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

ABSTRACT

MnO_x 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 MnO_x/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 NO_x process at 210°C were discussed based on the DRIFTS results. Furthermore, the NH3-SCR reaction pathways over MnO_x/MWCNTs catalysts were proposed. The results showed that NH₄⁺ species on Brønsted 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°C. Most of NO_x 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 NO_x ad-species could react with NH₄⁺ to form intermediate of NH₄N₂O₄ (a), NH₄NO₂ (a) or NH₄NO₃ (a), then to produce N₂ 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 NO_x ad-species to produce intermediate NH₂NO₂ or NH₂NO₃ which were unstable and would decompose into N₂ and H₂O.