Mechanistic Study of the Influence of the Surface Acidity on Lean NO2 Reduction by Propane in HZSM-5

This study focuses on the mechanism of lean NO x 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 Br\uf8nsted acid sites for a sample with a low SiO 2 /Al 2 O 3 ratio. The activity for NO x reduction and the selectivity for N 2 formation correlate well with the Br\uf8nsted acidity. Step-response experiments with NO 2 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 Br\uf8nsted acidity. In the NO 2 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 NO 2 indicate a fast reaction, where the amine and NO + species react over Br\uf8nsted acid sites. Hence, amine species are possible reaction intermediates in the lean reduction of NO 2 by saturated hydrocarbons, such as propane, over HZSM-5. \ua9 2005 Elsevier Inc. All rights reserved