Optimizing acid promoters of Ce-based NH3-SCR catalysts for reducing NOx emissions

WO3 and MoO3 are often used as acidic promoters to modify vanadium based and non-vanadium based composite oxide catalysts, meanwhile, the NH3-SCR performance of the catalysts could be evidently improved. Generally, WO3 and MoO3 are believed to play the same role in V2O5-based and other composite oxide NH3-SCR catalysts. Interestingly, our recent work found that there are significant differences between WO3 and MoO3 in improving the NH3-SCR performance of CeZrOx. The operating temperature window of WO3/CeZrOx (222−454 °C) is 76 °C wider than that of MoO3/CeZrOx (254−410 °C) for NH3-SCR reaction. WO3/CeZrOx exhibits higher NOx removal efficiency than MoO3/CeZrOx in the whole temperature range. The similarities and differences in the effects of the acid promoters WO3 and MoO3 on the NH3-SCR performance of Ce-based catalysts are systematically investigated by Vis-Raman, XRD, XPS, NH3/O2/NOx-TPD, H2-TPR, NO/NH3-oxidation and in situ DRIFTS. Noteworthy, stronger interaction between MoO3 and CeZrOx than WO3 and CeZrOx leads to less oxygen vacancies and Lewis acid sites on the surface of MoO3/CeZrOx as well as strong oxidation property, which inhibit the NOx adsorption but contribute to the peroxidation of NH3 on MoO3/CeZrOx, reflecting its poor SCR activity. This work provides an in-depth understanding of the intrinsic effects of WO3 and MoO3 on the NH3-SCR performance of CeZrOx-based catalysts.