Selective catalytic reduction of NOx with CH3OH, C2H5OH and C3H6 in the presence of O2 over Ag/Al2O3 catalyst:: Role of surface nitrate species

The involvement of the reaction of surface nitrate [NO3-(ads)] species with different reductants (C3H6, C2H5OH and CH3OH) in the selective catalytic reduction of nitrogen oxides (NOx) over a Ag/Al2O3 catalyst has been studied by in situ diffuse reflectance infrared Fourier transform (DRIFT) spectroscopy and temperature-programmed desorption (TPD). When an NO/O-2 mixture was exposed to a Ag/Al2O3 catalyst at 150 degrees C, three kinds of nitrate species (bridging, monodentate and bidentate) were observed by DRIFT. The thermal stability of the monodentate NO3-(ads) species was higher than that of the bridging and bidentate NO3-(ads) species, which was confirmed by DRIFT and TPD measurements. The monodentate NO3-(ads) species reacted effectively with C2H5OH and CH3OH in the presence of O-2 to form surface isocyanate [NCO(ads)] species at 250 degrees C, whereas the bridging and bidentate NO3-(ads) species reacted minimally. All NO3-(ads) species were largely unreactive with C3H6 in the presence of O-2 at temperatures below 250 degrees C. On the other hand, the order of reactivity in the reduction of NO, at temperatures below 350 degrees C was in good agreement with that of both the reactivity of the monodentate NO3-(ads) species and the ease of NCO(a) formation (C2H5OH > CH3OH > C3H6). Based on these findings, the involvement of the reactivity of NO3-(ads) species and the formation of NCO(ads) species in the selective reduction of NOx are discussed.