Acidity of unsupported and silica-supported vanadia, molybdena, and titania as studied by pyridine adsorption

Abstract

The acidic properties of unsupported titania, vanadia, and molybdena, and the corresponding silica-supported metal oxides were studied. The number of acid sites was investigated by thermogravimetry of adsorbed pyridine. The types of acid sites were examined by infrared spectroscopy of adsorbed pyridine. Both Lewis and Brønsted acid sites were present on the surfaces of V 2O 5 and MoO 3, while Lewis acid sites were found exclusively for TiO 2. When supported on silica, vanadia and molybdena species showed a greater number of acid sites than the corresponding unsupported oxides. The oxidized surfaces of these materials possessed both Lewis and Brønsted acid sites. In contrast, Lewis sites predominated on the surfaces of the reduced samples. Silica-supported titania showed strong Lewis acidity, analogous to unsupported titania. The addition of water vapor was observed to generate Brønsted acid sites for the oxidized forms of silica-supported titania, vanadia, and molybdena. This behavior was not observed for the reduced forms of these materials. Pauling's electrostatic bond strength rules are shown to be useful to predict whether Brønsted acid sites exist on the surfaces of metal oxides.