Selective Reduction of NOx by Hydrocarbons over Supported Silver Catalysts
The NOx reduction activities of various supported silver catalysts were measured. Among the tested catalysts, Ag/Al2O3 showed the highest activity. Though Ag/ferrierite showed rather higher activities at low temperatures, the activities were almost the same with that of ferrierite itself. Other catalysts had lower activities. Reactivities of hydrocarbons had large effect on the NOx reduction over the Ag/Al2O3 catalyst at low temperatures. Hydrocarbons which have longer chain were liable to react at lower temperatures and were able to reduce NOx more effectively. Unsaturated hydrocarbons showed higher reactivity and gave higher NOx reduction rate than saturated ones. The effects of contact time and the concentrations of inlet NO, propene, oxygen, and water vapor on the reduction of NOx over the Ag/Al2O3 catalyst were also investigated. The reduction rate of NOx below 450°C was greatly decreased with the decrease of contact time. The reduction of NOx was promoted at higher propene/NO ratios. At constant propene/NO ratio, the proportion of propene which was used for NOx reduction increased with the increase of concentrations of propene and NO.The consumption of propene by oxygen was accelerated at high concentrations of oxygen. The effect of water vapor was large at low temperatures a small amount of water inhibited the reaction of propene and decreased the reduction rate of NOx significantly below 400°C.