Reverse Conversion Treatment of Gaseous Sulfur Trioxide Using Metastable Sulfides from Sulfur-Rich Flue Gas

Sulfur trioxide (SO₃) is an unstable pollutant, and its removal from the gas phase of industrial flue gas remains a significant challenge. Herein, we propose a reverse conversion treatment (RCT) strategy to reduce S(VI) in SO₃ to S(IV) by combining bench-scale experiments and theoretical studies. We first demonstrated that metastable sulfides can break the S-O bond in SO₃, leading to the re-formation of sulfur dioxide (SO₂). The RCT performance varied between mono- and binary-metal sulfides, and metastable CuS had a high SO₃ conversion efficiency in the temperature range of 200-300 °C. Accordingly, the introduction of selenium (Se) lowered the electronegativity of the CuS host and enhanced its reducibility to SO₃. Among the CuSe_1-S composites, CuSe_0.3S_0.7 was the optimal RCT material and reached a SO₂ yield of 6.25 mmol/g in 120 min. The low-valence state of selenium (Se^2-/Se^1-) exhibited a higher reduction activity for SO₃ than did S^2-/S^1-; however, excessive Se doping degraded the SO₃ conversion owing to the re-oxidation of SO₂ by the generated SeO₃^2-. The density functional theory calculations verified the stronger SO₃ adsorption performance (E_ads = -2.76 eV) and lower S-O bond breaking energy (E_a = 1.34 eV) over CuSe_0.3S_0.7 compared to those over CuS and CuSe. Thus, CuSe_1-S can serve as a model material and the RCT strategy can make use of field temperature conditions in nonferrous smelters for SO₃ emission control.