Theoretical Studies On The Group VB/VIB Transition Metal Oxide Clusters

Transition metal oxides (TMOs) have attracted considerable interest dueto their wide applications in catalysis, catalyst support, sensor materials,electrochromic devices and so forth. For example, group VB/VIB metaloxides are convenient and efficient catalysts for the direct (single-pot)conversion of alkanes to the corresponding carboxylic acids, alcohols andketones. The understanding of their chemical and physical properties at themicroscopic level will help us to design the materials with desired properties.Studies on the gas-phase clusters may build a bridge to this goal. Molecularclusters of TMOs have been considered as models for surfaces and catalyticreaction mechanism. Both experimental and theoretical approaches have beenemployed to investigate the TMO clusters. The compositions and chargestates of gas-phase clusters can be well controlled through experimentalconditions. With the aid of quantum chemical calculation, we are able tostudy these clusters more elaborately.In this dissertation, the theoretical studies on a series of group VB/VIBtransition metal and their oxide clusters are presented, i.e., the trinuclear M3On-/0(M=Nb, n=0-2; M=Ta, n=0-8) and the tetranuclear W4On-/0(n=10-13)clusters. A summary of our work is given as following:For the trinuclear niobium oxide clusters, Nb3On/0(n=0-2), DFTcalculations were used to investigate the structural and electronic properties,chemical bonding and their sequential oxidation. We found that Nb3O2possesses a low symmetry C1(1A) structure, which contains a bridging and aterminal O atom. The terminal Nb=O unit is common in niobia catalysts andthe Nb3O2cluster with a Nb=O unit may be viewed as a molecular model forthe catalytic sites or the initial oxidation of a Nb surface.As for the heavier element of group VB, i.e., tantalum, its bare metalcluster Ta3/0was investigated using density functional theory calculations.Multiple d-orbital aromaticity was found for the Ta3ground-state,commensurate with its highly symmetric D3hstructure. A detailed molecularorbital analysis was performed to elucidate the chemical bonding in Ta3-. Then, theoretical study on the sequential oxidation behaviors oftritantalum oxide clusters, Ta3On-/0(n=1-8), were performed using densityfunctional theory calculations. The evolutions of geometric and electronicstructures were elaborated for the clusters, Ta3On-/0(n=1-8).We also reported the theoretical calculations for a series of tetratungstenoxide clusters, W4On-/0(n=10-13). The evolutions of geometric andelectronic structures were presented. We showed that W4O11contains alocalized W3+site, which can readily react with O2to form the W4O13cluster.Molecular orbital analyses were performed to analyze the chemical bonding inthe tetratungsten oxide clusters and to elucidate their electronic and structuralevolution.