Deposition of carbon species on the surface of metal: As a poison or a promoter for the long-term stability of Ni/SiO2 methanation catalyst?

•Trace pyrrolic nitrogen containing carbon can be deposited on Ni(1 1 1) planes.•Pyrrolic nitrogen containing carbon makes more active sites available on Ni(1 1 1).•Nitrogen containing carbon modified Ni/SiO2 has better stability in methanation.

The effect of carbon deposition on the long-term stability of the Ni/SiO2 catalyst during the methanation reaction was studied in this work. For a conventional Ni/SiO2 catalyst prepared by the impregnation method, it was found that the water-gas shift reaction (WGSR) as one side reaction took place on Ni(1 0 0) planes predominately and followed the carboxyl mechanism under the present condition. The inhabitation of this WGSR could produce the carboxyl and carbonyl species on surface which initiated the deactivation of this conventional impregnation derived Ni/SiO2 catalyst. Comparatively, the co-impregnation of nickel nitrate and dicyandiamide molecules on SiO2 support did not markedly change the defect sites on the metallic Ni0 particles, however, it was favorable to deposit trace pyrrolic nitrogen containing carbon on Ni(1 1 1) planes. Such carbon deposition allowed the electron transfer from pyrrolic nitrogen atoms to Ni(1 1 1) planes, leading to more active sites available for the reaction on this Ni(1 1 1). This effect facilitated the occurrence of WGSR on Ni(1 1 1) rather than Ni(1 0 0) planes and significantly reduced the deposition of carboxyl and carbonyl species on this carbon pre-deposited Ni/SiO2 catalyst. As a result, the co-impregnation method derived Ni/SiO2 catalyst has better long-term stability than the conventional Ni/SiO2 catalyst in the methanation reaction.