A sensitive method for determining dehydrogenation probabilities at a metal surface

We report a sensitive new approach for tracking surface hydrogen coverage using the novel high-repetition rate implementation of velocity-resolved kinetics in an investigation of methanol-d4 dehydrogenation on Pt(111) to CO and D2. The reactant beam of methanol-d4 had an incidence kinetic energy of 0.46 eV, and the reaction was studied over a surface temperature range of 700-950 K. The reaction probability of methanol dehydrogenation was determined by probing the second-order reaction kinetics of D2 and HD formation, which is directly correlated to the surface hydrogen coverage. Under the conditions of our study, the methanol dehydrogenation probability was found to be low (∼10-3), and the lack of strong temperature dependence is consistent with dehydrogenation via a direct mechanism. Under conditions that favored methanol clustering in the incident methanol beam, the dehydrogenation probability increases significantly at low surface temperatures, which suggests a precursor-mediated mechanism. The possible role of cluster formation in enhancing the trapping probability of methanol is discussed.