(91e) Reaction of Methylene On α-Cr2O3(1012): A Prototype for Surface Carbene Coupling Reactions On Non-Reducible Oxides

CH₂I₂ has been used to examine the reaction of methylene on á-Cr₂O₃(1012). XPS shows that C-I bond breaking occurs at temperatures lower than 250 K, and indicates that both methylene and I adatoms bind at surface Cr cation sites. Thermal desorption experiments indicate multiple reaction routes for surface methylene including dehydrogenation to H₂ and surface carbon, hydrogenation to CH₃ and methane and coupling to ethene in two separate reaction channels with different activation barriers. Since surface Cr cations on Cr₂O₃(1012) expose predominantly one coordination vacancy, the coupling reaction indicates that methylene surface diffusion likely plays an important role in the coupling reaction.

Density functional theory (DFT) has been used to examine the diffusion and coupling of methylene. DFT indicates that methylene binds preferentially to surface Cr cations as seen experimentally with XPS, but also predicts a local minimum at bridge sites between neighboring coordinately-unsaturated surface cations and anions. Two possible diffusion paths have been identified: (1) a lower-barrier route involving direct cation-to-cation diffusion across a surface trough, and (2) a higher-barrier route involving indirect cation-to-cation diffusion through the bridging methylene species. These predicted barriers to diffusion are in good agreement with experimental activation barriers to coupling observed in thermal desorption experiments.