Influence of TiO2 structure on metal-support interactions in Rh/TiO2 catalysts probed by propylene hydrogenation and other techniques

Abstract

Metal-support interactions between rhodium and nanoparticles of anatase (ANP), rutile (RNP), and brookite (BNP) TiO2 with similar average particle sizes were investigated. The low rhodium loading (0.4 % by weight) yielded 1.5–2.0 nm Rh particles and assured maximum Rh-TiO2 interactions. Under these closely matched conditions, the oxidation state of rhodium was shown to vary with TiO2 structure plus particle size and reducing conditions. After reduction at 200 °C, more metallic rhodium was present at the surface of the anatase TiO2 support with the larger particle sizes (15 nm) compared with Rh on brookite or rutile TiO2, or smaller anatase TiO2 nanoparticles, and this likely explains why the Rh/TiO2-ANP is the most active catalyst in the hydrogenation of propene. A high temperature reduction (500 °C) resulted in migration of TiOx over rhodium on anatase and brookite, but not on rutile TiO2 supports where instead strong electronic interactions dominated. This study reveals the importance of considering TiO2 structure and particle size, active metal particle size, as well as shape and stability of the TiO2 support when investigating metal-support interactions. The electronic properties of Rh depend sensitively on the TiO2 structure, particle size and stability, and vary significantly with pretreatment conditions.