| With the rapid development of China’s economy and the acceleration of industrialization, the consumption of coal resources continues to increase, and the emission of NOx increases year by year. As a result, serious air pollution has been triggered, such as photochemical smog, acid deposition, ozone depletion of stratospheric and so on. Selective catalytic reduction by ammonia (NH3-SCR) as a low cost, high efficiency technology has been widely used in NOx control field of stationary sources. Currently, the most widely used commercial SCR catalyst is V2O5-MoO3/WO3-TiO2 catalysts, which is active in the temperature range of 300-400℃. However, after the dust-removal and desulfurization equipment, the temperature of the flue gas from fixed source is rather low (below 200℃). Therefore, the design and development of catalysts with high low-temperature activity is of important practical significance. In view of the above situation, Mn-TiO2 and W-Mn-TiO2 composite metal oxide catalysts were synthesized by one-pot co-precipitation method, and the NH3-SCR reactivity was tested under different conditions in present study. Varieties of characterizations were adopted to investigate the influence of physical-chemical properties on the catalytic activity of the catalysts. In addition, in situ infrared spectroscopy was used to clarify the behaviors of reaction intermedium and the reaction mechanism over the catalysts. The specific research contents are as follows:(1) The Mn-TiO2 mixed oxide catalysts were synthesized by one-pot coprecipitation method, and the influence of precipitant and Mn content on NH3-SCR activity was also investigated. The results showed that Mn-TiO2 catalyst (Mn/(Mn+Ti)= 0.5) with ammonium carbonate as the precipitant exhibited high catalytic acitivity, and the NOx conversion could reach more than 90% in the temperature range of 80-260℃. Compared with MnOx and TiO2 catalysts, the formation of Mn-O-Ti mixed bond induced more surface defects, and increased the amount of adsorbed oxygen species on the catalyst surface simultaneously. Therefore, the NH3-SCR performance of Mn-TiO2 catalyst was significantly enhanced.(2) The W-Mn-TiO2 mixed oxide catalysts with high low-temperature activity were synthesized by one-pot coprecipitation method. The addition of W led to the further enhancement of NH3-SCR activity, and the operation temperature window was broadened at the same time. Among the catalysts, the W(0.25)-Mn(0.25)-TiO2 catalyst presented the best catalytic activity, which exhibited more than 90% NOx conversion within 60-400℃. Various physic-chemical characterisations revealed that the concentration of Mn4+ species and the chemical adsorbed oxygen on the catalyst surface were enhanced by W doping, which promoted the oxidation of NO to NO2, and thus improving the reaction SCR activity.(3) In situ FTIR was used to investigate the NH3-SCR reaction mechanism of W(0.25)-Mn(0.25)-TiO2 catalyst. Compared with Mn-TiO2 and W-TiO2 catalysts, there were more Lewis and Br(?)nsted acid sites presented on the surface of W(0.25)-Mn(0.25)-TiO2 catalyst. In addition, the reaction intermedium with high catalytic activity were generated, and the intermediate (-NH2) adsorbed on Lewis acid sites and the monodentate nitrate were the main active species in the NH3-SCR reaction. The mechanism on the W(0.25)-Mn(0.25)-TiO2 catalyst mainly followed the E-R and the L-H mechanism. |
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