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Further investigations on high temperature methanation catalysts are not only desirable but also necessary. With such catalysts, the overall heat efficiency of current coal gasification-SNG processes could be improved. Moreover, these catalysts can also be used in a chemical heat pipe system, which is one of the efficient energy transfer systems.The thermal stability of Ni-Mo alloy catalysts supported on zirconia for high temperature methanation was tested at 650°C, 80kg/cm2 and 15, 000hr-1 GHSV for several weeks using a feed mixture containing 15% of carbon monoxide, 45% of hydrogen and 40% of methane. Shown in Table 2 are four kinds of zirconium compounds, monoclinic zirconia, zirconyl hydroxide, zirconyl carbonate and stabilized, cubic zirconia, used as support materials. The results of the thermal stability tests are shown in Figs. 1-4, in which the methane concentration at temperatures lower than the thermal stability test temperature, 650°C, is represented as a measure of the catalytic activity at that temperature. As a result, Ni-Mo-ZrO2-1 maintained its high activity in the temperature range of from 400 to 500°C for 4 weeks in spite of the rapid decrease in its initial activity. On the other hand, the activity of Ni-Mo-ZrO2-2 and that of Ni-Mo-ZrO2-4 were low initially but they increased with progress of the test, but their catalytic activities were lower than the activity of Ni-Mo-ZrO2-1 even at the end of the test. For Ni-Mo-ZrO2-3, its catalytic activity was low throughout the test. Thus, Ni-Mo-ZrO2-1 supported on monoclinic zirconia was found most active among the catalysts tested.The result of X-ray diffraction analyses on these catalysts are shown in Figs. 5-8 and Table 2. The diffraction peaks identifiable with Ni-Mo alloy and metallic molybdenum were observed for all of the catalysts tested after hydrogen reduction at 700°C. The Mo content in the Ni-Mo alloy decreased with progress of the test, as shown in Fig. 10. Metallic Mo disappeared and Mo2C appeared at the same time.The crystal forms of zirconia in Ni-Mo-ZrO2-1 and Ni-Mo-ZrO2-4 did not change during the test. On the other hand, the amorphous zirconia in Ni-Mo-ZrO2-2 and Ni-Mo-ZrO2-3 before hydrogen reduction, appeared as tetragonal and monoclinic zirconia after reduction.As to the carbon content of the catalysts, no increase was observed except the rapid increase on the first day of the test for all catalysts as shown in Fig. 9. If all of the Mo atoms in the catalysts had been changed to a mixture of Mo2C and Ni-Mo alloy containing 10 atomic % of Mo, the calculated carbon content of the catalysts tested would be 1.0% by weight; thus, the observed value of 1-2% would not be considered large.According to the results above, monoclinic zirconia is considered to be the best support material for the high temperature methanation catalysts investigated. The reason may be attributed to the less extent of interaction between the monoclinic zirconia possessing covalent nature and the nickel and molybdenum oxides in the calcined catalyst. On the other hand, for other three catalysts, a greater extent of interaction between zirconium compounds and the nickel and molybdenum oxides is expected from the results described below. One of the results is the increase in activity in the early stages of the test. This may indicate that incompletely reduced oxides by hydrogen treatment might be rereduced during the thermal stability test. The other is that the catalysts were amorphous before they were subjected to the hydrogen treatment.
- 公益社団法人 石油学会の論文
公益社団法人 石油学会 | 論文
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