Surface chemistry of methyl radicals on OTMo(100) surfaces

The chemistry of CH₃ radicals on oxygen-modified Mo(100) surfaces (O/Mo(100)) has been studied using temperature-programmed desorption (TPD) and high-resolution electron energy loss spectroscopy (HREELS). Gas-phase CH₃ radicals were produced by pyrolysis of azomethane and dosed on O/Mo(100) at a surface temperature of 320 K. In TPD, O/Mo(100) with θ_O = 1.4 monolayer (ML) produces exclusively CH₄ and CO, but O/Mo(100) with θ_o = 0.9 and 0.4 ML produce significant amounts of C₂₊ alkenes in addition to CH₄ and CO. HREELS shows that the CH₃ groups are bound to surface Mo atoms, not to surface oxygen. On 1.4 ML-O, the CH₃ groups are stable at 320 K and have a symmetry lower than C_3ν. On 0.9 ML-O and 0.4 ML-O, some CH₃ groups decompose to methylene groups, which react with intact CH₃ groups to form surface alkyl groups. The surface species at 320 K appear to be controlled by the preadsorbed oxygen coverage, depending on whether θ_o ≤ 1 ML or θ_o > 1 ML. CH₄ is formed via hydrogenation of CH₃ groups by surface hydrogen that is a product of CH₃ decomposition. C₂₊ alkene products are formed by β-hydrogen elimination of surface alkyl groups. When atomic iodine is coadsorbed on O/Mo(100), the alkene yield in TPD is significantly reduced.