Insight into the reaction mechanism over PMoA for low temperature NH3-SCR: A combined In-situ DRIFTs and DFT transition state calculations

Phosphomolybdic acid catalyst (PMoA/TiO2) is a promising catalyst for selective catalytic reduction of NOx with NH3 (NH3-SCR) due to its strong acidity and excellent redox property. This work presents the NH3-SCR reaction mechanism by In-situ diffuse reflectance Infrared Fourier Transform Spectroscopy (In-situ DRIFTs) and density functional theory (DFT). In-situ DRIFTs results indicated that the NH3-SCR performance over PMoA/TiO2 followed both Eley-Rideal (E-R) and Langmuir-Hinshelwood (L-H) mechanisms. The reaction pathway, intermediate, transition state and energy barrier over PMoA to complete NH3-SCR reaction were calculated by DFT. The results showed that the catalytic cycle includes foundational reaction (NH3 + NO reaction) and regenerative reaction (NH3 + NO2 reaction). NH2, NH2NO, HNNOH and HO2NNH species were the key intermediates. In the foundational reactions, NO2 played an important role in the removal of remaining H atoms. The NH3 dissociation on Lewis acid site, the internal hydrogen transfer on Brunsted acid site and the formation of HO2NNH species were the rate-controlling steps. The catalytic cycle of NH3-SCR over PMoA consists of standard SCR and fast SCR.