Selective Catalytic Reduction Of NO_x By NH₃ Over Manganese-Based Catalysts

The selective catalytic reduction of NOx by NH3 (NH3-SCR) is the most widely commercialized technology for NOx removal. Catalyst is the core of the technology. In this study, we investigated the catalytic performance of Mn-promoted V2O5/TiO2 catalyst and WO3-promoted Mn-Zr mixed oxide catalyst. It was found that VMnTi and WMnZr catalysts exihibted excellent NH3-SCR catalytic activity, high selectivity of N2, high resistance to H2O and SO2, wide temperature window. Seravals methods including BET, XRD, H2-TPR, XPS were used to characterize the structure of catalysts. And the in situ diffuse reflectance infrared transform spectroscopy (DRIFTS) was used to reavel the NH3-SCR reacton mechanism of Mn and WO3 promoted catalysts.VMnTi catalysts were prepared by impregnation method. The addition of Mn to VTi catalyst significantly enhanced the catalytic ativity for NH3-SCR below 400℃. It showed 90% NOX conversion at 250-400℃. And the optimal content of Mn is 2% by weight. The characterization results showed that the redox cycle (V4++Mn4+(?)V5++Mn3+) on VMnTi catalyst played a key role for the high catalytic performance. In addition, the redox cycle promoted the adsorption and activation of NH3 and NO, forming reactive intermediates (NH4+,coordinated NH3, NO2, monodentate nitrate species), further promoting the NH3-SCR to proceed.The added WO3 greatly enhanced the catalytic activity of Mn-Zr mixed oxide catalyst. WMnZr catalyst showed over 90% NOx conversion in the temperature range of 250-400℃, at a space velocity of 128,000 h-1. The catalyst also showed high resistance to H2O and SO2. At 300℃, it showed 90% NOX conversion. The characterization results showed highly dispersed WO3 can not only promote the formation of more acid site on the catalyst surface, but also inhibit the adsorption of stable nitrate species, releasing more active sites available for NH3 adsorption. Moreover, the synergetic effect between W and Mn improves the redox property of the catalyst, thus enhancing the activity of catalyst for NH3-SCR.