Sequential production of H_2 and CO over supported Ni catalysts

摘要

Methane decomposition into H_2 and carbon nanofibers at 823 K and subsequent gasification of the carbon nanofibers with CO_2 into CO at 923 K were performed over supported Ni catalysts (Ni/SiO_2, Ni/TiO_2 and Ni/Al_2O_3). Supported Ni catalysts were deactivated for CH_4 decomposition with time on stream due to deposition of a large amount of carbon nanofibers. Subsequent contact of CO_2 with carbon nanofibers on the deactivated catalysts resulted in the formation of CO with a conversion of the carbons higher than 95%. In addition, gasification with CO_2 regenerated the activity of supported Ni catalysts for CH_4 decomposition, indicating that H_2 formation through CH_4 decomposition and CO formation through gasification with CO_2 could be carried out repeatedly. Conversions of carbon nanofibers into CO were kept higher than 95% in the repeated gasification over all the catalysts, while change in the catalytic activity for CH_4 decomposition with the repeated cycles depended on the kind of catalytic supports. Catalytic activity of Ni/SiO_2 for CH_4 decomposition was high at early cycles, however, the activity decreased gradually with the repeated cycles. On the other hand, Ni/TiO_2 and Ni/Al_2O_3 showed high activity for CH_4 decomposition and the activity was kept high during the repeated cycles. These changes of catalytic activities for CH_4 decomposition could be explained by changes in particle sizes of Ni metal, i.e. Ni metal particles in Ni/SiO_2 aggregated into ones larger than 150 nm with the repeated cycles, while the particle sizes of Ni metal in Ni/TiO_2 and Ni/Al_2O_3 remained at an effective range for CH_4 decomposition (60-100 nm).