Theoretical Investigation of the Mechanism of the Selective Catalytic Reduction of Nitrogen Dioxide with Ammonia on H-Form Zeolites and the Role of Nitric and Nitrous Acids as Intermediates

Abstract

The selective catalytic reduction (SCR) of NO2 with ammonia and the role of nitrous and nitric acids as intermediates have been investigated on a portion of the H-ZSM5, which contains 5T atoms by using the density functional theory, representing H-form zeolites. For the NO2-SCR, it is most likely that, first, NO2 reacts as as-N2O4 with ammonia, forming nitrosamine and nitric acid. The latter dehydrates in a self-reaction to N2O5, which reacts with ammonia by recycling nitric acid and producing nitramide. Nitramide decomposes in a push−pull mechanism to N2O and H2O, similar to the decomposition of nitrosamine to nitrogen and water. Nitrous and nitric acids are stable intermediates in the SCR reacting network and strongly adsorb to the active site. They can be considered to be the main surface species besides NH3 and H2O and are mainly produced from the reaction of NOx with ammonia. Their most important sink is the dehydration in a self-reaction or with each other. The hydrolysis of NOx as the source of the...