Methanol reactions over oxygen-modified re surfaces: Influence of surface structure and oxidation

The reactions of methanol over planar and faceted oxygen-covered Re(12 (3) over bar1), and over a thin-film oxide on Re(12 (3) over bar1), are studied using temperature-programmed desorption (TPD), high-resolution X-ray photoelectron spectroscopy (HRXPS), and low-energy electron diffraction (LEED). These surfaces are potential model catalysts for the selective oxidation of methanol to methylal [dimethoxymethane, CH(2)(CH(3)O)(2)]. Re(12 (3) over bar1) is chosen because it is a morphologically unstable substrate that develops nanometer scale facets when precovered with oxygen and annealed, thus providing a suitable surface to investigate structure-reactivity relationships in methanol decomposition. Reaction pathways for methanol over the O-covered planar and faceted surfaces are qualitatively similar, proceeding through competing pathways of dehydrogenation to CH(2)O and CO and nonselective decomposition to yield H(2) and CO that is formed by recombination of C((ads)) with surface-bound oxygen. The selectivity toward methanol dehydrogenation products is not affected by morphological differences between the planar and faceted surfaces. However, the activity of the O-modified surfaces decreases progressively in the following order: planar O/Re > faceted O/Re > thin-film oxide/Re, with the thin-film oxide relatively inactive toward methanol reaction. The differences in activity may be attributed to changes in local geometric structure, the oxidation state, and the local order of Re-O species formed on the O-modified Re surfaces. We discuss these results in the context of the elementary steps in the catalytic oxidation of methanol to methylal.