The effects of tungsten and hydrothermal aging in promoting NH_3-SCR activity on V_2O_5/WO_3-TiO_2 catalysts

摘要

The fresh catalysts of V2O5-TiO2 (VTi) and V2O5/WO3-TiO2 (VWTi) were prepared via an impregnation method. The hydrothermal aged samples were obtained at 750 degrees C for 24 h under airflow with 10 vol% water vapor. Compared to the fresh catalysts, the catalytic performance of the aged catalysts (VTi-A and VWTi-A) has been significantly improved. Particularly, VWTi-A displayed the best activity over a wide temperature range (NOx conversion: 90% @ 250-550 degrees C). Structural characterizations indicated that the surface area of the aged catalysts was dramatically decreased, and the morphologies of catalysts also changed. The X-ray photoelectron spectroscopy (XPS), H-2 temperature-programmed reduction (H-2-TPR), and Raman results demonstrated that hydrothermal aging greatly influenced the migration and agglomeration of vanadium species. As a result, numerous active polymeric VOx species were emerged on the surface of catalysts. More importantly, XPS results revealed that more low valence vanadium species (V4+ and V3+) was produced on the VWTi-A catalysts than that of VTi-A after hydrothermal aging. These low valence vanadium species resulted in considerably higher SCR activity than the high valence vanadium species (V5+). Consequently, both polymeric surface VOx species and low valence vanadium species determined the enhanced low-temperature SCR behavior of the aged catalysts.