Simultaneous selective catalytic reduction (SCR) of NO_x and N2O over Ag/ZSM-5 - Catalytic studies and mechanistic implications

摘要

The selective catalytic reduction (SCR) of NO_x and N2O using propene as the reducing agent was investigated over Ag/ZSM-5 catalyst samples of different surface acidities and metal loadings (1.5-6 wt.%). The kind of silver species generated in the materials was identified by UV/vis spectroscopy. In the absence of Bronsted acidity, such as in Na-ZSM-5, mainly metallic silver nanoparticles are formed, while in its presence the dominating species are silver ions and ionic silver clusters. Only in H-ZSM-5-based catalyst samples with higher metal loadings do metallic silver clusters and particles appear as well. Especially the latter species improves the CO2 selectivity of the investigated SCR reactions. Both, silver ions and ionic silver clusters, were found to be active in the SCR of NO_x. For this reason, the temperature window in which nitrogen oxides are reduced effectively to N2 is very broad and comprises approximately 300 °C. Maximum conversion was found to be 46% at 600 °C. In contrast, the N2O reduction appeared to be unaffected by the certain kind of silver species available. As long as silver ions or clusters (ionic and metallic) are present, a quantitative conversion of N2O can be achieved at temperatures above 450 °C. The presence of N2O exerted furthermore a positive effect on the NO_x reduction in a simultaneous SCR of both components. Assumingly, the increase in the NO_x conversion can be traced back to an enhanced activation of propene in the presence of nitrous oxide due to the formation of oxygenated hydrocarbon species, which are known to be more active in deNO_x reactions.