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TiO2 system
Surface Science 197 (1988) L281-L286 North-Holland, Amsterdam
L281
SURFACE SCIENCE LETTERS
AN INVESTIGATION BY ANGULAR RESOLVED X-RAY PHOTOELECTRON SPECTROSCOPY OF STRONG METAL-SUPPORT INTERACTION (SMSI) IN THE Pt/TiO2 SYSTEM Koji TAMURA, Ugo BARDI * and Yoshimasa NIHEI Institute of Industrial Science, University of Tokyo, 7-22-1 Roppongi, Minato-kg Tokyo 106, Japan Received 18 November 1987; accepted for publication 17 December 1987
Angular resolved XPS measurements were performed on platinum particles deposited on the TiO2(100) surface. High temperature reduction at 733 K in hydrogen at atmospheric pressure caused the appearance of lower binding energy states in the titanium 2p region. These states corresponded to the f.~rination of Ti 2+ and Ti 3+ ions on the surface. From the angular dependencies of the relative intensities of the Ti 2p and Pt 4f peaks, it appeared that the reduced Ti species were located in the outermost surface region, on the surface of the Pt particles.
After the first report by Tauster et al. [1] on the strong metal-support interaction (SMSI) effect occurring for platinum supported on titanium dioxide after high temperature reduction in hydrogen, a large number of studies have been performed in order to elucidate the cause of the suppression of H 2 and CO chemisorption in the SMSI state. Although models based on chemical [2] or electronic [3,4] effects have been proposed, the generally accepted model involves the migration of substrate moieties on the metal surface ("decoration" or "migration" model) [5,6]. In this model, it is commonly assumed that the moieties are formed from reduced titanium oxide. Ti3+-type oxides have been detected by TEM and ESR in catalysts formed out of Pt supported on titania after reduction in hydrogen [7-9]. However, surface sensitive techniques, and in particular XPS, appear better suited to prove the presence of reduced titanium moieties on the surface of the Pt particles. ]In the literature on the SMSI-type system, a number of works [10-13] reported that no reduced Ti spec;es could be detected by XPS. In contrast, in other studies, the detection of titanium suboxide species by XPS [14-17] or by other surface analytical techniques [18,19] was claimed, in most of these studies, the specification on the exact nature of the suboxide was insufficient, and the reduced Ti ions were identified as Ti 2+ in ref. [19] and as Ti 3+ in ref. [17]. * Present address: Dipartimento d~ CNmica, Universitfi di Firenze, 50121 Florence, Italy.
0039-6028/88/$03.50 © Elsevier Science Pub!ishers B.V. (Nortb-Holland Physics Publishing Divi~ic,~
L282
K. Tamura et ai. / A R X P S investigation of S M S l in Pt / Tit),
Given the contradictory claimes in the existing literature, we felt that a re-examination of the Pt/TiO2 system was necessary, with the objective of determining with certainty the presence and the nature of the reduced Ti ions formed on the surface upon reduction in H 2. In order to enhance the surface sensitivity for XPS measurements, we examined a system which is composed of evaporated platinum on the fiat surface of a TiO 2 single crystal. By angle resolved experiments on this system, information from different detection depths from the surface can be obtained. By using grazing electron take-off angles, the signal from species located in the outermost surface region can be enhanced. In order to further increase the sensitivity to species located on the platinum deposit, we examined a relatively thick Pt film to reduce the substrate contribution to the overall signal. Experiments were performed in a vacuum system capable of base pressures in the low 10-iv Torr range. This system was equipped with a hemi-spherical electron analyzer and a conventional A1 K a source. The acceptance angle of the analyzer entrance lens along the polar direction was less than + 1.3 °. The sample was held by a manipulator capable of both azimuthal and polar rotation. The sample was a TiO 2 single crystal with a surface cut and polished along the (100) plane. Cleaning of the sample in vacuum was performed by cycles of Ar ion bombardment and annealing, until no impurities were detected by XPS. Platinum was deposited by thermal evaporation from a filament. The thickness of the deposit was estimated from the relative intensities of the Pt 4f and Ti 2p transitions, assun~ng an exponential attenuation of the photoelectrons as a function of the path in the solid and mean free paths obtained from ref. [20]. Reduction was performed in situ by introducing H 2 at atmospheric pressure in the sample chamber and by annealing the sample with an infrared lamp at 773 K for 15 min. The hydrogen gas was subsequently evacuated, and the vacuum conditions were restored without exposing the sample to air before XPS measurements. Fig. la shows the Ti 2p spectrum for the TiO2(100) surface before Pt deposition. Tiffs spectrum shows that the initial surface contained only Ti 4+ ions, i.e. no reduced titanium species were present. Platinum was deposited onto this surface to an average thickn~.ss equal to 22.4,. In order to elirrfinate temperature effects from the, high t,~,-p~,-~m-,~ r,~d,,c.fi,-,, in hvclrc~o~n the sample was annealed in vacuum at 823 K for 20 rrfin. Tiffs treatment caused a reduction of the platinum signal intensity which could be attributed mainly to the coalescence of Iffghly dispersed particles into large particles [21], and in small part also to diffusion into the bulk substrate. The Ti 2p spectrum after the hydrogen gas treatment, i.e. in " S M S I " conditions, is shown in fig. lb for normal exit angle. A peak in the low binding energy side could be detected, indicating the formation of reduced Ti species. A fitting of the experimental Ti 2p spectrum was performed by summing
K. Tamura et al. / ARXPS investigation of SMSI in Pt / TiO 2
L283
Ti 2p
b
m .,4 m
m 4J
m o 4~ e-
a
465
460
Binding
455
Energy
/ eV
Fig. 1. Ti 2p spectra for (a) clean annealed TiO2(100 ) surface before Pt deposition, (b) after deposition of platinum, ~_,mealed in vacuum and successive reduction in H 2 at atmospheric pressure at 773 K.
curves which were obtained from the clean surface so as to represent different surface states. The best fit was obtained when three ionic states were assumed. Fig. 2 shows the analytical result obtained at a polar angle of 70 ° relative to the surface normal. According to the results reported in refs. [22,23] the binding energy found for these three states corresponded well to those of Ti02 (T{4+~ Ti.CJ~ (Ti3+~ and TiCJ (Ti 2+ ) Table 1 shows the variation of the relative intensity of each titanium component as a function of take-off angle. From these results, it can be seen that the lower bindhag energy states increase in intensity for more grazing collection angles, which indicates that the reduced species were located over the TiO z component. We note that small contributions from low binding energy states were also observed on the surface after the vacuum annealing treatment, i.e. before annealing in hydrogen. However, the signal from the reduced titanium states in this case, was not enhanced as the detection angie increased. Exposure of the reduced surface to X---
I~"
----k
--
..~
X---
l: r
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
K. Tamura et al. / A R X P S investigation of S M S I in Pt / TiO 2
L284
ri 2p
7O
"2
- -
,m
465
460 Binding Energy
45'5
/ eV
Fig. 2. Fitted spectrum of the reduced Pt/TiO2 surface showing the contribution of Ti 4+, Ti 3+ and Ti 2+ species.
20 Torr oxygen at 673 K diminished the T i 3+ and Ti ~+ states as a result of the re-oxidation of the suboxide. Finally, in table 2, we report the observed intensity ratio of the Pt 4f and of the Ti 2p suboxide components (Ti 3+ and Ti 2+) as a function of take-off angle. In thege results, the contribution by the diffraction effect was corrected. From these results, it was obvious that the intensity ratio decreased as the detection angle increased. This tendency indicated that the titanium suboxide was present on the surface of the platinum particles. These results were considered to be consistent with the '°decoration" model which predicted the migration of the reduced oxide moieties on the surface of the Pt particles. Table 1 Relative intensity (percentage) of ionic states versus take-off angle Angle from normal to the surface (deg)
Ti z +
Ti 3+
Ti4 +
0 50 70
13 22 22
5 9 16
82 76 62
K. Tamura et al. / A R X P S investigation of S M S l in Pt / TiO 2
L285
Table 2
Intensity ratio of Pt 4f and sum of Ti 2p Ti 3+ and Ti 2+ peaks versus take-off angle Angle from surface normal (deg)
Ratio
0 50 70
36.0 35.5 29.7
Our study clearly showed that suboxides were formed on the surface of the Pt/TiO2 sample after the reduction in hydrogen and that Ti 2+ was the major reduced component, although Ti 3+ was also present. In comparing our results with those of the existing literature, it is necessary to take into account the different relative sensitivities to surface species. Thus, the comparison of our XPS results with those obtained by TEM or ESR [7-9] may not be significant. It seems clear, as discussed in ref. [24], that the Ti 3+ state detected by ESR and TEM was located principally in the TiO2 bulk rather than in the surface region. These techniques would not be able to detect a very small amoant of Ti 2+ located only in the outermost surface region, which could be observed, however, by XPS at grazing take-off angle. More significant is the comparison of our results with those obtained by XPS or other surface techniques [10-19]. In this area, several authors reported that no reduced Ti ions whatsoever could be observed [10-13]. In other cases, evidence for the presence of titanium suboxJde was reported [14-19]. Although the reduced species were, in general, not clearly identified, an examination of the XPS spectrum of the Ti 2p region showed that the majority of the reduced species was Ti 3+, rather than Ti 2+ [14]. Also, reports [16,17] based on a siml01e enlargement of the Ti peaks (rather than the appearance of a new peak, as i~ the results of fig. 1 in the present work), seem to indicate the detection of a majority of Ti 3+ ions. In contrast, in ref. [19] the species were identified as Ti 2+ i o n s only. To account for the different resuks reported in the above-cited papers [10-19] and in the present work, it should be taken into account that the surface sensitivity of XPS may vary considerably depending on the experimental conditions, in our work, the use of grazing detection and of a reiativeiy thick Pt fihn permitted a significant reduction of the contribution to the overall signal from species located in the bulk or in the subsurface. Thus, differences in the proportions of the detected Ti 2+ and Ti 3+ or even the lack of detection, may actually depend on a different "probing depth" of the analysis rather than on a different surface composition. Our results therefore point out the importance of a high surface sensitivity to obtain a reliable characterization of the species which play an important role in detern~rfing the properties of SMSI-type systems. Alternatively, it may be possible teat SMSI
L286
K. Tamura et al. / ARXPS investigation of SMSI in Pt / Ti02
conditions can be attained independently of the exact stoichiometry of the titanium suboxide species formed, so that the proportion of Ti 3+ and Ti 2+ ions on the surface may not be a critical factor and may depend on the differences in the Pt coverage or in the conditions of the treatment.
References [1] S.J. Tauster, S.C. Fung and R.L. Garten, J. Am. Chem. Soc. 100 (1978) 170. [2] P.G. Menon and G.F. FromenL Vol. 11 of Studies in Surface Science and Catalysis (Elsevier, Ams*erdam, 1982) p. 171. [3] S.J. Tauster and SJ. Fung, J. Catalysis 55 (1978) 29. [4] J,M. Hermann, J. Catalysis 89 (1984) 404. [5] D.E. Resasco and G.L. Hailer, J. Catalysis 82 (1983) 279. [6] J. Santos, J. Phillips and J.A. Dumesic, J. Catalysis 81 (1983) 147. [7] R.T.K. Baker, E.B. Prestidge and R.L. Garten, J. Catalysis 56 (1979) 170. [8] R.T.IC Baker, E.B. Prestidge and R.L. Garten, J. Catalysis 59 (1979) 390. [9] T. Huizinga and R. Prins, J. Phys. Chem. 85 (1981) 2156. [10] S.H. Chien, B.N. Shelimov, D.E. Pesasco, E.L. Lee and G.L. Hailer, J. Catalysis 77 (1982) 301. [11] B.A. Sexton, A.E. Hughes and K. Foger, 2. Catalysis 77 (1982) 85. [12] W.A. Hongli, T. Sheng, X. Maosong, X. Guoxing and G. Xiexian, in: Metal-Support and Metal-Additive Effects in Catalysis, Eds. B. lmelik et al. (Elsevier, Amsterdam, 1982) p. 19. [13] B.H. Chela and J.M. White, J. Phys. Chem. 86 (1982) 3534. [14] S.C. Fung, J. Catalysis :9 (1982) 225. [15] T. Huizinga and R. Prins, in: Metal-Support and Metal-Additive Effects in Catalysis, Eds. B. Imelik et al. (Elsevier Amsterdam, 1982) p. 11. [16] H.R. Sedeghi and V.E Henrich, J. Catalysis 87 (1984) 279. [17] Y.W. Chuag, G. Xiong and C.C. Kao, .i. Catalysis 85 (1984) 237. [18] D,N. Belton, Y.M. Sun and J.M. White, J. Phys. Chem. 88 (1984) 5172. [19] S. Taka~ani and Y.W. Chung, J. Catalysis 90 (1984) 75. [20] D.IL ,Penn, J. Electron Spectrosc. Related Phenomena 9 (1976) 29. [21] K. Tamura, M. Kudo, M. Owari and Y. Ni,hei, Chem. Letters (1986) 1921. [22] N.R, Armstrong and R.K. Quirm, Surface Sci. 67 (1977) 451. [23] D.J. Dwyer, S,D. Cameron and J. Gland, Surface Sci 159 (1985) 430. [24] B.H Chen and J.M. White, J. Phys. Chem. 87 (1983) 1327.
L281
SURFACE SCIENCE LETTERS
AN INVESTIGATION BY ANGULAR RESOLVED X-RAY PHOTOELECTRON SPECTROSCOPY OF STRONG METAL-SUPPORT INTERACTION (SMSI) IN THE Pt/TiO2 SYSTEM Koji TAMURA, Ugo BARDI * and Yoshimasa NIHEI Institute of Industrial Science, University of Tokyo, 7-22-1 Roppongi, Minato-kg Tokyo 106, Japan Received 18 November 1987; accepted for publication 17 December 1987
Angular resolved XPS measurements were performed on platinum particles deposited on the TiO2(100) surface. High temperature reduction at 733 K in hydrogen at atmospheric pressure caused the appearance of lower binding energy states in the titanium 2p region. These states corresponded to the f.~rination of Ti 2+ and Ti 3+ ions on the surface. From the angular dependencies of the relative intensities of the Ti 2p and Pt 4f peaks, it appeared that the reduced Ti species were located in the outermost surface region, on the surface of the Pt particles.
After the first report by Tauster et al. [1] on the strong metal-support interaction (SMSI) effect occurring for platinum supported on titanium dioxide after high temperature reduction in hydrogen, a large number of studies have been performed in order to elucidate the cause of the suppression of H 2 and CO chemisorption in the SMSI state. Although models based on chemical [2] or electronic [3,4] effects have been proposed, the generally accepted model involves the migration of substrate moieties on the metal surface ("decoration" or "migration" model) [5,6]. In this model, it is commonly assumed that the moieties are formed from reduced titanium oxide. Ti3+-type oxides have been detected by TEM and ESR in catalysts formed out of Pt supported on titania after reduction in hydrogen [7-9]. However, surface sensitive techniques, and in particular XPS, appear better suited to prove the presence of reduced titanium moieties on the surface of the Pt particles. ]In the literature on the SMSI-type system, a number of works [10-13] reported that no reduced Ti spec;es could be detected by XPS. In contrast, in other studies, the detection of titanium suboxide species by XPS [14-17] or by other surface analytical techniques [18,19] was claimed, in most of these studies, the specification on the exact nature of the suboxide was insufficient, and the reduced Ti ions were identified as Ti 2+ in ref. [19] and as Ti 3+ in ref. [17]. * Present address: Dipartimento d~ CNmica, Universitfi di Firenze, 50121 Florence, Italy.
0039-6028/88/$03.50 © Elsevier Science Pub!ishers B.V. (Nortb-Holland Physics Publishing Divi~ic,~
L282
K. Tamura et ai. / A R X P S investigation of S M S l in Pt / Tit),
Given the contradictory claimes in the existing literature, we felt that a re-examination of the Pt/TiO2 system was necessary, with the objective of determining with certainty the presence and the nature of the reduced Ti ions formed on the surface upon reduction in H 2. In order to enhance the surface sensitivity for XPS measurements, we examined a system which is composed of evaporated platinum on the fiat surface of a TiO 2 single crystal. By angle resolved experiments on this system, information from different detection depths from the surface can be obtained. By using grazing electron take-off angles, the signal from species located in the outermost surface region can be enhanced. In order to further increase the sensitivity to species located on the platinum deposit, we examined a relatively thick Pt film to reduce the substrate contribution to the overall signal. Experiments were performed in a vacuum system capable of base pressures in the low 10-iv Torr range. This system was equipped with a hemi-spherical electron analyzer and a conventional A1 K a source. The acceptance angle of the analyzer entrance lens along the polar direction was less than + 1.3 °. The sample was held by a manipulator capable of both azimuthal and polar rotation. The sample was a TiO 2 single crystal with a surface cut and polished along the (100) plane. Cleaning of the sample in vacuum was performed by cycles of Ar ion bombardment and annealing, until no impurities were detected by XPS. Platinum was deposited by thermal evaporation from a filament. The thickness of the deposit was estimated from the relative intensities of the Pt 4f and Ti 2p transitions, assun~ng an exponential attenuation of the photoelectrons as a function of the path in the solid and mean free paths obtained from ref. [20]. Reduction was performed in situ by introducing H 2 at atmospheric pressure in the sample chamber and by annealing the sample with an infrared lamp at 773 K for 15 min. The hydrogen gas was subsequently evacuated, and the vacuum conditions were restored without exposing the sample to air before XPS measurements. Fig. la shows the Ti 2p spectrum for the TiO2(100) surface before Pt deposition. Tiffs spectrum shows that the initial surface contained only Ti 4+ ions, i.e. no reduced titanium species were present. Platinum was deposited onto this surface to an average thickn~.ss equal to 22.4,. In order to elirrfinate temperature effects from the, high t,~,-p~,-~m-,~ r,~d,,c.fi,-,, in hvclrc~o~n the sample was annealed in vacuum at 823 K for 20 rrfin. Tiffs treatment caused a reduction of the platinum signal intensity which could be attributed mainly to the coalescence of Iffghly dispersed particles into large particles [21], and in small part also to diffusion into the bulk substrate. The Ti 2p spectrum after the hydrogen gas treatment, i.e. in " S M S I " conditions, is shown in fig. lb for normal exit angle. A peak in the low binding energy side could be detected, indicating the formation of reduced Ti species. A fitting of the experimental Ti 2p spectrum was performed by summing
K. Tamura et al. / ARXPS investigation of SMSI in Pt / TiO 2
L283
Ti 2p
b
m .,4 m
m 4J
m o 4~ e-
a
465
460
Binding
455
Energy
/ eV
Fig. 1. Ti 2p spectra for (a) clean annealed TiO2(100 ) surface before Pt deposition, (b) after deposition of platinum, ~_,mealed in vacuum and successive reduction in H 2 at atmospheric pressure at 773 K.
curves which were obtained from the clean surface so as to represent different surface states. The best fit was obtained when three ionic states were assumed. Fig. 2 shows the analytical result obtained at a polar angle of 70 ° relative to the surface normal. According to the results reported in refs. [22,23] the binding energy found for these three states corresponded well to those of Ti02 (T{4+~ Ti.CJ~ (Ti3+~ and TiCJ (Ti 2+ ) Table 1 shows the variation of the relative intensity of each titanium component as a function of take-off angle. From these results, it can be seen that the lower bindhag energy states increase in intensity for more grazing collection angles, which indicates that the reduced species were located over the TiO z component. We note that small contributions from low binding energy states were also observed on the surface after the vacuum annealing treatment, i.e. before annealing in hydrogen. However, the signal from the reduced titanium states in this case, was not enhanced as the detection angie increased. Exposure of the reduced surface to X---
I~"
----k
--
..~
X---
l: r
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
K. Tamura et al. / A R X P S investigation of S M S I in Pt / TiO 2
L284
ri 2p
7O
"2
- -
,m
465
460 Binding Energy
45'5
/ eV
Fig. 2. Fitted spectrum of the reduced Pt/TiO2 surface showing the contribution of Ti 4+, Ti 3+ and Ti 2+ species.
20 Torr oxygen at 673 K diminished the T i 3+ and Ti ~+ states as a result of the re-oxidation of the suboxide. Finally, in table 2, we report the observed intensity ratio of the Pt 4f and of the Ti 2p suboxide components (Ti 3+ and Ti 2+) as a function of take-off angle. In thege results, the contribution by the diffraction effect was corrected. From these results, it was obvious that the intensity ratio decreased as the detection angle increased. This tendency indicated that the titanium suboxide was present on the surface of the platinum particles. These results were considered to be consistent with the '°decoration" model which predicted the migration of the reduced oxide moieties on the surface of the Pt particles. Table 1 Relative intensity (percentage) of ionic states versus take-off angle Angle from normal to the surface (deg)
Ti z +
Ti 3+
Ti4 +
0 50 70
13 22 22
5 9 16
82 76 62
K. Tamura et al. / A R X P S investigation of S M S l in Pt / TiO 2
L285
Table 2
Intensity ratio of Pt 4f and sum of Ti 2p Ti 3+ and Ti 2+ peaks versus take-off angle Angle from surface normal (deg)
Ratio
0 50 70
36.0 35.5 29.7
Our study clearly showed that suboxides were formed on the surface of the Pt/TiO2 sample after the reduction in hydrogen and that Ti 2+ was the major reduced component, although Ti 3+ was also present. In comparing our results with those of the existing literature, it is necessary to take into account the different relative sensitivities to surface species. Thus, the comparison of our XPS results with those obtained by TEM or ESR [7-9] may not be significant. It seems clear, as discussed in ref. [24], that the Ti 3+ state detected by ESR and TEM was located principally in the TiO2 bulk rather than in the surface region. These techniques would not be able to detect a very small amoant of Ti 2+ located only in the outermost surface region, which could be observed, however, by XPS at grazing take-off angle. More significant is the comparison of our results with those obtained by XPS or other surface techniques [10-19]. In this area, several authors reported that no reduced Ti ions whatsoever could be observed [10-13]. In other cases, evidence for the presence of titanium suboxJde was reported [14-19]. Although the reduced species were, in general, not clearly identified, an examination of the XPS spectrum of the Ti 2p region showed that the majority of the reduced species was Ti 3+, rather than Ti 2+ [14]. Also, reports [16,17] based on a siml01e enlargement of the Ti peaks (rather than the appearance of a new peak, as i~ the results of fig. 1 in the present work), seem to indicate the detection of a majority of Ti 3+ ions. In contrast, in ref. [19] the species were identified as Ti 2+ i o n s only. To account for the different resuks reported in the above-cited papers [10-19] and in the present work, it should be taken into account that the surface sensitivity of XPS may vary considerably depending on the experimental conditions, in our work, the use of grazing detection and of a reiativeiy thick Pt fihn permitted a significant reduction of the contribution to the overall signal from species located in the bulk or in the subsurface. Thus, differences in the proportions of the detected Ti 2+ and Ti 3+ or even the lack of detection, may actually depend on a different "probing depth" of the analysis rather than on a different surface composition. Our results therefore point out the importance of a high surface sensitivity to obtain a reliable characterization of the species which play an important role in detern~rfing the properties of SMSI-type systems. Alternatively, it may be possible teat SMSI
L286
K. Tamura et al. / ARXPS investigation of SMSI in Pt / Ti02
conditions can be attained independently of the exact stoichiometry of the titanium suboxide species formed, so that the proportion of Ti 3+ and Ti 2+ ions on the surface may not be a critical factor and may depend on the differences in the Pt coverage or in the conditions of the treatment.
References [1] S.J. Tauster, S.C. Fung and R.L. Garten, J. Am. Chem. Soc. 100 (1978) 170. [2] P.G. Menon and G.F. FromenL Vol. 11 of Studies in Surface Science and Catalysis (Elsevier, Ams*erdam, 1982) p. 171. [3] S.J. Tauster and SJ. Fung, J. Catalysis 55 (1978) 29. [4] J,M. Hermann, J. Catalysis 89 (1984) 404. [5] D.E. Resasco and G.L. Hailer, J. Catalysis 82 (1983) 279. [6] J. Santos, J. Phillips and J.A. Dumesic, J. Catalysis 81 (1983) 147. [7] R.T.K. Baker, E.B. Prestidge and R.L. Garten, J. Catalysis 56 (1979) 170. [8] R.T.IC Baker, E.B. Prestidge and R.L. Garten, J. Catalysis 59 (1979) 390. [9] T. Huizinga and R. Prins, J. Phys. Chem. 85 (1981) 2156. [10] S.H. Chien, B.N. Shelimov, D.E. Pesasco, E.L. Lee and G.L. Hailer, J. Catalysis 77 (1982) 301. [11] B.A. Sexton, A.E. Hughes and K. Foger, 2. Catalysis 77 (1982) 85. [12] W.A. Hongli, T. Sheng, X. Maosong, X. Guoxing and G. Xiexian, in: Metal-Support and Metal-Additive Effects in Catalysis, Eds. B. lmelik et al. (Elsevier, Amsterdam, 1982) p. 19. [13] B.H. Chela and J.M. White, J. Phys. Chem. 86 (1982) 3534. [14] S.C. Fung, J. Catalysis :9 (1982) 225. [15] T. Huizinga and R. Prins, in: Metal-Support and Metal-Additive Effects in Catalysis, Eds. B. Imelik et al. (Elsevier Amsterdam, 1982) p. 11. [16] H.R. Sedeghi and V.E Henrich, J. Catalysis 87 (1984) 279. [17] Y.W. Chuag, G. Xiong and C.C. Kao, .i. Catalysis 85 (1984) 237. [18] D,N. Belton, Y.M. Sun and J.M. White, J. Phys. Chem. 88 (1984) 5172. [19] S. Taka~ani and Y.W. Chung, J. Catalysis 90 (1984) 75. [20] D.IL ,Penn, J. Electron Spectrosc. Related Phenomena 9 (1976) 29. [21] K. Tamura, M. Kudo, M. Owari and Y. Ni,hei, Chem. Letters (1986) 1921. [22] N.R, Armstrong and R.K. Quirm, Surface Sci. 67 (1977) 451. [23] D.J. Dwyer, S,D. Cameron and J. Gland, Surface Sci 159 (1985) 430. [24] B.H Chen and J.M. White, J. Phys. Chem. 87 (1983) 1327.