Study on Low-Temperature SCR Denitration Mechanisms of Manganese-Based Catalysts with Different Carriers

Micro-physicochemical characteristics and low-temperature SCR activities of the Mn–Ce–Cr catalysts on different carriers were investigated with SEM, XRD, XPS, FTIR, and denitration experiments. Mn–Ce–Cr catalysts carried on TiO2 and ZrO2, and composite carrier containing Al2O3 and TiO2 had visible element interactions on the surfaces, and Mn presented the mixed valences of Mn3+ and Mn4+. Mn3+ was transferred to Mn4+ due to the oxidation processes of Ce3+ to Ce4+ and Cr3+ to Cr6+, and the Oα/Oβ ratio decreased during the SCR process. Compared with single carriers such as TiO2 or ZrO2, the catalysts on the composite carriers of Al2O3 and TiO2 had better pore structures and higher fractions of Mn4+, Ce3+, Cr6+, and chemisorbed oxygen. It could also absorb the coordination-state NH3 well, especially the higher activity l-acid sites during the SCR process, and contribute to the formation of composite oxide MnxTi1−xO on Mn–Ce–Cr/Al2O3+TiO2 catalyst. All the above factors had positive effects on the low-temperature SCR. However, Co-doping in Mn–Ce–Cr catalysts could not improve the pore structures or promote the dispersions of Mn–Ce–Cr active substances on the carrier surface. Groups such as nitrates and nitrites produced by NO adsorption would hinder the adsorption of NH3 and low-temperature SCR. Mn–Ce–Cr/Al2O3+TiO2 catalyst had high low-temperature SCR activity, while Mn–Ce–Cr/ZrO2 catalyst was the most unstable with the lowest denitration efficiency. Moreover, for the scrapped catalyst from coal-fired power plants, it could still be used as the carrier of the Mn–Ce–Cr catalyst, and its SCR characteristics were much better than the above catalysts, especially at broader temperature range.