monolayer. Thick Cu(JJ1) Overlayer on Smooth Ru(001). It is known that Cu deposited on Ru(001) forms an epitaxial (1 11) overlayer upon annealing.8 In this work a thick overlayer of Cu was deposited at 100 K on the smooth Ru(001) surface. Since Ru was not detected by AES, it is estimated that the overlayer consisted of a t least 10 atomic layers of copper. Figure 1 shows the UV photoemission spectra of Xe adsorbed on the Cu overlayer. Prior to xenon adsorption, the Cu overlayer was annealed at 650 K for 10 min. Since the intensities of the spectral lines do not increase further with increasing xenon exposure above 11 langmuirs, the 14-langmuir spectrum represents complete Xe monolayer coverage. It is noteworthy that the Xe 5p3/2 line begins to split after an exposure of 5 langmuirs, which corresponds to a coverage of 0.3, whereas without any annealing such a splitting was not observed even after exposures up to 14 langmuirs. In a prior experiment in which the Cu overlayer had been annealed at 520 K for 10 min, the splitting was observed after an exposure of 8 langmuirs. The 5p3 splitting indicates that the adsorbed Xe atoms are in contact with other Xe atoms, forming islands on the flat Cu(ll1) surface. The Xe 5pIl2binding energy increases slightly with increasing exposure from 7.1 eV at zero coverage (extrapolated) to 7.3 eV at complete monolayer coverage. Such a small dependence of binding energy on coverage is usually observed. The low and high xenon coverages correspond t o two-dimensional gas and solid phases, respectively. When the surface is in contact with Xe at a sufficiently high pressure, a second layer of physisorbed Xe atoms is formed. The (26) Holloway, P. H. J . Vac. Sci. Technol. 1975, 12, 1418. (27) Seah, M. P.;Dench, W. A. Surf. Interface Anal. 1979, I, 2.
Physisorbed Xe Atoms on Ru-Cu Surfaces
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The Journal of Physical Chemistry, Vol. 91, No. 9, 1987 2339 I
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Figure 2. Comparison of a Xe 51)3/2,1/2photoemission spectrum for Xe atoms adsorbed on a thick Cu(ll1) overlayer after a Xe exposure of 14 langmuirs with a spectrum obtained while Xe was present at a pressure of 3 X lo-' Torr (L = langmuirs). The former is characteristicof a Xe monolayer, while the latter shows the effect of a second layer of physisorbed Xe atoms.
photoemission spectrum is then different from the spectrum characteristic of one monolayer of Xe atoms, as illustrated in Figure 2. The lower spectrum in Figure 2 was obtained for a monolayer of Xe atoms, which was formed after a Xe exposure of 14 langmuirs a t a pressure between lo-* and lo-' Torr. The upper spectrum was obtained with Xe present at a pressure of 3 X lo-' Torr during the measurement. This spectrum contains features arising from the Xe atoms in both layers. The lines at binding energies of approximately 6.4, 6.7, and 8.0 eV are due to Xe atoms in the second layer, while the line at approximately 7.4 eV and the poorly resolved features at binding energies in the range of 5.6-6.2 eV are due to Xe atoms in the first layer. Xe atoms adsorbed on an epitaxial Cu( 111) monolayer on Ru(OO1) are expected to have a higher binding energy than those on a thick C u ( l l 1 ) overlayer. The sum of the Xe 5p binding energy and the work function of the surface is constant for many metal surfaces.'* Since the work function of a Cu(11 1) monolayer on Ru(001) is 0.3 eV lower than that of the (1 1 1) surface plane of a copper crystal,14 the expected 5pl/2 binding energy of Xe atoms adsorbed on the former surface is 7.4 eV at zero coverage and 7.6 eV at complete monolayer coverage. The latter value is very close to the value observed when copper is present as a partial monolayer on the smooth Ru(001) surface, as will be seen from the data presented herein. Ru(001)Surfaces. In a photoemission spectrum for xenon physisorbed on a smooth Ru(OO1) surface, the 5p, and 5p3j2lines have maxima at energies of approximately 6.9 and 5.6 eV at monolayer coverage.28 These lines are very evident in Figure 3 in a spectrum for xenon physisorbed on the rough Ru(001) surface after 14-langmuir exposure to xenon, and also in a spectrum obtained while the system was open to xenon at a pressure of lo-' Torr. In spectra obtained after low xenon exposures (1 langmuir or lower), the lines are not observed. Instead, the spectra consist of lines which are centered at energies of approximately 7.3 and 5.9 eV, which are identified with the 5p1j2 and 5p3j2lines of xenon adsorbed on defect sites in the ruthenium surface. According to a line shape analysis, these lines do not grow when the exposure is increased above 2 langmuirs, which indicates that the defect sites are filled. After 2-langmuir exposure, the lines associated with a smooth Ru(001) surface become evident, and after 3langmuir exposure, they are the major lines. The sequential appearance and growth of the different lines with increasing xenon exposure indicate that xenon atoms adsorbed (28) Wandelt, K.; Hulsc, J.; Kilppers, J. Surf. Sci. 1981, 104, 212.
4.5
8.5 8.6 Blndlng Energy (eV)
Figure 3. Xe 5p3/2,1/2photoemission spectra for Xe atoms adsorbed on a rough Ru(001) surface prepared by bombardment with argon ions. The spectra were obtained after various Xe exposures, except for one spectrum obtained while Xe was present at a pressure of lo-' Torr. The dose of argon ions was equivalent to 0.5 monolayer. An exposure of 14 langmuirs corresponds to complete monolayer coverage (L = langmuirs).
4.72
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Figure 4. Xe 5p3p,,12photoemission spectra for Xe atoms adsorbed on a surface prepared by vapor deposition of 0.4 monolayer of copper onto a smooth Ru(001) surface maintained at 100 K. The surface was annealed at 520 K prior to Xe adsorption. The spectra were obtained after various Xe exposures, except for one spectrum obtained with Xe present at a pressure of lo-' Torr (L = langmuirs). on the Ru(001) surface at 60 K are mobile and that they preferentially occupy the defect sites. The intensities of the lines at 7.3 and 5.9 eV observed at the lowest exposures were found to increase with increasing degree of argon ion bombardment and to decrease upon annealing. These findings are consistent with the assignment of these lines to xenon atoms adsorbed on the defects. CulRu(001) Surfaces. Photoemission spectra were obtained for xenon adsorbed on Cu/Ru surfaces consisting of submonolayer amounts of copper on Ru(OO1). Two samples were investigated, Cu/smooth Ru(001) and Cu/rough Ru(001) with copper coverages of 0.4 and 0.3, respectively. The determination of these coverages will be discussed later. The samples were annealed at various temperatures up to 520 K. Spectra were obtained after various xenon exposures subsequent to each annealing. Figure 4 shows the photoemission spectra of Xe atoms adsorbed on the Cu/smooth Ru(001) surface which was annealed at 520 K for 10 min. The spectra taken after Xe exposures up to 14 langmuirs clearly show three Xe 5plI2lines at about 6.8, 7.3, and 7.7 eV. The lines at 6.8 and 7.7 eV are assigned to Xe atoms
Kim et al.
2340 The Journal of Physical Chemistry, Vol. 91, No. 9, 1987 (8)
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Figure 5. (a) Deconvolution of the 1 1-langmuir spectrum from Figure 4 (L = langmuirs). (b) Schematic representation of Xe adsorption sites.
adsorbed on the exposed part of the Ru(OO1) surface and on Cu( 11 1) islands, respectively. The line at 7.3 eV is assigned to Xe atoms associated with Ru-Cu mixed sites such as those present a t edges of Cu islands. Spectra on the unannealed surface did not show the line at 7.7 eV, indicating that the Cu islands were formed as a result of the annealing at 520 K. It is interesting to note that xenon atoms are adsorbed on Ru and Ru-Cu mixed sites in preference to being adsorbed on Cu islands. Thus, Xe atoms adsorbed during exposures up to 3 langmuirs are not present on the Cu islands. Only after an exposure of 5 langmuirs is the line assigned to xenon on Cu islands observed. The 11-langmuir spectrum can be fitted very well with the three sets of 5p3/2,1/2lines shown in Figure 5. Since an exposure of 11 langmuirs leads to an almost complete Xe monolayer, the Xe/Ru, Xe/Ru-Cu, and Xe/Cu line intensities provide a measure of the exposed Ru(OO1) area, the total perimeter of the Cu islands, and the Cu-covered area, respectively. According to the line shape analysis, the Cu coverage is 0.4. The Auger method gave a lower value, 0.15, for the coverage. Although the difference between the two values is large, a precise knowledge of the coverage is not of major importance here. We adopt the higher value obtained from the line shape analysis. Copper coverages on this basis, rather than Auger values, are used throughout the presentation and discussion of results on the Cu/Ru surfaces. If we assume that the Cu islands are circular in shape, their size (assuming they are uniform) can be determined from the Xe/Ru-Cu and Xe/Cu line intensities. W e estimate that the Cu islands consist of about 200 Cu atoms. The conclusion that the copper is present as two-dimensional flat Cu( 11 1) islands is supported by the presence of the Xe 5p3/2 doublet in the 11langmuir spectrum (see Figure 5). As discussed earlier, the Xe 5p3,2 doublet is shown by Xe atoms which form islands on a flat Cu substrate. After the spectra shown in Figure 4 were obtained, the xenon was removed from the surface by increasing the temperature to 150 K. The Cu/smooth Ru(001) surface was then cooled to 100 K and bombarded with 3-keV argon ions. The dose of argon ions was equivalent to about one-half of a monolayer. The bombardment created defects on the bare Ru(OO1) surface. As a result of the bombardment, 30% of the C u was removed, as was determined by Auger electron spectroscopy. The resulting surface was cooled to 60 K for readsorption of xenon and for obtaining xenon photoemission spectra as a function of Xe exposure. The spectra showed two Xe 5p1/*lines centered at 6.8 and 7.3 eV. The
6.55
8.55
Binding Energy (CV)
Figure 6. Xe 5p3/2,1/2photoemission spectra for Xe atoms adsorbed on a surface prepared by vapor deposition of 0.3 monolayer of copper onto a rough Ru(001) surface (L = langmuirs). The surface was annealed at 520 K prior to Xe adsorption. The spectra were obtained after various Xe exposures. One spectrum was also obtained with Xe present at a pressure of lo-' Torr.
first was assigned to Xe atoms on unperturbed Ru(001) sites, and the second to Xe atoms on Ru(001) defect sites and on Ru-Cu mixed sites. The assignment of the latter line was based on its growth mode in addition to its binding energy. It was dominant after exposures of 0.5, 1, and 2 langmuirs. After further exposures, its intensity continued to increase, but it became a minor line. The results indicated that xenon atoms adsorbed during exposures up to 2 langmuirs occupied mainly the Ru(001) defect sites. Additional xenon atoms adsorbed during further exposures occupied the unperturbed Ru(001) sites and also the Ru-Cu sites. Unlike the original Cu/smooth Ru(001) surface, the roughened surface did not show a line at 7.7 eV. This result indicated that Cu islands were not present on the ruthenium after the argon ion bombardment. The 7.7-eV line is also absent from photoemission spectra for xenon adsorbed on the specimen prepared by deposition of copper on the initially roughened Ru(001) surface. As indicated earlier, we refer to this specimen as Cu/rough Ru(001). It was annealed at 520 K for 10 min prior to xenon adsorption at 60 K. The only 5p1/2lines observed in the spectra are those at 6.8 and 7.3 eV, as shown in Figure 6. Thus, the Cu atoms preferentially occupy defect sites and do not form Cu( 11 1) islands. Since the copper coverage is 0.3 and since about 15% of the rough Ru(001) surface can be ascribed to defects, there are about two Cu atoms per defect. The defect density and copper coverage were estimated from a line shape analysis of the spectra in Figures 3 and 6, respectively. The line at 7.3 eV in Figure 6 is too intense to be assigned solely to Xe atoms on Ru defect sites (compare Figures 3 and 6). In addition, its growth mode is different. That is, its intensity increases continuously for xenon exposures up to 11 langmuirs, unlike that of the line at 7.3 eV for xenon on Ru defects in Figure 3. Moreover, it differs significantly from the line at 7.3 eV in the spectra for the Cu/smooth Ru(001) surface in Figure 4. At full surface coverage by xenon, it is more intense. Also, its intensity relative to that of the line at 6.8 eV exhibits a different dependence on xenon exposure, as can be seen readily from a comparison of Figures 4 and 6. In Figure 6 we note that it is surpassed in intensity by the line at 6.8 eV only after a xenon exposure of 3 langmuirs, whereas in Figure 4 the line at 7.3 eV is already less intense than the line at 6.8 eV after an exposure of 1 langmuir. These results indicate more extensive interaction of adsorbed Xe atoms with Ru-Cu mixed sites on the Cu/rough Ru(001) surface than on the Cu/smooth Ru(001) surface. The simplest explanation for this is a higher concentration of Ru-Cu mixed sites on the rough Ru(001) surface, as a consequence of the more highly dispersed nature of the Cu on this surface. The
Physisorbed Xe Atoms on Ru-Cu Surfaces
The Journal of Physical Chemistry, Vol. 91, No. 9, 1987 2341
Torr
10-8 Torr
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5x10-' 3x104 4.0
6.0 Binding Energy
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Figure 7. Xe 5p3/2,1/2photoemission spectra for Xe atoms adsorbed on a clean Ru powder at 80 K. The spectra were obtained with Xe present at various pressures. The spectrum taken at a pressure of 1 X 10" Torr
corresponds to complete monolayer coverage by Xe. higher degree of dispersion of the copper is presumably due to the presence of the defects, which interfere with the formation of large Cu islands. Ru Powder Surface. Spectra of xenon physisorbed on the ruthenium powder are shown in Figure 7 . Prior to the experiments, the powder was annealed at 650 K. The spectra were obtained with xenon present at various pressures. The spectrum taken at 1Od Torr corresponds to complete monolayer coverage of the ruthenium by xenon. At the lowest pressure, the dominant lines are Xe 5pIl2and 5p312lines at 7.3 and 5.9 eV, respectively, as was also observed for the rough Ru(OO1) surface after the lowest Xe exposure. The powder surface would be expected to consist of more than one crystal plane. That is, nonbasal planes would be present in addition to the Ru(001) basal plane. Since the 5p binding energies of xenon physisorbed on a metal crystal are higher for the less densely packed planes,'* one would expect them to be higher on the nonbasal planes of ruthenium than on the (001) plane. The only lines in Figure 7 which appear at energies higher than those characteristic of the smooth Ru(001) plane are those at 7.3 and 5.9 eV, which have already been identified with defect sites on Ru(001). We suggest that these lines may also contain contributions from Xe atoms adsorbed on nonbasal planes of the ruthenium. This would mean that the strength of interaction of xenon with sites on nonbasal planes is comparable to that with the Ru(001) defect sites. According to a line shape analysis of the spectrum in Figure 7, the regular sites on Ru(001) account for about 70% of the total surface. The remaining sites are attributed to defects on Ru(001) and sites on the nonbasal planes. Such a high contribution from the basal plane is possible if primary particles share mainly nonbasal planes in the formation of aggregates, as can be inferred from transmission electron microscopy data on the powder. Then the exposed surface could consist primarily of terraces of Ru(OO1) and steps, the latter being associated with exposed parts of the nonbasal planes joining primary particles. Cu/Ru Powder Surface. Spectra of xenon physisorbed on the surface existing after deposition of copper on the ruthenium powder are shown in Figure 8. The powder was kept at a temperature of 100 K during the deposition and was subsequently annealed a t 650 K prior to the low-temperature xenon adsorption. The copper coverage was estimated to be 0.5. The spectra in Figure 8 were obtained with xenon present at various pressures. They bear a much closer resemblance to the spectra in Figure 6 for the Cu/rough Ru(001) surface than to those in Figure 4 for the Cu/smooth Ru(001) surface. The presence or absence of a line at 7.7 eV is the feature of primary interest here. In common with the spectra for the Cu/rough Ru(001) surface in Figure 6, the spectra in Figure 8 show no
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Blndlng Energy
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(eV)
Figure 8. Xe 5p3/2,1/2 photoemission spectra for Xe atoms adsorbed on a surface prepared by vapor deposition of 0.5 monolayer of copper onto the Ru powder used in obtaining the data in Figure 7 . The Xe atoms were adsorbed at 80 K. The spectra were obtained with Xe present at various pressures. The spectrum taken at a pressure of 10" Torr cor-
responds to complete monolayer coverage. evidence of a line at 7.7 eV. In marked contrast, such a line is clearly evident in the spectra for the Cu/smooth Ru(001) surface in Figure 4, at least after xenon exposures of 5 langmuirs and higher. Since this line is identified with the presence of copper islands on the ruthenium surface, we conclude that the Cu/Ru powder surface, like the Cu/rough Ru(001) surface, does not exhibit such islands. Only on the smooth Ru(001) surface are the copper islands observed. Discussion The way in which copper is distributed over the surface of ruthenium is sensitive to the detailed nature of the surface. On a smooth (001) plane of ruthenium, submonolayer amounts of copper after mild annealing (520 K) give rise to the formation of copper islands, as seen in the present work for a copper coverage of approximately 40% of a monolayer. However, the presence of a significant concentration of defects on the (001) plane seriously interferes with the formation of copper islands, as can be seen by the disappearance of the line at a binding energy of approximately 7.7 eV in a 5p photoemission spectrum of xenon adsorbed on the surface. When a submonolayer amount of copper is deposited on a polycrystalline ruthenium powder and subsequently annealed, the copper does not form islands such as those observed on a smooth Ru(001) plane. The distribution of the copper on the ruthenium surface is much closer to that observed on a rough Ru(001) surface, as evidenced by the absence of the line a t 7.7 eV in the xenon photoemission spectrum. The ruthenium powder has surface sites similar to the defects on the rough Ru(001) surface. The nature of the distribution of copper on a ruthenium surface could influence the catalytic properties. If a reaction required a site consisting of a large array of ruthenium atoms, and the ruthenium was extensively covered (say 50%) by copper, the probability of finding such an array with no copper on it would be much lower if the copper were randomly distributed as atoms or small groups of atoms than if it were present as large islands on the surface. The randomly distributed copper (if it were inactive for the reaction) would therefore be much more effective in inhibiting catalytic activity. Consequently, the copper present at the surface of small Ru-Cu aggregates or clusters could have an effect on catalytic activity which is very different from that observed when copper is deposited on the smooth (001) surface of a ruthenium single crystal. In recent studies of the hydrogenolysis of ethane to methane, Peden and GoodmanI6 reported that the deposition of copper on the (001) plane of a ruthenium single crystal had a much smaller
J. Phys. Chem. 1987, 91, 2342-2347
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inhibiting effect on hydrogenolysis activity than has been observed for actual Ru-Cu catalysts by other workers?-3,29-30If the required site for hydrogenolysis comprises an array of ruthenium atoms rather than a single atom,4 it would appear that differences in the manner in which the copper is distributed on the ruthenium surface could be an important factor in accounting for the different magnitudes of the effects. One might also consider this matter from the point of view that defect sites on a ruthenium surface could be more active than normal sites, whether or not the reaction requires a large array (29)R u m , A . J.; Haller, G. L.; Oliver, J. A,; Kemball, C. J. Catal. 1983, 84,297. ( 3 0 ) Haller, G. L.; Resasco, D. E.; Wang, J. J . Catal. 1983,84, 477.
of ruthenium atoms in a site. If there is preferential interaction of copper with the defects, as can be inferred from the results of the present work, it would be reasonable for copper to have a different effect on the catalytic activity of ruthenium when defects are present. Consequently, the effect of copper on catalytic activity could well be different for a ruthenium powder than it is for a smooth (001) plane of ruthenium. The results of the present investigation clearly demonstrate the value of ultraviolet photoemission spectroscopy of physisorbed xenon as a probe of surfaces of interest in catalysis. Since it is sensitive to localized properties of the surface, it is a powerful probe. for the detection of surface heterogeneity. It can therefore make a useful contribution in studies of the structure sensitivity of surface-catalyzed reactions.
Sensitlzation of TiO, in the Visible Light Region Using Zinc Porphyrins K. Kalyanasundaram, N. Vlachopoulos, V. Krishnan,+A. Monnier,t and M. Gratzel* Institut de Chimie Physique, Ecole Polytechnique Federale, CH- 101 5 Lausanne, Switzerland (Received: November 13, 1986)
Efficient charge injection from the excited state of a zinc porphyrin to the conduction band of Ti02 has been observed during the visible light irradiation of Ti02 electrodescoated with a film of [tetrakis(4-~arboxyphenyl)prphyrinato]zinc(II)(ZnTPPC). The mechanism of the charge injection process has been studied in dye-adsorbed Ti02dispersions and colloids, using steady-state and time-resolved photolysis techniques. The process is extremely sensitive to the pH. The charge injection occurs primarily from the singlet excited state of the porphyrin, that too only from the dyes that are adsorbed on the electrode.
Introduction Sensitization of stable, large bandgap semiconductors in the visible light using dyes has been a long sought, continuing goal in the area of photochemical solar energy conversion in many laboratories.lS2 The reported overall quantum efficiencies span almost 2 orders of magnitude. Progress in this area has been hampered by the limited knowledge available on the mechanism of the charge injection processes and on the factors that control them. Electrochemical techniques such as potential modulation3 and use of rotating ring disk electrodes (RRDE)4 have been of immense help in deciphering some of the mechanistic details. Sensitization processes investigated on TiOz electrodes and dispersions include dyes such as phthalocyanines,5 R ~ ( b p y ) , ~and + derivatives,6 ~hlorophyllin,~ and 8-hydroxyquinoline complexes.* With the availability of small (colloidal) semiconductor particles amenable to fast kinetic studies, we9 and others1° recently have been investigating the mechanism of such dye-sensitization processes using steady-state and laser photolysis techniques. Resonance raman spectroscopy1 and microwave absorption techniques'* have also been recently demonstrated to be useful. The present study involves the use of a water-soluble anionic porphyrin, [tetrakis(4-carboxyphenyl)porphyrinato]zinc(II) (abbreviated hereafter as ZnTPPC) as a sensitizer to study electron injection from the dye excited state(s) to the conduction band of the semiconductor Ti02. Precise information is availableI3 on the ground- and excited-state properties of the dye as well as the absorption spectra of its monoreduced and monooxidized radical forms, facilitating identification of the transient species formed during the photoreaction. Photocurrent measurements under visible light illumination on the porphyrin-coated polycrystalline TiOl electrodes revealed very efficient charge injection from an 'Visiting Professor from the Department of Inorganic and Physical Chemistry, Indian Institute of Science, Bangalore 560 012,India. *Institut de Chimie Mintrale, Analytique et AppliquC, Universitt de GenEve, CH-1211, Switzerland.
0022-3654/87/2091-2342$01.50/0 , I
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excited state of the dye (incident photon to current conversion efficiencies exceeding 40% at the Soret absorption maximum of the porphyrin). By a combined use of steady-state and time-resolved photolysis techniques on dye-coated colloidal T i 0 2 dispersions, the mechanism of the process is shown to involve the singlet excited state of the dye, that too only from the dyes that are adsorbed onto the electrode.
Experimental Section Materials. Preparation of the Polycrystalline Ti02Electrode. Ti02 (anatase) electrodes were prepared by thermal decomposition of titanium ethanolate solutions, deposited on the cross section (1)Gerischer, H.; Willig, F. Top Curr. Chem. 1976,61,31. (2)Watanabe, T.;Fujishima, A,; Honda, K. In Energy Resources Through Photochemistry and Catalysts; Gritzel, M., Ed.;Academic: New York, 1983; Chapter 11. (3)Matsumura, M.; Mitsuda, K.; Tsubomura, H. J . Phys. Chem. 1983, 87, 5248. (4)Watanabe, T.;Fujishima, A.; Honda, K. Chem. Lett. 1978,735. ( 5 ) (a) Fan, F. R. F.; Bard, A. J. J . Am. Chem. SOC.1979,101,6139.(b) Giraudeau, A.; Fan, F. R. F.; Bard, A. J. J. Am. Chem. Soc. 1980,102,5137. (6) (a) Borgarello, E.;Kiwi, J.; Pelizetti, E.; Visca, M.; Gratzel, M. J . Am. Chem. Soc. 1983,103,6423.(b) Hashnoto, K.; Kawai, T.; Sakata, T.Nouv. J . Chim. 1983,7, 249. (c) Furlong, D. N.; Wells, D.; Sasse, W. H. F. J . Phys. Chem. 1986,90, 1107. (d) Kiwi, J. Chem. Phys. Lett. 1981,83,594. (7)Kamat, P. V.; Fox, M. A. Chem. Phys. Lett. 1983,102,379. (8)Houlding, V.; Gritzel, M. J . Am. Chem. SOC.1983,105, 6595. (9)(a) Moser, J.; Gratzel, M. J . Am. Chem. SOC.1984,106, 6557. (b) Moser, J.; Gratzel, M.; Sharma, D. K.; Serpone, N. Helu. Chim. Acta 1985, 68,168. (10)Kamat, P. V.; Chauvet, J.-P.; Fessenden, R. W. J. Phys. Chem. 1986, 90, 1389. (11)Rossetti, R.; Brus, L. E. J . Am. Chem. SOC.1984,106,4336. (12) Fessenden, R. W.; Kamat, P. V. Chem. Phys. Lett. 1986,106,4336. (13)(a) Kalyanasundaram, K.; Neumann-Spallart, M. J . Phys. Chem. 1982,86,5163. (b) Neumann-Spallart, M.; Kalyanasundaram, K. Z . Naturforsch. 1981,36B,596. (c) Neta, P. J . Phys. Chem. 1981.85,3678. (d) Richoux, M.-C.; Harriman, A. J . Chem. SOC.,Faraday Trans. 1 1982,78, 1873. (e) Harriman, A.; Porter, G.; Walters, P. Zbid. 1983,79, 1335.
0 1987 American Chemical Society
