Methane Decomposition into Hydrogen and Carbon Nanofibers over Supported Pd−Ni Catalysts:  Characterization of the Catalysts during the Reaction

Abstract

The addition of Pd into Ni/SiO2 improved the catalytic activity and life for methane decomposition into hydrogen and carbon nanofibers. Thus, Pd-added Ni/SiO2 (denoted as Pd−Ni/SiO2) was characterized by measuring the Ni and Pd K-edge XANES and EXAFS and TEM images. Methane decomposition on Ni/SiO2 or Pd/SiO2 formed carbon nanofibers with Ni or Pd metal particles at the tips of them. Ni metal in Ni/SiO2 was transformed into Ni carbides at the deactivation stage of the catalyst for methane decomposition. Pd metal particles in Pd/SiO2 were fragmented into smaller ones during methane decomposition. On the other hand, Pd−Ni/SiO2 produced carbon nanofibers of different shapes than those on Ni/SiO2 and Pd/SiO2, i.e., branched carbon nanofibers. The local structure and average crystallite size of Pd−Ni alloys in Pd−Ni/SiO2 did not change appreciably during methane decomposition, but the shape of alloy particles changed gradually. The number of facets on Pd−Ni alloy particles that precipitated carbon atoms increased during methane decomposition. This explains the increase in the catalytic life as well as the formation of branched carbon nanofibers.