Mechanistic Investigation On Reactions Of Molybdenum(Doping)Oxide Clusters With CO

Molybdenum(doping)oxides have a wide range of applications as catalysts,catalyst supports,catalyst promoters and so forth,due to their unique physicochemical properties.Gas-phase clusters can be used as molecular model to gain the effective information at the molecular level and provide the fundamental insight into reaction mechanisms,which is helpfull for designing better catalysts with high activity and selectivity.In this dissertation,we report the structures and reactivity on molybdenum(doping)oxides.A summary of the work is given as following:1.The structures and reactivity of molybdenum oxides(MoO3)n+(n=1-4)are studied using density functional theory(DFT).Our calculations show that all the positively charged molybdenum oxide clusters(MoO3)n+(n=1-4)are doublet with a single unpaired electron located on the terminal oxygen atom,which forms an oxygen radical center.The oxygen-radical center on transition metal oxide is highly reactive.CO is used as a molecular probe to study their reactivity.The detailed mechanisms for CO oxidation reactions are investigated at the B3LYP level.The reactions of CO with(MoO3)n+(n=1-4)all start with a CO-adsorption process;the radical oxygen is delivered to CO forming a CO2 unit along with electron transfer;finally the CO2 unit desorbs.The overall reaction processes are barrierless and exothermic for these positively charged molybdenum oxide clusters(MoO3)n+(n=n 1-4)towards CO.The monomer MoO3+ is found to be of highest reactivity for CO oxidation reaction,and its low-coordination is likely to be the key factor.2.Density functional theory(DFT)calculations are carried out to search the global minimums for a series of heteronuclear clusters MMon-1O3n0/-(M = V,Nb,Ta;n=2-4).Theoretical results show that all the neutrals are doublet ground states,and the anion clusters are closed-shell.All the clusters of MMon-1O3n0/-(M = V,Nb,Ta;n=2-4)possess ring structures,except for NbMo2O9-and TaMo2O9-,whose global minimums are chained structures.Generalized Koopmans' theorem is applied to predict the vertical detachment energies(VDEs)and simulate the photoelectron spectra(PES)of anionic ground states.The calculated electron detachment energies for the anions are relative high,which suggest the corresponding neutral clusters are strong oxidizers.Spin density analyses reveal that all the neutral clusters have the oxygen radical center located on dopant atom(V,Nb,Ta),and they can be used as potential molecular models to investigate radical center in relevant oxide systems.3.Based on the theoretical study of electronic and structural properties of the MMon-1O3n0/-(M = V,Nb,Ta;n=2-4)clusters,density functional theory(DFT)calculations are carried out to investigate the reactivity of the MMon-1O3n(M = V,Nb,Ta;n=2-4)clusters.They share similarities in the reaction mechanisms with molybdenum oxygen clusters(MoO3)n+.The reactions all start by CO attacking the dopant atom(vanadium,niobium,tantalum).Subsequently,CO2 subunit is formed by O atom transfer process.Finally,CO2 desorbs.Comparing the potential energy surface with that of the iso-electron clusters(MoO3)n+(n=2-4),we found that the oxidation reactivity can be enhanced by replacing molybdenum by doping vanadium.