Cr Doping MnOx Adsorbent Significantly Improving Hg0 Removal and SO2 Resistance from Coal-Fired Flue Gas and the Mechanism Investigation

A series of binary Mn-based materials (Fe-MnOx, Cu-MnOx, Cr-MnOx, and Mo-MnOx) were prepared by the coprecipitation method, and the mechanism for elemental mercury (Hg0) removal of Cr-MnOx in the absence and presence of SO2 was investigated. The physicochemical structure properties of the fresh as well as spent Cr-MnOx samples were well characterized by the methods of N2 physisorption, XRD, SEM-EDS, FTIR, H2-TPR, XPS, and TPD. The experiment results revealed that the loading of Cr into/onto MnOx yielded above 90% of Hg0 removal efficiency in the presence of SO2 at 120 °C. However, the appearance of SO2 would lead to the formation of chromium sulfate/sulfite and manganese sulfate/sulfite although much superior SO2 resistance was obtained. According to the N2 physisorption and XRD characterization results, Cr doping can enlarge the BET surface area of MnOx when Cr was doped into/onto MnO2, and the MnCrO4 mixed oxide material was produced. Moreover, the H2-TPR result exhibited that Cr loading into/onto MnOx enhanced the redox property of Cr-MnOx mixed oxides and many more easily reduced species appeared in the mixture. The larger surface area and an abundance of reactive species could benefit from excellent performances of Hg0 removal and superior SO2 resistance although the DFT calculation showed almost the same adsorption energies of Hg0 on MnO2 (110) and Cr-MnO2 (110) surfaces. The DFT calculation exhibited that the adsorption energy of SO2 on the surface of Cr-MnO2 (110) was much lower than that on MnO2 (110), implying that Cr doping into MnO2 would decrease the adsorption behavior of SO2, thereby improving its SO2 resistance performance.