Support effect of Mn-based catalysts for gaseous elemental mercury oxidation and adsorption

Abstract

Mn-Based catalysts with a Mn loading of 4 wt% were prepared using an impregnation method. Their Hg⁰ removal efficiencies were in the following order: 4% Mn/MK10 (montmorillonite K 10) > 4% Mn/SiO₂ > 4% Mn/TiO₂ > 4% Mn/Al₂O₃. Results from various characterization techniques, such as BET, XRD, STEM, H₂-TPR, Hg⁰-TPD and XPS, suggested that the species, morphologies, and distributions of MnO_x led to the different performances in Hg⁰ removal. The support effect on Hg⁰ adsorption and oxidation was studied. For 4% Mn/Al₂O₃ and 4% Mn/SiO₂, physical adsorption dominates Hg⁰ removal at low temperature (150 °C), while chemical adsorption is more important at high temperature (350 °C). At both low and high temperatures, chemical adsorption and oxidation play leading roles in 4% Mn/TiO₂ and 4% Mn/MK10, respectively. The effect of MnO_x species and their morphologies on Hg⁰ oxidation was investigated. Amorphous MnO₂ benefits Hg⁰ oxidation at both low and high temperatures, while amorphous Mn₂O₃ only facilitates Hg⁰ oxidation at high temperature. Hg⁰ oxidation on Mn-based catalysts follows a Mars–Maessen mechanism where the lattice oxygen of MnO_x reacts with the absorbed Hg⁰.