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

Phosphomolybdic acid catalyst (PMoA/TiO) is a promising catalyst for selective catalytic reduction of NO with NH (NH-SCR) due to its strong acidity and excellent redox property. This work presents the NH-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 NH-SCR performance over PMoA/TiO 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 NH-SCR reaction were calculated by DFT. The results showed that the catalytic cycle includes foundational reaction (NH + NO reaction) and regenerative reaction (NH + NO reaction). NH, NHNO, HNNOH and HONNH species were the key intermediates. In the foundational reactions, NO played an important role in the removal of remaining H atoms. The NH dissociation on Lewis acid site, the internal hydrogen transfer on Brønsted acid site and the formation of HONNH species were the rate-controlling steps. The catalytic cycle of NH-SCR over PMoA consists of standard SCR and fast SCR.