Surface Species Formed During CO and CO 2 Hydrogenation over Rh/TiO 2 (W6+) Catalysts Investigated by FTIR and Mass-Spectroscopy

Abstract

Surface species formed over Rh/TiO 2 (W 6+) catalysts during CO and CO 2 hydrogenation were investigated by in situ FTIR spectroscopy and transient techniques. It was found that at least four carbon-containing species, namely, linear CO, bridged CO, active carbon (C α) and less-active carbon (C β), exist on Rh crystallites, while formate and/or carbonate and/or hydrocarbonate species are on the TiO 2 support. A reduction of CO coverage by 25 ∼ 40% and a shift of the linear CO band to higher wavenumber values by 10 ∼ 14 cm −1 were observed under CO and CO 2 hydrogenation upon doping the TiO 2 carrier with small amounts of W 6+ cations (< 1 at.%). This implies that the Rh-CO bond is significantly weakened by doping, presumably due to an alteration of the electronic structure of the TiO 2 carrier, which modifies the electronic state of surface Rh via electronic interactions at the metal-support interface. The concentrations of the C α and C β species on Rh crystallites and carbon-containing species on the support were also found to be influenced by doping. While the concentration of the C α species (including CH x species) was reduced and that of the C β species was enhanced with increasing W 6+ dopant content, the concentration of the carbonate and/or hydrocarbonate species on the carrier exhibited a maximum at a W 6+ content of 0.11 ∼ 0.22 at.%. It is reasoned that hydrogen chemisorption is favored on the doped catalysts, as the CO coverage and the strength of the Rh-CO bond are significantly reduced. This has twofold consequences which result in a large increase of the CO and CO 2 hydrogenation activity, as previously observed over the Rh/TiO 2 (W 6+) catalysts: there is an increase in the concentration of surface hydrogen participating in the rate-determining step and an enhancement of the formation of Rh carbonyl hydride species, through which the reaction proceeds via a route of lower activation energy (H-assisted CO dissociation).