A reason for the structure-insensitive catalytic activity of Ni(100) and Ni(111) surfaces for the methanation reaction of CO

Abstract

The structure-insensitive methanation reaction, CO + 3H 2 → CH 4 + H 2O, on Ni(111) and Ni(100) surfaces (1, 2), is rationalized on the basis of the structure of carbidic carbon intermediates. Accumulation of carbidic carbon intermediates on the Ni(100) surface results in a (2 × 2)p4g overlayer, and its hydrogenation proceeds at a rate almost equal to that of a steady-state methanation reaction. In contrast to the carbide overlayer on the Ni(100) surface, the LEED pattern of the carbide overlayer on the Ni(111) surface is too complex to be solved. A single-domain carbide on a Ni(111) surface accidentally obtained by the segregation of carbon allowed us to deduce the structure of the carbide overlayer on the Ni(111) surface. It was shown that the carbide overlayer on Ni(111) has exactly the same arrangement of carbon atoms as that of the (2 × 2)p4g structure on the Ni(100) surface. In addition, the carbide overlayer undergoes decomposition at 685 K on Ni(100), Ni(110), and Ni(111) surfaces. Therefore, we conclude that the accumulation of carbidic intermediates creates an identical surface carbide on Ni(100) and Ni(111) surfaces. This may be a reason for the structure-insensitive catalysis, because the methanation may be catalyzed by this surface carbide on Ni(100) and Ni(111) surfaces.