The mechanism of methane formation from the Reaction between graphite and hydrogen

Monolayer etch pits formed on the basal plane of graphite by hydrogen are hexagonal in shape and are bounded by {101l} zigzag edge faces. The reaction is also studied by using extended Hückel molecular orbital (EHMO) calculations with geometry optimization, where H atoms are added to the edge surface carbon and these carbon atoms are allowed to deform from the original positions corresponding to the graphite structure to reach the equilibrium positions. The EHMO/geometry optimization results indicate that the zigzag edge face is more reactive than the {112l} armchair face before and after one H is chemisorbed on the surface atom. However, a reversal in the relative reactivity occurs after the second H addition; the CC bond on the armchair face becomes weaker and carbon atoms become more reactive for the third H chemisorption. Breakage of CC bonds takes place upon the third H addition, and this step is the rate-limiting step for CH₄ formation.