Selective catalytic reduction of NO_x 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 (NO_x) 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/O₂ mixture was exposed to a Ag/Al₂O₃ catalyst at 150 °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 O₂ to form surface isocyanate [NCO(ads)] species at 250 °C, whereas the bridging and bidentate NO₃⁻(ads) species reacted minimally. All NO₃⁻(ads) species were largely unreactive with C₃H₆ in the presence of O₂ at temperatures below 250 °C. On the other hand, the order of reactivity in the reduction of NO_x at temperatures below 350 °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 NO_x are discussed.