Reduction of N2O by CO over Fe- and Cu-BEA zeolites: An experimental and computational study of the mechanism

Abstract

The catalytic reduction of N2O by CO over two types of modified beta zeolite (BEA) catalysts, i.e. Fe modified beta (Fe-BEA) and Cu modified beta (Cu-BEA), was investigated. Catalytic tests for N2O direct decomposition and reduction by CO over Fe-BEA prepared by liquid ion exchange method were performed through a fixed-bed reactor. The experimental results showed that N2O conversion is apparently improved using CO as reductant; Fe-BEA starts to reduce N2O from 513K; a complete N2O conversion is achieved at 650K; and the required temperature for this reaction is reduced by ca. 150K. The reaction mechanism over Fe-BEA was investigated by in situ FTIR and DFT methods with 5T [(Si(OH)3)4AlO4(Fe) or (FeO)] cluster models. For Cu-BEA, a 5T [(Si(OH)3)4AlO4(Cu)] cluster model was established to optimize the geometries of reactants, transition states, and products. Two different reaction routines, namely the redox mechanism and the associative mechanism, have been investigated for the reduction of N2O by CO over both Fe- and Cu-BEA. It was found that for Fe-BEA, the energy barrier over the [Fe]+ active site is much lower than that over [FeO]+ active site; the redox mechanism is preferred for both Fe- and Cu-BEA; and the energy barrier on [Cu]+ active site is higher than that on [Fe]+ active site, indicating that Fe-BEA is superior to Cu-BEA for the reduction of N2O by CO.