Computational studies of catalytic active site properties and reactions at the metal–oxide interface

Abstract

In this thesis work, the geometric and electronic structures of metal–oxide catalysts were studied using density functional theory. The studied systems were zirconia-supported metal atoms and clusters, and ReOx-modified rhodium. Various aspects of these metal–oxide systems were investigated, including the metal– oxide interaction and interfacial properties, the structural variation and dynamics of the supported clusters, and the reducibility and acidity of the oxide components. The dissociation of water over the metal–oxide interface and the hydrodeoxygenation of glycerol on ReOx-modified Rh were used as model reactions. It was shown that small Pt and Rh clusters on zirconia exhibit unique interfacial reaction sites, producing non-scaling behavior in the interfacial water splitting reaction. Less stable cluster isomers were found to dissociate water more exothermically due to the stronger binding of the dissociated fragments. The challenges of simulating the dynamics of such clusters using constant-temperature DFT-MD were investigated, highlighting the necessity of tight SCF convergence and proper thermostatting to avoid anomalies such as temperature gradients. The metal-enhanced reducibility of monoclinic zirconia was studied using a variety of adsorbed single transition metal atoms, with Ir and Pt providing the strongest enhancement. To account for the origin of the enhancement, the metal– oxide and metal–vacancy binding were investigated in detail, with a focus on the charge transfer and covalent interactions. Finally, the metal-acid bifunctional ReOx–Rh catalyst was found to acid-catalyze the dehydroxylation of glycerol, with a competitive metal-catalyzed pathway possibly explaining the experimentally observed poor selectivity. The same catalyst was found unable to acidcatalyze the ring opening of glycidol, pointing toward a ring-size effect in solid acid catalysis.