15% MnOx/5% WO3/ TiO2 catalyst for low-temperature selective catalytic reduction of NOx was synthesized by an impregnation method. NH3-SCR mechanism was explored systematically and possible pathway was proposed through in situ FT-IR study on microscopic transient and steady states under various conditions of single or mixed gas feed with multiple pre-treatment. The results revealed that NH3-SCR reaction over the 15%MnOx/5%WO3/TiO2 catalyst proceeded mainly by the Eley-Rideal mechanism with the Langmuir-Hinshclwood pathway only at certain temperature. NH3 adsorbed on the Lewis acid sites of the catalyst surface was the main source of reductant; however

more NH4+ adsorbed on the Brønsted acid sites of the catalyst surface involved in the NOx reduction with increase of reaction temperature. The adsorption and activation of NH3 was key step for the whole SCR reaction. NO2 easily reacted with adsorbed NH3 with stronger affinity to catalyst surface than NO. The bidentate nitrate

which formed considerably at oxidation active centers of the catalyst by NO adsorption

didn't participate in NH3-SCR reaction but obstructed adsorption and activation of NH3. O2 could promote NH3-SCR by blocking formation of bidentate nitrate. The adsorption of NO and NH3 on the catalyst surface was competitive at room temperature

which NO adsorption was higher than NH3 at O2 absence. The absorbed NH3 could be massively activated and reacted with NOx upon temperature reached to 100℃.