Paper SS-ThM2
Reaction Chemistry of Methyl and Methylene Species on Cr₂O₃(0001)

Thursday, November 3, 2011, 8:20 am, Room 107

The reactions of methyl (CH₃) and methylene (CH₂) surface species formed from the dissociation of halogenated methanes have been studied with temperature programmed desorption over Cr₂O₃(0001). Methyl fragments undergo a rate-limiting dehydrogenation to methylene and produce methane (CH₄) and ethylene (CH₂=CH₂) as the primary products. Comparison to earlier results for the Cr₂O₃(1012) surface show that methyl dehydrogenation is a structure-insensitive reaction, with the barrier to dehydrogenation being insensitive to the surface cation coordination number and/or local Cr–O site pair geometry. The ethylene formed from the dehydrogenation of CH₃ is associated with the surface diffusion of methylene species and subsequent coupling (C-C bond formation) reactions.

Separate studies of the reaction of methylene show that surface diffusion and coupling to ethylene can occur at lower temperatures than required for CH₃ dehydrogenation. The diffusion-limited coupling of CH₂ appears to be a structure-sensitive reaction, with the barrier to surface diffusion about 25 kJ/mol higher on Cr₂O₃(0001) than the (1012) surface.