Mechanistic study of the influence of surface acidity on lean NO2 reduction by propane in HZSM-5

This study focuses on the mechanism of lean NOx reduction by propane, over acidic zeolites (HZSM-5), and the influence of surface acidity. In situ FTIR measurements of ammonia adsorption indicate a higher number of Brønsted acid sites for a sample with a low SiO2/Al2O3 ratio. The activity for NOx reduction and the selectivity for N2 formation correlate well with the Brønsted acidity. Step-response experiments with NO2 and propane show the formation of surface-bound NO+, isocyanate, unsaturated hydrocarbons, and amine species. Formation of the latter two seems to be closely related to the Brønsted acidity. In the NO2 reaction with propane, the NO+ species seem to play a vital role, probably reacting with carbenium ions (from propane cracking) to form isocyanates, which may be hydrolysed to amine species. Step-response experiments with isopropylamine and NO2 indicate a fast reaction, where the amine and NO+ species react over Brønsted acid sites. Hence, amine species are possible reaction intermediates in the lean reduction of NO2 by saturated hydrocarbons, such as propane, over HZSM-5.