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

The selective catalytic reduction (SCR) of NO 2 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 NO 2 -SCR, it is most likely that, first, NO 2 reacts as as -N 2 O 4 with ammonia, forming nitrosamine and nitric acid. The latter dehydrates in a self-reaction to N 2 O 5, which reacts with ammonia by recycling nitric acid and producing nitramide. Nitramide decomposes in a push−pull mechanism to N 2 O and H 2 O, 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 NH 3 and H 2 O and are mainly produced from the reaction of NO x with ammonia. Their most important sink is the dehydration in a self-reaction or with each other. The hydrolysis of NO x as the source of the acids and the subsequent reaction with ammonia as the sink is less likely to happen. The onset of ammonia oxidation affects the system in terms of dehydration reactions of the acids with nitroxyl (HNO). For the NO 2 -SCR, this leads to the reduction of the N 2 O/N 2 ratio of the final products. For the investigations of the reaction pathways, the crossing of potential energy surfaces was considered, if necessary. This work completes together with recent results the establishment of a complex reaction network, combining SCR and SCO catalyzed by the Brønsted acid of H-form zeolites, which is in agreement with the experimental literature.