Catalytic reduction of NO by C3H6 over Rh/TiO2 catalysts: Effect of W6+-cation doping of TiO2 on morphological characteristics and catalytic performance

Abstract

The effect of W6+-cation doping of TiO2 on its physicochemical characteristics has been examined and related to the catalytic performance of dispersed rhodium crystallites for the selective catalytic reduction (SCR) of NO by propylene in the presence of excess oxygen. It was found that doping of TiO2 with small amounts of W6+ cations results in materials with increased surface areas and anatase-to-rutile contents, in a manner which depends on dopant concentration and calcination temperature. The acidity of the doped supports, measured by adsorption of ammonia, increases with increasing W6+-cation concentration and goes through a maximum for dopant contents of ca. 0.45at.% W6+. The catalytic performance of supported rhodium for the SCR of NO by propylene is significantly improved over the doped carriers and varies with W6+-content in a way which is qualitatively similar to that of acidity. Results are explained evoking the beneficial effect of acidity on adsorption and partial oxidation of propylene, which results in faster regeneration of the catalytically active Rh0 sites, and the dopant-induced modifications of the electronic structure of rhodium crystallites which result in higher dissociation rates of adsorbed NO. Under the experimental conditions employed, the conversion of NO over the 0.5%Rh/TiO2(0.45% W6+) catalyst is significantly improved and reaches 52%, compared to 38% over the undoped one. A further increase of conversion to 65% is achieved with increasing Rh loading to 1.5wt.%. Measurements of the specific reaction rates show that the turnover frequency (TOF) of the reaction is only affected by electronic-type modifications and not by the geometric properties of the active sites.