The computational studies on the chemistry of titanium dioxide nanoparticles
The chemistry of TiO_2 and SiO_2 nanoclusters is studied using computational methods. The potential energy surfaces (PESs), thermochemistry of the intermediates, and the reaction paths for the initial steps of the hydrolysis of TiCl_4 were calculated. Transition state theory and RRKM unimolecular rate theory are used to predict the rate constants. Clustering energies and heats of formation are calculated for neutral clusters, and the calculated heats of formation were used to study condensation reactions. The reaction energy is substantially endothermic if more than 2 HCl are eliminated. The calculations show that the reported values for ΔH_f^0(TiOCl_2) should be remeasured. Transition metal oxides such as TiO_2 can be used as photocatalysts to control chemical transformations for energy production. An important applications for TiO_2 is its use to photochemically split water to produce H_2 and O_2. The PES for splitting water on the ground and first excited state surfaces of (TiO_2)_n (n=1-4) nanoparticles have been studied up through the coupled cluster CCSD(T)/complete basis set level. Water is readily split to form hydroxyl groups without the need for a photon. Experimental measurements of the photoconversion of ketones (C(O)RR') on the rutile TiO_2 (110) surface show that one can eliminate R or R'. The bond dissociation energies of R=CH_3 and a wide range of R' for the gem-diols CRR'(OH)_2 were calculated at the density functional theory (DFT) and G3(MP2) levels. The calculated bond dissociation energies are in excellent agreement with the experimental values. The calculations show that most of the photodissociation processes are under thermodynamic control except for R'=CF_3. X-ray photoelectron spectroscopy (XPS) and DFT electronic structure calculations were used to study the average formal oxidation state of silicon in fumed silica (CAB-O-SIL®). The results show that the average surface oxidation state of the silicon in fumed silica is predominantly +1 and suggest a notably less hydrophilic character for CAB-O-SIL® than the oxides of silicon with Si in the formal +3 and +4 oxidation states. Once the +3 oxidation state is formed, water on the silica surface facilitates the conversion of the Si^+3 to the Si^+4 oxidation state.