Computational studies of the catalytic reactions of group ivb and vib transition metal oxide clusters

Abstract

Computational chemistry approaches have been used to study the reactivity of Group IVB and VIB transition metal oxide clusters. The hydrolysis of MCl₄ (M = Zr, Hf) as the initial steps on the way to form zirconia and hafnia nanoparticles has been studied with density functional theory (DFT) and coupled cluster [CCSD(T)]theory. Instead of the direct production of MOCl₂ and HCl or MO₂ and HCl, the hydrolysis reaction starts with the formation of oxychlorohydroxides followed by the release of HCl due to the large endothermicities associated with the direct path to form gas phase MO₂. The formation of MO₂ nanoparticles by the high temperature oxidation method is complicated and is associated with the potential production of a wide range of intermediates. The interaction between H₂O and small (MO₂)n (M = Ti, Zr, Hf, n = 1&minus;4) nanoclusters has been studied for the first step to understand the reaction mechanism of photocatalytic water splitting with the presence of (MO₂)n as catalysts. Both the singlet and the first excited potential energy surfaces (PESs) are studied. The hydrolysis reactions begin with the formation Lewis acid-base adducts followed by proton transfer from H₂O to the nanclusters. The reactions are highly exothermic with very small activation energies. Thus, H₂O should readily decompose to generate two OH groups on (MO2)n nanoclusters. The generation of H₂ and O₂ starting from the hydroxides formed in the hydrolysis step has been studied with the same computational methods as used for the hydrolysis study. The water splitting reactions prefer to take place on the first excited triplet potential energy surface (PES) due to its requirement of less energy than that on the singlet PES. A low excess potential energy is needed to generate 2H₂ and O₂ from 2H₂O if the endothermicity of the reaction is overcome on the first excited triplet PES using two visible photons. Hydrogen generation occurs via the formation of an M&minus;H containing intermediate and this step can be considered to be a proton coupled, electron transfer (PCET) reactions with one or two electrons being transferred. Oxygen is produced by breaking two weak M&minus;O bonds on the triplet PES. Ethanol (CH3CH2OD) conversions on cyclic (MO3)3 (M = Mo, W) clusters have been studied experimentally with temperature programmed desorption and computationally with both DFT and CCSD(T) methods. The addition of two alcohol molecules is required to match experiment. The reaction begins with the elimination of water with the formation of an intermediate of dialkoxy species for further reaction. The dehydration reaction proceeds through a &beta; hydrogen transfer to a terminal M<super>VI</super> = O atom without the involvement of a redox process. The dehydrogenation reaction is through an &alpha; hydrogen transfer to an Mo<super>VI</super> = O with redox involved or a W<super>VI</super> avoiding redox. The same computational methods have been used to study the other alcohol species such as methanol, n-propanol and isopropanol. The reactions with single, double and triple alcohols per M3O9 cluster have been studied. The dehydrogenation and dehydration for single alcohol reactions is via a common intermediate of metal hydroalkoxide formed by the dissociation of alcohol. The dehydration is through a &beta; hydrogen transfer to OH group. The lowest energy pathway for dehydrogenation is the same for different alcohols in both single and double alcohol reactions. Three alcohols involved condensation reaction may lower the reaction barrier tremendously by the sacrifice of an alcohol to form a metal hydroalkoxide, a strong gas phase Brønsted acid. This is a Brønsted acid driven reaction different from dehydrogenation and dehydration reactions governed by the Lewis acidity of the metal center and its reducibility.