Study On Mn-based Catalysts And Mechanism For Selective Catalytic Reduction Of NO_X At Low-temperature

Selective catalytic reduction of NOx by ammonia (NH₃-SCR) is an effective way to remove NOx from stationary source. However, low-temperature NH₃-SCR (<150℃) is still a stringent challenge all over the world. In this work, two types of Mn-based catalysts, supported MnOX/TiO₂ and unsupported MnOX catalysts, were developed and characterized by BET,XRD,XPS and DRIFTS methods.The activities of each kind of catalyst were investigated in detail under the simulated emission conditions of stationary source. Furthermore, the mechanism of NH₃-SCR at low-temperatures was investigated based on the activity tests and catalyst characterizations. The main results were summarized as follows:The supported MnOX/TiO₂ catalyst prepared by co-precipitation method showed good performance for NH₃-SCR at low temperature. NOx conversion could be 70% at 100℃, and nearly 100% in the temperature range of 150²00℃. The precursors showed significant effects on the low temperature activities of MnOX/TiO₂ catalysts. The MnOX/TiO₂ from Manganese Acetate precursor had better low temperature activity than the one from Manganese Nitrate precursor. The higher activity of the former catalyst is possibly due to the high surface Mn concentration and amorphous Mn2O3 species.The unsupported MnOX catalysts, prepared by rheological phase reaction method, showed excellent activity at low-temperature. The NOX conversion could be 60% at 50℃and nearly 100% at 80℃. The results of characterizations revealed that the MnOX catalyst is consisted of MnO₂ crystallite with a diameter of about 10nm. The size of the catalyst crystallite is well coincided with the parameter ofε-MnO₂. DRIFTS results revealed that NH₃ molecules were adsorbed on the Lewis acid and Br?nsted acid sites to form coordinated NH₃ and NH4+ ions species on unsupported MnOX catalysts at low temperature (<150℃), and both of them participated in SCR reactions. NO molecules were also adsorbed on the MnOX catalysts as surface adsorbed NO₂, bridging nitro-nitrito and bidentate nitrate species. The surface adsorbed NO₂ and bridging nitro-nitrito species could take part in the SCR reactions while the bidentate nitrate could not be reduced by ammonia. It was concluded that the adsorption and activation of NH₃ is the first step of SCR reaction and the oxidation of NO to surface adsorbed NO₂ and bridging nitro-nitrito species is the determined step of SCR process. The probable reaction pathway of low temperature NH₃-SCR was proposed based on the experimental results.