Superior ZnO-tolerant ability of the CeOx-WO3 mixed oxide catalysts for the selective catalytic reduction of NOx with NH3

This article demonstrates the superior ZnO tolerance of the CeO_x-WO₃ (CeW) mixed oxide, using the commercialized V₂O₅-WO₃/TiO₂ (VW/Ti) catalyst as a reference. Both fresh CeW and VW/Ti catalysts exhibited >90 % NO_x conversion at temperatures above 300 °C. However, Upon ZnO introduction, the NO_x conversion of both catalysts displayed a declining trend. Notably, at a ZnO loading of 7 wt%, the CeW catalyst maintained >80 % NO_x conversion, while the VW/Ti catalyst showed nearly 0 % NO_x elimination under the same conditions. Characterizations results revealed that loading 3 wt% resulted in a 79 % loss of acid sites on the VW/Ti catalyst. This significant reduction in acidity hindered NH₃ utilization for NO_x reduction, overweighing the positive effects of enhanced NH₃ activation through improved oxidative capacity. Additionally, NO_x adsorption on the ZnO-poisoned catalyst surface formed inert nitrate species, which covered active sites, thereby explaining the severe ZnO-induced deactivation of the VW/Ti composite. In contrast, the ZnO-poisoned CeW catalyst retained moderate acidity, preserving 79 % of its acid sites for NH₃ adsorption. This substantial retention of acid sites ensured the effective progression of NO_x elimination reactions, accounting for the satisfactory ZnO resistance of the CeW catalyst. These finding provide valuable insights for addressing the challenge of stable catalyst operation under ZnO-rich conditions.