Quantitative study of the NH3-SCR pathway and the active site distribution over CeWOx at low temperatures

Abstract The reaction and adsorption amounts of NO, NO 2 , and NH 3 and the reaction pathway of the selective catalytic reduction of NO x with NH 3 (NH 3 -SCR) at low temperatures in the absence and presence of H 2 O on CeWO x were quantitatively determined by a transient response method (TRM). The distribution of the active sites was quantitatively determined based on the results of TRM, X-ray photoelectron spectroscopy and temperature programmed desorption. NO x adsorbed on O vacancies and oxidized Ce sites, with surface concentrations of 40–63 and 20 μmol g −1 respectively, while NH 3 adsorbed on [Ce 4+ ]-OH and W species, at 61–70 and 90–99 μmol g −1 respectively. Both the reaction between NO and NH 4 NO 3 to form N 2 , H 2 O and NO 2 , and reaction between nitrite and NH 3 to form N 2 and H 2 O were present. H 2 O weakened NH 3 adsorption, decreased the adsorption amount at 150 °C, and inhibited the reaction between NO and NH 4 NO 3 /adsorbed NH 3 , impeding NH 3 -SCR on CeWO x .