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 NH(2)NO is formed in gas phase, and then is decomposed into N(2) and H(2)O over H-form zeolites. In the first stage, the formation of NH(2)NO may occur via two routes: (1) NO is directly oxidized by O(2) to NO(2), and then NO(2) combines with NO to form N(2)O(3), which reacts with NH(3) to produce NH(2)NO; (2) when NO(2) exceeds NO in the content, NO(2) associates with itself to form N(2)O(4), and then N(2)O(4) reacts with NH(3) to produce NH(2)NO. 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 NO(2) 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.

Understanding Catalytic Reactions over Zeolites: A Density Functional Theory Study of Selective Catalytic Reduction of NOx by NH3 over Cu-SAPO-34

“…44,45 Currently, a clear mechanism for NO oxidation to NO 2 on the Cu ion is under debate and have not been achieved yet. Some previous studies stated that NO was oxidised by O 2 in the gas phase or in pores of zeolites (O 2 + 2NO −−→ 2 NO 2 ), 18 but it is a three-molecule reaction thus its contribution to the overall NO oxidation is limited. We suggest that the elementary reaction here is…”

“…Many previous studies showed it a relatively easily process in the B-site of zeolites by a hydrogen push-pull mechanism. 18,21 We here investigated its decomposition on both Brønsted acid and Lewis acid site, finding that the Brønsted site is indeed more favourable, and all intermediates and transition states were identified (Fig. S5).…”

“…6 that the configuration of NH 2 NO will change several times by the transferring of H atom, which was well recognised by previous studies. 18,49 We firstly tried this process on the L-site (Fig. S4), but the energy barrier of the first step of NH 2 NO decomposition in L-acid, the proton transferring from N to O, was extremely high (2.11 eV).…”

“…4 Some questions remain to be answered: i) how NO is activated and what is the role of O 2 ; ii) whether the redox cycle of Cu 2+ and Cu + is involved and what is the redox mechanism; iii) whether both Brønsted acid and Lewis acid collectively catalyse the SCR process. To elucidate these questions, several theoretical attempts have been made by Li et al 6,18 and Bruggemann et al 21,22 on H-form and Fe exchanged zeolites with finite cluster models, and some possible reaction pathways were identified. Very recently, Paolucci et al 16 proposed a detailed NH 3 -SCR mechanism on Cu-SSZ-13 with both operando X-ray absorption experiments and density functional theory (DFT) calculations.…”

“…4 Another interesting phenomenon is that fast SCR (equation 2) reacts more rapid than standard SCR (equation 1), of which the intrinsic reason is still under debate. Some studies 18,19 suggested that in fast SCR, NO and NO 2 would bind in gas phase to form N 2 O 3 , followed by a hydrolysis to nitrous acid (N 2 O 3 + H 2 O −−→ 2HONO). After that, NH 3 can easily react with HONO to form NH 4 NO 2 which readily decompose to N 2 and H 2 O under reaction condition.…”