Effect of the calcination temperature of cerium–zirconium mixed oxides on the structure and catalytic performance of WO3/CeZrO2 monolithic catalyst for selective catalytic reduction of NOx with NH3

Abstract

A series of WO₃/CeZrO₂ catalysts, prepared at different calcination temperatures (400, 500, 600 and 700 °C) of cerium–zirconium mixed oxides (CeZrO₂) for the selective catalytic reduction of NO_x with ammonia (NH₃-SCR), were investigated via various characterizations, such as N₂ physisorption, XRD, Raman, NH₃-TPD, DRIFTS, XPS and H₂-TPR. The catalytic performance of NH₃-SCR was remarkably promoted by modestly increasing the calcination temperature of CeZrO₂: WO₃/CeZrO₂-500 possessed the lowest light-off temperature (173 °C) and complete conversion temperature (205 °C), while W/CeZrO₂-600 could achieve greater than 90% NO_x conversion in a broad temperature range of 220–455 °C. The characterization results indicated that modest enhancement of the calcination temperature of CeZrO₂ was beneficial to stabilizing the structure of the catalysts. The largest amount of Lewis acid sites, Ce³⁺ and surface active oxygen species, as well as strong redox properties of WO₃/CeZrO₂-500 should together contribute to its better low-temperature deNO_x activity. Moreover, increasing the calcination temperature of cerium–zirconium mixed oxides resulted in the enhancement of Brønsted acid sites, which was responsible for the widened operation temperature window. Therefore, WO₃/CeZrO₂ serial catalysts with appropriate calcination treatment of CeZrO₂ would be a good choice for the removal of NO_x emitted from diesel engines.