Unraveling the role of oxygen vacancies in enhancing the SO2-resistance of α-MnO2 towards elemental mercury removal

Abstract

Enhancing SO₂-resistance has long been a persistent challenge for Mn-based catalytic adsorbents in the field of Hg⁰ removal. Herein, a straightforward strategy is proposed to mitigate SO₂ toxicity via regulating oxygen vacancy (Ov) content. Defective α-MnO₂ is prepared by impregnation with sodium borohydride, and systematic tests are performed to verify its anti-sulfur performance. Characterizations reveal that the impregnation causes little change in phase composition and intrinsic structure, but have an intuitive impact on surface species and Ov. Experiments confirm that the increased Ov not only increases the removal efficiency by 25 % in a 6 % O₂ + 800 ppm SO₂ atmosphere, also preserves 5-hour anti-poisoning stability, attributing to the weakened competitive adsorption and less sulfate deposition, due to the accelerated generation and migration of oxidative species, and enhanced oxidation ability. Theoretical calculations clarify: (i) the adsorption energy of SO₂ at the same sites is significantly reduced after introducing Ov, weakening the inhibition of SO₂; (ii) on defective surface, adsorbed SO₃* generated by the reaction of adsorbed oxygen with SO₂ is more easily desorbed, preventing sulfate accumulation and maintaining the active sites for Hg⁰ oxidation. These findings unravel the underlying mechanism by which Ov enhances the SO₂-resistance of α-MnO₂, offering a practical solution for ameliorating the sulfur tolerance.