The selective catalytic reduction of nitrogen oxides by methane on noble metal-loaded sulfated zirconia

The selective catalytic reduction of NOx by methane on noble metal-loaded sulfated zirconia (SZ) catalysts was studied. Ru, Rh, Pd, Ag, Ir, Pt, and Au-loaded sulfated zirconia catalysts were compared with the intact sulfated zirconia. For the NO–CH4–O2 reaction, Ru, Rh, Pd, Ir, and Pt showed promotion effect on NOx reduction, while for the NO2–CH4–O2 reaction, only Rh and Pd showed promotion effect. Over intact and Rh, Pd, Ag, and Au-loaded sulfated zirconia, NOx conversion in NO2–CH4–O2 reaction was significantly higher than that in NO–CH4–O2 reaction, while clear difference was not observed over Ru, Ir, and Pt-loaded sulfated zirconia. Comparison of [NO2]/([NO]+[NO2]) in the effluent gases in NO–O2 and NO2–O2 reactions showed that Ru, Ir, and Pt has high activity for NO oxidation under the reaction conditions. These facts suggest that effects of these metals toward NOx reduction by methane can be categorized into the following three groups: (i) low activity for NO oxidation to NO2, and high activity for NO2 reduction to N2 (Pd, Rh); (ii) high activity for NO oxidation to NO2, and low activity for NO2 reduction to N2 (Ru, Ir, Pt); (iii) low activity for both reactions (Ag, Au). To confirm these suggestions, combination of these metals were investigated on binary or physically-mixed catalysts. The combination of Pd or Rh with Pt or Ru gave high activity for the selective reduction of NOx by methane.