NH3-SCR of NO over M/ZSM-5 (M = Mn, Co, Cu) catalysts: An in-situ DRIFTS study

M/ZSM-5 (M = Mn, Co, Cu) samples were prepared by impregnation, and the catalytic performance of these samples in NH3-SCR of NO was tested at 150-400 degrees C. Cu/ZSM-5 was more active than Mn/ZSM-5, and Co/ZSM-5 was the least active in NH3-SCR of NO. The nature and evolution of surface species were probed by in-situ DRIFTS. Among NH3-derived adsorbed species, coordinated NH3, NH4+, and amide (-NH2) species were found to be relevant for NH3-SCR. Among NOx-derived adsorbed species, bridged nitrate was the main active species. Bridged nitrate existed in larger quantities and was thermally more stable on Cu/ZSM-5 than on Mn/ZSM-5 and Co/ZSM-5, in line with the highest activity of Cu/ZSM-5. On the other hand, bidentate nitrate was inert towards NH3-SCR at low temperatures, whereas it became active at high temperatures. Both Eley-Rideal (E-R) and Langmuir-Hinshelwood (L-H) mechanisms were applicable to Mn/Z at 150 degrees C, but only E-R mechanism was identified over Mn/Z at 300 degrees C. As for Co/Z and Cu/Z, both pathways existed simultaneously at 150 and 300 degrees C.