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

Abstract

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 NO_x 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.