Influence of WO_3 doping on properties of MnO_x/TiO_2 catalyst for low-temperature selective catalytic reduction of NO_x by NH_3

Series of MnOx/WO3/TiO2 with different WO3 content were prepared by a one-step impregnation method and BET surface area measurement(BET), X-ray diffraction(XRD), H2-temperature programmed reduction(H2-TPR), High Resolution Transmission Electron Microscope(HRTEM), and in situ Fourier transform infrared spectroscopy(FT-IR) were employed to study their physico-chemisitry properties. It is found that the addition of appropriate amount of WO3can improve remarkably the catalytic activity of Mn-based catalysts. Particularly, the 15% MnOx/5%WO3/TiO2 exhibites excellent low-temperature catalytic performance with above 85% NO conversion in the wide temperature range of 80-240 ℃. The combination of characterization techques indicate that the addition of 5% WO3 can effectively improve the dispersion of active components and the reducibility as well as enhance the amount and strength of the surface acid sites, especially the Br?nsted acid sites, and promote the formation of the active intermediate(—NH2). Furthermore, manganese oxide existing in the form of Mn2O3 but MnO2 begin to appear and predominate with the WO3 increasing. A huge amount of monodentate and bidentate coordination nitrates are formed when the 5% WO3-doped catalyst is merely exposed to the pure NO,which occupy the active center of oxidation, thus impeding the absorption and activation of NH3. These materials are irreversible at the low-temperature window, hence blocking the follow-up reaction.