DRIFTS study of a Ce–W mixed oxide catalyst for the selective catalytic reduction of NOx with NH3

Abstract

A systematic study has been conducted on the reactivity of the selective catalytic reduction of NO_x with NH₃ (NH₃-SCR) in a wide range of NO/NO₂ feed ratios (from 0 : 1 to 1 : 0) over a CeWO_x catalyst prepared by a homogeneous precipitation method. By using in situ diffuse reflectance infrared Fourier transform spectroscopy (in situ DRIFTS), the roles of NO and NO₂ at low temperatures during the fast SCR reaction have been revealed. NO₂ adsorption results in the formation of surface nitrates, which participate in the NH₃-SCR reaction through two pathways: one path where the nitrates react with NH₃ to form ammonium nitrate (NH₄NO₃), then NO reduces NH₄NO₃ below its melting point to form N₂ (the NH₄NO₃ path); the other path where the surface nitrates are reduced by NO to form active nitrite species that further react with NH₃ to produce N₂ (the nitrate path). “The NH₄NO₃ path” and “the nitrate path” contributed simultaneously to the standard and the fast SCR reaction at low temperatures. Both the surface nitrates and NO in the gas phase were suggested to be necessary for the excellent performance of the fast SCR reaction over the CeWO_x catalyst.