Ceria–zirconia modified MnOx catalysts for gaseous elemental mercury oxidation and adsorption

Abstract

A series of MnO_x/CeO₂ (Mn/Ce), MnO_x/ZrO₂ (Mn/Zr), and MnO_x/Ce_0.75Zr_0.25O₂ (Mn/CZ) catalysts prepared by an impregnation method were tested for their ability to catalyse the oxidation of Hg⁰ at relatively low temperature (423 K). Various characterization techniques, namely, Brunauer–Emmett–Teller (BET) surface area analysis, X-ray diffraction (XRD), Raman spectroscopy (RS), X-ray photoelectron spectroscopy (XPS), and H₂-temperature programmed reduction (H₂-TPR) were employed to understand the structural, surface, and redox properties of the prepared catalysts. Specific aspects of the catalysis of Hg⁰ oxidation that were investigated included the influence of MnO_x loading (5, 15, and 25%) and the influence of HCl and O₂. Among the catalysts tested, the 15Mn/CZ catalyst achieved the best Hg⁰ oxidation performance (~83% conversion of Hg⁰ to Hg²⁺) in the presence of HCl and O₂. The higher activity of the 15Mn/CZ catalyst was most likely due to the presence of more oxygen vacancies, enhanced Mn⁴⁺/Mn⁴⁺ + Mn³⁺ + Mn²⁺ ratio and more surface adsorbed oxygen, which were proved by XRD, BET, Raman, and XPS. H₂-TPR results also show that the strong interaction between the Ce_0.75Zr_0.25O₂ support and MnO_x improved the redox properties significantly as compared to pure CeO₂ and ZrO₂ supported MnO_x catalysts.