Structural Change of Ni Species in Ni/SiO2 Catalyst during Decomposition of Methane

The structural change of Ni species in a Ni/SiO2 catalyst during methane decomposition into hydrogen and carbon was investigated by X-ray diffraction (XRD), scanning electron microscopy, and Ni K-edge X-ray absorption near-edge spectroscopy/extended X-ray absorption fine structure (XANES/EXAFS). The Ni/SiO2 catalyst decomposed methane actively into hydrogen and filamentous carbon at initial periods of the reaction. The catalytic activity for the reaction decreased with time on stream of methane and finally the catalyst was deactivated completely. Prior to the contact of methane with the Ni/SiO2 catalyst, Ni species on the catalyst were present as Ni metal mainly. XRD analyses showed that Ni metal particles were aggregated as soon as the Ni/SiO2 came into contact with methane. After this initial aggregation, the structure of the Ni metal did not change appreciably when the Ni/SiO2 was decomposing methane actively. Ni K-edge XANES/EXAFS suggested the formation of some nickel carbide species on the catalyst when significant deactivation of the catalyst was observed.