Reactive oxygen sites at MoO3 surfaces: Ab initio cluster model studies

The electronic structure and bonding of geometrically inequivalent surface oxygens is examined for MoO3(010) and (100) surfaces where the local electronic structure is obtained from ab initio density functional theory (DFT-LCGTO) cluster calculations. The clusters are chosen as finite sections of the ideal MoO3 surface where cluster embedding is achieved by bond saturation with hydrogens, yielding clusters up to Mo7O30H18. Local charging, bond orders, and electrostatic potentials of the surface clusters depend weakly on cluster size, suggesting general validity for the extended surface. The difference in electronic structure between the (010) and (100) surface is found to be mainly due to the different atom arrangement, while local atom charging and binding properties are surface-independent. Terminal molybdenyl oxygens experience the smallest negative charging and form double bonds with the adjacent Mo centers. Asymmetric bridging oxygens are slightly more negative and similar in their binding scheme to molybdenyl oxygens. Symmetric bridging oxygens become most negative and form single bonds with the two neighboring Mo centers. Electrostatic potentials determined from cluster charge distributions show broad negative minima above the terminal oxygens while there are no minima above bare Mo metal centers which can affect stabilization and binding of adparticles at the MoO3 surfaces.