Structure sensitivity and crystallite size change of nickel during methanation of CO/H 2 on nickel-silica catalysts

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In the methanation of carbon monoxide on a series of nickel-silica catalysts the activity per unit surface area of nickel was found to be strongly dependent on the mean nickel crystallite size (0.5–13 nm). The specific activity shows a maximum value at a crystallite size of about 4 nm. During the methanation of carbon monoxide in a differential flow reactor at 470–520 K. the magnetization of a catalyst sample measured at low field strength was found to increase with time, which indicates a growth of larger crystallites at the expense of smaller ones. Additional evidence for this fact arises from the measurement of the metal surface area, which during methanation decreases with time, and from high field magnetization measurements, which show that the crystallite size distribution shifts to larger sizes. During these changes the activity of the sample remains almost constant, although small crystallites disappear. This suggests that the larger crystallites are particularly active in the reaction. The dependence of the specific activity on the crystallite size supports this finding, as also in these experiments small crystallites were less active than larger ones. This finding might suggest that on nickel the methanation of carbon monoxide requires an ensemble size most abundantly found on larger crystallites. A high partial pressure of carbon monoxide and a low temperature favour the crystallite size changes. This suggests that the formation of nickel tetracarbonyl is important as intermediate for the transport of nickel from small to larger crystallites.