Mechanism of graphite hydrogenation catalyzed by nickel

Monolayer channeling of Ni particles (less than 2000 Å diameter) on the basal plane of graphite was studied for the catalyzed C H₂ reaction (producing methane). The preferred wetting of Ni on the {101l} zigzag edge of graphite determines the channel orientation and is also the driving force for channeling. The reaction follows these sequential steps: dissolution of carbon at monolayer step/Ni interface, diffusion of carbon in Ni, reaction of carbon with chemisorbed hydrogen at the Ni/gas interface. The last step (the surface reaction) is the rate-limiting step. The close similarity between the turnover rates of the surface reaction for monolayer channeling, deep-layer channeling, bulk reaction (using mixed Ni C), and methanation (from CO + H₂) suggests that the surface reaction is the common rate-limiting step. Moreover, Ni is approximately two orders of magnitude more active than Pt for all modes of action except for methanation, where CH₄ is not the only product from Pt.