The Investigation of Catalytic Methanol Oxidation on Pt(111) and Pd(111) by X-Ray Photoelectron Spectroscopy and Mass-Spectrometry

Oxidation and dehydrogenation of methanol over Pt(111) and Pd(111) single-crystals were studied using in situ X-ray photoelectron spectroscopy (XPS) and mass-spectrometry. It was found that for both metals the methanol dehydrogenation proceeds via two routes: dehydrogenation to CO and hydrogen, and decomposition of methanol with C–O bond scission [1–3]. The rate of the second route is several times lower, however, carbon production in this case leads to formation of carbon deposits, which block the surface and hinder the catalytic reactions. Even in the presence of O2 in the gas phase, the main route of methanol conversion over Pd is dehydrogenation to CO and H2. Hydrogen is partially oxidized to water, and CO is oxidized to CO2. The reactions start at temperature above 450 K when surface carbon depositions are removed by oxygen. In contrast, over Pt in presence of O2, the main reaction products are CO2 and water. Reaction also starts above 450 K when the surface carbon deposits are removed. We suppose that the reaction comes via two stages: at first, methanol dehydrogenates to CO and H2, and then total oxidation of these intermediates occurs. At the same time we detected by in situ XPS the formation of formates on the Pt surface. It means that methanol over Pt partially oxidize via non-CO-involved route when the formation and following decomposition of surface formates lead to CO2 and H2 yield. The difference in product distribution over Pt and Pd in the methanol oxidation is in a good agreement with the fact that Pt is more active in the CO oxidation than Pd. In both cases, the active state in the methanol oxidation is Pt or Pd in the metallic state.