To investigate the mechanism of low temperature selective catalytic reduction (SCR) of NOx by NH3, highly dispersed MnOx/SAPO-34 catalyst was prepared by an improved molecularly designed dispersion (MD) method and evaluated at 100-300 degrees C. The 20 wt.% MnOx/SAPO-34 showed both high SCR activity and N-2 selectivity over a broad temperature window of 150-300 degrees C. Also, the catalyst was characterized by X-ray diffraction (XRD) and field emission scanning electron microcopy (FSEM), which indicated that the active components of Mn were in a highly dispersed state on SAPO-34 support. Moreover, the possible SCR mechanism over MnOx/SAPO-34 was studied by in situ Fourier transform infrared (FT-IR). The in situ FT-IR results revealed that -NH2 species were the key intermediates for NOx removal and bridged nitrate species were the active intermediates in the SCR reaction. At 200 degrees C, two possible reaction pathways were proposed. The first was that gas phase NO could react with -NH2 species intermediates and the formed NH2NO, producing N-2 and H2O. The second was that NH3 ad-species or -NH2 species intermediates were all able to react with NOx ad-species. The possible intermediates NH4NO2 and NH2NO were unstable and would decompose into N-2 and H2O. (C) 2016 Elsevier B.V. All rights reserved.