Effect of hierarchical element doping on the low-temperature activity of manganese-based catalysts for NH3-SCR

A series of metals doped Mn-Me-M-x (M-x = Fe, Cu, Ce; M x = Ce, Mg, Co, Ni, W) catalyst were synthesized by a coprecipitation method for low-temperature selective catalytic reduction of NOx with NH3. The results showed that among the binary oxides catalysts, the Mn-Fe catalyst exhibited the best low-temperature activity, and more than 80 % NOx conversion in a reaction temperature range of 150-200 degrees C. and the ternary oxide catalysts Mn-Fe-Ce (0.1) prepared on this basis can reach about 95 % NOx conversion and excellent the tolerance of SO2 and H2O at the same condition. The activity test results indicate that Ce-modification has a clear promoting effects on low temperature activity and resistance to SO2 and/or H2O of the Mn-Fe catalyst. The effect of element doping on physiochemical properties of catalyst were investigated by XRD, BET, XPS, NH3-TPD and H-2-TPR. The results show that there is a strong interaction between Ce, Co or Ni and Mn, Fe oxides, resulting in high dispersibility and reduced crystallinity of these oxides. In addition, the results always prove doping Ce could not only prominently modify and optimize the structure of mixed metal oxides, but also increase the relative content of Mn4+, Mn3+, Fe3+ and surface oxygen (O-alpha) of Mn-Fe catalyst. Moreover, The specific surface area of the catalyst was increased and its surface acidity was strengthened after Ce doping, which is beneficial to increase the adsorption of NH3. Finally, The typical Eley-Rideal mechanism over Mn-Fe-Ce (0.1) were proposed via the in situ DRIFTs experiments.