Adsorption, decomposition and oxidation of methanol on alumina supported palladium particles

Abstract

We have investigated the adsorption, decomposition and oxidation of methanol on a well-defined supported Pd model catalyst, utilizing a combination of molecular beam methods, reflection absorption IR spectroscopy (RAIRS) and temperature-programmed desorption (TPD). The Pd model catalyst is prepared under ultrahigh-vacuum (UHV) conditions on a well-ordered Al₂O₃ film grown on NiAl(110). In previous studies, this model system has been characterized in detail with respect to its geometric and electronic structure. On the alumina support, two molecular adsorption states of methanol are distinguished by RAIRS and TPD. Moreover, we can differentiate between adsorption on the Pd particles and on the alumina support, enabling us to follow surface diffusion from the alumina film to the Pd particles during the adsorption process. Upon heating, methanol partially desorbs from the Pd particles and partially undergoes decomposition, with a reaction probability that is sensitively dependent on the initial methanol coverage. At 100 K, preadsorbed CO suppresses methanol adsorption on the Pd particles, whereas preadsorbed oxygen reduces the reaction probability. As a first intermediate, methoxy species are formed, which are stable up to temperatures of 200 K. Isotope exchange experiments indicate that a fast equilibrium is established between molecular methanol and methoxy species and that both species are rapidly exchanged with the gas phase. Further decomposition of methanol proceeds via two competing reaction pathways. The dominant pathway is dehydrogenation to CO, followed by CO₂ formation in the presence of oxygen. Adsorbed oxygen has a pronounced inhibiting effect on the rate of decomposition. As a second pathway, we observe slow breakage of the carbon–oxygen bond, leading to formation of carbon and hydrocarbon species.