Studies Of Some Early Transition Metal Oxide Clusters:Electronic Structures And Mechanistic Investigation On Reactions With Small Molecules

Early transition metal oxides have wide applications as catalysts or catalyst supports in many chemical processes for their diverse physicochemical properties.The structures of bulk catalysts are generally complex for the existence of various active sites.Therefore,it’s a challenge to identify catalytic active sites and determine the detailed reaction mechanisms.The well-defined property of gas-phase cluster makes it an ideal model to provide molecular-level insight on the complex chemical reaction processes and the information of the structure-reactivity relationships for metal oxide catalysts.The molecular-level understanding on reaction mechanisms will find use in designing and developing better catalysts of high activity and selectivity.This thesis presents a theoretical study on several early transition metal oxide clusters,with particular emphasis on the mechanistic inverstigation on reactions with small molecules.A summary of the work is given as following:1.Density functional theory(DFT)calculations are employed to investigate the reactivity of tungsten oxide clusters towards carbon monoxide.Extensive structural searches show that all the ground-state structures of the(WO3)n+(n = 1-4)contain an oxygen radical center with a lengthened W-O bond which is highly active in the oxidation of carbon monoxide.Energy profiles are calculated to determine the reaction mechanisms and evaluate the effect of cluster size.The monomer WO3+ has the highest reactivity among the stoichiometric clusters of different sizes(WO3)n+(n =1-4).The reaction mechanisms for CO with mono-nuclear stoichiometric tungsten oxide clusters of different charge(WO3-/0/+)are also studied to clarify the influence of charge state.Our calculated results show the ability to oxidize CO gets weaker from WO3+ to WO3-as the negative charge accumulates progressively.2.The reaction mechanisms of H2 generation by titanium oxide ions Ti2O4+/-reacting with H2O are studied by DFT calculations.It’s found that Ti2O4+/-are both doublet.The extra electron is located on a Ti atom for Ti2O4-,while the Ti2O4+ contains a Ti-O·oxygen radical.Energy profiles are calculated to determine the reaction mechanisms and evaluate the effect of charge state.The anionic and cationic Ti2O4+/-share similar processes of H2 generation via splitting water:H2O molecules are firstly dissociated on Ti2O4+/-forming-OH groups;Ti-H ’and Ti-OOH groups are then formed by intra-molecular hydrogen transfer;H2 is produced via the interaction of these two groups and finally desorbed.3.Based on the previous work,the bimetal oxide clusters TiMO4(M = Sc,V)are investigated to compare with their isoelectronic species Ti2O4+/-.It’s revealed that ScTiO4 contains a Sc-O·oxygen radical,and the unpaired electron is located on the V atom of TiVO4.The reaction mechanisms of H2 generation from water splitting by TiMO4(M = Sc,V)are studied by calculating the energy profiles.The doping atoms Sc/V are found to be less reactive for H2 production than Ti.4.Density functional theory(DFT)calculations are carried out to investigate the structural and electronic properties of bare tri-tungsten clusters(W3,W3-,W32-)and tri-tungsten oxide clusters W3Ox-/0(x = 1,2).Generalized Koopmans’ theorem is applied to predict the vertical detachment energies and simulate the photoelectron spectra(PES)for W3Ox-(x = 0-2)clusters.Extensive DFT calculations are performed in search of the lowest energy structures for both the anions and the neutrals.The bare tri-tungsten clusters are predicted to be triangular structures with D3h(3A1’),C2v(1A1)and D3h(1A1’)symmetry for W3,W3-and W32-,respectively.For W3O-and W3O clusters,the oxygen atom occupies the terminal site,while the next added oxygen atom is found to be a bridging one in both W3O2-and W3O2 clusters.Molecular orbital analyses are carried out to elucidate the chemical bonding of these clusters and provide insights into the sequential oxidation from W3-to W3O2-.Partial σ-and δ-aromaticity are revealed in the neutral W3(D3h,3A1’),while the anion W32-(D3h,1A1’)possesses only 8-aromaticity.