New insight into selective catalytic reduction of nitrogen oxides by ammonia over H-form zeolites: a theoretical study

Density functional theory calculations were carried out to investigate the reaction mechanism of selective catalytic reduction of nitrogen oxides by ammonia in the presence of oxygen at the Brønsted acid sites of H-form zeolites. The Brønsted acid site of H-form zeolites was modeled by an aluminosilicate cluster containing five tetrahedral (Al, Si) atoms. A low-activation-energy pathway for the catalytic reduction of NO was proposed. It consists of two successive stages: first NH2NO is formed in gas phase, and then is decomposed into N2 and H2O over H-form zeolites. In the first stage, the formation of NH2NO may occur via two routes: (1) NO is directly oxidized by O2 to NO2, and then NO2 combines with NO to form N2O3, which reacts with NH3 to produce NH2NO; (2) when NO2 exceeds NO in the content, NO2 associates with itself to form N2O4, and then N2O4 reacts with NH3 to produce NH2NO. The second stage was suggested to proceed with low activation energy via a series of synergic proton transfer steps catalyzed by H-form zeolites. The rate-determining step for the whole reduction of NOx is identified as the oxidation of NO to NO2 with an activation barrier of 15.6 kcal mol−1. This mechanism was found to account for many known experimental facts related to selective catalytic reduction of nitrogen oxides by ammonia over H-form zeolites.