Abstract
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°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°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 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 NOx ad-species to produce intermediate NH2NO2 or NH2NO3 which were unstable and would decompose into N2 and H2O.
Highlights
Selective catalytic reduction (SCR) of NOx with NH3 is a well-established technology to abate nitrogen oxides from stationary source (Bosch, 1988; Forzatti, 1996)
We had successfully developed MnOx supported on multi-walled carbon nanotubes (MWCNTs) catalysts which showed high active for low-temperature selective catalytic reduction (SCR) (Wang et al, 2012)
We proposed two possible reaction pathways that are significant to understand the behaviour of the formed surface compounds, active intermediates and elucidate the potential mechanism for the catalysts with high low-temperature activity in respect to the SCR reaction cycle
Summary
Selective catalytic reduction (SCR) of NOx with NH3 is a well-established technology to abate nitrogen oxides from stationary source (Bosch, 1988; Forzatti, 1996). Due to the drawbacks of these catalysts, such as high catalytic temperature, narrow temperature window and toxicity of vanadium by SO2 or dust, there has been a continuing effort to develop a highly low-temperature (≤ 250°C) efficient, stable, environmentalfriendly SCR catalysts for the removal of NOx (Lou et al, 2003., Qi et al, 2004; Jiang et al, 2009; Yang et al, 2012; Liu et al, 2013). It has been reported that Mn-based catalysts, such as MnOx/TiO2 (Jiang et al, 2009), MnOx/activated carbon fiber (Pasel et al, 1998), and MnOx/activated carbon/ceramic (Tang et al, 2007), have presented good catalytic activity in SCR reaction with excess oxygen at low temperature. We had successfully developed MnOx supported on multi-walled carbon nanotubes (MWCNTs) catalysts which showed high active for low-temperature SCR (Wang et al, 2012)
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