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al2o3 catalysts on the partial oxidation and autothermal reforming of methane
Natural Gas Conversion VIII F.B. Noronha, M. Schmal, E.F. Sousa-Aguiar (Editors) © 2007 Published by Elsevier B.V.
409
The performance of Pt/CeZrO2/Al2O3 catalysts on the partial oxidation and autothermal reforming of methane Vanessa B. Mortolaa, Juan A. C. Ruizb, Diego da S. Martineza, Lisiane V. Mattosb, Fábio B. Noronhab, Carla E. Horia a
Faculdade de Engenharia Química - Universidade Federal de Uberlândia, Av. João Naves de Ávila, 2160/Bloco 1K, Uberlândia-MG 38400-902, Brazil b Laboratório de Catálise, Instituto Nacional de Tecnologia/MCT, Av. Venezuela, 82/518, Centro, Rio de Janeiro-RJ 21081-312, , Brazil
Abstract The goal of this work was to evaluate the effect of the support calcination temperature (1073, 1173 and 1273 K) on the Pt/CeZrO2/Al2O3 performance on the partial oxidation and autothermal reforming of methane. The sample calcined at 1273 K was less stable on the partial oxidation of methane probably due to the smaller contact between mixed oxide and Pt on the alumina surface, which favors the coke deposition. Moreover, for all samples, the addition of water led to a better activity and stability on autothermal reforming of methane. 1. Introduction The partial oxidation and autothermal reforming of natural gas are advantageous alternatives in the syngas production when compared with steam reforming largely used today, since they are exothermic processes and, consequently the energy costs are minimized [1]. The transformation of methane in syngas, through reforming process, happens at high temperatures, usually above 1073 K, and still, coke can be formed in secondary reactions. Previous works showed that Pt/CeZrO2/Al2O3 catalysts present higher activity and stability on partial oxidation of methane, due to the coke removal mechanism promoted by mixed oxide [2,3].
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V.B. Mortola et al.
The aim of this work is to understand the influence of the support calcination temperatures (1073, 1173 and 1273 K) in the physical, chemical and catalytic properties of Pt/Ce0.5Zr0.5O2/Al2O3 catalysts on the partial oxidation and autothermal reforming of methane reactions. 2. Experimental The Al2O3 was calcined at 1073, 1173 and 1273 K, under air flow for 6 h. After that, CeZrO2 was added to the alumina by incipient wetness impregnation method with an aqueous solution of cerium (IV) ammonium nitrate (Aldrich) and zirconium nitrate (MEL Chemicals) in order to obtain a 14wt.% of Ce0.5Zr0.5O2. This loading was chosen because it is the theoretical value necessary to form a monolayer of ceria on the surface of the alumina. Finally, the supports were aged in a muffle furnace for 4 h at 1073, 1173 and 1273 K. Platinum was added to the supports by incipient wetness impregnation, using an aqueous solution of H2PtCl6 (Aldrich). After the impregnation with 1.5 wt.% of platinum, the samples were dried at 373 K, and calcined under air (50 ml/min) at 673 K for 2 h. The samples were characterized through BET surface area, Xray diffraction (XRD), CO2 infrared spectroscopy, oxygen storage capacity (OSC), temperature programmed reduction (TPR) and the metallic dispersion was evaluated using the cyclohexane dehydrogenation reaction. Details about experimental conditions are presented elsewhere [2,3]. Partial oxidation (POX) and autothermal reforming (ATR) of methane were performed in a quartz reactor at atmospheric pressure. Prior to the reaction, the catalyst was reduced under H2 at 773 K for 1 h and then heated to 1073 K under N2. The reactions were carried out at 1073 K and WHSV = 260 h-1. For partial oxidation of methane, it was used a CH4/O2 ratio of 2.0. The autothermal reforming of methane was carried out using a CH4/O2 and H2O/CH4 ratio of 2.0 and 0.2, respectively. The exit gases were analyzed using a gas chromatograph equipped with a thermal conductivity detector. 3. Results and Discussion Increasing calcination temperature from 1073 to 1273 K decreased the BET surface area of the alumina samples from 96 to 38 m2/g. For Pt/CeZrO2/Al2O3 catalysts, the results obtained in the BET analysis (Table 1) also showed that the increase of temperature caused a decrease in the surface area of the samples. This can be attributed to the alumina sintering processes. The OSC values (Table 1) strongly decreased when the calcination temperature increased from 1073 K to 1173 K, although there was not a significant change in the OSC value when the temperature was increased to 1273 K. The TPR profiles of all the samples (not shown) exhibited two reduction regions: one at lower temperature (440 - 670 K) that presented the highest H2 consumption and is related to the reduction of Pt oxide and superficial CeZrO2, and another region, at higher
411
The performance of Pt/CeZrO2 /Al2O3 catalysts
temperature (>1200 K), which may be attributed to the reduction of bulk CeZrO2 [4]. The values of H2 consumption during TPR experiments (Table 1) agree with BET and OSC results and show a decrease of reducibility with the increase of calcination temperature. This is probably associated to the sintering of CeZrO2 particles, which makes harder to remove bulk oxygen from the inside of these particles [5]. Figure 1 shows the X-ray diffraction patterns for all the catalysts. For the samples calcined at 1073 and 1173 K, it was possible to identify the presence of γ-Al2/O3 phase. However, for the sample submitted to 1273 K, an additional phase, possibly α-Al2/O3, was also present. Table 1. Results of BET, OSC, TPR, DCeO2 and Pt dispersion for Pt/CeZrO2/Al2O catalysts.
Calcinations (K)
BET area (m2/g)
1073 1173 1273
85 70 37
OSC (μmoles de O2/gcat) 458 237 279
TPR (μmoles de H2/gcat) 700 564 421
DCeO2 (nm) 8 9 12
Pt Dispersion (%) 47 41 39
The three XRD profiles also exhibit lines related to ceria cubic phase shifted to higher 2θ values. This could be verified with a slow scanning speed between 2θ = 27° and 32° (not shown) for all the samples. The peak characteristic of ceria cubic phase (JCPDS-4-0593) shifted from 28.6° to 28.8°, for the catalyst submitted to 1073 K, and to 2θ = 28.97° for sample calcined at 1173 K. This result indicates that zirconium was introduced into the ceria lattice [2,3]. On the other hand, the sample calcined at 1273 K showed peaks related to a tetragonal zirconia phase, which suggests the presence of two phases: one rich phase in ceria and another one rich in zirconia. The compositions of these solid solutions were calculated using the procedure described by Kozlov et al. [6]. The results showed that the samples treated at 1073 and 1173 K contain 25 and 46% of Zr incorporated into the cerium oxide lattice while the sample calcined at 1273 K presented two peaks with different compositions: Ce0.875Zr0.125O2 (2θ = 28.8°) and Ce0.125Zr0.725O2 (2θ = 30°). These results are in agreement with Yao et al. [7] who identified the formation of solid solutions with heterogeneous compositions for samples aged at 1273 K and zirconia loadings above 10%. CeO2 average particles sizes (DCeO2) were calculate through Scherrer equation using Ce(111) reflection (Table 1). The results showed that the increase in the calcination temperature from 1073 K to 1173 K did not cause a significant change in the particle size. However, the sample treated at 1273 K showed larger particles, which is consistent with the results reported by Yao et al. [7].
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V.B. Mortola et al.
Figure 1: X-ray diffraction patterns of Pt/CeZrO2/Al2O3 catalysts aged at 1073, 1173 and 1273 K.
The values of Pt dispersion (Table 1) are in agreement with those obtained in previous reports [2,3] and decreased with the increase of the calcination temperature. The degree of coverage of the alumina surface was evaluated through the optical density of the band at 1235 cm-1 calculated through adsorbed CO2 infrared data [8]. In fact, the optical density of the band at 1235 cm-1 is a measurement of the interaction of CO2 with the hydroxyl groups of the alumina. For catalysts treated at higher calcination temperature (1273 K) this measurement is inaccurate because of the removal of the hydroxyl groups after the exposure to this high temperature and then it was not performed. The results obtained by adsorbed CO2 infrared data showed a decrease on the degree of alumina coverage by the ceria-based oxide from 80.7 to 66.2 % when the aging temperature is increased from 1073 to 1173 K. Figure 2 shows the CH4 conversion on POX reaction. The samples calcined at 1073 and 1173 K presented similar performance during the reaction. It was observed a slight deactivation during 24 h for both samples. However, Pt/CeZrO2/Al2O3 calcined at 1273 K strongly deactivated along the reaction. The CO selectivity decreased while the CO2 formation increased during the reaction (not shown) for all samples. However, this effect was more accentuated for the catalyst calcined at 1273 K. These results can be explained by the twostep mechanism of partial oxidation of methane [2]. The first step comprehends the methane combustion, producing CO2 and H2O. In the second step, syngas is produced through CO2 and steam reforming of unreacted methane. The support takes part in the CO2 dissociative adsorption close to the metallic particle, suppling oxygen to the metal surface. When the support does not exhibit OSC, carbon deposits around the metal particle and CO2 dissociation is inhibited. In the case of Pt/CeZrO2/Al2O3
413
The performance of Pt/CeZrO2 /Al2O3 catalysts
catalysts, a high degree of coverage of the alumina by the CeZrO2 oxide provides a good interaction of the metal with the mixed oxide, promoting the carbon removal mechanism of the metallic surface [2]. In this work, the increase of CO2 formation during the reaction indicates that the second step (CO2 reforming) is being inhibited. This effect was more significant for the sample calcined at 1273 K, although the metal dispersion and OSC values for this catalyst are similar to the ones obtained for the sample calcined at 1173 K. Therefore, the smallest stability of the catalyst calcined at 1273 K could be related to a low degree of coverage of the alumina by the mixed oxide. Although it was impossible to measure the degree of coverage of the alumina for this sample by FTIR, the XRD analysis showed that the calcination of the support at 1273 K strongly increased average particle size of the ceria based oxide. This fact probably decreased the contact between platinum particles and this mixed oxide on the surface of alumina. Recently, it was shown that a Pt/Al2O3 catalyst suffers a strong deactivation in POX, due to the absence of the metallic particle cleaning mechanism [9]. Then, the coke removal mechanism of the metallic particle in the Pt/CeZrO2/Al2O3 catalyst calcined at 1273 K is not favored, causing the catalyst deactivation.
CH4 Conversion (%)
100 90 80 70 60 50 40 30 20 10 0
1073K 1173K 1273K
0
4
8
12 16 Time (h)
20
24
Figure 2: CH4 conversion on partial oxidation of methane over Pt/CeZrO2/Al2O3 catalysts calcined at 1073, 1173 and 1273 K.
Concerning the ATR reaction (Figure 3), the sample calcined at 1073 K presented a slight deactivation only at the beginning of the reaction, while the other samples were very stable during the reaction. Furthermore, the CH4 conversions were higher than those observed on partial oxidation of methane for all the samples. The higher values of CH4 conversion can be associated to the occurrence of steam reforming of methane due to the addition of water. CeZrO2 support plays the same role on the stability of the catalysts observed on POX reaction. Moreover, the better stability of the catalysts on ATR is also related to the introduction of water, which avoids the carbon deposition.
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CH4 Conversion (%)
100 90 80 70 60 50 40 30 20 10 0
1073 K 1173 K 1273 K
0
4
8
12 16 Time (h)
20
24
Figure 3: CH4 conversion on autothermal reforming of methane over Pt/CeZrO2/Al2O3 catalysts.
4. Conclusions The Pt/CeZrO2/Al2O3 catalysts calcined at 1073 and 1173 K presented a higher stability than the sample calcined at 1273 K on partial oxidation of methane. According to the XRD results, the sample calcined at 1273 K showed particle agglomeration. This probably caused a smaller contact between mixed oxide and Pt on the alumina surface and consequently the coke removal mechanism was less efficient for this sample. Furthermore, the activity and the stability of Pt/CeZrO2/Al2O3 catalysts were higher on autothermal reforming of methane. The addition of water leads to the occurrence of steam reforming of methane and avoids the carbon deposition. References 1. J.R. Rostrup-Nielsen, Catal Today 71 (2002) 243. 2. P.P. Silva, F.A. Silva, H.P. Souza, A.G. Lobo, L.V. Mattos, F.B. Noronha, C.E. Hori, Catal. Today 101 (2005) 31. 3. P.P. Silva, F.A. Silva, L.S. Portela, L.V. Mattos, F.B. Noronha, C.E. Hori, Catal. Today 107-108 (2005) 734. 4. H.C. Yao, Y.F. Yao, J. Catal. 86 (1984) 256. 5. J.P. Cuiff, G. Blanchard, O. Touret, A. Seigneurin, M. Marczi, E. Quéméré, SAE paper 970463 (1996). 6. A.I. Kozlov, D.H. Kim, A. Yezerets, P. Andersen, H.H. Kung, M.C. Kung, J. Catal. 209 (2002) 417. 7. M.H. Yao, R.J. Baird, F.W. Kunz, T.E. Hoost, J. Catal. 166 (1997) 67. 8. R.Frety, P.J.Lévy, V. Perrichon, V.Pitchon, M.M.Chevrier, C.Gauthier, F.Mathis, Stud.Surf.Sci.Catal. 96 (1995) 405. 9. L.V. Mattos; E.R. Oliveira; P.D. Resende; F.B. Noronha; F.B. Passos Catal. Today 77 (2002) 245.
409
The performance of Pt/CeZrO2/Al2O3 catalysts on the partial oxidation and autothermal reforming of methane Vanessa B. Mortolaa, Juan A. C. Ruizb, Diego da S. Martineza, Lisiane V. Mattosb, Fábio B. Noronhab, Carla E. Horia a
Faculdade de Engenharia Química - Universidade Federal de Uberlândia, Av. João Naves de Ávila, 2160/Bloco 1K, Uberlândia-MG 38400-902, Brazil b Laboratório de Catálise, Instituto Nacional de Tecnologia/MCT, Av. Venezuela, 82/518, Centro, Rio de Janeiro-RJ 21081-312, , Brazil
Abstract The goal of this work was to evaluate the effect of the support calcination temperature (1073, 1173 and 1273 K) on the Pt/CeZrO2/Al2O3 performance on the partial oxidation and autothermal reforming of methane. The sample calcined at 1273 K was less stable on the partial oxidation of methane probably due to the smaller contact between mixed oxide and Pt on the alumina surface, which favors the coke deposition. Moreover, for all samples, the addition of water led to a better activity and stability on autothermal reforming of methane. 1. Introduction The partial oxidation and autothermal reforming of natural gas are advantageous alternatives in the syngas production when compared with steam reforming largely used today, since they are exothermic processes and, consequently the energy costs are minimized [1]. The transformation of methane in syngas, through reforming process, happens at high temperatures, usually above 1073 K, and still, coke can be formed in secondary reactions. Previous works showed that Pt/CeZrO2/Al2O3 catalysts present higher activity and stability on partial oxidation of methane, due to the coke removal mechanism promoted by mixed oxide [2,3].
410
V.B. Mortola et al.
The aim of this work is to understand the influence of the support calcination temperatures (1073, 1173 and 1273 K) in the physical, chemical and catalytic properties of Pt/Ce0.5Zr0.5O2/Al2O3 catalysts on the partial oxidation and autothermal reforming of methane reactions. 2. Experimental The Al2O3 was calcined at 1073, 1173 and 1273 K, under air flow for 6 h. After that, CeZrO2 was added to the alumina by incipient wetness impregnation method with an aqueous solution of cerium (IV) ammonium nitrate (Aldrich) and zirconium nitrate (MEL Chemicals) in order to obtain a 14wt.% of Ce0.5Zr0.5O2. This loading was chosen because it is the theoretical value necessary to form a monolayer of ceria on the surface of the alumina. Finally, the supports were aged in a muffle furnace for 4 h at 1073, 1173 and 1273 K. Platinum was added to the supports by incipient wetness impregnation, using an aqueous solution of H2PtCl6 (Aldrich). After the impregnation with 1.5 wt.% of platinum, the samples were dried at 373 K, and calcined under air (50 ml/min) at 673 K for 2 h. The samples were characterized through BET surface area, Xray diffraction (XRD), CO2 infrared spectroscopy, oxygen storage capacity (OSC), temperature programmed reduction (TPR) and the metallic dispersion was evaluated using the cyclohexane dehydrogenation reaction. Details about experimental conditions are presented elsewhere [2,3]. Partial oxidation (POX) and autothermal reforming (ATR) of methane were performed in a quartz reactor at atmospheric pressure. Prior to the reaction, the catalyst was reduced under H2 at 773 K for 1 h and then heated to 1073 K under N2. The reactions were carried out at 1073 K and WHSV = 260 h-1. For partial oxidation of methane, it was used a CH4/O2 ratio of 2.0. The autothermal reforming of methane was carried out using a CH4/O2 and H2O/CH4 ratio of 2.0 and 0.2, respectively. The exit gases were analyzed using a gas chromatograph equipped with a thermal conductivity detector. 3. Results and Discussion Increasing calcination temperature from 1073 to 1273 K decreased the BET surface area of the alumina samples from 96 to 38 m2/g. For Pt/CeZrO2/Al2O3 catalysts, the results obtained in the BET analysis (Table 1) also showed that the increase of temperature caused a decrease in the surface area of the samples. This can be attributed to the alumina sintering processes. The OSC values (Table 1) strongly decreased when the calcination temperature increased from 1073 K to 1173 K, although there was not a significant change in the OSC value when the temperature was increased to 1273 K. The TPR profiles of all the samples (not shown) exhibited two reduction regions: one at lower temperature (440 - 670 K) that presented the highest H2 consumption and is related to the reduction of Pt oxide and superficial CeZrO2, and another region, at higher
411
The performance of Pt/CeZrO2 /Al2O3 catalysts
temperature (>1200 K), which may be attributed to the reduction of bulk CeZrO2 [4]. The values of H2 consumption during TPR experiments (Table 1) agree with BET and OSC results and show a decrease of reducibility with the increase of calcination temperature. This is probably associated to the sintering of CeZrO2 particles, which makes harder to remove bulk oxygen from the inside of these particles [5]. Figure 1 shows the X-ray diffraction patterns for all the catalysts. For the samples calcined at 1073 and 1173 K, it was possible to identify the presence of γ-Al2/O3 phase. However, for the sample submitted to 1273 K, an additional phase, possibly α-Al2/O3, was also present. Table 1. Results of BET, OSC, TPR, DCeO2 and Pt dispersion for Pt/CeZrO2/Al2O catalysts.
Calcinations (K)
BET area (m2/g)
1073 1173 1273
85 70 37
OSC (μmoles de O2/gcat) 458 237 279
TPR (μmoles de H2/gcat) 700 564 421
DCeO2 (nm) 8 9 12
Pt Dispersion (%) 47 41 39
The three XRD profiles also exhibit lines related to ceria cubic phase shifted to higher 2θ values. This could be verified with a slow scanning speed between 2θ = 27° and 32° (not shown) for all the samples. The peak characteristic of ceria cubic phase (JCPDS-4-0593) shifted from 28.6° to 28.8°, for the catalyst submitted to 1073 K, and to 2θ = 28.97° for sample calcined at 1173 K. This result indicates that zirconium was introduced into the ceria lattice [2,3]. On the other hand, the sample calcined at 1273 K showed peaks related to a tetragonal zirconia phase, which suggests the presence of two phases: one rich phase in ceria and another one rich in zirconia. The compositions of these solid solutions were calculated using the procedure described by Kozlov et al. [6]. The results showed that the samples treated at 1073 and 1173 K contain 25 and 46% of Zr incorporated into the cerium oxide lattice while the sample calcined at 1273 K presented two peaks with different compositions: Ce0.875Zr0.125O2 (2θ = 28.8°) and Ce0.125Zr0.725O2 (2θ = 30°). These results are in agreement with Yao et al. [7] who identified the formation of solid solutions with heterogeneous compositions for samples aged at 1273 K and zirconia loadings above 10%. CeO2 average particles sizes (DCeO2) were calculate through Scherrer equation using Ce(111) reflection (Table 1). The results showed that the increase in the calcination temperature from 1073 K to 1173 K did not cause a significant change in the particle size. However, the sample treated at 1273 K showed larger particles, which is consistent with the results reported by Yao et al. [7].
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V.B. Mortola et al.
Figure 1: X-ray diffraction patterns of Pt/CeZrO2/Al2O3 catalysts aged at 1073, 1173 and 1273 K.
The values of Pt dispersion (Table 1) are in agreement with those obtained in previous reports [2,3] and decreased with the increase of the calcination temperature. The degree of coverage of the alumina surface was evaluated through the optical density of the band at 1235 cm-1 calculated through adsorbed CO2 infrared data [8]. In fact, the optical density of the band at 1235 cm-1 is a measurement of the interaction of CO2 with the hydroxyl groups of the alumina. For catalysts treated at higher calcination temperature (1273 K) this measurement is inaccurate because of the removal of the hydroxyl groups after the exposure to this high temperature and then it was not performed. The results obtained by adsorbed CO2 infrared data showed a decrease on the degree of alumina coverage by the ceria-based oxide from 80.7 to 66.2 % when the aging temperature is increased from 1073 to 1173 K. Figure 2 shows the CH4 conversion on POX reaction. The samples calcined at 1073 and 1173 K presented similar performance during the reaction. It was observed a slight deactivation during 24 h for both samples. However, Pt/CeZrO2/Al2O3 calcined at 1273 K strongly deactivated along the reaction. The CO selectivity decreased while the CO2 formation increased during the reaction (not shown) for all samples. However, this effect was more accentuated for the catalyst calcined at 1273 K. These results can be explained by the twostep mechanism of partial oxidation of methane [2]. The first step comprehends the methane combustion, producing CO2 and H2O. In the second step, syngas is produced through CO2 and steam reforming of unreacted methane. The support takes part in the CO2 dissociative adsorption close to the metallic particle, suppling oxygen to the metal surface. When the support does not exhibit OSC, carbon deposits around the metal particle and CO2 dissociation is inhibited. In the case of Pt/CeZrO2/Al2O3
413
The performance of Pt/CeZrO2 /Al2O3 catalysts
catalysts, a high degree of coverage of the alumina by the CeZrO2 oxide provides a good interaction of the metal with the mixed oxide, promoting the carbon removal mechanism of the metallic surface [2]. In this work, the increase of CO2 formation during the reaction indicates that the second step (CO2 reforming) is being inhibited. This effect was more significant for the sample calcined at 1273 K, although the metal dispersion and OSC values for this catalyst are similar to the ones obtained for the sample calcined at 1173 K. Therefore, the smallest stability of the catalyst calcined at 1273 K could be related to a low degree of coverage of the alumina by the mixed oxide. Although it was impossible to measure the degree of coverage of the alumina for this sample by FTIR, the XRD analysis showed that the calcination of the support at 1273 K strongly increased average particle size of the ceria based oxide. This fact probably decreased the contact between platinum particles and this mixed oxide on the surface of alumina. Recently, it was shown that a Pt/Al2O3 catalyst suffers a strong deactivation in POX, due to the absence of the metallic particle cleaning mechanism [9]. Then, the coke removal mechanism of the metallic particle in the Pt/CeZrO2/Al2O3 catalyst calcined at 1273 K is not favored, causing the catalyst deactivation.
CH4 Conversion (%)
100 90 80 70 60 50 40 30 20 10 0
1073K 1173K 1273K
0
4
8
12 16 Time (h)
20
24
Figure 2: CH4 conversion on partial oxidation of methane over Pt/CeZrO2/Al2O3 catalysts calcined at 1073, 1173 and 1273 K.
Concerning the ATR reaction (Figure 3), the sample calcined at 1073 K presented a slight deactivation only at the beginning of the reaction, while the other samples were very stable during the reaction. Furthermore, the CH4 conversions were higher than those observed on partial oxidation of methane for all the samples. The higher values of CH4 conversion can be associated to the occurrence of steam reforming of methane due to the addition of water. CeZrO2 support plays the same role on the stability of the catalysts observed on POX reaction. Moreover, the better stability of the catalysts on ATR is also related to the introduction of water, which avoids the carbon deposition.
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V.B. Mortola et al.
CH4 Conversion (%)
100 90 80 70 60 50 40 30 20 10 0
1073 K 1173 K 1273 K
0
4
8
12 16 Time (h)
20
24
Figure 3: CH4 conversion on autothermal reforming of methane over Pt/CeZrO2/Al2O3 catalysts.
4. Conclusions The Pt/CeZrO2/Al2O3 catalysts calcined at 1073 and 1173 K presented a higher stability than the sample calcined at 1273 K on partial oxidation of methane. According to the XRD results, the sample calcined at 1273 K showed particle agglomeration. This probably caused a smaller contact between mixed oxide and Pt on the alumina surface and consequently the coke removal mechanism was less efficient for this sample. Furthermore, the activity and the stability of Pt/CeZrO2/Al2O3 catalysts were higher on autothermal reforming of methane. The addition of water leads to the occurrence of steam reforming of methane and avoids the carbon deposition. References 1. J.R. Rostrup-Nielsen, Catal Today 71 (2002) 243. 2. P.P. Silva, F.A. Silva, H.P. Souza, A.G. Lobo, L.V. Mattos, F.B. Noronha, C.E. Hori, Catal. Today 101 (2005) 31. 3. P.P. Silva, F.A. Silva, L.S. Portela, L.V. Mattos, F.B. Noronha, C.E. Hori, Catal. Today 107-108 (2005) 734. 4. H.C. Yao, Y.F. Yao, J. Catal. 86 (1984) 256. 5. J.P. Cuiff, G. Blanchard, O. Touret, A. Seigneurin, M. Marczi, E. Quéméré, SAE paper 970463 (1996). 6. A.I. Kozlov, D.H. Kim, A. Yezerets, P. Andersen, H.H. Kung, M.C. Kung, J. Catal. 209 (2002) 417. 7. M.H. Yao, R.J. Baird, F.W. Kunz, T.E. Hoost, J. Catal. 166 (1997) 67. 8. R.Frety, P.J.Lévy, V. Perrichon, V.Pitchon, M.M.Chevrier, C.Gauthier, F.Mathis, Stud.Surf.Sci.Catal. 96 (1995) 405. 9. L.V. Mattos; E.R. Oliveira; P.D. Resende; F.B. Noronha; F.B. Passos Catal. Today 77 (2002) 245.