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High performance of Ni-substituted calcium aluminosilicate for partial oxidation of methane into syngas
Catalysis Communications 8 (2007) 1735–1738 www.elsevier.com/locate/catcom
High performance of Ni-substituted calcium aluminosilicate for partial oxidation of methane into syngas Katsuya Sato a, Satoru Fujita b, Kenzi Suzuki c, Toshiaki Mori a
a,*
Department of Frontier Materials Chemistry, Faculty of Science and Technology, Hirosaki University, Bunkyo-cho, Hirosaki 036-8561, Japan b Toyota Central R&D Laboratories, Inc., Nagakute-cho, Aichi-gun, Aichi 480-1192, Japan c Division of Environmental Research/EcoTopia Science Institute, Nagoya University, Furo-cho, Chikusa-ku, Nagoya 464-8603, Japan Received 24 November 2006; received in revised form 28 January 2007; accepted 2 February 2007 Available online 12 February 2007
Abstract Newly synthesized calcium aluminosilicate, in which oxide anions were occluded, was tested as a catalyst for partial oxidation of CH4 into CO and H2. Substitution of a part of Ca2+ with Ni2+ resulted in a marked increase in CH4 conversion (93% at 800 C) and in CO and H2 selectivities (94% and 95%, respectively), while Cr3+, Co3+, Fe3+ and Cu2+ slightly increased CH4 conversion with a considerable decrease in CO and H2 selectivities. Such a high performance may be attributed to the ability of Ni-substituted sample for CH4 steam reforming appearing during CH4 partial oxidation. 2007 Elsevier B.V. All rights reserved. Keywords: Calcium aluminosilicate; Metal-substitution; CH4 partial oxidation; Syngas
1. Introduction Syngas, a mixture of CO and H2, is mainly produced from steam reforming of hydrocarbons (in case of CH4, CH4 + H2O ! CO + 3H2) [1,2]. Partial oxidation of CH4 (POM; CH4 + 1/2O2 ! CO + 2H2) is another route to produce syngas. The H2/CO ratio of 2 in POM syngas is favorable for its further utilization for methanol and Fischer–Tropsch syntheses. Among catalysts active for POM, supported Ni has been studied most extensively [3–21]. Both steam reforming and POM accompany a serious coke deposition, which leads to catalyst deactivation and also to plugging of the gas flow. In steam reforming, coking is avoided by maintaining the steam/carbon ratio much larger than the stoichiometric ratio. However, such methodology is not applicable to POM, because the increase in the O2/CH4 ratio is in favor of the complete oxidation of CH4 to CO2 and H2O. It is, therefore, of primary
*
Corresponding author. Tel./fax: +81 172 39 3565. E-mail address: [email protected] (T. Mori).
1566-7367/$ - see front matter 2007 Elsevier B.V. All rights reserved. doi:10.1016/j.catcom.2007.02.006
importance to develop a high performance anti-coking POM catalyst [7,10,11,17,19,21]. The authors have prepared calcium aluminosilicate, the mineralogical name of which is mayenite (Ca12Al10Si4O35), and revealed its catalytic performance [22–30]. It was found 2 that oxide anions ðO 2 ; O2 Þ were occluded in micropores of mayenite [24,25,29] and a part of Ca2+ could be substituted with other metal ions, by which the combustion activity was highly improved [28–30]. Such oxide anions occluded may positively play an anti-coking role in POM. Therefore, the catalytic activity of metal-substituted mayenite is examined. 2. Experimental Hydrogarnet (Ca3Al2(SiO4)0.8(OH)8.8), a precursor of mayenite, was synthesized in advance from hydrothermal reaction of the stoichiometric mixture of powders of CaO, Al2O3, and amorphous SiO2 at 200 C for 15 h. Then, it was converted to mayenite via calcination at 900 C for 2 h. Metal-substituted mayenites were obtained by calcining metal-substituted hydrogarnet similarly synthesized. Details of the preparation procedure of these mayenites
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K. Sato et al. / Catalysis Communications 8 (2007) 1735–1738
Fig. 1. Schematic drawings of hydrogarnet (a) and mayenite (b). In hydrogarnet, SiO4 tetrahedron, AlO6 octahedron and CaO12 dodecahedron are linked with one another by sharing corner oxygen atoms. In mayenite, each AlO4 tetrahedron (T1) is three-dimensionally linked with one another by sharing corner oxygen atoms. Central Al3+ in some tetrahedra is replaced with Si4+ to form (Al, Si)O4 (T2) tetrahedron. Oxide anions (indicated by O 2 here) are located in mayenite.
were reported in the literatures [22–30]. Unless otherwise described, the metal/Ca atomic ratio in metal-substituted mayenite was 0.0714, 0.04, 0.0344, 0.025, and 0.05 for Cr-, Fe, Co-, Ni-, and Cu-mayenite, respectively. Except for Ni-substitution, each value was the ratio at which the highest activity was obtained for NO reduction with C3H6 [29]. It should be described that conversion from hydrogarnet to mayenite produces free metal oxide together with CaO, because hydrogarnet transforms to mayenite by releasing excess CaO according to the following equation (see Fig. 1): 5Ca3 Al2 ðSiO4 Þ0:8 ðOHÞ8:8 ! Ca12 Al10 Si4 O35 þ 3CaO ð1Þ
þ 22H2 O 2+
XRD revealed the substitution of a part of Ca ions in mayenite with other metal ions [29]. The oxide anions located in the prepared mayenites was confirmed by both EPR and Raman spectroscopic measurements [29]. For comparison, supported Ni catalysts were prepared by an impregnation method using mayenite, a- and c-Al2O3 as a catalyst support. Metal loading was 1.2 wt%. The catalytic activity was tested by using a conventional flow-type apparatus with a U-shaped quartz reactor (i.d., 4.8 mm/) under atmospheric pressure. 100 mg of catalyst (particle size, 300–600 lm) was placed in the reactor with no diluents (length of the catalyst bed, ca. 7 mm). Prior to reaction, it was heated in a flow of O2 at 500 C for 1 h. The flow rate of reactant gases (CH4/O2 = 2) was 60 ml/ min, unless otherwise described. Considering the apparent density of mayenite of 0.5 g/ml, the space velocity (SV) was 1.8 · 104 h1. No diluents were added to the reactant gases. Exit gases were analyzed by gas chromatography (see Fig. 2). Steam reforming of CH4 was similarly carried out by replacing the reactant from O2 to H2O with the H2O/ CH4 ratio of 3.
Fig. 2. Effect of the kind of metal ions on partial oxidation of CH4 into syngas at 800 C.
3. Results and discussion The effect of the kind of substituting metal ions on the POM activity of mayenite was examined at 800 C. Fig. 1 summarizes the results. Mayenite itself was active for POM with CH4 conversion of 20% and both CO and H2 selectivities of ca. 50%. Substitution with Cr3+, Co3+, Fe3+, and Cu2+ slightly increased CH4 conversion with a considerable decrease in the selectivities of both CO and H2 to <20%, indicating that combustion of CH4 to CO2 and H2O preferentially occurred on these mayenites. Substitution with Ni2+, on the other hand, resulted in a marked increase in CH4 conversion (93%) with excellent selectivities of both CO and H2 (94% and 95%, respectively). Such a high performance was maintained for 12 h, suggesting a negligible coke formation and also a negligible sintering. It is known that POM often generates hot spots in the catalyst bed [4,6]. Such hot spots may be generated more
K. Sato et al. / Catalysis Communications 8 (2007) 1735–1738
Fig. 3. Effect of the temperature on partial oxidation of CH4 into syngas over Ni-mayenite. SV = 1.8 · 104 h1. Dotted line: increasing temperature. Solid line: decreasing temperature.
readily under the present experimental conditions, because no diluents were used both in the catalyst and in the reactant gases. However, no deactivation of Ni-mayenite and CH4 conversion and both CO and H2 selectivities below each equilibrium value (see Fig. 3) suggest a negligible generation of the hot spots in this catalyst. Further study is necessary to clarify the reason for the absence of the hot spots. Over Ni-mayenite, POM was carried out by sequentially increasing and decreasing the reaction temperature and the results are shown in Fig. 3. Below 600 C, the activity was very low and the product was only CO2 and H2O, suggesting that complete oxidation of CH4 occurs. At 700 C, on the other hand, CH4 conversion abruptly rose to 78% with CO selectivity of 88% and H2 selectivity of 89%. At 800 C, CH4 conversion and CO and H2 selectivities further increased to 93%, 94%, and 95%, respectively. When the temperature was decreased afterward, reproducible results were obtained at the temperature down to 700 C. Below 700 C, however, CH4 conversion and both CO and H2 selectivities gradually decreased, respectively, down to 41%, 32%, and 29% at 500 C and did not retrace the pathways observed during the initial temperature increase. It is mentioned that CH4 conversion and CO and H2 selectivities were below the respective equilibrium limits [18]. The observed CO and H2 selectivities indicated that POM was predominant at 600 C, while complete oxidation of CH4 to CO2 and H2O prevailed at 500 C. The absence of retracing the pathways in the lower temperature range has also been observed on supported Ni catalysts
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[3,6,12,20]. Following explanation similar to reported ones is also possible for our Ni-mayenite. While in the fresh Nimayenite, all the Ni is in the oxidized state, responsible for complete oxidation of CH4, it is reduced to the metallic state at 700 C during POM. A considerable part of Ni remains reduced during the temperature decrease to 500 C, to which the POM activity is attributed. To obtain more information about the absence of retracing the pathways in our catalyst, steam reforming of CH4 was attempted over Ni- and Cu-mayenite. No activity was observed on both fresh Ni- and Cu-mayenite at 700 C, because both Ni and Cu are in the oxidized state in the fresh samples. After POM was conducted at 700 C, Ni-mayenite exhibited the activity for steam reforming reaction: CH4 conversion attained to 61%. For Cu-mayenite, on the other hand, exposing to a mixture of CH4 and O2 at 700 C did not convert it active for CH4 steam reforming. These findings lead us to conclude that during POM at 700 C, Ni-mayenite was reduced to metallic Ni, active for steam reforming, and that the difference in the POM activity between Ni- and Cu-mayenite is attributed to the activity for steam reforming different from each other. The lack of the POM activity of fresh Ni-mayenite at 500 and 600 C should result from a difficult reduction to metallic Ni state at these temperatures. For POM, two mechanisms have been proposed: the combustion-reforming mechanism [3,4,6] and the direct POM or the pyrolysis mechanism [5,8,13–16]. The explanation described above seems to be in favor of the former mechanism, while the latter mechanism may explain the results of Cu-mayenite, because Cu-mayenite has no ability to catalyze steam reforming of CH4. The latter mechanism may also explain the results of unsubstituted and Cr-, Co-, and Fe-mayenites, because these mayenites are considered not to have an ability to catalyze steam reforming of CH4. Table 1 summarizes the POM activity of Ni-mayenites with various Ni/Ca atomic ratios at 800 C. All Ni-mayenites examined exhibited a very high CH4 conversion and CO and H2 selectivities. It is interesting to note that the introduction of a small amount of Ni to mayenite can convert it to a high performance POM catalyst. For comparison, POM was carried out over supported Ni catalysts. Fig. 4 shows the results. Obviously, CH4 conversion was higher on Ni-mayenite than on supported Ni catalysts for all SV examined, although the stable POM activity was also maintained on all the supported Ni catalysts. The ability of Ni-mayenite to catalyze both Table 1 Effect of the Ni/Ca atomic ratio on partial oxidation of CH4 into syngas over Ni mayenite at 800 C Ni/Ca atomic ratio
CH4 conversion (%)
CO selectivities (%)
H2 selectivities (%)
0 0.025 0.05 0.15
19.8 93.4 89.7 89.9
52.0 94.2 92.5 93.8
53.3 95.2 92.2 93.6
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References
Fig. 4. Effect of the space velocity on partial oxidation of CH4 into syngas over various Ni catalysts at 800 C.
combustion and steam reforming of CH4 is a possible cause for its excellent performance for POM. It is mentioned that the POM activity of our mayenite-supported Ni was slightly lower than that of C12A7-supported Ni [21]. Metal loading different from each other (1.2 wt% vs. 5 wt%) may explain the difference. 4. Conclusion The catalytic activity of unsubstituted and Cr-, Co-, Ni-, and Cu-substituted mayenites were examined toward POM. Among them, Ni-substituted one was most active with an excellent selectivity for this reaction and more active than supported Ni catalysts. During POM at >700 C, the ability to catalyze CH4 steam reforming appeared on Ni-mayenite. It is considered that such a high performance is attributed to the abilities of Ni-substituted one to catalyze both CH4 combustion and CH4 steam reforming.
[1] D.L. Trimm, Catal. Rev. Sci. Eng. 16 (1977) 155. [2] J.R. Rostrup-Nielsen, in: J.R. Anderson, M. Boudart (Eds.), Catalysis: Science and Technology, vol. 5, Springer Verlag, New York, 1984. [3] D. Dissanayake, M.P. Rosynek, K.C.C. Kharas, J.H. Lunsford, J. Catal. 132 (1991) 117. [4] W.J.M. Vermeiren, E. Blomsma, P.A. Jacobs, Catal. Today 13 (1992) 427. [5] V.R. Choudhary, A.M. Rajput, B. Prabhakar, J. Catal. 139 (1993) 326. [6] D. Dissanayake, M.P. Rosynek, J.H. Lunsford, J. Phys. Chem. 97 (1993) 3644. [7] J.B. Claridge, M.L.H. Green, S.C. Tsang, A.P.E. York, A.T. Ashcroft, P.D. Battle, Catal. Lett. 22 (1993) 299. [8] Y.H. Hu, E. Ruckenstein, J. Catal. 158 (1996) 260. [9] M.A. Goula, A.A. Lemonidou, W. Grunett, M. Baern, Catal. Today 32 (1996) 148. [10] T. Hayakawa, H. Harihara, A.G. Andersen, K. Suzuki, H. Yasuda, T. Tsunoda, S. Hamakawa, A.P.E. York, Y.S. Yoon, M. Shimizu, K. Takehira, Appl. Catal. A 149 (1997) 391. [11] V.R. Choudhary, V.H. Rane, A.M. Rajput, Appl. Catal. A 162 (1997) 235. [12] F. van Looij, J.W. Geus, J. Catal. 168 (1997) 154. [13] V.R. Choudhary, B.S. Uphade, A.S. Mamman, J. Catal. 172 (1997) 281. [14] A.A. Lemonidou, A.E. Stambouli, G.J. Tjatjopoulos, I.A. Vasalos, Catal. Lett. 43 (1997) 235. [15] E. Ruckenstein, Y.H. Hu, Ind. Eng. Chem. Res. 37 (1998) 1744. [16] E. Ruckenstein, Y.H. Hu, Appl. Catal. A 183 (1999) 85. [17] W. Chu, W. Yang, L. Lin, Catal. Lett. 74 (2001) 139. [18] W.S. Dong, K.-W. Jun, H.-S. Roh, Z.-W. Liu, S.-E. Park, Catal. Lett. 78 (2002) 215. [19] K. Takehira, T. Shishido, M. Kondo, J. Catal. 207 (2002) 307. [20] J.H. Jun, T.-J. Lee, T.H. Lim, S.-W. Nam, S.-A. Hong, K.J. Yoon, J. Catal. 221 (2004) 178. [21] S. Yang, J.N. Kondo, K. Hayashi, M. Hirano, K. Domen, H. Hosono, Appl. Catal. A. 277 (2004) 239. [22] S. Fujita, K. Suzuki, M. Ohkawa, Y. Shibasaki, T. Mori, Chem. Mater. 13 (2001) 2523. [23] S. Fujita, K. Suzuki, T. Mori, J. Mater. Cycles Waste Manag. 4 (2002) 70. [24] S. Fujita, K. Suzuki, M. Ohkawa, T. Mori, Y. Iida, Y. Miwa, H. Masuda, S. Shimada, Chem. Mater. 15 (2003) 255. [25] S. Fujita, M. Ohkawa, K. Suzuki, H. Nakano, T. Mori, H. Masuda, Chem. Mater. 15 (2003) 4879. [26] K. Suzuki, S. Fujita, T. Mori, Trans. Mater. Res. Soc. Jpn. 29 (2004) 2163. [27] S. Fujita, K. Suzuki, T. Mori, H. Masuda, J. Eur. Ceram. Soc. 25 (2005) 3479. [28] S. Fujita, K. Suzuki, T. Mori, Catal. Lett. 86 (2003) 139. [29] K. Sato, J. Iritani, R. Miyamoto, S. Fujita, K. Suzuki, M. Ohkawa, T. Mori, Stud. Surf. Sci. Catal. 158 (2005) 2001. [30] K. Sato, M. Yamaguchi, S. Fujita, K. Suzuki, T. Mori, Catal. Commun. 7 (2006) 132.
High performance of Ni-substituted calcium aluminosilicate for partial oxidation of methane into syngas Katsuya Sato a, Satoru Fujita b, Kenzi Suzuki c, Toshiaki Mori a
a,*
Department of Frontier Materials Chemistry, Faculty of Science and Technology, Hirosaki University, Bunkyo-cho, Hirosaki 036-8561, Japan b Toyota Central R&D Laboratories, Inc., Nagakute-cho, Aichi-gun, Aichi 480-1192, Japan c Division of Environmental Research/EcoTopia Science Institute, Nagoya University, Furo-cho, Chikusa-ku, Nagoya 464-8603, Japan Received 24 November 2006; received in revised form 28 January 2007; accepted 2 February 2007 Available online 12 February 2007
Abstract Newly synthesized calcium aluminosilicate, in which oxide anions were occluded, was tested as a catalyst for partial oxidation of CH4 into CO and H2. Substitution of a part of Ca2+ with Ni2+ resulted in a marked increase in CH4 conversion (93% at 800 C) and in CO and H2 selectivities (94% and 95%, respectively), while Cr3+, Co3+, Fe3+ and Cu2+ slightly increased CH4 conversion with a considerable decrease in CO and H2 selectivities. Such a high performance may be attributed to the ability of Ni-substituted sample for CH4 steam reforming appearing during CH4 partial oxidation. 2007 Elsevier B.V. All rights reserved. Keywords: Calcium aluminosilicate; Metal-substitution; CH4 partial oxidation; Syngas
1. Introduction Syngas, a mixture of CO and H2, is mainly produced from steam reforming of hydrocarbons (in case of CH4, CH4 + H2O ! CO + 3H2) [1,2]. Partial oxidation of CH4 (POM; CH4 + 1/2O2 ! CO + 2H2) is another route to produce syngas. The H2/CO ratio of 2 in POM syngas is favorable for its further utilization for methanol and Fischer–Tropsch syntheses. Among catalysts active for POM, supported Ni has been studied most extensively [3–21]. Both steam reforming and POM accompany a serious coke deposition, which leads to catalyst deactivation and also to plugging of the gas flow. In steam reforming, coking is avoided by maintaining the steam/carbon ratio much larger than the stoichiometric ratio. However, such methodology is not applicable to POM, because the increase in the O2/CH4 ratio is in favor of the complete oxidation of CH4 to CO2 and H2O. It is, therefore, of primary
*
Corresponding author. Tel./fax: +81 172 39 3565. E-mail address: [email protected] (T. Mori).
1566-7367/$ - see front matter 2007 Elsevier B.V. All rights reserved. doi:10.1016/j.catcom.2007.02.006
importance to develop a high performance anti-coking POM catalyst [7,10,11,17,19,21]. The authors have prepared calcium aluminosilicate, the mineralogical name of which is mayenite (Ca12Al10Si4O35), and revealed its catalytic performance [22–30]. It was found 2 that oxide anions ðO 2 ; O2 Þ were occluded in micropores of mayenite [24,25,29] and a part of Ca2+ could be substituted with other metal ions, by which the combustion activity was highly improved [28–30]. Such oxide anions occluded may positively play an anti-coking role in POM. Therefore, the catalytic activity of metal-substituted mayenite is examined. 2. Experimental Hydrogarnet (Ca3Al2(SiO4)0.8(OH)8.8), a precursor of mayenite, was synthesized in advance from hydrothermal reaction of the stoichiometric mixture of powders of CaO, Al2O3, and amorphous SiO2 at 200 C for 15 h. Then, it was converted to mayenite via calcination at 900 C for 2 h. Metal-substituted mayenites were obtained by calcining metal-substituted hydrogarnet similarly synthesized. Details of the preparation procedure of these mayenites
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Fig. 1. Schematic drawings of hydrogarnet (a) and mayenite (b). In hydrogarnet, SiO4 tetrahedron, AlO6 octahedron and CaO12 dodecahedron are linked with one another by sharing corner oxygen atoms. In mayenite, each AlO4 tetrahedron (T1) is three-dimensionally linked with one another by sharing corner oxygen atoms. Central Al3+ in some tetrahedra is replaced with Si4+ to form (Al, Si)O4 (T2) tetrahedron. Oxide anions (indicated by O 2 here) are located in mayenite.
were reported in the literatures [22–30]. Unless otherwise described, the metal/Ca atomic ratio in metal-substituted mayenite was 0.0714, 0.04, 0.0344, 0.025, and 0.05 for Cr-, Fe, Co-, Ni-, and Cu-mayenite, respectively. Except for Ni-substitution, each value was the ratio at which the highest activity was obtained for NO reduction with C3H6 [29]. It should be described that conversion from hydrogarnet to mayenite produces free metal oxide together with CaO, because hydrogarnet transforms to mayenite by releasing excess CaO according to the following equation (see Fig. 1): 5Ca3 Al2 ðSiO4 Þ0:8 ðOHÞ8:8 ! Ca12 Al10 Si4 O35 þ 3CaO ð1Þ
þ 22H2 O 2+
XRD revealed the substitution of a part of Ca ions in mayenite with other metal ions [29]. The oxide anions located in the prepared mayenites was confirmed by both EPR and Raman spectroscopic measurements [29]. For comparison, supported Ni catalysts were prepared by an impregnation method using mayenite, a- and c-Al2O3 as a catalyst support. Metal loading was 1.2 wt%. The catalytic activity was tested by using a conventional flow-type apparatus with a U-shaped quartz reactor (i.d., 4.8 mm/) under atmospheric pressure. 100 mg of catalyst (particle size, 300–600 lm) was placed in the reactor with no diluents (length of the catalyst bed, ca. 7 mm). Prior to reaction, it was heated in a flow of O2 at 500 C for 1 h. The flow rate of reactant gases (CH4/O2 = 2) was 60 ml/ min, unless otherwise described. Considering the apparent density of mayenite of 0.5 g/ml, the space velocity (SV) was 1.8 · 104 h1. No diluents were added to the reactant gases. Exit gases were analyzed by gas chromatography (see Fig. 2). Steam reforming of CH4 was similarly carried out by replacing the reactant from O2 to H2O with the H2O/ CH4 ratio of 3.
Fig. 2. Effect of the kind of metal ions on partial oxidation of CH4 into syngas at 800 C.
3. Results and discussion The effect of the kind of substituting metal ions on the POM activity of mayenite was examined at 800 C. Fig. 1 summarizes the results. Mayenite itself was active for POM with CH4 conversion of 20% and both CO and H2 selectivities of ca. 50%. Substitution with Cr3+, Co3+, Fe3+, and Cu2+ slightly increased CH4 conversion with a considerable decrease in the selectivities of both CO and H2 to <20%, indicating that combustion of CH4 to CO2 and H2O preferentially occurred on these mayenites. Substitution with Ni2+, on the other hand, resulted in a marked increase in CH4 conversion (93%) with excellent selectivities of both CO and H2 (94% and 95%, respectively). Such a high performance was maintained for 12 h, suggesting a negligible coke formation and also a negligible sintering. It is known that POM often generates hot spots in the catalyst bed [4,6]. Such hot spots may be generated more
K. Sato et al. / Catalysis Communications 8 (2007) 1735–1738
Fig. 3. Effect of the temperature on partial oxidation of CH4 into syngas over Ni-mayenite. SV = 1.8 · 104 h1. Dotted line: increasing temperature. Solid line: decreasing temperature.
readily under the present experimental conditions, because no diluents were used both in the catalyst and in the reactant gases. However, no deactivation of Ni-mayenite and CH4 conversion and both CO and H2 selectivities below each equilibrium value (see Fig. 3) suggest a negligible generation of the hot spots in this catalyst. Further study is necessary to clarify the reason for the absence of the hot spots. Over Ni-mayenite, POM was carried out by sequentially increasing and decreasing the reaction temperature and the results are shown in Fig. 3. Below 600 C, the activity was very low and the product was only CO2 and H2O, suggesting that complete oxidation of CH4 occurs. At 700 C, on the other hand, CH4 conversion abruptly rose to 78% with CO selectivity of 88% and H2 selectivity of 89%. At 800 C, CH4 conversion and CO and H2 selectivities further increased to 93%, 94%, and 95%, respectively. When the temperature was decreased afterward, reproducible results were obtained at the temperature down to 700 C. Below 700 C, however, CH4 conversion and both CO and H2 selectivities gradually decreased, respectively, down to 41%, 32%, and 29% at 500 C and did not retrace the pathways observed during the initial temperature increase. It is mentioned that CH4 conversion and CO and H2 selectivities were below the respective equilibrium limits [18]. The observed CO and H2 selectivities indicated that POM was predominant at 600 C, while complete oxidation of CH4 to CO2 and H2O prevailed at 500 C. The absence of retracing the pathways in the lower temperature range has also been observed on supported Ni catalysts
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[3,6,12,20]. Following explanation similar to reported ones is also possible for our Ni-mayenite. While in the fresh Nimayenite, all the Ni is in the oxidized state, responsible for complete oxidation of CH4, it is reduced to the metallic state at 700 C during POM. A considerable part of Ni remains reduced during the temperature decrease to 500 C, to which the POM activity is attributed. To obtain more information about the absence of retracing the pathways in our catalyst, steam reforming of CH4 was attempted over Ni- and Cu-mayenite. No activity was observed on both fresh Ni- and Cu-mayenite at 700 C, because both Ni and Cu are in the oxidized state in the fresh samples. After POM was conducted at 700 C, Ni-mayenite exhibited the activity for steam reforming reaction: CH4 conversion attained to 61%. For Cu-mayenite, on the other hand, exposing to a mixture of CH4 and O2 at 700 C did not convert it active for CH4 steam reforming. These findings lead us to conclude that during POM at 700 C, Ni-mayenite was reduced to metallic Ni, active for steam reforming, and that the difference in the POM activity between Ni- and Cu-mayenite is attributed to the activity for steam reforming different from each other. The lack of the POM activity of fresh Ni-mayenite at 500 and 600 C should result from a difficult reduction to metallic Ni state at these temperatures. For POM, two mechanisms have been proposed: the combustion-reforming mechanism [3,4,6] and the direct POM or the pyrolysis mechanism [5,8,13–16]. The explanation described above seems to be in favor of the former mechanism, while the latter mechanism may explain the results of Cu-mayenite, because Cu-mayenite has no ability to catalyze steam reforming of CH4. The latter mechanism may also explain the results of unsubstituted and Cr-, Co-, and Fe-mayenites, because these mayenites are considered not to have an ability to catalyze steam reforming of CH4. Table 1 summarizes the POM activity of Ni-mayenites with various Ni/Ca atomic ratios at 800 C. All Ni-mayenites examined exhibited a very high CH4 conversion and CO and H2 selectivities. It is interesting to note that the introduction of a small amount of Ni to mayenite can convert it to a high performance POM catalyst. For comparison, POM was carried out over supported Ni catalysts. Fig. 4 shows the results. Obviously, CH4 conversion was higher on Ni-mayenite than on supported Ni catalysts for all SV examined, although the stable POM activity was also maintained on all the supported Ni catalysts. The ability of Ni-mayenite to catalyze both Table 1 Effect of the Ni/Ca atomic ratio on partial oxidation of CH4 into syngas over Ni mayenite at 800 C Ni/Ca atomic ratio
CH4 conversion (%)
CO selectivities (%)
H2 selectivities (%)
0 0.025 0.05 0.15
19.8 93.4 89.7 89.9
52.0 94.2 92.5 93.8
53.3 95.2 92.2 93.6
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References
Fig. 4. Effect of the space velocity on partial oxidation of CH4 into syngas over various Ni catalysts at 800 C.
combustion and steam reforming of CH4 is a possible cause for its excellent performance for POM. It is mentioned that the POM activity of our mayenite-supported Ni was slightly lower than that of C12A7-supported Ni [21]. Metal loading different from each other (1.2 wt% vs. 5 wt%) may explain the difference. 4. Conclusion The catalytic activity of unsubstituted and Cr-, Co-, Ni-, and Cu-substituted mayenites were examined toward POM. Among them, Ni-substituted one was most active with an excellent selectivity for this reaction and more active than supported Ni catalysts. During POM at >700 C, the ability to catalyze CH4 steam reforming appeared on Ni-mayenite. It is considered that such a high performance is attributed to the abilities of Ni-substituted one to catalyze both CH4 combustion and CH4 steam reforming.
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