Mechanism of Methane Formation by Catalytic Hydrogenation of Carbon

Abstract

Metallic catalyst being active for the methanation reaction of a synthetic gas are in general active for the direct hydrogenation of carbon to methane. It was found that the rate of formation of methane from catalyst-impregnated carbon decreased rapidly as the formation reaction proceeded (Fig. 1) In this paper, the formation of methane from cobalt-impregnated carbon was studied precisely. The rate of the formation of methane was prohibited by the deposition of carbon on active sites by the decomposition of methane or by Boudouard reaction. By removing the deposited carbon with hydrogen, the activity for the hydrogenation of bulk carbon was recovered (Fig. 7). The hydrogenation of cobalt-impregnated carbon on which ¹³C was deposited gave more than 85% of ¹³CH₄ at the initial stage in the recovery process (Figs. 8, 9).
These results strongly suggest that the diffusion of carbon to active sites is slow step in the hydrogenation of carbon, and that the mean free path of carbon until it arrives at the active sites which fulfill the structural prerequisites for hydrogenation of carbon depends on pressure of hydrogen. During the hydrogenation of carbon, accordingly, active sites have little carbon at the initial stage of the hydrogenation reaction, but are gradually covered by carbon as incresing the partial pressure of methane, which results in retarding the hydrogenation of bulk carbon.