Revealing the effect of paired redox-acid sites on metal oxide catalysts for efficient NOx removal by NH3-SCR.

Understanding the nature of active sites on metal oxide catalysts in the selective catalytic reduction (SCR) of NO by NH3 (NH3-SCR) is a crucial prerequisite for the development of novel efficient NH3-SCR catalysts. In this work, two CeO2-based SCR catalyst systems with diverse acidic metal oxides-CeO2 interfaces, i.e., Nb2O5-CeO2 (Nb2O5/CeO2 and CeO2/Nb2O5) and WO3-CeO2 (WO3/CeO2 and CeO2/WO3), were prepared and used to reveal the relationship between NH3-SCR activity and surface acidity/redox properties. In combination with the results of the NH3-SCR activity test and various characterizations, it was found that the NH3-SCR performance of Nb2O5-CeO2 and WO3-CeO2 catalysts was highly dependent on the strong interactions between the redox component (CeO2) and acidic component (Nb2O5 or WO3), as well as the amount of paired redox-acid sites. From a quantitative perspective, an activity-surface acidity/redox property relationship was proposed. For both Nb2O5-CeO2 and WO3-CeO2 catalysts systems operated at the more concerned low-temperature range (200 °C), the NH3-SCR activity in low NOx conversion region (< 40%) was mainly dominated by the surface acidity of catalysts, while the NH3-SCR activity in high NOx conversion region (> 40%) was more determined by redox properties.