Platinum-catalyzed hydrogenation of graphite: Mechanism studied by the rates of monolayer channeling

Monolayer channeling on the basal plane of graphite was studied for the Pt-catalyzed CH₂ reaction using gold decoration transmission electron microscopy, at temperatures below 1050 °C, H₂ pressures below 1 atm, and Pt particle sizes below 5000 Å. The particle movement is driven by the adhesion forces between Pt and the {101̄1} zigzag edge plane of graphite. Larger particles channel faster but the gasification rate per Pt surface area is the same for all sizes. The rate is independent of H₂ partial pressure. The kinetic data suggest that the rate-limiting step is the reaction on the Pt surface between surface carbon and chemisorbed hydrogen at their equilibrium amounts. A comparison of the monolayer channeling data and those on multilayer channeling and bulk measurement indicates that there exists a similarity between the mechanisms. A further comparison of the kinetic data for the catalyzed CH₂ reaction and for the methanation reaction (from CO + H₂) suggests the likelihood of a common rate-limiting step.