Model catalyst studies on vanadia particles deposited onto a thin-film alumina support.: 2.: Interaction with carbon monoxide

The interaction of CO molecules with a model catalyst system formed by alumina-supported vanadia particles was studied utilizing infrared (IR) spectroscopy. A thin, well-ordered alumina film grown on the NiAL(110) alloy surface was used as a support oxide. The structural characterization of this model system has been performed previously by applying scanning tunneling microscopy, X-ray photoelectron spectroscopy, and IR spectroscopy. Exposure to CO at 90 K results in a complex adsorption behavior as a function of vanadia particle size. At high vanadia coverages, only one CO species can be detected that interacts with vanadyl groups (V=O) present at the. surface of the particles. At lower coverages, however, three further species were observed. Their existence seems to be correlated with either defect sites or with special sites provided at the vanadia-alumina interface. Furthermore, particles in this coverage regime are capable of adsorbing a maximum amount of CO molecules. Regarding the reactivity of our model system, we found only molecular desorption of CO upon thermal treatment. However, exposing the CO covered particles to low-energy electrons renders a reaction possible. A portion of the adsorbed CO gets oxidized to CO2 via a redox mechanism, i.e., by a transfer of lattice oxygen. According to our experiments, the vanadyl oxygen does not seem to be the species that is consumed during the course of the reaction.