REACTIVE OXYGEN SITES AT MOO3 SURFACES - AB-INITIO CLUSTER MODEL STUDIES

The electronic structure and bonding of geometrically inequivalent sur face oxygens is examined for MoO3(010) and (100) surfaces where the lo cal electronic structure is obtained from ab initio density functional theory (DFT-LCGTO) cluster calculations. The clusters are chosen as f inite sections of the ideal MoO3 surface where cluster embedding is ac hieved by bond saturation with hydrogens, yielding clusters up to Mo7O 30H18. Local charging, bond orders, and electrostatic potentials of th e surface clusters depend weakly on cluster size, suggesting general v alidity for the extended surface. The difference in electronic structu re between the (010) and (100) surface is found to be mainly due to th e different atom arrangement, while local atom charging and binding pr operties are surface-independent. Terminal molybdenyl oxygens experien ce the smallest negative charging and form double bonds with the adjac ent 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 t wo neighboring Mo centers. Electrostatic potentials determined from cl uster charge distributions show broad negative minima above the termin al oxygens while there are no minima above bare Mo metal centers which can affect stabilization and binding of adparticles at the MoO3 surfa ces.