where ? (or |–) stands for the substrate surface; (s) denotes the surface species; (g) is the gas phase; k1 is the surface reaction rate. Because the generated methane can be removed by the inert gas, the surface reaction can be assumed irreversible and rate-limiting [13]. Besides, according to the study of Elliott et al. [8], the subse- quent surface reaction can also occur when one methyl neighbors combine at least two surface hydroxyl groups, which can be described by,