- Email: [email protected]
Methane activation on alumina supported platinum, palladium, ruthenium and rhodium catalysts
Studies in Surface Science and Catalysis, volume 147 X. Bao and Y. Xu (Editors) 9 Elsevier B.V. All rights reserved.
643
Methane activation on alumina supported platinum, palladium, ruthenium and rhodium catalysts Ruth L. Martins a, Maria A. Baldanza a, Mariana M.V.M. Schmal*a'b
Souza
a
and Martin
aNUCAT/PEQ/COPPE, Universidade Federal do Rio de Janeiro, C.P. 68502, 21945-970, Rio de Janeiro, Brazil. *E-mail" [email protected] bEscola de Quimica, Universidade Federal do Rio de Janeiro, C.P. 68542, 21940-900, Rio de Janeiro, Brazil. ABSTRACT Methane activation was conducted on Pt, Pd, Ru and Rh alumina supported catalysts. Simultaneously with the gas chemisorption, H2, CO2 and CO were evolved from all the catalyst surfaces. The H2 evolution resulted from the association of hydrogen atoms provided from dissociative chemisorption of methane. CO and CO2 evolution was explained by the partial migration of carbon adspecies, from metal surface to the metal-support interface, followed by their interaction with the alumina 02- ions. 1. INTRODUCTION The direct methane conversion into more valuable chemicals remains a challenge for catalyst researchers. By far, the most promising route has been the oxidative coupling [1, 2] although the parallel formation of carbon oxides still results into poor selectivities to C2 products. Another approach is the two-step reaction sequences, involving transition metal catalysts and hydrogen. The dissociative chemisorption of methane occurs above 373K and produces three forms of carbon, C~, C~ and Cv, which are distinguishable by temperature they react with hydrogen. High flow rates of methane [3-6], membrane-assisted process [7] and hydrogen acceptors [8] are alternatives used experimentally to ensure the metal surface free from hydrogen adspecies, but indeed, result in a very low conversion. In this paper it was reported a comparative study about the interaction of C H 4 with 5% of transition metal loaded catalysts supported on alumina. In an attempt to go more deeply into the mechanism of the methane interaction with the catalyst surfaces, two types of experiments were performed:
644 "in situ" Infrared measurements of methane adsorption and surface reaction of hydrogen with carbon adspecies formed when methane pulses were chemisorbed at several temperatures.
2. EXPERIMENTAL 2.1. Catalyst Preparation The Pt, Pd and Ru catalyst, with 5% (w/w) of metal, were supplied by Acros Organics and were used as such. The Rh catalyst was prepared by wet point impregnation of an alumina provided by Davison Grace, with a RhC13.xH20 solution (Aldrich). The dried catalyst was calcined up to 673K in a flow of synthetic air (50 cm 3 min-1). 2.2. Methane chemisorption and reaction of carbon adspecies with hydrogen Infrared experiments were conducted using a Fourier Transform spectrometer, Perkin Elmer 2000, and self supported wafers (9.8 mg r -2 thickness) by using a Pyrex cell with CaF2 windows. During the sample pretreatment and gas adsorption the cell was attached to a vacuum glass system. The spectral domain was between 4000 and 1000 cm -1 with a 4 cm -1 resolution. The samples were reduced "in situ" with pure hydrogen at 673 K for one hour, followed by evacuation up to 10-5 Torr at the same time and temperature. After cooling down to room temperature the infrared spectrum was recorded and used as background for the other adsorption experiments. Adsorption studies were conducted by exposing the reduced wafers to 10 Tort of methane at room temperature, followed by heating the wafer to 373, 473 and 573K. Experiments with C02 chemisorption were also conducted on the reduced wafers, by exposing them to 30 Torr of CO2, followed by evacuation up to 105 Torr. The activated chemisorption of methane followed by reaction of the carbon adspecies with hydrogen were conducted in a TPD/TPR 2900 Micromeritics equipment using a Balzers quadrupole mass spectrometer as detector. All the catalysts (100 mg) were reduced "in situ" with hydrogen (50 cm 3 rain -~) in a ramp of 2K min -1 up to 673K, staying at this temperature overnight. The hydrogen was swept from the surface by flowing He at 673K for one hour, then the sample temperature was reduced to the reaction temperature, and the catalyst surface was exposed to three pulses of methane (44,6gmols each) in flowing He, followed by three pulses of hydrogen (44,6 gmols each). Each plotted point in Figures 2-5, represents the average of the three pulses.
645
3. RESULTS AND DISCUSSION 3.1. Infrared experiments Infrared spectra of methane chemisorbed on reduced wafer of Pt/A1203 at different temperatures, are exhibited in the Fig. 1. At 228K, a band at 1304cm -1 was observed and assigned as symmetric CH 3 bending of methane, in the gas phase [9]. As the temperature increased, bands at 1652, 1437 and 1229 cm -~ were formed and were interpreted as asymmetric stretch of unidentate and bidentate carbonate species [9]. The spectrum e, obtained by exposition of the just reduced catalyst to 30 Tort of CO2, at room temperature, followed by vacuum up to 104 Torr, gave support to the above interpretation. Similar spectra were obtained for the other alumina metal loaded catalysts, suggesting that methane was dehydrogenated at the metal surface, and the resulted carbon adspecies migrate to the metal-support interface, interacting with 0 2. ions of the support and forming carbonated adspecies. In a previous work [10] it was reported similar behavior when Pt/ZrO2 catalyst was exposed to methane at different temperatures. Bands at 1613, 1564, 1441 and 1229 cm -~ were interpreted as bicarbonate and carbonate species formed when carbon adspecies provided from dehydrogenation of methane at Pt ~ migrate toward metal-support interface and interact with the zirconia OH and O 2- ions. 3.2. Methane Chemisorption and reaction of C adspecies with hydrogen Simultaneously with the methane pulses, H2, CO2 and CO were evolved from all the catalyst surfaces as detected by the mass spectrometer. The H2 evolution resulted from the association of hydrogen atoms provided from dissociative chemisorption of methane, as indicated by reactions (1) and (2): 1,0] .--. 0,8 :5 II~
0,6
"-
0,4
1652
1437
1304 1229
d
c a
< 0,2
0,0
2000
cm
-1
Fig, 1. Pt/A1203 reduced at 773K: a, CH4 chemisorption at 228K; wafer heated to: b, 373K; c, 473K; d, 573K; e, CO2 chemisorption at 228K on reduced wafer.
646
H4
CHx.I+H
C spillover carbonated species Infrared absorptions:
Mx Oy I
1652, 1437, 1229 cm Scheme 1. Spiltover of C adspecies from metal surface to the metal-support interface, followed by interaction with OH and/or O2-ions of the support.
CH4 + 2* ---~CH3 ads nt- Hads
(1)
Had~ + Hads --+H2, where * represents a Pt ~ Pd ~ Rh ~ or Ru ~ sites.
(2)
C02 and CO evolutions can be understood by the partial migration of carbon
adspecies, formed at metal surface, to the metal-support interface followed by their interaction with the alumina 0 2- ions, as proposed in the Scheme 1. Part of this carbonated species still remains attached to the alumina surface, as was seen in the Infrared spectra exhibited at Fig. 1. The carbon adspecies which remain attached to the metal surfaces, were hydrogenated when the catalyst was submitted to pulses of H2. Only methane was observed, in amount detectable by the mass spectrometer. Figures 2-4 represent the methane activated chemisorptions followed by the hydrogenation of the carbon adspecies, exhibited at Fig. 5, from the different metal loaded catalysts. - - I - - 5% Pt/AI~O 3 --O-- 5% PdtAI203 5% Ru/AI20 s - - V - - 5 % Rh/AI 0
t ' - - ~ ,
450
500
,
550
--I---0---&-[ --V--
9 . ~ ~'
,
600
,
650
700
Temperature, K
Fig. 2.pmols of reacted CH4
4s
500
550
600
650
700
Temperature, K
Fig. 3.pmols of H2 evolved during CH4 chemisorption.
450
9
,
,5OO
5% 5% 5% 5%
.
Pt I AI203 Pd / AI203 Ru t AI203 Rh t AI203
,
55O
.
/y
,
6OO
.
,
65O
.
Temperature, K
Fig. 4. pmols of CO evolved during CH4 chemisorption.
647
I - - 5% --O-- 5% --&-- 5% --V--5%
4,0 3,5
--I---O---&---V--
Pt / AI203 Pd / AI203 Ru / AI203 t - - V ~ V Rh/AI~O~
/
3,0
5% 5% 5% 5%
Pt / AI203 Pd / AI203 Ru / AI20 a Rh / AI203
/
2,5 ('~
"6 I~
o
2,0
O
1,5-
"6 9-1
1,5-
1,0
E=
1,0-
0,5
0,50,0-0,5 450
9
5;0
5;o
6;0
6;0
7;o
Temperature, K
Fig. 5. pmols of CO2 evolved during methane chemisorption.
450
5;0
9
5;0
I-
6;0
I
9
6;0
7;0
Temperature, K
Fig. 6. pmols of CH4 evolved by reaction of CHx adspecies with H2.
Despite their low reactivities, Rh and Ru were the most active catalysts, as observed by the data of Fig. 2. Pt loaded catalyst was the less active. In general, the methane conversion increases as the temperature of the reaction increases for all catalysts. The H2 evolution also increases with the temperature of the reaction except for Rh, at 673K. However, it cannot be ruled out the possibility of hydrogen spillover from metal particles onto the support, when the temperature of the reaction was greater then 613K [11 ]. The profiles of CO evolution show the same tendency for increasing with the temperature of reaction. Ru and Rh loaded alumina showed a minimum for CO evolution at 553 K while Pd loaded alumina showed a little maximum at the same temperature, and since 599K, Pd, Ru an Rh catalysts exhibited a linear increase of CO evolution with the temperature of the reaction. Pt loaded alumina showed a different behavior, being the less effective for CO production, which was detected, in the gas phase, only at 673K. For CO2 evolution, Rh loaded catalyst exhibited a maximum at 593K, with a similar profile for H2 evolution showed in Fig. 2 Ru loaded catalyst exhibited two maxima at 553 and 633K, respectively. Pd and Ru loaded catalysts presented similar behavior with respect to CO2 evolution, and Pt loaded catalyst was the less selective. The presence of carbon oxides species were supported by Infrared experiments where carbonated species were observed at support surface. Rhodium catalyst was also the best metal for hydrogenation of carbon adspecies. Probably, for this catalyst, two types of carbonaceous material were formed during methane chemisorption, being hydrogenated at different temperatures (at 513K and above 600K). Pt and Ru catalysts exhibited similar behavior with respect to hydrogenation of carbon adspecies, and Pd showed a little maximum for CH4 evolution at 553K.
648 4. C O N C L U S I O N S Activated chemisorption of methane takes place at Pt, Pd, Ru and Rh supported catalysts with simultaneous evolutions of H2, CO and CO2. Carbon oxides were believed to result by the migration of part of carbon adspecies from the active metal sites toward the support where their interaction with OH and/or O 2- ions takes place. Pt catalyst was the less selective for the carbon oxides evolutions and carbonated adspecies formation. Rh was the most active metal for methane activation and also for stabilizing carbon adspecies in different structures (carbidic and amorphous carbon) suitable for hydrogenation around 500K and at temperatures higher than 599K. Pt and Ru catalysts showed similar behavior with respect to hydrogenation of carbon adspecies being much less active than Rh catalyst. The same is truth for Pd catalyst, which presented a little maximum for methane evolution at 553K. As no higher hydrocarbon species were detected it can be concluded that the operation conditions used in this work (pulses of methane in flowing helium) were not suitable for carbon nucleation needed for the formation of higher species.
REFERENCES [ 1] G.E. Kelleer and M.M. Bhasin, J. Catal. 73 (1982) 9. [2] J.H. Lunsford, Angew Chem. Int. Ed. Engl. 34 (1995) 970. [3] M. Belgued, A. Amariglio, P. Pardja, H. Amariglio, J. Catal. 159 (1996) 441. [4] M. Belgued, A. Amariglio, P. Pardja, H. Amariglio, J. Catal. 159 (1996) 449. [5] P. Par6ja, S. Molina, A. Amariglio, H. Amariglio, Appl. Catal. 168 (1998) 289. [6] M. Belgued, A. Amariglio, L. Lefort, P. Par6ja, H. Amariglio, J. Catal. 161 (1996) 282. [7] O. Gamier, J. Shu, B.P.A. Grandjean, Ind. Eng. Chem. Res. 36 (1997) 553. [8] P. Par6ja, M. Mercy, J.C. Gachon, A. Amariglio, H. Amariglio, Ind. Eng. Chem Res. 38 (1999) 1163. [9] L.H. Little in Infrared Spectra of Adsorbed Species, Academic Press, London, New York, 1966. [10] R.L.Martins, M.M.V.M. Souza, M.A. Baldanza, M. Schmal, Proceedings of the 18 th North American Catalysis Society Meeting, P-314, June 2003. [11] J.M. Hermann, P. Pichat in Studies in Surface Science and Catalysis, vol 17, Spillover of Adsorbed Species, G.M. Pajonk, S.J. Teichner, J.E. Germain, Editors, Elsevier Amsterdam, Oxford, New York, Tokyo, 1983.
643
Methane activation on alumina supported platinum, palladium, ruthenium and rhodium catalysts Ruth L. Martins a, Maria A. Baldanza a, Mariana M.V.M. Schmal*a'b
Souza
a
and Martin
aNUCAT/PEQ/COPPE, Universidade Federal do Rio de Janeiro, C.P. 68502, 21945-970, Rio de Janeiro, Brazil. *E-mail" [email protected] bEscola de Quimica, Universidade Federal do Rio de Janeiro, C.P. 68542, 21940-900, Rio de Janeiro, Brazil. ABSTRACT Methane activation was conducted on Pt, Pd, Ru and Rh alumina supported catalysts. Simultaneously with the gas chemisorption, H2, CO2 and CO were evolved from all the catalyst surfaces. The H2 evolution resulted from the association of hydrogen atoms provided from dissociative chemisorption of methane. CO and CO2 evolution was explained by the partial migration of carbon adspecies, from metal surface to the metal-support interface, followed by their interaction with the alumina 02- ions. 1. INTRODUCTION The direct methane conversion into more valuable chemicals remains a challenge for catalyst researchers. By far, the most promising route has been the oxidative coupling [1, 2] although the parallel formation of carbon oxides still results into poor selectivities to C2 products. Another approach is the two-step reaction sequences, involving transition metal catalysts and hydrogen. The dissociative chemisorption of methane occurs above 373K and produces three forms of carbon, C~, C~ and Cv, which are distinguishable by temperature they react with hydrogen. High flow rates of methane [3-6], membrane-assisted process [7] and hydrogen acceptors [8] are alternatives used experimentally to ensure the metal surface free from hydrogen adspecies, but indeed, result in a very low conversion. In this paper it was reported a comparative study about the interaction of C H 4 with 5% of transition metal loaded catalysts supported on alumina. In an attempt to go more deeply into the mechanism of the methane interaction with the catalyst surfaces, two types of experiments were performed:
644 "in situ" Infrared measurements of methane adsorption and surface reaction of hydrogen with carbon adspecies formed when methane pulses were chemisorbed at several temperatures.
2. EXPERIMENTAL 2.1. Catalyst Preparation The Pt, Pd and Ru catalyst, with 5% (w/w) of metal, were supplied by Acros Organics and were used as such. The Rh catalyst was prepared by wet point impregnation of an alumina provided by Davison Grace, with a RhC13.xH20 solution (Aldrich). The dried catalyst was calcined up to 673K in a flow of synthetic air (50 cm 3 min-1). 2.2. Methane chemisorption and reaction of carbon adspecies with hydrogen Infrared experiments were conducted using a Fourier Transform spectrometer, Perkin Elmer 2000, and self supported wafers (9.8 mg r -2 thickness) by using a Pyrex cell with CaF2 windows. During the sample pretreatment and gas adsorption the cell was attached to a vacuum glass system. The spectral domain was between 4000 and 1000 cm -1 with a 4 cm -1 resolution. The samples were reduced "in situ" with pure hydrogen at 673 K for one hour, followed by evacuation up to 10-5 Torr at the same time and temperature. After cooling down to room temperature the infrared spectrum was recorded and used as background for the other adsorption experiments. Adsorption studies were conducted by exposing the reduced wafers to 10 Tort of methane at room temperature, followed by heating the wafer to 373, 473 and 573K. Experiments with C02 chemisorption were also conducted on the reduced wafers, by exposing them to 30 Torr of CO2, followed by evacuation up to 105 Torr. The activated chemisorption of methane followed by reaction of the carbon adspecies with hydrogen were conducted in a TPD/TPR 2900 Micromeritics equipment using a Balzers quadrupole mass spectrometer as detector. All the catalysts (100 mg) were reduced "in situ" with hydrogen (50 cm 3 rain -~) in a ramp of 2K min -1 up to 673K, staying at this temperature overnight. The hydrogen was swept from the surface by flowing He at 673K for one hour, then the sample temperature was reduced to the reaction temperature, and the catalyst surface was exposed to three pulses of methane (44,6gmols each) in flowing He, followed by three pulses of hydrogen (44,6 gmols each). Each plotted point in Figures 2-5, represents the average of the three pulses.
645
3. RESULTS AND DISCUSSION 3.1. Infrared experiments Infrared spectra of methane chemisorbed on reduced wafer of Pt/A1203 at different temperatures, are exhibited in the Fig. 1. At 228K, a band at 1304cm -1 was observed and assigned as symmetric CH 3 bending of methane, in the gas phase [9]. As the temperature increased, bands at 1652, 1437 and 1229 cm -~ were formed and were interpreted as asymmetric stretch of unidentate and bidentate carbonate species [9]. The spectrum e, obtained by exposition of the just reduced catalyst to 30 Tort of CO2, at room temperature, followed by vacuum up to 104 Torr, gave support to the above interpretation. Similar spectra were obtained for the other alumina metal loaded catalysts, suggesting that methane was dehydrogenated at the metal surface, and the resulted carbon adspecies migrate to the metal-support interface, interacting with 0 2. ions of the support and forming carbonated adspecies. In a previous work [10] it was reported similar behavior when Pt/ZrO2 catalyst was exposed to methane at different temperatures. Bands at 1613, 1564, 1441 and 1229 cm -~ were interpreted as bicarbonate and carbonate species formed when carbon adspecies provided from dehydrogenation of methane at Pt ~ migrate toward metal-support interface and interact with the zirconia OH and O 2- ions. 3.2. Methane Chemisorption and reaction of C adspecies with hydrogen Simultaneously with the methane pulses, H2, CO2 and CO were evolved from all the catalyst surfaces as detected by the mass spectrometer. The H2 evolution resulted from the association of hydrogen atoms provided from dissociative chemisorption of methane, as indicated by reactions (1) and (2): 1,0] .--. 0,8 :5 II~
0,6
"-
0,4
1652
1437
1304 1229
d
c a
< 0,2
0,0
2000
cm
-1
Fig, 1. Pt/A1203 reduced at 773K: a, CH4 chemisorption at 228K; wafer heated to: b, 373K; c, 473K; d, 573K; e, CO2 chemisorption at 228K on reduced wafer.
646
H4
CHx.I+H
C spillover carbonated species Infrared absorptions:
Mx Oy I
1652, 1437, 1229 cm Scheme 1. Spiltover of C adspecies from metal surface to the metal-support interface, followed by interaction with OH and/or O2-ions of the support.
CH4 + 2* ---~CH3 ads nt- Hads
(1)
Had~ + Hads --+H2, where * represents a Pt ~ Pd ~ Rh ~ or Ru ~ sites.
(2)
C02 and CO evolutions can be understood by the partial migration of carbon
adspecies, formed at metal surface, to the metal-support interface followed by their interaction with the alumina 0 2- ions, as proposed in the Scheme 1. Part of this carbonated species still remains attached to the alumina surface, as was seen in the Infrared spectra exhibited at Fig. 1. The carbon adspecies which remain attached to the metal surfaces, were hydrogenated when the catalyst was submitted to pulses of H2. Only methane was observed, in amount detectable by the mass spectrometer. Figures 2-4 represent the methane activated chemisorptions followed by the hydrogenation of the carbon adspecies, exhibited at Fig. 5, from the different metal loaded catalysts. - - I - - 5% Pt/AI~O 3 --O-- 5% PdtAI203 5% Ru/AI20 s - - V - - 5 % Rh/AI 0
t ' - - ~ ,
450
500
,
550
--I---0---&-[ --V--
9 . ~ ~'
,
600
,
650
700
Temperature, K
Fig. 2.pmols of reacted CH4
4s
500
550
600
650
700
Temperature, K
Fig. 3.pmols of H2 evolved during CH4 chemisorption.
450
9
,
,5OO
5% 5% 5% 5%
.
Pt I AI203 Pd / AI203 Ru t AI203 Rh t AI203
,
55O
.
/y
,
6OO
.
,
65O
.
Temperature, K
Fig. 4. pmols of CO evolved during CH4 chemisorption.
647
I - - 5% --O-- 5% --&-- 5% --V--5%
4,0 3,5
--I---O---&---V--
Pt / AI203 Pd / AI203 Ru / AI203 t - - V ~ V Rh/AI~O~
/
3,0
5% 5% 5% 5%
Pt / AI203 Pd / AI203 Ru / AI20 a Rh / AI203
/
2,5 ('~
"6 I~
o
2,0
O
1,5-
"6 9-1
1,5-
1,0
E=
1,0-
0,5
0,50,0-0,5 450
9
5;0
5;o
6;0
6;0
7;o
Temperature, K
Fig. 5. pmols of CO2 evolved during methane chemisorption.
450
5;0
9
5;0
I-
6;0
I
9
6;0
7;0
Temperature, K
Fig. 6. pmols of CH4 evolved by reaction of CHx adspecies with H2.
Despite their low reactivities, Rh and Ru were the most active catalysts, as observed by the data of Fig. 2. Pt loaded catalyst was the less active. In general, the methane conversion increases as the temperature of the reaction increases for all catalysts. The H2 evolution also increases with the temperature of the reaction except for Rh, at 673K. However, it cannot be ruled out the possibility of hydrogen spillover from metal particles onto the support, when the temperature of the reaction was greater then 613K [11 ]. The profiles of CO evolution show the same tendency for increasing with the temperature of reaction. Ru and Rh loaded alumina showed a minimum for CO evolution at 553 K while Pd loaded alumina showed a little maximum at the same temperature, and since 599K, Pd, Ru an Rh catalysts exhibited a linear increase of CO evolution with the temperature of the reaction. Pt loaded alumina showed a different behavior, being the less effective for CO production, which was detected, in the gas phase, only at 673K. For CO2 evolution, Rh loaded catalyst exhibited a maximum at 593K, with a similar profile for H2 evolution showed in Fig. 2 Ru loaded catalyst exhibited two maxima at 553 and 633K, respectively. Pd and Ru loaded catalysts presented similar behavior with respect to CO2 evolution, and Pt loaded catalyst was the less selective. The presence of carbon oxides species were supported by Infrared experiments where carbonated species were observed at support surface. Rhodium catalyst was also the best metal for hydrogenation of carbon adspecies. Probably, for this catalyst, two types of carbonaceous material were formed during methane chemisorption, being hydrogenated at different temperatures (at 513K and above 600K). Pt and Ru catalysts exhibited similar behavior with respect to hydrogenation of carbon adspecies, and Pd showed a little maximum for CH4 evolution at 553K.
648 4. C O N C L U S I O N S Activated chemisorption of methane takes place at Pt, Pd, Ru and Rh supported catalysts with simultaneous evolutions of H2, CO and CO2. Carbon oxides were believed to result by the migration of part of carbon adspecies from the active metal sites toward the support where their interaction with OH and/or O 2- ions takes place. Pt catalyst was the less selective for the carbon oxides evolutions and carbonated adspecies formation. Rh was the most active metal for methane activation and also for stabilizing carbon adspecies in different structures (carbidic and amorphous carbon) suitable for hydrogenation around 500K and at temperatures higher than 599K. Pt and Ru catalysts showed similar behavior with respect to hydrogenation of carbon adspecies being much less active than Rh catalyst. The same is truth for Pd catalyst, which presented a little maximum for methane evolution at 553K. As no higher hydrocarbon species were detected it can be concluded that the operation conditions used in this work (pulses of methane in flowing helium) were not suitable for carbon nucleation needed for the formation of higher species.
REFERENCES [ 1] G.E. Kelleer and M.M. Bhasin, J. Catal. 73 (1982) 9. [2] J.H. Lunsford, Angew Chem. Int. Ed. Engl. 34 (1995) 970. [3] M. Belgued, A. Amariglio, P. Pardja, H. Amariglio, J. Catal. 159 (1996) 441. [4] M. Belgued, A. Amariglio, P. Pardja, H. Amariglio, J. Catal. 159 (1996) 449. [5] P. Par6ja, S. Molina, A. Amariglio, H. Amariglio, Appl. Catal. 168 (1998) 289. [6] M. Belgued, A. Amariglio, L. Lefort, P. Par6ja, H. Amariglio, J. Catal. 161 (1996) 282. [7] O. Gamier, J. Shu, B.P.A. Grandjean, Ind. Eng. Chem. Res. 36 (1997) 553. [8] P. Par6ja, M. Mercy, J.C. Gachon, A. Amariglio, H. Amariglio, Ind. Eng. Chem Res. 38 (1999) 1163. [9] L.H. Little in Infrared Spectra of Adsorbed Species, Academic Press, London, New York, 1966. [10] R.L.Martins, M.M.V.M. Souza, M.A. Baldanza, M. Schmal, Proceedings of the 18 th North American Catalysis Society Meeting, P-314, June 2003. [11] J.M. Hermann, P. Pichat in Studies in Surface Science and Catalysis, vol 17, Spillover of Adsorbed Species, G.M. Pajonk, S.J. Teichner, J.E. Germain, Editors, Elsevier Amsterdam, Oxford, New York, Tokyo, 1983.