Mechanistic study on the enhanced SO₂-tolerant ability of CuCl₂-promoted MnO_x-CeO_x mixed oxide for Hg⁰ removal

SO2, a typical component in coal-fired flue gas, always results in a decreased Hg-0 removal efficiency of the catalysts/adsorbents. Therefore, developing a catalyst/adsorbent with satisfactory sulfur resistance constitutes an important step in achieving the goal of mercury emission reduction. In this study, we modified MnOx-CeOx mixed oxide with CuCl2, aiming to enhance its SO2-tolerant ability and reveal the relevant promotional mechanisms. As 200 ppm SO2 was introduced, MnOx-CeOx suffered from deactivation with its Hg-0 removal efficiency decreasing from similar to 100% to similar to 80%. With increasing the SO2 concentration, this deactivation phenomenon was more pronounced. This was because SO2 had a greater affinity to MnOx-CeOx, which would drive the adsorbed Hg-0 away from the active sites. Besides, SO2 was able to rob the oxygen atoms from HgO formed on the MnOx-CeOx surface, which increased the Hg-0 concentration in the working gas and thus explained the decreased Hg-0 removal efficiency with SO2. Given the doping of CuCl2, the highly active Cl- favored the efficient oxidation of Hg-0, making the adverse factors arising from the Hg-0-SO2 competitive behaviors less influencing. Moreover, the produced HgCl2 tended to emit into the working gas instead of being stabilized on the material surface, which could be free from the reduction to Hg-0 by SO2. As a result, CuCl2-promoted MnOx-CeOx exhibited superior sulfur resistance; >95% Hg-0 removal efficiency was well-retained during the 320-min deactivation cycle. This result was vital to the stable operation of the MnOx-CeOx-based materials in coal-fired flue gas.