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Dispersion and Electronic Structure of Ti02-Supported Cobalt and Cobalt Oxide Y. Shao,? W. Chen,? E. Wold,* and J. Paul*’? Physics III, KTHIRoyal Institute of Technology, 100 44 Stockholm, Sweden, and Applied Physics, NTHINorwegian University of Technology, 7034 Trondheim, Norway Received June 16, 1993. I n Final Form: October 12,199P Results from a surface study of titania-supported cobalt are presented. The data show a clear interaction between the Ti(ll0) support and the overlayer, as a function of thermal treatments and oxidation state. Two model substrates, fully oxidized and partially reduced TiOz, were impregnated with metallic cobalt or cobalt oxide via dry chemical methods. The resulting four combinations, TiO~-olCo,Ti&-o/CoO, TiOP-r/Co,and TiOz-rICoO,were compared with reference spectra for the isolated constituents by means of electron spectroscopic measurements following annealing to subsequently higher temperatures. We employ a new ‘fingerprint” technique to analyze the data, thereby allowing us to utilize all available peak positions for a better understanding of chemical interactions and dispersion changes. From our data we can conclude that only metallic cobalt influences the chemical state of titania. The dispersion of COO changes at moderate temperatures, 500-700 K, but this is restricted to coalescence of the overlayer. The overlayer oxide is less stable on the partially reduced support than on fully oxidized titania. Metallic cobalt, as deposited or from decomposition of COO,diffuses into titania at above 700 K on TiOz-o and above 500 K on TiO2-r. This interdiffusion is accompanied by a reduction of the support. The lower stability of COOon the partially reduced support is accompanied by a stabilization of cobalt in the surface region and we conclude that the driving force for the destabilization may be the formation of a bimetallic coating. This anomalous stabilization of Co near the surface is only observed for thick overlayers of cobalt metal on oxidized titania and for COO on TiO2-r. 1. Introduction Cobalt is tomorrow’s preferred Fischer-Tropsch catalyst. This is motivated by a favorable product distribution in combination with a relatively high price for alternative metals. Commercial catalysts are being developed by all major petroleum c0mpanies.l Based on previous experience, different supports are chosen, some of which are reducible. A reducible or semiconducting support in combination with cobalt or cobalt oxide increases the chances of large dispersion changes as a result of thermal or chemical treatments. A different but costly alternative is to use titania-coated alumina as a rigid support, yet with good wetting toward the active cobalt catalyst.2 Co/TiOz is a documented SMSI ~ a t a l y s t .The ~ ~ strong ~ cobalt-support interaction may document itself as bimetallic oxides, dispersion changes, or chemical interaction. Electronic interaction in biphasic oxide/oxide or oxide/ metal systems has often been suggested but rarely seen. Model experiments on Pt/TiOa have shown that SMSI effects are caused by dispersion changes related to Ti4+ Ti3+ reduction at the surface, but electronic perturbations have not been unambiguously d o c ~ m e n t e d . ~ ~ ~ Co/TiOz model experiments have to cover the extremes from a fully oxidized to a partially reduced support in combination with likewise different states of oxidation of the catalytic metal. This postulate is supported not only by observed redox properties but also by variations in preparation techniques and related performance differ-
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* T o whom correspondence may be addressed. t KTH/Royal Institute. of Technology.
* NTH/Norwegian University of Technology.
e Abstract published in Aduance ACS Abstracts, December 1, 1993. (1)Haggin, J. Chem. Eng. News 1991, 69 (17), 22. (2)Stranick, M. A.; Houalla, M.; Hercules, D. M. J . Catal. 1987,106, 362. (3)ACS Symp. Ser. 1986, No. 29.9. (4) Materials Science Monographs 47, Surface and Near-Surface Chemistry of Ozide Materials; Nowotny, J.,Dufour, L. C., Eds.;Elsevier: Amsterdam, 1988.
ences when put on stream. Conventional impregnation methods with a mixture of Ti02 particles and cobalt nitrate in solution as well as elaborate techniques based on the hydrolysis of a solution of a titanium alkoxide and cobalt nitrate are always followed by calcination and reduction steps. The final catalyst is markedly influenced by the temperatures and chemical activities of each step. We synthesize models by dry physical deposition techniques which allow us to prepare the substrate and the overlayer independently at low temperature and gradually raise the temperature. Photoemission is our prime and sensitive spectroscopyto map the ‘surface phase diagram” of Co-Ti-0. Our starting point is reference spectra for the clean substrates, fully oxidized (TiOz-o) and partially reduced titania (TiOz-r), and bulk quantity overlayers, metallic cobalt (Co),and cobalt oxide (COO).Our models of titania covered by cobalt are then analyzed with reference to these starting points. Finally we discuss how our models compare with the impregnated bulk catalyst and what conclusions can be drawn about these materials from our results. 2. Experimental Section
The Ti02(110) crystal was cut, oriented, and polished by Metal Crystals & OxidesLtd. Platinum was evaporated on the back side which then was brazed to a 0.05-mm platinum sheet with 0.01-mm gold foil.6 The brazing procedure results in partial reduction of Ti02 but the crystal was then further reduced in H2 at 1150 K for 6 h. Tantalum wires for resistive heating were spot welded to the Pt sheet to ensure rapid heating of the sample. This procedure together with the good thermal conductivity through the gold film ensures accurate control of the sample temperature as monitored by a thermocouple in direct contact with the Pt sheet. ( 5 ) Paul, J. Manuscript in preparation.
0743-7463/94/2410-0178$04.50/00 1994 American Chemical Society
TiOp!hpported Co and COO
The sample was cleaned in situ by repeated cycles of argon sputtering (2.2 kV, 25mA, 300K, lOmin), annealing (900 K, 10 min), and oxidation (900 K, 10-6 Torr, 5 min). Cleanliness was verified with photoemission spectroscopy. Cobalt was evaporated from a hot filament made of Co and W wires, twisted together. Elemental composition of the condensed film was again confirmed with photoemission spectroscopy. The TiO2-0 substrate was obtained by heating the Ti02 crystal to 900 K in 0 2 (10-6 Torr) for 10 min after Ar+ sputtering at 300 K for 10 min. The partially reduced substrate, TiO2-r, was prepared by sputtering without annealing. Metallic cobalt was obtained by evaporating the metal on the substrate at 300 K. Finally cobalt oxide overlayers were synthesized by Co evaporation in 0 2 (104 Torr) with the substrate kept at 300 K for thin and at 480 K for thick, bulklike films. Our catalyst models were prepared by combinations of either of the above two substrates and two overlayers. Our "standard" coverage was around one monolayer as judged from the attenuation (approximately 50 % ) of electrons photoemitted from the titania substrate and assuming, without further evidence, homogeneous growth at 300 K. For TiOz/Co we also used a "thick" overlayer (>5 atomic layers) again as judged from the reduced intensity from the titania substrate. Finally, "bulk" quantities of Co and COOcould be annealed without any observed emission from the underlying substrate. This means a minimum thickness of 10 layers at any point. We employed four standard annealing temperatures: 300,500, 700, and 900 K but all measurements were done after recooling to 300 K. The sample was cleaned between each set of measurements, as described above. Spectra were also obtained from oxidized cobalt as a reference. All measurements were done at normal emission with a hemispherical analyzer in an ultrahigh vacuum endstation connected to beamline 22 at MAXLAB synchrotron laboratory and all electron energies are given as positive binding energies (Eb)with reference to the Fermi level (Ef).We identified four regions which together gave adequate information for cobalt overlayers on TiO2. These regions were Eb = 0-80 eV measured with hv = 150 eV for the valence band and the 029, TiSp, Ti3s, and co3p orbitals, Eb = 450-480 eV with hv = 620 eV for the Ti2p core level, Eb = 520-540 eV with hv = 620 eV for Ols, and finally Eb = 765-805 eV with hv = 860 eV for C02p. The Fermi edge was also measured with each photon energy for calibration of energy scales. Depth profile measurements are in progress and will be reported elsewhere.6
Langmuir, Vol. 10, No. 1, 1994 179 Reference Spectra
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3. Results 3.1. ReferenceSpectra. Section 3.1 presents reference spectra for the two different substrate preparations, fully oxidized and partially reduced TiO2, and the two different overlayers, metallic cobalt and cobalt oxide, used in this study. We will point out several characteristic spectral features which allow us to determine the state of the substrate as well as the overlayer. In subsequent sections these fingerprints will be used to determine electronic changes caused by substrate-overlayer interactions. The fingerprints are summarized in section 3.1.4. Only a limited number of photoemission spectra for complex and reference spectra of cobalt oxides are titanate, CoTiOa or CozTiOd, will be published elsewhere.6 Sections 3.1.5-6 present data for clean TiO2-r and bulk COOas a function of annealing temperature. These data
are necessary to judge if an observed change for titania coated by thin overlayers is a result of substrate-overlayer interactions or simply a property of the isolated species. Fully oxidized Ti02 and bulk metallic cobalt do not undergo any electronic changes as a result of annealing. 3.1.1. Substrates. Figure 1shows characteristic spectra for fully oxidized and partially reduced TiO2. The condition of the titanium oxide can be determined by the density of states at the Fermilevel, the profile of the valence band, and the shapes of the 02s, Ti3p, Tias, Ti2p, and 0 1 s peaks, i.e., a total of seven spectral features. Partial reduction multiplies the number of occupied states at the Fermi level (Figure la). Similar effects are introduced by sputtering, chemical reduction, or metal deposition.9-12 The shape and position of the valence band change in the characteristic way shown in Figure la.gJ0J2 Parts a and b of Figure 1 reveal that the 02s and 01s peaks become slightly asymmetric as a result of partial reduction.' Finally, parts a and c of Figure 1indicate that the Ti3p, TiBs, and, in particular, the Ti2p peaks broaden or split.7J2 3.1.2. Overlayers. Figure 2 shows characteristic spectra for bulk quantities of metallic cobalt and cobalt
(6) Chen, W.; Shao, Y.; Paul, J. Work in progress. (7) Sayers, C. N.; Armstrong, N. R. Surf. Sci. 1978,77,301. (8)Mchtyre, N.S.; Johnston, D. D.; Coataworth, L. L.; Davidson, R. D.; Brown, J. R. Surf. Interface Anal. 1989,14,66.
(9)Chung, Y.W.; Lo, W. J.; Somorjai, G. A. Surf. Sci. 1977,64,588. (10)Lo, W. J.; Chung, Y. W.; Somorjai, G. A. Surf. Sci. 1978,71,199. (11) Henrich, V. E.; Kurtz, R. L.Phys. Rev. B 1981,23,6280. (12)Sadeghi, H. R.;Henrich, V. E. J. Catal. 1988,109,l.
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180 Langmuir, Vol. 10, No. 1, 1994 ~
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Figure 2. Photoemission spectra of metallic cobalt and cobalt oxide, COO.
oxide. The cobalt oxidation state is determined by the density of states at the Fermi level, the contour of the valence band, and the shapes of the 029, co3p, 018, and C02p peaks, Le., a total of six spectral features. Of these features the DOS(Ef), the oxygen peaks, and the co3p peaks are partially overlapping with the fingerprints of titania but the remaining peaks are sufficient for an accurate measurement of both substrate and overlayer, for our catalyst models. The formation of cobalt oxide lowers the DOS(Ef) as obvious from Figure 2a. The position and shape of the valence band are also different for metallic cobalt and cobalt oxide. We note that the slight overlap between the valence bands of Ti02 and cobalt oxide does not limit the possibility to determine the state of each component. For metallic cobalt no such overlap has to be considered. Metallic cobalt and cobalt oxide can also be distinguished by shifts and broadening of the co3p and C02p peaks (Figure 2a,c). Cobalt forms severaloxides but previous work gives that only two, COO and Co304, are likely to form at the surface.13-17 Unfortunately no reference data exist for ~
~
(13)Chuang,T. J.; Brundle, C. R.; Rice, D. W. Surf.Sci. 1976,59,413. (14)Shen, Z.X.; Lindberg, P. A. P.; Shih, C. K.; Spicer, W. E.; Lindau, I. Phpica C 1989,162-164,1311. (15)Castro, G.R.; Kltppem, J. Surf. Sci. 1982,123,456. (16)Jugnet, Y.;Tran Minh Duc, J. Phys. Chem. Solids 1979,40,29. (17)Kim, K. S. Phys. Rev. B 1976,1I,2177.
cobalt oxide formed under conditions identical to ours. Nevertheless, existing data show that the shape and position of the valence band are different for the two oxides.13-17 The top of the valence band is located 1eV below the Fermi level for Cos04 and 2 eV below EFfor Coo, and the shake-up peak of Co2p and the satellite at around 10 eV are relatively higher for COO than for co30~.15~16~18 This clearly identifies the cobalt oxide in this work as COO. The intensities of other features of the valence band are more difficult to compare directly, since the shape also changes with photon energy.14J6 A comparison between Figures 2a,b and la,b shows that the Ole and 02s peaks of cobalt oxide are shifted to lower binding energy as compared to titania.7~8~13~17~19~20 3.1.3. Valence Band. Figure 3 shows the valence bands in detail. The bands are only partially overlapping which makes a deconvolution feasible. A high DOS at the Fermi level can only be explained by metallic cobalt or partially reduced titania. Moreover, the shape of the valence band is different for all four components and the peak at 10 eV is unique for COO. 3.1.4. Fingerprints. The following fingerprints will be used in subsequent sections to identify the state of the substrate and the overlayer: i. Fermi edge, high DOS comes from metallic Co or from TiOz-r (Figures l a and 2a); ii. Valence band, shape is different for all components (Figure 3); iii. 02s and Ols,position and shape for all components but metallic Co (Figures l b and 2b); iv. Ti3p, position and shape for titania (Figure la); v. Ti2p,position and shape for titania (Figure IC);vi. Ti3s, position and shape for titania, note overlap with Co 3p (Figure la); vii. co3p, position and shape for Co and COO(Figure 2a); viii. C02p, position and shape for Co and COO (Figure 2c). 3.1.6. Temperature Stability of TiOz-r. Figure 4 shows temperature-induced changes in photoemission spectra of partially reduced TiOz. The sample is stable upon annealing at 530 K. At temperatures up to 700 K, the characteristic features for TiOz-r are slightly altered or decreased in intensity. These slight alterations are much more pronounced after annealing to 900K. This could be attributed to oxygen diffusion from the bulk, possibly connected to oxidation of Tis+ Ti4+and reordering of
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(18)Lee, G.; Oh, S. J. Phys. Rev. B 1991,43,14674. (19)Brundle, C.R.; Chuang, T. J.; Rice, D. W. Surf. Sci. 1976,60,286. (20) Chuang, T. J.; Brundle, C. R.; Rice, D. W. Surf. Sci. 1976,59,413.
TiOz-Supported Co and COO
Langmuir, Vol. 10, No. 1, 1994 181 Bulk COO
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the surface.21*22 In addition, the maximum of the valence band shifts to lower binding energy during annealing at 970 K, due to decreased titanium coordination of oxygen atom^.^^?^^ Similar effects of annealing on the valence band of sputtered Ti02 were also observed by KaoZ4and Tait.22 Tait et al. found that the valence band spectra and the Ti3+ (Ti3d) peak height at the Fermi level of sputtered Ti02 are unaffected by further heating, once annealed at 900 K.22 Similar results should be expected for other regions of the spectra. The finite concentration of surface defects linked to oxygen deficiencies does not seem to be removed by heating alone,22 but Figure 4 shows that oxidation at high temperature leads to fully oxidized TiOz. 3.1.6. Temperature Stability of COO. The bulk cobalt oxide is not observed when Co is evaporated on the fully oxidized Ti02 at 300 K in 0 2 (2 X lV Torr), but only when the substrate is kept at 485 K during evaporation. Upon annealing to 700 K the spectra indicate coalescence and the presence of small quantities of metallic cobalt (Figure 5). The highest temperature displayed in Figure 5 is 700 K. The results for higher temperatures deviate (21) Henrich, V. E.;Dresaelhaue, G.;Zeiger, H. J. Phys. Rev. Lett. 1976,36, 1335. (22) Tait, R. H.;Kasowski, R. V. Phys. Reu. B 1979,20, 5178. (23) Kasoweki, R. V.;Tait, R. H. Phys. Rev. B 1979,20, 5168. (24) Kao, C.C.;Tsai, S. C.; Bahl, M. K.; Chung, Y. W. Surf. Sci. 1980, 96,1.
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from those of pure COO due to the influence of the substrate. Annealing to 920 K completely removes the COO overlayer due to decomposition of the oxide. The resulting metallic Co diffuses into the substrate as will be discussed in section 3.2. Hence, the changes observed at 920 K do not reflect the properties of bulk COObut SMSI related dispersion changes for titania supported cobalt. The decomposition of the cobalt oxide is, however, unambiguous from the spectral changes following annealing. This is further discussed below. 3.2. Annealing of Catalyst Models. 3.2.1. Oxidized Ti02 with Cobalt Oxide. Figure 6 presents photoemission spectra from a monolayer of cobalt evaporated on the fully oxidized Ti02 substrate at 300 K in 0 2 (10-8 Torr). The fingerprint of COO is clearly visible and no traces of metallic cobalt can be observed at this temperature. The intensity of COO weakens with increasing annealing temperature, while the emission from the substrate becomes stronger. COOis finally removed by heating the sample to 900 K and some reduced titanium sites are produced in the process. The double 0 1 s peak, resulting from emission from COO and TiO2, respectively, shifts toward the latter one as a result of the annealing. 3.2.2. Oxidized Ti02 with Cobalt Metal. The photoemission spectra of a monolayer of Co metal, evaporated in uucuo on the fully oxidized Ti02 surface at 300 K and
Shao et al.
182 Langmuir, Vol. 10, No.1, 1994 OxidizedTIQ with '&ball oxide Annealing elfact
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annealed a t successively higher temperatures, are shown in Figure 7. Metallic cobalt develops the high DOS at Ef and suppresses the emission from the substrate. Differ-
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ences in peak shapes may also be observed. Small amounts of C o o may be formed during evaporation, but it is difficult to discern if this comes from the substrate or from the background contamination. These features disappear upon annealing to 500 K. After the sample was heated to 900 K, some new Ti3+ was found by comparison with our fingerprint spectra (Table l),indicating the occurrence of stronger interaction between substrate and Co layer. Simultaneously, the intensities of the spectra for the substrate are significantly
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Figure 8. Photoemission spectra of oxidized titania covered by a thick (>6atomic layers) film of cobalt metal as a function of annealingtemperature. All spectrawere measured after recooling to 300 K. enhanced and the emission from Co is markedly diminished. The dispersion of the Co particles has obviously changed, due to encapsulation by or diffusion into the titania substrate. Figure 8 shows the corresponding results for a thick Co film condensed at room temperature. The Colayer is thick enough to fully attenuate the Ti signals from the substrate. It is significant that many reduced Ti species are formed after heating to 900 K and it is clear that the number of reduced sites increases with the amount of deposited cobalt. 3.2.3. Reduced Ti02 with Cobalt Oxide. Figure 9 displays photoemission spectra of Co evaporated on a partially reduced Ti02 substrate a t 300 K in 0 2 (1o-STorr).
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Figure 9. Photoemission spectra of partially reduced titania covered by a monolayer of cobalt oxide as a function of annealing temperature. All spectra were measured after recooling to 300 K.
COO forms on the substrate during evaporation, simultaneously with the conversion of some reduced titania species to Ti4+. When the sample is heated to 500 K, significant amounts of COOdecompose into metallic Co, which rationalizes the lower intensity of the peaks for COO and the higher intensity of the peaks for metallic cobalt. As previously mentioned, some reduced titania is also formed. After annealing a t 700K, the COOpeaks disappear and some new reduced TiS+ species appear, as seen by the increase in the integrated peak area from reduced titania and by the shoulders on the Ti2p and Ti3p peaks. This reveals strong interaction between cobalt oxide and titania. Upon further annealing a t 900 K, the intensity of the newly
Shao et al.
184 Langmuir, Vol. 10, No. 1, 1994 Reduced“4Kith Cobalt Metal
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Figure 10. Photoemission spectra of partially reduced titania covered by a monolayer of cobalt metal aa a functionof annealing temperature. All spectra were measured after recooling to 300 K. reduced Ti species decreases somewhat but Co is still observable. 3.2.4. Reduced Ti02 with Cobalt Metal. Figure 10 shows spectra of Co evaporated on the partially reduced Ti02 surface at 300 K in vacuum, followed by heat treatments. Only metallic cobalt forms on the partially reduced Ti02 surface during evaporation. The presence of cobalt enhances the DOS at the Fermi level and decreases the intensity of emission from the substrate, without any formation of the new Ti3+sites. At the same time, the valence band, initially bent down for the partially reduced TiOz, relative to that for the fully oxidized TiOz, bends up again toward Ef.No effect on the shoulder of
the Ti2p peak is observed upon annealing at 700 K, in contrast with the case for the bare substrate. A new peak at about 1.45 eV appears and it seems to be correlated with the relative decrease in the emission at Efand also with the shift of the maximum of the valence band to lower binding energy. These changes again clearly indicate that interaction between cobalt and the substrate occurs, probably from reduced titania in contact with Co. According to the theoreticalresults, sucha Co-Ti bond should be partially ionic with Co negatively charged.2se26 This will be discussed in section 4. Further annealing at 900 K decreases the intensity of emission from Co and erases the peak at 1.45 eV. Other peak shapes are changed in a similar way for the substrate alone. 4. Discussion 4.1. Chemical Interactionand Dispersion Changes. Our analysis of the catalyst models will be based on the fingerprints for the separate constituents. Certain spectral features are well correlated and vary coherently on a scale from a fully oxidized to a partially reduced system. For titania these features are the increased intensity of the highest occupied orbitals, the shift of the valence band, the broadening of the Ti3p, TiBs, and 01s peaks, and the altered profile of the Ti2p profile (Figure 1). The corresponding changes for CoO/Co are again the DOS(s),, the valence band, the co3p orbital, and finally the Co2p profile (Figure 2). A lack of correlation among the above features indicates the formation of new phases. 4.1.1. TiO2-o/CoO. Oxidized titania does not undergo any rapid changes as a result of annealing alone, which means that observed changes on the substrate must be caused by chemical interaction with the overlayer (Figure 5 and Results). Different temperature regimes can be identified for the annealing of a COO overlayer on fully oxidized titania. Redistribution, often synonymous with coalescence, and, at an increasingrate, decompositionof the overlayer, occurs between 500 and 700 K. Redistribution is apparent from the increased intensity of the titania signal in Figure 6 and decomposition from a minor contribution from Co metal. The 01s as well as the valence band intensities are dominated by a superposition of the signals from titania and cobalt oxide. Below 700 K no spectral changes from titania can be observed but only an increased TiO2:CoO ratio. This reveals a significant change of the dispersion of the overlayer but also that the overlayer does influence the chemical state of the substrate. An increased ratio without accompanied electronic changes points an island formation as a possible explanation. The redistribution is also accompanied by decomposition of some COO into metallic cobalt and atomic oxygen. The increased intensity a t the Fermi level comes from Co metal rather than from reduced titania, since no correlated changes of the Ti2p signal could be found at these temperatures. Between 700 and 900 K complete decomposition of the COOoccurs and the Co metal diffuses into titania. To summarize theTiO2-o/CoO system, we conclude that coalescence occurs between 500 and 700 K, which gives a lowered dispersion of the overlayer. Some decomposition also takes place but with no apparent effect on the substrate. The oxide decomposes above 700 K and the metal diffuses into titania in a SMSI-like behavior. The decomposition of COO is a property of this phase alone, with no apparent influence by the substrate. Cobalt promotes partial reduction of the titania support. (25) Haberlandt, H.; Ritschl, F. J. Phys. Chem. 1986,90,4322. (26) Horsley, J. A. J. Am. Chem. SOC.1979, 101, 2870.
TiO2-Supported Co and COO 4.1.2. TiOz-o/Co. Oxidized titania and cobalt do not undergo any changes as a result of heat treatments alone, so any observed change must be due to substrate-overlayer interactions (see Results). Cobalt metal stays metallic at all temperatures up to 900 K. This is apparent from the Co2p level and from the high density of states a t the Fermi level, i.e. from the highest occupied orbitals. Likewise only small effects are observed on the support and the only significant change comes after annealing above 700 K, when some Ti3+sites are detected (Figure 7d). The number of reduced sites is proportional to the heating temperature as summarized in Table 1. Partial reduction of clean titania can be observed after prolonged heating in uacuo, but metallic cobalt promotes this reaction. The partially reduced Substrate lowers the dispersion of the catalyst, as seen from the altered titania to Co ratio. Continued annealing induces changes in titania. At temperatures above 700 K Co metal alters the Ti02 as obvious from the Ti2p, Ti3p, and 01s orbitals, and also from comparisons with the reference compounds. No related shifts of the co3p and C02p peaks are observed. The slight asymmetry of the 01s peak comes from oxygen atoms in contact with cobalt. This is obvious since the changes are not correlated with the changes of the Ti2p and Ti3p peaks, and thus not related to titania. The same shift is apparent from our cobalt model systems and it goes away by heating to 500 K or well below the temperature for modification of titania. Heating above 700 K again induces diffusion of cobalt into the bulk substrate or migration of titania atop the cobalt. Oxygen exposure cannot regenerate the above special type of oxygen atoms, since the dispersion of the metal has completely changed. When a very small amount of Co was evaporated on the fully oxidized Ti02 surface, the Co2+ could be detectable as the clear appearance of the small peak around 10 eV in the valence band, in addition to the metallic cobalt. But Co2+ could not be detected for larger coverages of Co because titania will not oxidize an overlayer of Co metal except for a possible small number of sites in the surface region. A possible explanation is that Co2+forms only at the initial stage of Co evaporation. The formation of Co2+ is probably due to electron transfer from metallic Co to titania and this transfer offers an alternative explanation for the asymmetry of the 01s peak a t low exposures. Theoretical calculations on the direction of the electron transfer between the fully oxidized titania support and the metal verify the existence of such electron transfer.25 Substrate-overlayer electronic interaction is obvious from the above spectroscopic changes and also offers a likely explanation for the observed dispersion changes. Dispersion changes and electronic interaction go hand in hand, as detected by electron spectroscopy, although ideally one would like to follow the former with tunneling microscopy. Thick overlayers of cobalt apparently follow a slightly different route. Figure 5 gives the data for a thick film (>5 monolayers). The valence band as well as the co3p and 2p orbitals show that significant amounts of metallic cobalt are left in the surface region after annealing a t 900 K. At the same time many Ti3+ sites are created, as comparedwith a freshly sputtered surface,and the number of sites is roughly proportional to the amount of Co (Figure ad). This is in contrast to the traditional picture of the SMSI state, where the metal is inaccessible and encapsulated by the 0xide.m The explanation for this behavior (27) Belton, D. N.; Sun, Y. M.; White, J. M. J.Phys.Chem. 1984,88, 6172.
Langmuir, Vol. 10, No. 1, 1994 185 is unclear but it could be related to the formation of a new and stable phase at the interface. Thermal desorption experiments could not detect any Co desorption below 900 K. To summarize we observe different behaviors for thin and thick overlayers of metallic cobalt on oxidized titania. This overlayers stay metallic and only undergo slight dispersion changes with little or no effect on the substrate below 700 K. Above this temperature the metal readily diffuses into the bulk, thereby inducinga partial reduction of the substrate. A thick metallic overlayer induces a partial reduction of the titania, the number of sites being proportional to the amount of cobalt. Noteworthy is that Co metal is stabilized in the surface region. 4.1.3. TiOz-r/CoO. Both the substrate and the overlayer are sensitive to thermal treatments (Figures 4 and 5). This means that special care has to be taken to deduce any interaction between the support material and the catalyst. Bulk COOdecomposes at around 700 K and the resulting oxygen atoms will oxidize some trivalent titanium ions to fully coordinated tetravalent species. On oxidized Ti02 this temperature is lowered to between 500 and 700 K and on reduced titania to around 500 K. This is apparent from the large increase of the DOS at the Fermi level in Figure 9a and from the small change in the titania signal (Figure 9d). The cobalt metal will induce partial reduction of the titania support already at 700 K. Again cobalt has to be in contact with the Ti3+ sites. This points at an electronic mechanism for the strong interaction. Finally we note that this is the second system for which cobalt is stabilized in the surface region and for which diffusion into the bulk is prevented, the first being thick overlayers of metallic cobalt on oxidized titania. To summarize COOon reduced titania, we find that the overlayer is stable to around 500 K when decomposition leads to the formation of metallic cobalt. Co reduces TiOz-r at 700 K and the interaction stabilizes Co at the surface. 4.1.4. TiOz-r/Co. Only the reduced titania phase changeswith temperature, a bulk phase of metallic cobalt is stable up to 900 K (Figure 4 and Results). The cobalt overlayer is unaffected by annealing and stays constant to 700 K (Figure lo), but a slight increase in the number of reduced titania sites can be detected (Figure 10d). In agreement with previous measurements on reduced titania we could not detect any metal ions from the supported metal.25p28At 700 K a drastic change occurs and the cobalt metal appears to diffuse rapidly into the bulk, leaving a less reduced titania surface behind! This is an anomalous behavior and the Iogic explanation is that the reduced titania must be in contact with the metal and that the zone of SMSI state is found below the surface. Electron spectroscopyonly probes a few atomic layers. 4.2. Other Titania Supported Metal Systems. The strong interaction points at an electronic mechanism and possibly at electron transfer between the two phases. This has been discussed for other systems which show SMSI behavior. The experimentalevidence for electronic bonding in the system Rh/TiOz-r was given by Sadeghi.12 RhTi bonding is believed to be part of the thermodynamic driving force which leads to encapsulation of rhodium during SMSI. Other systems that have been investigated are Ti02/VF9 Ti02/Cu,3° TiOz/A1,3l Ti0z/FeF2 TiOd ~
(28) Deng, J.; Wang,D.; Wei, X.;%ai, R.; Wang,H.Surf. Sci. 1991, 249, 213. (29) Zhang,Z.; Henrich, V. E. Surf. Sci. 1992, 277, 263. (30) Diebold, U.; Pan,J. M.;Madey, T. E. Phys.Rev. B 1993,47,3868.
186 Langmuir, Vol. 10,No. 1, 1994 and Ti02/Ni.N The ionic component introduced by electron transfer from reduced Ti3+ species to metal atoms may be essential in order to obtain a strong bonding since the covalent bonding alone is not sufficient to account for the strength of the metal-support interaction.26 Other calculations performed for reduced titania-metal systems favor the above s u g g e ~ t i o n . ~It~ was ? ~ ~found that the electron transfer is directed to the metal from a reduced support, the driving force being the occupied surface states in the band gap acting as electron donors. In our case we attribute the interaction between reduced titania and cobalt metal at 700 K to Co-Ti bonding with Co at oxygen ion vacancies. Apparently this bonding is localized but the state of cobalt is unclear. The overlayer-titania bonding documents itself in the dispersion of the deposited metal. For aluminum and iron this means that the metals are oxidized on both stoichiometric and defect-rich s~pports.3~93~ The situation is reverse for copper, which interacts weakly with all forms of titania, leading to poor dispersion and coalescence of the metal.30 The interaction with vanadium is dependent on the pretreatment of the support and the oxidation state of the o ~ e r l a y e r .Metallic ~~ vanadium interacts strongly with oxidized titania but weakly with partially reduced titania. No strong interaction is observed for vanadium oxide on stoichiometric titania. We note that the present system, titania-supported cobalt, covers the full regime from weak to strong interaction, depending on the oxidation state of the overlayer as well as of the substrate. 4.3. Formation of Bimetallic Oxides. Oxidation of titanium-cobalt alloys leads to the formation of simple as well as complex oxides.3w The simple oxides of cobalt, COOand Co304, are reducible in U ~ C U Oand in a reducing gas atmosphere. Both oxides are useful for catalyst impregnation and can easily be converted into metallic cobalt at modest temperatures. The titanium oxides can be either catalytically active or passive, depending on the oxidation condition^.^^ Ti02 can readily be partially reduced and will then participate in redox cycles in the catalyzed reaction. A trivalent titanium oxide, formed from the metal by excessive heating during calcination, is catalytically inactive and will only serve as a passive support with no SMSI behaviorS41 Bimetallic oxides may also form, both as a result of oxidation of a binary metal alloy and during posttreatment after impregnation of a titania support. The latter situation is addressed in this paper. Anomalous behavior, possibly explained by the formation of bimetallic oxides, is seen for thick overlayers of cobalt metal on oxidized titania and for cobalt metal on TiO2-r. Bimetallic oxides have not been spectroscopically identified, but the staPt,33-36
(31) Dake, L. S.; Lad, R. J. Surf. Sci. 1993, 289, 297. (32) Pan, J. M.; Madey, T. E. J. Vac. Sci. Technol., in press (33) Beard, B. C.; Ross, P. N. J.Phys. Chem. 1986,90, 6811.
(34)Dwyer, D. J.; Robbins, J. L.; Cameron, S. D.; Dudash, N.; Hardenbergh, J. InStrongMetal-SupportInteractions; ACS Symposium Series 298; Baker, R. T. K., Tauster, S. J., Dumesic, J. A., Eds.; American Chemical Society Washington, DC, 1986. (35) Asenio, M. C.; Kerkar, M.; Woodruff, D. P.; deCarvalho, A. V.; Fernhdez, A.; Gondez-Elipe, A. R.; Femhdez-Garcia, M.; Conesa, J. C.Surf. Sei. 1992, 273, 31. (36) Kao, C. C.; Tsai, S. C.; Bahl, M. K.; Chung, Y. W.; Lo, W. J. Surf. Sci. 1980, 95, 1. (37) Viswanathan, B. in Advances in Catalysis Science &Technology; Proceedings of the 7th National Symposium on Catalysis, Feb 6-8,1985, B a r d a , India, p 63. (38) Nevitt, M. V. Trans. Metall. SOC. AZME 1960,218, 327. (39) Garrett, S. J.; Egdell, R. G.; Riviere, J. C. J. Chem. SOC., Faraday Trans. 1991,87, 2756. (40) Garett, S. J.;Egdell, R. G.; Rivihre,J. J.Electron Spectrosc.Relat. Phenom. 1990,54/55,1065. (41) Chen, W.; Cameron, S.; Tillborg, H.; Nilason, A.; Hammar, M.; Tarnevik, C.; Paul, J. Work in progress.
Shao et al.
bilization of cobalt in the surface region points is a likely explanation. The most common bimetallic oxides are cobalt titanate, CoTiO3 and C02Ti04,4~ Conventional preparation leads to Co304 formation, as seen from a mixture of Co2+and Co3+ and CoA1204 formation has been observed on an alumina supportm4 4.4. Implications for Catalyst Preparation. Electronic and chemical interaction between cobalt and the titania support leads to large dispersion and thus performance changes of the catalyst. These effects are pronounced for this oxide on oxide system since both phases may change their state of oxidation when put on stream after activation or regeneration processes. Welldispersed tiny cobalt oxide clusters, formed on the Ti02 support after the calcination of a Co/TiOz catalyst prepared by the alkoxide technique, may dissolve in a CO/H2 a t m ~ s p h e r e .Preparation ~~ techniques will also influence the possibility to redisperse Co particles by high-temperature reduction.M The Co particle size increases with reduction temperature. At 700 “C, when the phase transformation of Ti02 from anatase to rutile occurs, cobalt particles redisperse to individual crystallite^.^^ Arco and Rives concluded, after a study of the bulk catalyst, that the final states of Co/TiOzand CosOdTiOznot only depend on the thermal and reduction/oxidation treatments but also on the order of such treatments.& Nevertheless, these changes are often reversible in contrast to the above discussed formation of “irreducible” bimetallic phase~.~g The present work hints at possible ways to understand and avoid irreversible and erroneous preparation techniques without costly coatings for improved wetting.2 It is worth noting that the intensity of Co emission is significantlydiminished accompanyingthe drastic increase in the emission from the substrate after heating sample to 900 K, as shown for the TiO2-o/Co and TiOz-r/Co systems (Figures 7 and 10). Since cobalt did not desorb at this temperature, one of the possible reasons for the lower intensity could be that the cobalt particles redispersed to much finer particles a t 900 K. Baker et al. found that the mean particle size of Pt evaporated on thin films of Ti02 increased with the temperature up to 500 OC, but platinum particles decomposed to smaller ones at 600 OC indicating the redispersion.60s1 Redispersion of cobalt particles to individual crystallites was observed, for the Co/TiOZ catalyst prepared by the alkoxidetechnique, when the catalyst was reduced at 700 0C,M)Jj2i.e. at the temperature where phase transformation of Ti02 from anatase to rutiletakes place. No redispersion was observed at this temperature for the catalyst prepared by the conventional impregnation method and reduction at 700 0C.s2 These observations show that redispersion of metal ~
_
_
(42) Inagaki, M.; Masuda, Y.; Shibata, C.; Naka, S. J. Inorg. Nucl. Chem. 1974,36, 2623. (43) Criado, J. J.; Macias, B.; Martin, C.; Rives, V. J. Mater. Sci. 1986, 20, 1427. (44) Garbowski,E.; Guenin, M.; Marion, M. C.; Primet, M. Appl. Catal. 1990,64,209. (45) Tanabe, S.; Ida, T.; Teuiki, H.; Ueno, A.; Kotera, Y.; Tohji, K.; Udagawa, Y. Chem. Lett. 1984, 1271. (46) Takasaki,S.;Takahashi,K.;Suzuki,H.;Sato,Y.;Ueno,A.;Kotera, Y . Chem. Lett. 1983,265. (47) Takasaki, 5.; Suzuki, H.; Takaluwhi, K-; Tanabe, S.; Ueno, A.; Kotera, Y. J. Chem. SOC., Faraday Trans. 1 1984,80,803. (48) Del Arco, M.; Rives, V. J. Mater. Sci. 1986,21, 2938. (49) Tohji, K.; Udagawa, Y.; Tanabe, S.;Ida, T.;Ueno, A. J. Am. Chem. SOC. 1984,106,5172. (50) Takasaki, S.; Takahashi, K.; Suzuki, H.Chem. Lett. 1983,256. (51) Baker, R. T. K.; Prestridge, E. B.; Garten, R. L. J. Catal. 1979, 56,390. (62) Takasaki, S.; Suzuki, H.; Takahashi, K.; Tanabe, S.; Ueno, A,; Kotera, Y. J. Chem. SOC.,Faraday Trans. I 1984,80,803.
TiOa-Supported Co and COO
Langmuir, Vol. 10, No. 1, 1994 187
particles does not depend on the phase transformation of Ti02 from anatase to rutile.
Conclusions Models of titania-supported cobalt were synthesized with dry chemical methods in an ultrahigh vacuum system and characterized with photoemission spectroscopy. Four different models were studied, (i) TiOz-o/CoO, (ii) TiOzo/Co, (iii) TiOz-r/CoO, and (iv) TiOz-r/Co, where -0 indicates a fully oxidized sample and -r apartiallyreduced sample. The samples were prepared at 300 K and annealed at 500, 700, and 900 K. All overlayers are less stable on the reduced than on the oxidized support. COO is destabilized by titania and decomposes between 500 and 700 K, depending on the support preparation. Cometal, either as deposited or from the decomposition of the oxide, diffuses into titania at temperatures above 700 K. The strong metal-support interaction leads to partial reduction Ti4+ Ti3+. The interaction is local and the result of direct electronic bonding between reduced sites and metal atoms.
-
All systems show SMSI behavior as judged by the distribution of cobalt but two systems, thick films of metallic cobalt on TiOz-o and TiOz-r/Co, display anomalous dispersion with the metal stabilized in the surface region even after prolonged annealing at 900 K. We propose the formation of bimetallic oxides as a possible explanation. The conclusion from our data must be that both the support and the overlayer play an active role in a CO hydrogenation catalyst and that unwanted bimetallic oxides can form at modest temperatures on a reduced support. Calcination prior to impregnation would thus be advantageous to prevent the formation of bimetallic phases. Acknowledgment. We acknowledge discussions with J. Robbins from EXXON R&E, Annandale, NJ, the help of M. Nygren, Stockholm University, in reducing the samples in hydrogen, and the assistance of R. Nyholm, J. N. Andersen, and the other staff at MAXLAB, Lund. We also thank C. M. Pradier for her kind discussions on the surface chemistry of reducible oxides.
