MnOx-CeO2-ZrO2/gamma-Al2O3 catalyst was prepared by the sol-gel method and then investigated on the selective catalytic reduction (SCR) of NOx (NO and NO2) by NH3. The catalytic activity was enhanced by the addition of Zr in the broad temperature range of 100-400 degrees C. This catalyst could also be well-behaved under high GHSV and in the presence of SO2 and H2O. The XRD and XPS results showed that the Zr4+ ions incorporated into the CeO2 crystal lattice and formed the solid solution, resulting that the BET surface area and pore volume of this catalyst increased obviously. The detailed SCR mechanism was researched using the in situ DRIFTS. It showed that NO could be adsorbed on the catalyst and formed different kinds of nitrates at 150 degrees C, which almost converted to the stable bidentate and bridged nitrates in the presence of O-2. NH3 could be adsorbed on both Lewis sites (coordinated NH3 species) and Bronsted sites (NH4-) at 150 degrees C. These adsorption products were enhanced by introducing O-2. The results of the SCR reaction on the catalyst indicated that the reaction route could follow the Eley-Rideal mechanism as NH3 pre-adsorbed on the catalyst, it could also follow the Langmuir-Hinshelwood mechanism when NO pre-adsorbed on the catalyst. (C) 2014 Elsevier B.V. All rights reserved.