Hydrothermal Stability of CeO2-WO3-ZrO2 Mixed Oxides for Selective Catalytic Reduction of NOx by NH3.

CeO2-WO3-ZrO2 mixed oxides were prepared by the homogeneous precipitation method for the selective catalytic reduction of NOx with NH3 (NH3-SCR). The effects of hydrothermal aging on the catalytic performances of CeO2-WO3-ZrO2 were investigated. The results showed that CeO2-WO3-ZrO2 catalyst exhibited excellent NH3-SCR activity for removal of NOx and hydrothermal stability. After hydrothermal aging at 850 °C for 16 h, the optimum CeO2-WO3-ZrO2 catalyst could still realize 80% NOx conversion at 300-500 °C even under a high gas hourly space velocity of 250 000 h-1. The structural properties, redox ability, surface species, and acidity of fresh and hydrothermally aged CeO2-WO3-ZrO2 catalysts were characterized by N2-physisorption, XRD, Raman, H2-TPR, XPS, NH3-TPD, and in situ DRIFTS. The characterization results showed that decreases of 89% of the surface area and 71% of the NH3 storage capacity as well as new phase formation occurred for the CeO2-WO3-ZrO2 sample after hydrothermal aging at 850 °C for 16 h. The activity of hydrothermally aged CeO2-WO3-ZrO2 was mainly attributed to the retention of redox-acid sites and their interaction due to the formation of Ce-Zr solid solutions and Ce4W9O33.