Extraordinary Deactivation Offset Effect of Arsenic and Calcium on CeO₂-WO₃ SCR Catalysts

An extraordinary deactivation offset effect of calcium and arsenic on CeO₂-WO₃ catalyst had been found for selective catalytic reduction of NO with NH₃ (NH₃-SCR). It was discovered that the maximum NO _x conversion of As-Ca poisoned catalyst reached up to 89% at 350 °C with the gaseous hourly space velocity of 120 000 mL·(g·h)^-1. The offset effect mechanisms were explored with respect to the changes of catalyst structure, surface acidity, redox property and reaction route by XRD, XPS, H₂-TPR, O₂-TPD, NH₃-TPD and in situ Raman, in situ TG, and DRIFTS. The results manifested that Lewis acid sites and reducibility originating from CeO₂ were obviously recovered, because the strong interaction between cerium and arsenic was weakened when Ca and As coexisted. Meanwhile, the CaWO₄ phase generated on Ca poisoned catalyst almost disappeared after As doping together, which made for Brønsted acid sites reformation on catalyst surface. Furthermore, surface Ce⁴⁺ proportion and oxygen defect sites amount were also restored for two-component poisoned catalyst, which favored NH₃ activation and further reaction. Finally, the reasons for the gap of catalytic performance between fresh and As-Ca poisoned catalyst were also proposed as follows: (1) surface area decrease; (2) crystalline WO₃ particles generation; and (3) oxygen defect sites irreversible loss.