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 CuSe1-xSx composites, CuSe0.3S0.7 was the optimal RCT material and reached a SO2 yield of 6.25 mmol/g in 120 min. The low-valence state of selenium (Se2-/Se1-) exhibited a higher reduction activity for SO3 than did S2-/S1-; however, excessive Se doping degraded the SO3 conversion owing to the re-oxidation of SO2 by the generated SeO32-. The density functional theory calculations verified the stronger SO3 adsorption performance (Eads = -2.76 eV) and lower S-O bond breaking energy (Ea = 1.34 eV) over CuSe0.3S0.7 compared to those over CuS and CuSe. Thus, CuSe1-xSx can serve as a model material and the RCT strategy can make use of field temperature conditions in nonferrous smelters for SO3 emission control.