Mechanistic insight into selective catalytic combustion of HCN over Cu-BEA: influence of different active center structures

Abstract

HCN being a highly toxic N-containing volatile organic compound (VOCs) poses great threat to human living environment. Selective catalytic combustion of HCN (HCN-SCC) over metal modified zeolite catalysts has attracted great attention due to related high efficiency and excellent N₂ selectivity. In the present work, three types of 24T-Cu-BEA models with different active centers of single [Cu]⁺, double [Cu]⁺, and [Cu–O–Cu]²⁺ were constructed for HCN-SCC mechanism simulations based on density functional theory (DFT). DFT simulation results revealed that HCN-SCC followed an oxidation mechanism over double [Cu]⁺ through an intermediate of NCO, wherein the synergistic effects of double [Cu]⁺ active centers were clearly observed, resulting in a significantly lowered energy barrier (1.6 kcal mol⁻¹) during HCN oxidation into NCO. However, an oxidation mechanism (HCN oxidized into NH radical and CO₂ through intermediate of HNCO) combining with a hydrolysis mechanism (NH radical hydrolyses into NH₃) occurred over single [Cu]⁺ and [Cu–O–Cu]²⁺, wherein the NH₂ hydrolysis to NH₃ step was regarded as the rate determining step with an energy barrier of 72.3 and 74.3 kcal mol⁻¹, respectively. Finally, Mulliken charge transfer (CT) analysis was conducted, based on which the electric properties of different active centers were well illustrated.