Mechanisms of Medium- and Low-Temperature Denitration by Combined CO + NH3 on Mn–Ce/AC Catalysts and Their SO2 Poisoning.

The mechanisms of medium- and low-temperature denitrations using selective catalytic reduction (SCR) by combined CO + NH3 on Mn–Ce/activated carbon (AC) catalysts and their poisoning by SO2 were investigated. Several Mn–Ce/AC catalysts prepared and then poisoned by SO2 were used in the study. The physicochemical properties of the catalysts were systematically characterized before and after poisoning, and the mechanism of SO2 poisoning was proposed. SO2 poisoning considerably affects the Mn–Ce/AC catalyst, decreasing its denitration rate from 98% to 47.9% and N2 selectivity from 96 to 85%. This can be attributed to several phenomena. SO2 poisoning increases the surface roughness of the catalyst and results in the formation of Mn(SO4)2, other sulfates, and other substances, which block the catalyst pores and reduce the content of the C and O elements on the catalyst surface as well as its specific surface area, consequently decreasing the adsorption area available for the reaction gas. Moreover, the crystallinity of MnOx on the surface of the catalyst increases, decreasing the contents of Mn4+, Ce4+ and chemosorbed oxygen. In addition, the surface functional groups, such as –COOH, –OH, and C=O, are destroyed, reducing the adsorption sites of the reaction gas and the adsorption capacity of the catalyst, consequently inhibiting the SCR reaction. Ce can reduce the effects of SO2 poisoning on the catalyst because Ce4+ can improve its oxygen storage capacity, increase the adsorption sites for the reaction gas, and react with SO2, preventing sulfate formation and pore blockage. However, excessive Ce loading causes metal agglomeration on the catalyst surface, reducing its denitration efficiency. After SO2 poisoning, the contents of Mn4+, Mn3+, and Ce4+ decreased in the Mn -Ce/AC catalysts, inhibiting the synergistic reaction between Mn and Ce, which decreased the number of oxygen vacancies and Oβ, consequently inhibit the reaction's conversion to fast selective catalytic reduction. Thus, SO2 destroyed the functional groups; occupied the oxygen vacancies and active site; and destroyed the –COOH, O–H, and C=O functional groups on the surface of the Mn–Ce/AC catalyst, reducing the acidity and number of active sites on the catalyst surface. [ABSTRACT FROM AUTHOR]