(702g) Investigation on the Role of Acidity for NH3 Selective Catalytic Reduction of NO on Mo/Fe2O3 single Atom Catalyst

Selective catalytic reduction with ammonia (NH₃-SCR) is one of the effective strategies for treating NO_x from the vehicular exhaust.¹ The redox properties of the catalyst are crucial in determining the NH₃-SCR reaction mechanism and kinetics. This investigation is focused on modelling and analysing the NH₃-SCR reaction on single-atom Mo on Fe₂O₃ catalyst using Density Functional Theory (DFT) calculations to understand the influence of its acid-redox properties on the reaction mechanisms and kinetics. Single-atom Mo on Fe₂O₃ catalyst was modelled based on experimental data available in the literature.² Specifically, a single Mo atom is substituted on the top and sub-surface layer of the α-Fe₂O₃(001) surface. The optimised catalyst model with sub-surface layer Mo substitution containing one H on the surface oxygen had geometric features in agreement with reported X-ray Absorption Fine Structure (XAFS) data. Furthermore, Bader charge analysis showed that the oxidation state of Mo was 4.85, which is consistent with the experimental value.² Ab initio thermodynamics analyses predicted that the oxygen-rich state of Mo on α-Fe₂O₃(001) surface is the most stable in an oxygen environment representative of vehicle exhaust gas. On the Mo on α-Fe₂O₃(001) catalyst, potential reaction pathways such as those involving NO oxidation and those involving oxidative dehydrogenation of NH₃ were investigated. In the pathway involving oxidative dehydrogenation of NH₃ to form NH₂, NO is preferentially adsorbed on the Fe sites adjacent to the Mo atom, which facilitates the easy coupling and formation of an NH₂NO intermediate with NH₂. This intermediate undergoes dehydrogenation to form an NHNO intermediate, which further undergoes intramolecular H transfer to form N₂ and OH without the concomitant formation of N₂O. This sequence of reactions ultimately enhances the N₂ selectivity. In routes involving NO oxidation to NO₂, the NH₂-NO₂ coupling step is unfavorable and N₂O is a surface intermediate and a potential byproduct.