PNNL

Abstract

We report a photoelectron spectroscopy and density functional theory (DFT) study on the electronic and structural properties of Nb3-, Nb3O-, Nb3O2-, and the corresponding neutrals. Well-resolved photoelectron spectra are obtained for the anion clusters at different photon energies and are compared with DFT calculations to elucidate their structures and chemical bonding. We find that Nb3 - possesses a C2V (3A2) structure, and Nb3 is a scalene Cs (2A'') triangle. Both Nb3O- and Nb3O are found to have C2V structures, in which the O atom bridges two Nb atoms in a Nb3 triangle. The ground-state of Nb3O2 - is found surprisingly to be a low symmetry C1 (1A) structure, which contains a bridging and a terminal O atom. Molecular orbital analyses are carried out to understand the structures and bonding of the three clusters and provide insights into the sequential oxidation from Nb3- to Nb3O2-. The terminal NbdO unit is common in niobia catalysts, and the Nb3O2- cluster with a NbdO unit may be viewed as a molecular model for the catalytic sites or the initial oxidation of a Nb surface.