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CeO2
Studies in Surface Science and Catalysis 130 A. Corma, F.V. Melo, S. Mendioroz and J.L.G. Fierro (Editors) 9 2000 Elsevier Science B.V. All fights reserved.
1349
Effect of Sulfur on the Oxygen Storage/release Capacity of Rh/CeO2 and Rh/CeO2-ZrO2 Model TWCs Marta Boaro, Carla de Leitenburg, Giuliano Dolcetti and Alessandro Trovarelli Dipartimento di Scienze e Tecnologie Chimiche, Universita di Udine, via del Cotonificio 108, 33100 Udine, Italy
In the present work we have examined the effect of sulfur in CO oxidation carried out under redox conditions over Rh supported on ceria and ceria-zirconia. Presence of SO2 (30-70 ppm) is shown to suppress the oxygen storage activity at low temperature of both ceria and ceria-zirconia based catalysts due to formation of bulklike sulfate species. However mixed oxides better recover catalytic activity after regeneration. It appeared that the presence of ZrO2 facilitates removal of sulfate species under reductive atmospheres, while no differences are found following regeneration under oxidizing conditions. This carries important implications for use of these materials in TWC formulations.
1. INTRODUCTION Ceria has an important role in the removal of noxious compounds from gaseous stream originating either from stationary or mobile sources and it is present as a component of fluid catalytic cracking catalysts (FCC)(1) and three-way catalysts (TWC)(2). In the latter, the main role of ceria is to provide oxygen buffering capacity during rich/lean oscillation of exhaust gases; however, ceria contributes to a number of other catalytic functions. In particular, it seems that ceria is also involved in the formation and storage of sulfates due to sulfur impurities present in the fuels (3). These impurities are oxidatively adsorbed on ceria with its simultaneous reduction to CeO2.x and subsequently they decompose to SO2 or H2S under reducing conditions (4-7). At present, the textural stability of ceria is not high enough to meet the requirements of several gas-phase catalytic applications. Therefore, much effort has been directed in recent years to find catalyst formulations which can enhance the thermal stability of ceria without diminishing its specials features, such as the redox properties. These characteristics can be obtained by doping ceria with transition and rare-earth metal oxide like Zr, La and Pr (8). In particular, ceria-zirconia solid
1350 solutions have been used in the formulations of car exhaust catalysts since the early 1990s (9). They show a higher thermal resistance compared with conventional CeO2containg TWC, a higher reduction efficiency of the redox couple Ce4§ 3§ and a good oxygen storage/release capacity (10-13). However, a few fundamental studies exist on the sulfation of these materials (7,14), although this is a key issue in the evaluation of catalyst formulations. In these studies it was shown that formation of both surface and bulklike sulfate species occurs either in the presence or in the absence of the metal, and these species of different thermal stability can be partially removed after vacuum treatment at high temperature. With the present study we want to gain further information on sulfur adsorption in these materials and especially on the impact of sulphur on the oxygen storage capacity.
2. EXPERIMENTAL
CeO2 and CeO2-ZrO2 solid solutions were provided by Rhodia and the characterization of these samples was recently reported in the literature (15). Catalysts containing Rh were prepared by incipient wetness impregnation of the oxides previously calcined at 1173K with an aqueous solution of RhC13-3H20, followed by drying (373K for 15 h) and calcination at 773 K for 2 h. The specific surface area was measured by the BET method (see Table 1) and the effective Rh loading was checked by atomic adsorption spectrometry. Prior to catalytic testing the Rh-supported catalysts were treated in situ under H2 for one hour at 473 K. Table1 : characteristics of samples used in this study. Sample composition SA (m2/g) Rh/CZ100 Rh/CeO2 38 Rh / CZ80 Rh/Ce0.8Zr0.202 49 Rh / CZ50 Rh / Ce0.sZr0.sO2 53 Rh / CZ 15 Rh / Ce0.152Zr0.84802 55 Rh/CZ0 Rh/ZrO2 8
Rh(wt%) 0.50 0.59 0.52 0.41 0.45
The oxygen storage measurements and the deactivation tests were carried out in a microreactor system operating under steady-state and oscillating conditions, and will be described elsewhere (16). Under cycling operating mode the catalyst (0.1g) was treated with an oscillating feedstream (frequency 0.5 Hz, total flow 0.12 N1/min) containing alternately CO (4% in He) and 02 (2% in He). Conversion to CO2 was taken as an estimate of oxygen storage activity. Catalyst deactivation was performed at 473K by adding SO2 to the feedstream (30-70 ppm) and measuring the degree of CO conversion vs. time with a gas chromatograph equipped with a TCD detector.
1351 The characterization of the spent catalysts were carried out by combined TPR/TPO/TPD. They were performed by heating the spent catalysts (from 298K up to 1373K, 10K/rain) in a stream of H2 (5% in Ar), 02 (1% in He), and He respectively, with a total flow of 35ml/min. The reaction products, such as SO2 and H2S, released during the course of temperature programmed measurements were determined by means of mass spectrometry (Omnistar GDS-300 Balzers). FT-IR measurements were carried out with a FTS-40 instrument (Biorad).
3. RESULTS AND DISCUSSION In the presence of Rh, both ceria and ceria-zirconia solid solutions underwent a rapid and strong deactivation (Fig. la) and the conversion drops to less than 10% of the initial value in one hour of reaction. The deactivation rate is different; for ZrO2rich oxides the activity drops slowly with time, while a faster deactivation is observed with Rh/CZ100 and Rh/CZ80. However, after 1 h of reaction similar activities are found for all samples. The content of sulfur in the spent catalysts is near the theoretical value as expected if all the SO2 were adsorbed. This corresponds to ca. 0.01 gS/gcat and this value does not depend on the content of zirconium in solid solutions, in agreement with results reported by Bazin et al. (13) who did not found difference in the sulfur adsorption capacity of ceria and ceria-zirconia in the presence of oxygen. It seems therefore that the presence of Zr does not interfere with the adsorption properties. lOOI -~
~
o
N
=
~
80 60-
~ 40 ~.
,.
0~9
~ 20!
9
0
10 20 30 40 50 60 70 80 90 100 0 1'0 2'0 3'0 4'0 5'0 6'0 7'0 8'0 9'0 100 Time (min.) Time (min.) Fig. 1. Oxygen storage activity vs time in the presence of 70 p p m of 902 before (a) and after regeneration under reducing conditions (b): Rh/CZ100 ( , ) , Rh/CZ80 (O),
Rh/CZ50 (A), Rh/CZ15 (U), Rh/CZ(J,). The nature of the species formed were investigated by FT-IR spectra collected on samples after deactivation. It seems that bulklike sulfate species are the majority either in Rh/CZ100 and Rh/CZ50, with a broad band at ca. 1120 cm 1. Only traces of sulfites and surface sulfate species were observed. It is likely that sulfites, if initially
1352 present, are transformed to sulfates under our reaction conditions (lh under CO/O2 at 473K), as observed for sulfites in Pt containing catalysts (7). The presence of mobile oxygen species in these materials, which can be easily released, may explain the formation of sulfates also in a slight reducing environment. The absence of surface sulfate species is more puzzling; they are generally observed after sulfation of high-surface area ceria samples both in the presence and in the absence of the metal (6), while they are not formed on low surface area samples. Our surface area is not high if compared to that reported in ref. 7 (38 m2/g vs 13); moreover, the formation of bulklike sulfates from surface sulfates is promoted by increasing the temperature (7,17). Both conditions, which apply to ours, can explain the absence of surface sulfates. The thermal stability of adsorbed sulfur species was studied on deactivated samples by heating under different atmospheres and monitoring the evolution of gaseous products. It is reported that sulfur species on zirconia and ceria-based catalysts are stable up to ca. 673K under vacuum (13). Our temperature programmed experiments reveal that the nature of the support does not influence the stability of the sulfur species adsorbed. In Fig. 2 are reported the temperature programmed traces of SO2 and H2S formed under oxidizing and reductive conditions respectively over Rh/CZ100 and Rh/CZS0.
ei o ,,.~176
."
250
450
650 850 10'50 Temperature (K)
~,. " ~.~
12'50 1450
Fig. 2. Temperature programmed profiles for Rh/CZ50 (a) and Rh/CZ100 (b)" SO2 signal from TPD (---), S O 2 signal from TPO (......), H2S signal from TPR (-). Desorption starts at around 700K regardless of the support used and the traces are qualitatively similar. However the differences are related to the amount of gases desorbed at different temperatures; particularly it can be evidenced that the integrated amount of H2S desorbed from Rh/CZ50 is higher at low temperature than the corresponding amount formed on Rh/CZ100 (Fig. 3). This indicates that under reductive conditions formation of H2S is facilitated on solid solutions and under
1353 these conditions removal of sulfates is enhanced at lower temperature. Therefore the presence of ZrO2 promotes the reduction of sulfates, in agreement with its known role in the promotion of Ce 4§reduction (9).
o
.o
a r~
/
680
RtdCZ100 730 780 830 Temperature(K)
880
Fig. 3. Formation of H2S during TPR experiments. Removal of sulfates results in recovery of the catalytic activity, and the activity level after regeneration is closely dependent on the conditions used. In accordance with TP data, regeneration under reductive or slightly reductive conditions (either H2 or CO/O2 environment) restores ca. 80% of oxygen storage activity on Rh/CZ50 and less than 50% (70% with CO/O2) on Rh/CZ100 (Fig. 4). 100 Rh/CZ100 o =
80 60
r~
40 oo 9 20
o
2.._1_[Z21
c d e f Fig. 4. Oxygen storage activity of fresh (a) and deactivated catalyst (b). Activity after treatment of the deactivated catalyst at 773 K for lh under H2 (c), CO/O2 (d) He (e) and air (f). a
b
c
d
e
f
a
b
Regeneration under inert atmosphere (He) or under oxidative condition does not lead to substantial reactivation with both catalysts. Similar features are observed with the other ceria-zirconia compositions (Fig. lb). After regeneration under hydrogen the recovery of Rh/CZ catalysts is higher and the deactivation profiles are qualitatively similar to those observed on the fresh catalysts.
1354 In summary, the formation of similar sulfate species on ceria and ceria-zirconia is observed, and the presence of ZrO2 does not strongly influence their composition and thermal stability. Instead, their regeneration is strongly influenced by the presence of ZrO2. The formation of intermediate cerium oxysulfides may be suggested (18). The higher recovery of solid solution should involve migration of oxygen and sulfur ions from bulk to surface, thus exploiting the intrinsic higher anion mobility of these catalysts. ACKNOWLEDGMENTS: The authors wish to thank Rhodia for providing samples
for this study. We are also grateful to dr. Sandro Recchia (Department of Chemistry, Universita degli Studi dell'Insubria) for determining Rh loading. REFERENCES
1. A. K. Bhattacharyya, G. M. Woltermann, J. S.Yoo, J. A. Karch and W. E. Cormier, Ind. Eng. Chem. Res., 27 (1988) 1356. 2. A. Trovarelli, Catal. Rev. Sci. Eng., 38 (1996) 439. 3. B. Harrison, A. F. Diwell and C. Hallett, Platinum Metal Review, 32 (1988) 73. 4. A. F. Diwell, S. E.Golunski, J. R. Taylor and T. J. Truex, Stud. Surf. Sci. Catal., 71 (1991) 417. 5. S. Lundgren, G. Spiess, O. Hjortsberg, E. Jobson, I. Gottberg and G. Smedler, Stud. Surf. Sci. Catal., 96 (1995) 763. 6. M. Waqif, P. Bazin, J. C. Lavalley and G. Blanchard, O. Touret, Appl. Cat: B: Env. 11 (1997) 193. 7. P. Bazin, O. Sour, J. C. Lavalley, G. Blanchard, V. Visciglio, and O. Touret, Appl. Catal. B: Env. 13 (1997) 265. 8. A. Trovarelli, C. de Leitenburg and G. Dolcetti, CHEMTECH, 27 (1997) 32. 9. J. Kaspar, P. Fornasiero and M. Graziani, Catal. Today, 50 (1999) 285. 10. P. Fornasiero, R. Di Monte, G. Ranga Rao, J. Kaspar, S. Meriani, A. Trovarelli and M. Graziani, J. Catal., 151 (1995) 168. 11. C. E. Hori, H. Permana, Ng, K. Y. Simon, A. Brenner, K. More, K. M. Rahmoeller and D. Belton,, Appl. Catal. B: Env. 16 (1998) 105. 12. A. Trovarelli, F. Zamar, J. Llorca, C. de Leitenburg, G. Dolcetti and J. Kiss, J. Catal. 169 (1997) 490. 13. H.-W. Jen, G.W. Graham, W. Chun, R.W. McCabe, J.-P. Cuif, S.E. Deutsch, and O. Touret, Catal. Today 50 (1999) 309. 14. P. Bazin, O. Saur, J. C. Lavalley, A. M. Le Govic and G. Blanchard, Stud. Surf. Sci. Catal., 116 (1998) 571. 15. G. Colon, M. Pijolat, F. Valdivieso, H. Vidal, J. Kaspar, E. Finocchio, M. Daturi, C. Binet, J. C. Lavalley, R.T. Baker, and S. Bernal, J. Chem. Soc. Faraday Trans., 94 (1998) 3717. 16. M. Boaro, G. Dolcetti and A. Trovarelli, manuscript in preparation. 17. J. Twu, C. J. Chuang, K. I. Chang, C. H. Yang and K. H. Chen, Appl. Catal. B: Env. 12 (1997) 309. 18. Y. L. Suponitskii, G. M. Kuz'micheva and A. A. Eliseev, Russ. Chem. Rev. 57 (1988) 209.
1349
Effect of Sulfur on the Oxygen Storage/release Capacity of Rh/CeO2 and Rh/CeO2-ZrO2 Model TWCs Marta Boaro, Carla de Leitenburg, Giuliano Dolcetti and Alessandro Trovarelli Dipartimento di Scienze e Tecnologie Chimiche, Universita di Udine, via del Cotonificio 108, 33100 Udine, Italy
In the present work we have examined the effect of sulfur in CO oxidation carried out under redox conditions over Rh supported on ceria and ceria-zirconia. Presence of SO2 (30-70 ppm) is shown to suppress the oxygen storage activity at low temperature of both ceria and ceria-zirconia based catalysts due to formation of bulklike sulfate species. However mixed oxides better recover catalytic activity after regeneration. It appeared that the presence of ZrO2 facilitates removal of sulfate species under reductive atmospheres, while no differences are found following regeneration under oxidizing conditions. This carries important implications for use of these materials in TWC formulations.
1. INTRODUCTION Ceria has an important role in the removal of noxious compounds from gaseous stream originating either from stationary or mobile sources and it is present as a component of fluid catalytic cracking catalysts (FCC)(1) and three-way catalysts (TWC)(2). In the latter, the main role of ceria is to provide oxygen buffering capacity during rich/lean oscillation of exhaust gases; however, ceria contributes to a number of other catalytic functions. In particular, it seems that ceria is also involved in the formation and storage of sulfates due to sulfur impurities present in the fuels (3). These impurities are oxidatively adsorbed on ceria with its simultaneous reduction to CeO2.x and subsequently they decompose to SO2 or H2S under reducing conditions (4-7). At present, the textural stability of ceria is not high enough to meet the requirements of several gas-phase catalytic applications. Therefore, much effort has been directed in recent years to find catalyst formulations which can enhance the thermal stability of ceria without diminishing its specials features, such as the redox properties. These characteristics can be obtained by doping ceria with transition and rare-earth metal oxide like Zr, La and Pr (8). In particular, ceria-zirconia solid
1350 solutions have been used in the formulations of car exhaust catalysts since the early 1990s (9). They show a higher thermal resistance compared with conventional CeO2containg TWC, a higher reduction efficiency of the redox couple Ce4§ 3§ and a good oxygen storage/release capacity (10-13). However, a few fundamental studies exist on the sulfation of these materials (7,14), although this is a key issue in the evaluation of catalyst formulations. In these studies it was shown that formation of both surface and bulklike sulfate species occurs either in the presence or in the absence of the metal, and these species of different thermal stability can be partially removed after vacuum treatment at high temperature. With the present study we want to gain further information on sulfur adsorption in these materials and especially on the impact of sulphur on the oxygen storage capacity.
2. EXPERIMENTAL
CeO2 and CeO2-ZrO2 solid solutions were provided by Rhodia and the characterization of these samples was recently reported in the literature (15). Catalysts containing Rh were prepared by incipient wetness impregnation of the oxides previously calcined at 1173K with an aqueous solution of RhC13-3H20, followed by drying (373K for 15 h) and calcination at 773 K for 2 h. The specific surface area was measured by the BET method (see Table 1) and the effective Rh loading was checked by atomic adsorption spectrometry. Prior to catalytic testing the Rh-supported catalysts were treated in situ under H2 for one hour at 473 K. Table1 : characteristics of samples used in this study. Sample composition SA (m2/g) Rh/CZ100 Rh/CeO2 38 Rh / CZ80 Rh/Ce0.8Zr0.202 49 Rh / CZ50 Rh / Ce0.sZr0.sO2 53 Rh / CZ 15 Rh / Ce0.152Zr0.84802 55 Rh/CZ0 Rh/ZrO2 8
Rh(wt%) 0.50 0.59 0.52 0.41 0.45
The oxygen storage measurements and the deactivation tests were carried out in a microreactor system operating under steady-state and oscillating conditions, and will be described elsewhere (16). Under cycling operating mode the catalyst (0.1g) was treated with an oscillating feedstream (frequency 0.5 Hz, total flow 0.12 N1/min) containing alternately CO (4% in He) and 02 (2% in He). Conversion to CO2 was taken as an estimate of oxygen storage activity. Catalyst deactivation was performed at 473K by adding SO2 to the feedstream (30-70 ppm) and measuring the degree of CO conversion vs. time with a gas chromatograph equipped with a TCD detector.
1351 The characterization of the spent catalysts were carried out by combined TPR/TPO/TPD. They were performed by heating the spent catalysts (from 298K up to 1373K, 10K/rain) in a stream of H2 (5% in Ar), 02 (1% in He), and He respectively, with a total flow of 35ml/min. The reaction products, such as SO2 and H2S, released during the course of temperature programmed measurements were determined by means of mass spectrometry (Omnistar GDS-300 Balzers). FT-IR measurements were carried out with a FTS-40 instrument (Biorad).
3. RESULTS AND DISCUSSION In the presence of Rh, both ceria and ceria-zirconia solid solutions underwent a rapid and strong deactivation (Fig. la) and the conversion drops to less than 10% of the initial value in one hour of reaction. The deactivation rate is different; for ZrO2rich oxides the activity drops slowly with time, while a faster deactivation is observed with Rh/CZ100 and Rh/CZ80. However, after 1 h of reaction similar activities are found for all samples. The content of sulfur in the spent catalysts is near the theoretical value as expected if all the SO2 were adsorbed. This corresponds to ca. 0.01 gS/gcat and this value does not depend on the content of zirconium in solid solutions, in agreement with results reported by Bazin et al. (13) who did not found difference in the sulfur adsorption capacity of ceria and ceria-zirconia in the presence of oxygen. It seems therefore that the presence of Zr does not interfere with the adsorption properties. lOOI -~
~
o
N
=
~
80 60-
~ 40 ~.
,.
0~9
~ 20!
9
0
10 20 30 40 50 60 70 80 90 100 0 1'0 2'0 3'0 4'0 5'0 6'0 7'0 8'0 9'0 100 Time (min.) Time (min.) Fig. 1. Oxygen storage activity vs time in the presence of 70 p p m of 902 before (a) and after regeneration under reducing conditions (b): Rh/CZ100 ( , ) , Rh/CZ80 (O),
Rh/CZ50 (A), Rh/CZ15 (U), Rh/CZ(J,). The nature of the species formed were investigated by FT-IR spectra collected on samples after deactivation. It seems that bulklike sulfate species are the majority either in Rh/CZ100 and Rh/CZ50, with a broad band at ca. 1120 cm 1. Only traces of sulfites and surface sulfate species were observed. It is likely that sulfites, if initially
1352 present, are transformed to sulfates under our reaction conditions (lh under CO/O2 at 473K), as observed for sulfites in Pt containing catalysts (7). The presence of mobile oxygen species in these materials, which can be easily released, may explain the formation of sulfates also in a slight reducing environment. The absence of surface sulfate species is more puzzling; they are generally observed after sulfation of high-surface area ceria samples both in the presence and in the absence of the metal (6), while they are not formed on low surface area samples. Our surface area is not high if compared to that reported in ref. 7 (38 m2/g vs 13); moreover, the formation of bulklike sulfates from surface sulfates is promoted by increasing the temperature (7,17). Both conditions, which apply to ours, can explain the absence of surface sulfates. The thermal stability of adsorbed sulfur species was studied on deactivated samples by heating under different atmospheres and monitoring the evolution of gaseous products. It is reported that sulfur species on zirconia and ceria-based catalysts are stable up to ca. 673K under vacuum (13). Our temperature programmed experiments reveal that the nature of the support does not influence the stability of the sulfur species adsorbed. In Fig. 2 are reported the temperature programmed traces of SO2 and H2S formed under oxidizing and reductive conditions respectively over Rh/CZ100 and Rh/CZS0.
ei o ,,.~176
."
250
450
650 850 10'50 Temperature (K)
~,. " ~.~
12'50 1450
Fig. 2. Temperature programmed profiles for Rh/CZ50 (a) and Rh/CZ100 (b)" SO2 signal from TPD (---), S O 2 signal from TPO (......), H2S signal from TPR (-). Desorption starts at around 700K regardless of the support used and the traces are qualitatively similar. However the differences are related to the amount of gases desorbed at different temperatures; particularly it can be evidenced that the integrated amount of H2S desorbed from Rh/CZ50 is higher at low temperature than the corresponding amount formed on Rh/CZ100 (Fig. 3). This indicates that under reductive conditions formation of H2S is facilitated on solid solutions and under
1353 these conditions removal of sulfates is enhanced at lower temperature. Therefore the presence of ZrO2 promotes the reduction of sulfates, in agreement with its known role in the promotion of Ce 4§reduction (9).
o
.o
a r~
/
680
RtdCZ100 730 780 830 Temperature(K)
880
Fig. 3. Formation of H2S during TPR experiments. Removal of sulfates results in recovery of the catalytic activity, and the activity level after regeneration is closely dependent on the conditions used. In accordance with TP data, regeneration under reductive or slightly reductive conditions (either H2 or CO/O2 environment) restores ca. 80% of oxygen storage activity on Rh/CZ50 and less than 50% (70% with CO/O2) on Rh/CZ100 (Fig. 4). 100 Rh/CZ100 o =
80 60
r~
40 oo 9 20
o
2.._1_[Z21
c d e f Fig. 4. Oxygen storage activity of fresh (a) and deactivated catalyst (b). Activity after treatment of the deactivated catalyst at 773 K for lh under H2 (c), CO/O2 (d) He (e) and air (f). a
b
c
d
e
f
a
b
Regeneration under inert atmosphere (He) or under oxidative condition does not lead to substantial reactivation with both catalysts. Similar features are observed with the other ceria-zirconia compositions (Fig. lb). After regeneration under hydrogen the recovery of Rh/CZ catalysts is higher and the deactivation profiles are qualitatively similar to those observed on the fresh catalysts.
1354 In summary, the formation of similar sulfate species on ceria and ceria-zirconia is observed, and the presence of ZrO2 does not strongly influence their composition and thermal stability. Instead, their regeneration is strongly influenced by the presence of ZrO2. The formation of intermediate cerium oxysulfides may be suggested (18). The higher recovery of solid solution should involve migration of oxygen and sulfur ions from bulk to surface, thus exploiting the intrinsic higher anion mobility of these catalysts. ACKNOWLEDGMENTS: The authors wish to thank Rhodia for providing samples
for this study. We are also grateful to dr. Sandro Recchia (Department of Chemistry, Universita degli Studi dell'Insubria) for determining Rh loading. REFERENCES
1. A. K. Bhattacharyya, G. M. Woltermann, J. S.Yoo, J. A. Karch and W. E. Cormier, Ind. Eng. Chem. Res., 27 (1988) 1356. 2. A. Trovarelli, Catal. Rev. Sci. Eng., 38 (1996) 439. 3. B. Harrison, A. F. Diwell and C. Hallett, Platinum Metal Review, 32 (1988) 73. 4. A. F. Diwell, S. E.Golunski, J. R. Taylor and T. J. Truex, Stud. Surf. Sci. Catal., 71 (1991) 417. 5. S. Lundgren, G. Spiess, O. Hjortsberg, E. Jobson, I. Gottberg and G. Smedler, Stud. Surf. Sci. Catal., 96 (1995) 763. 6. M. Waqif, P. Bazin, J. C. Lavalley and G. Blanchard, O. Touret, Appl. Cat: B: Env. 11 (1997) 193. 7. P. Bazin, O. Sour, J. C. Lavalley, G. Blanchard, V. Visciglio, and O. Touret, Appl. Catal. B: Env. 13 (1997) 265. 8. A. Trovarelli, C. de Leitenburg and G. Dolcetti, CHEMTECH, 27 (1997) 32. 9. J. Kaspar, P. Fornasiero and M. Graziani, Catal. Today, 50 (1999) 285. 10. P. Fornasiero, R. Di Monte, G. Ranga Rao, J. Kaspar, S. Meriani, A. Trovarelli and M. Graziani, J. Catal., 151 (1995) 168. 11. C. E. Hori, H. Permana, Ng, K. Y. Simon, A. Brenner, K. More, K. M. Rahmoeller and D. Belton,, Appl. Catal. B: Env. 16 (1998) 105. 12. A. Trovarelli, F. Zamar, J. Llorca, C. de Leitenburg, G. Dolcetti and J. Kiss, J. Catal. 169 (1997) 490. 13. H.-W. Jen, G.W. Graham, W. Chun, R.W. McCabe, J.-P. Cuif, S.E. Deutsch, and O. Touret, Catal. Today 50 (1999) 309. 14. P. Bazin, O. Saur, J. C. Lavalley, A. M. Le Govic and G. Blanchard, Stud. Surf. Sci. Catal., 116 (1998) 571. 15. G. Colon, M. Pijolat, F. Valdivieso, H. Vidal, J. Kaspar, E. Finocchio, M. Daturi, C. Binet, J. C. Lavalley, R.T. Baker, and S. Bernal, J. Chem. Soc. Faraday Trans., 94 (1998) 3717. 16. M. Boaro, G. Dolcetti and A. Trovarelli, manuscript in preparation. 17. J. Twu, C. J. Chuang, K. I. Chang, C. H. Yang and K. H. Chen, Appl. Catal. B: Env. 12 (1997) 309. 18. Y. L. Suponitskii, G. M. Kuz'micheva and A. A. Eliseev, Russ. Chem. Rev. 57 (1988) 209.