Dynamic Change of Active Sites of Supported Vanadia Catalysts for Selective Catalytic Reduction of Nitrogen Oxides.

Selective catalytic reduction of NO by ammonia (NH-SCR) on VO/TiO catalysts is a widely used commercial technology in power plants and diesel vehicles due to its high elimination efficiency for NO removal. However, the mechanistic aspects of the NH-SCR reaction, especially the active sites on the VO/TiO catalysts, are still a puzzle. Herein, using combined operando spectroscopy and density functional theory calculations, we found that the reactivity of the Lewis acid site was significantly overestimated due to its conversion to the Brønsted acid site. Such interconversion makes it challenging to measure the intrinsic reactivity of different acid sites accurately. In contrast, the abundant V-OH Brønsted acid sites govern the overall NO reduction rate in realistic exhaust containing water vapor. Moreover, the vanadia species cycle between V═O and V-OH during NO reduction, and the re-oxidation of V species to form V is the rate-determining step.