High activity of NH3-SCR at high temperature over W-Zr/ZSM-5 in the exhaust gas of diesel engine

Abstract

The conventional NH3-SCR catalysts always demonstrated a low activity at high temperature (≥600 °C), which is a common situation in the exhaust gas of diesel engine. A series of W-Zr-ZSM-5 (WZZ) catalysts were prepared by sol–gel method (SG), impregnation method (IM), and grinding method (GR), respectively, to achieve the goals for NO removal at high temperature. It found that WZZ-SG700 prepared by SG method demonstrated the highest NO conversion among the three preparation methods, exhibiting >90% NO conversion at 300–650 °C. The characteristics of the catalysts were analyzed by the techniques of XPS, SEM, H2-TPR, NH3-TPD, BET, XRD and in-situ DRIFTS. It found that the interaction between WO3 and ZrO2 significantly enhanced the surface acidity and redox property of the catalysts, which was beneficial to the performance at high temperature. Furthermore, the catalyst prepared by SG method (WZZ-SG700) had a higher surface enrichment of W/Zr on a higher BET surfaces area when compared with IM and GR methods. It resulted in the formation of a more acid site of the catalyst and a higher redox property, exhibiting a higher NO conversion especially at high temperature. In addition, WZZ-SG700 catalyst showed a high resistance of SO2 & H2O and a high N2 selectivity. The in-situ DRIFTS studies revealed that the NO removal over WZZ-SG700 catalyst conformed to L-H mechanism, and the species of monodentate nitrite, ad-NO2 species, NH4+, surface-adsorbed NH3 and amide (–NH2) were the main reaction intermediates appearing on the surface of the catalyst during the reaction. WZZ-SG700 demonstrated to be a promising catalyst for the diesel engine SCR system at high temperature.