Raman Spectral Evidence of Reactive Oxide Formation during Methanol Oxidation on Polycrystalline Rhodium at High Gas Pressures

Abstract

Clusters of Pt and Pd catalyze dimethyl ether (DME) combustion to CO2 and H2O at 400–600 K without detectable formation of byproducts. Rh clusters are less active and also form CO, HCHO, and CH3OH in this temperature range. On Pd, isotopic and kinetic studies have shown that DMEO2 reactions proceed via redox cycles limited by hydrogen abstraction from chemisorbed DME molecules without the involvement of methoxide intermediates in kinetically relevant steps, as shown previously on Pt clusters. Kinetic inhibition by H2O is much stronger on Pd than on Pt clusters, apparently because of weaker binding of chemisorbed oxygen (O*) and hydroxyl (OH*) groups on Pt than on Pd. H2O titrates vacancies (*) required to chemisorb DME molecules involved in kinetically relevant H-abstraction steps. DME combustion turnover rates (per exposed metal atom) on Pt, Pd, and Rh increased with increasing cluster size, but were not affected by the identity of the support (Al2O3, ZrO2). These size effects reflect the stronger binding of O* and OH* on smaller clusters, which contain surface atoms with fewer neighbors and greater coordinative unsaturation. The higher reactivity of Pt compared with Pd and Rh also reflects the weaker binding of O* on Pt surfaces and the higher density of vacancies and of DME intermediates interacting with such vacancies. These trends resemble those reported for CH4O2 on Pt and Pd clusters. They represent a general feature of reactions that require vacancies and the abstraction of H-atoms by basic oxygens on surfaces covered predominately by O* or OH* during catalysis.