In Situ DRIFTS Study of the Low Temperature Selective Catalytic Reduction of NO with NH3 over MnOx Supported on Multi-Walled Carbon Nanotubes Catalysts

MnOx supported on multi-walled carbon nanotubes (MWCNTs) catalysts were prepared by the pore volume impregnation method and used for low-temperature selective catalytic reduction (SCR) of NO with NH3. Based on the previous study, 10 wt.% loading MnOx/MWCNTs were then selected for investigation of the reaction mechanism by in situ Diffuse Reflectance Infrared Fourier Transform spectroscopy (in-situ DRIFTS). The important intermediates in the SCR of NOx process at 210 degrees C were discussed based on the DRIFTS results. Furthermore, the NH3-SCR reaction pathways over MnOx/MWCNTs catalysts were proposed. The results showed that NH4+ species on Bronsted acid sites and coordinate ammonia species on Lewis acid sites existed during the SCR reaction. NH4+ species was more active than coordinate ammonia species over the catalysts at 210 degrees C. Most of NOx ad-species would react with NH3 ad-species. However, nitrite species, bidentate and monodentate nitrates contributed to the SCR reaction over the catalysts mostly. Two possible reaction pathways were proposed. One was that NOx ad-species could react with NH4+ to form intermediate of NH4N2O4 (a), NH4NO2 (a) or NH4NO3 (a), then to produce N-2 and H2O as the final products. The other pathway was that NH3 was initially adsorbed on active site and NH2 was formed, then NH2 reacted with NOx ad-species to produce intermediate NH2NO2 or NH2NO3 which were unstable and would decompose into N-2 and H2O.