Alkane Oxidation on Rh(111) Single-Crystal Surfaces under High-Temperature, Short-Contact-Time Conditions: A Molecular Beam Kinetic Study

Abstract

The kinetics of the partial oxidation of alkanes with molecular oxygen on Rh(111) single-crystal surfaces were studied by using a collimated effusive molecular beam under ultrahigh vacuum conditions. These experiments were conceived to probe the details of the mechanism of these processes under high-temperature, short-contact-time conditions. It was determined that the primary products in these reactions are H2 and CO, the components of syngas, as it has been suggested by the so-called direct mechanism. The production of water is also detected, but at a slower rate because of the slow formation of the surface hydroxo intermediate involved. Carbon dioxide, on the other hand, is never produced, because carbon monoxide desorbs before having the opportunity to react further with adsorbed oxygen atoms. Both surface and subsurface atomic oxygen species form during the reaction, but only the more labile surface intermediate is relevant, blocking Rh sites for alkane activation and reacting to form CO and H2O.