Activating H₂S into Polysulfide by Citric Acid-Induced Surface-Active Oxygen in Cu-OMS-2 for Enhanced Oxidation of Hg⁰ at Ambient Temperature

Capturing mercury (Hg⁰) in raw natural gas is crucial for the stable operation of natural gas purification systems. However, achieving direct oxidation removal of Hg⁰ in a reducing atmosphere at ambient temperature presents a significant challenge. In this study, we designed a Cu-doped OMS-2 sorbent, synthesized with citric acid (CA) assistance, which demonstrated exceptional Hg⁰ removal performance in simulated natural gas at ambient temperature. The sorbent achieved 97.0% Hg⁰ removal efficiency under a space velocity of 72 × 10⁴ h^-1 when the CA ratio was 0.05. As a carboxyl-rich organic complexing and reducing agent, CA promoted the introduction and dispersion of Cu ions, thereby forming more Mn-O-Cu units. The resulting charge transfer of Mn-O-Cu units further facilitated oxygen vacancy (O_v) formation and lattice oxygen (O_latt) activation, promoting the transformation of gaseous H₂S into polysulfide (<i>S</i>_<i>n</i>^2-) species. The accumulation of electrons around active <i>S</i>_<i>n</i>^2- groups accelerated electron transfer with Hg⁰ via an Eley-Rideal mechanism. This work provides new insights into the design of highly oxygen-active sorbents under reducing conditions and contributes to a deeper understanding of the adsorption and oxidation mechanisms of Hg⁰.