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.