The interaction of gaseous D atoms with methyl bromide molecules adsorbed on Pt(111), hydrogen saturated Pt(111), and graphite monolayer covered Pt(111) surfaces was studied in order to elucidate the reaction mechanisms. The reaction kinetics at 85 K surface temperature were measured as a function of the methyl bromide precoverage by monitoring reaction products simultaneously with D atom exposure. On all substrates incoming atoms abstract the methyl group from adsorbed CH3Br via gaseous CH3D formation. In the monolayer regime of CH3Br/Pt(111) pure hot-atom phenomenology was observed in the rates. At multilayer targets the fluence dependence of the kinetics gets Eley–Rideal-like. With coadsorbed H present, the reaction of D with adsorbed methyl bromide revealed in addition to CH3D a CH4 product. This and simultaneous abstraction of adsorbed H via gaseous HD and H2 products clearly demonstrates that hot-atom reactions occur. At CH3Br adsorbed on a graphite monolayer on Pt(111) the abstraction kinetics of methyl was found to agree with the operation of an Eley–Rideal mechanism. These observations are in line with the expectation that hot-atoms do not exist on a C/Pt(111) surface but on Pt(111) and H/Pt(111) surfaces. The methyl abstraction cross-sections in the monolayer regime of methyl bromide were determined as about 0.25 Å2, irrespective of the nature of the substrate. This value is in accordance with direct, Eley–Rideal or hot-atom reactions.

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