Kinetics of the H218O/ H216O isotope exchange over vanadia–titania catalyst

Abstract

Study of oxygen exchange between vanadia–titania catalyst, H 2 O and O 2 has been performed in isothermal (200 °C) and thermo programmed (50–500 °C) conditions. Experiments using H 2 18 O, 18 O 2 and monolayer vanadia–titania catalyst placed to the plug-flow reactor were carried out. Significant values of the rate of isotope exchange between O 2 and the catalyst were observed at T > 450 °C, but catalyst reduction accompanied oxygen desorption into gas phase proceeded in this case. Unlike dioxygen, water can exchange quite readily its oxygen atoms with vanadia–titania catalyst even at room temperature. Surface vanadium sites coordinating the OH group show the highest activity in the oxygen exchange with water. The rate constant of oxygen exchange between adsorbed water molecule and V– OH is ca. 0.5 s −1 at 200 °C, with the activation energy close to zero. The rate of oxygen exchange with dehydrated vanadium complexes V– O– V and V O is much lower. The rate constant of this exchange is ca. 10 −3 s −1 at 200 °C, with the activation energy of ca. 70 kJ/mol. The interaction of adsorbed water with VO x species results in their slow hydrolysis to form the V– OH groups (characteristic time ca. 10 4 s), which recombination underlies the isotope exchange.