Boosting SO2-tolerance of Mn-doped CeO2-ZrO2 for Hg catalytic oxidation via NaOH etching

Abstract

Manganese-doped CeO2-ZrO2 solid solution (CZM) is a prominent sorbent for extracting Hg0 from flue gas, operating optimally around 150 °C, but impaired by SO2 exposure. In this research, we apply a hydrothermal method, employing a sodium hydroxide (NaOH) solution, to enhance CZM's SO2 resilience. The detrimental effect of SO2 on CZM primarily arises from the reduction of active oxygen species, diminishing Hg0 removal efficacy. After conducting a 10-hour test in the atmosphere comprising N2, 6 %O2, and 500ppmSO2, the mercury removal efficiency of the unmodified CZM significantly decreased from ∼99.5 % to ∼26.0 %. Conversely, the modified sorbents, designated as CZM-1M, demontrasted enhanced stability, with its mercury removal efficiency modestly reducing from ∼97 % to ∼60 %. We then conducted comprehensive characterizations of both sorbents to understand the mechanism behind this improved SO2 tolerance. In CZM-1M, we noted an enhanced conversion rate from Ce4+ to Ce3+, a key factor in boosting the production of active oxygen species, necessary for Hg0 oxidation. Additionally, the formation of SO3 - a result of the interaction between the abundant oxygen species and SO2 - emerged as a novel active site that further promoted Hg0 oxidation. This shift in active site offers a pathway for enhancing SO2 resistance in such material.