Synergistic effects of Ir–Au/TiO2 catalysts in the total oxidation of propene: influence of the activation conditions

Abstract

Iridium was added to the Au/TiO₂ system to try to enhance its catalytic activity in the reaction of propene oxidation, performed under conditions close to those used in the studies of decomposition of volatile organic compounds (1200 ppm propene and 9 vol% O₂ in He). Titania supported Ir–Au (Ir/Au = 1) was prepared by sequential deposition–precipitation with urea (DPU) of Ir then Au. The effect of the activation conditions (hydrogen or air at 400 °C) was investigated. The study of the activation conditions of Ir–Au/TiO₂ showed that activation under hydrogen at 400 °C generated a catalyst more active than the monometallic ones, while Ir–Au/TiO₂ activated in air remained as poorly active as Au/TiO₂. TEM characterization showed the formation of metallic particles of similar size (2–3 nm) in both monometallic Au/TiO₂ and bimetallic Ir–Au/TiO₂. Characterization especially by DRIFTS using CO as a probe molecule suggests the presence of Ir–Au interaction, IrO₂–Au⁰ interaction when the sample is calcined and Ir⁰–Au⁰ bimetallic particles when it is reduced. XPS and TPR characterization techniques showed that gold hinders to some extent the reoxidation of iridium in the reduced bimetallic Ir–Au/TiO₂ catalyst. The enhanced catalytic activity of the reduced bimetallic Ir–Au/TiO₂ catalyst is attributed to a surface Ir⁰–Au⁰ synergism.