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?

Abstract The effect of carbon deposition on the long-term stability of the Ni/SiO 2 catalyst during the methanation reaction was studied in this work. For a conventional Ni/SiO 2 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/SiO 2 catalyst. Comparatively, the co-impregnation of nickel nitrate and dicyandiamide molecules on SiO 2 support did not markedly change the defect sites on the metallic Ni 0 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/SiO 2 catalyst. As a result, the co-impregnation method derived Ni/SiO 2 catalyst has better long-term stability than the conventional Ni/SiO 2 catalyst in the methanation reaction.