A density functional theory study of site-specific methyl reaction on MoO3(010): The effects of methyl coverage

摘要

We report density functional theory calculations, using pseudopotentials with a plane wave basis, of methyl adsorbed on the MoO3(010) surface at 0.5 and 1.0 ML coverages. The MoO3 surface is modeled by a one-layer slab. Methyl is adsorbed most strongly over the terminal oxygen, where methoxy is formed. Over the bridging oxygens, qualitatively different results are obtained depending on the coverage of methyl. At 0.5 ML, methoxy is formed over the bridging oxygens, with an associated relaxation of the surface caused by a repulsion between the methyl and the terminal oxygen. For the 1.0 ML coverage, methyl is not stable and decomposes. Over the asymmetric bridging oxygen, methyl reacts to form formaldehyde and hydroxyl, while over the symmetric bridging oxygen a formyl species and water are produced. The hydroxyl and water are formed via hydrogen transfer to a terminal oxygen. The bonding and reaction of methyl over the various oxygen species are analyzed and discussed using charge density difference plots as well as the crystal orbital overlap projection (COOP). For the 0.5 ML methyl coverage, this analysis shows that the tilting of the terminal oxygen bond weakens this bond slightly. For the 1.0 ML methyl coverage, the COOP plots indicate that the decomposition of methyl over the bridging oxygens is not as stable as methyl over the terminal oxygen because the presence of the carbon fragments affects the electronic structure, and causes a weakening of the Mo-terminal oxygen bond. These results are discussed in the context of partial oxidation reactions. (C) 2000 American Institute of Physics. [S0021-9606(00)70521-6]. [References: 21]