Catalytic oxidation of Hg0 over Mn-doped CeO2-ZrO2 solid solution and MnOx/CeO2-ZrO2 supported catalysts: Characterization, catalytic activity and SO2 resistance

Abstract

MnOx-based oxides have been recognized as efficient catalysts for elemental mercury (Hg0) oxidation at low temperature from flue gas. CeO2 or CeO2-ZrO2 solid solution can enhance the catalytic activity of MnOx, so combining manganese and CeO2 or CeO2-ZrO2 is a promising way to obtain more highly effective catalysts. Two kinds of combination modes were considered: one was MnOx loaded on the CeO2-ZrO2 to form a supported catalyst (MnOx/CeO2-ZrO2, Mn/CZ), and the other was Mn-doped into the CeO2-ZrO2 solid solution to generate a new MnOx-CeO2-ZrO2 (CZM) solid solution catalyst. However, it was not yet known which catalyst prepared by the two combination modes has the better Hg0 oxidation performance. To compare their catalytic activities, the two kinds of catalysts with varying Mn mass content were synthesized by the similar method, and the Hg0 catalytic oxidation performances and the effects of SO2 were tested under the same conditions. The catalysts were well characterized by ICP, XRD, Raman, XPS, SEM and N2 adsorption test, and the results suggested that with the same mass fraction of Mn in the catalyst, the BET surface area and the active species concentration (Mn4+ and Oad) of the new MnOx-CeO2-ZrO2 solid solution catalyst were higher than those of the supported catalyst. Activity tests results showed that for the MnOx loaded samples, 18%Mn/CZ exhibited the best activity (64.58%)., while for the Mn doped solid solution, Ce0.525Zr0.175Mn0.3O2 exhibited the highest Hg removal efficiency (89.53%). The mass contents of Mn in 18%Mn/CZ and Ce0.525Zr0.175Mn0.3O2 were similar. The results suggested that the new solid solution MnOx-CeO2-ZrO2 was more promising for Hg0 oxidation. For both types of the catalysts, Mn4+ and surface active oxygen (Oad) were the active species in the Hg0 oxidation reactions. SO2 significantly inhibited the activity of MnOx/CeO2-ZrO2, because SO2 prevented the reaction between Hg0 and the active species (Mn4+ and Oad). The MnOx-CeO2-ZrO2 solid solution exhibited better SO2 resistance, because the SO2 just inhibited the reaction between Hg0 and Oad.