Dynamic transformation of small Ni particles during methanation of CO under fluctuating reaction conditions monitored by X-ray absorption spectroscopy

A 10 wt.-% Ni/Al$_{2}$O$_{3}$ catalyst with Ni particles of about 4 nm was prepared and applied in the methanation of CO$_{2}$ under dynamic reaction conditions. Fast phase transformations between metallic Ni, NiO and NiCO$_{3}$ were observed under changing reaction atmospheres using operando X-ray absorption spectroscopy (XAS). Removing H$_{2}$ from the feed gas and, thus, simulating a H$_{2}$ dropout during the methanation reaction led to oxidation of the active sites. The initial reduced state of the Ni particles could not be recovered under methanation atmosphere (H$_{2}$/CO$_{2}$ = 4); this was only possible with an effective reactivation step applying H$_{2}$ at increased temperatures. Furthermore, the cycling of the gas atmospheres resulted in a steady deactivation of the catalyst. Operando XAS is a powerful tool to monitor these changes and the behavior of the catalyst under working conditions to improve the understanding of the catalytic processes and deactivation phenomena.