Kinetics of carbon formation from CH4 + H2 on silica-supported nickel and Ni-Cu catalysts

Abstract

Steady-state rates of carbon formation on a silica-supported nickel catalyst exposed to CH4 + H2 gas mixtures have been measured for a range of gas compositions (partial pressure of methane, 20-80 kPa, and of hydrogen, 5-15 kPa) and temperatures in the range 723-863 K. The data are used to test various kinetic models suggested previously in the literature. It is shown that the data are in conflict with previous suggestions that the rate is, at a given temperature, solely a function of the carbon activity of the gas, but compatible with a kinetic model based on the elementary steps suggested by Grabke [Grabke. H. J., Ber. Bunsenges. Phys. Chem.69, 409 (1965)] to explain results for carbon formation on iron. However, the data cannot be explained by using the Grabke assumption that the dehydrogenation of surface methyl is rate-limiting. Also rate measurements for two silica-supported Ni-Cu catalysts with 1: 99 and 1: 9 Cu : Ni atom ratios, respectively, are reported. They show that the same Grabke-type kinetic model can explain the results at low carbon activity for the Ni-Cu catalysts, but that at the higher carbon activities, the rates for the Ni0.9Cu0.1 catalysts are higher than the model rates. The results for the Ni0.99Cu0.01 catalyst indicate that a small amount of Cu promotes the carbon formation.