Investigation of the liquid-phase sulfonation mechanism driven by plasma-generated radicals in sulfuric acid solution

A plasma-based carbon sulfonation method using dilute sulfuric acid was developed. However, the sulfonation efficiency achieved by this approach remains limited. In our previous studies, hydroxyl (OH) radicals generated by plasma discharge were shown to react with sulfur-containing species and introduce sulfur functional groups at defect sites on the carbon surface. Among these species, sulfate radicals (•SO₄⁻) are hypothesized to play a key role in the sulfonation process. The detection of •SO₄⁻ under plasma–sulfuric acid conditions remain challenging due to their short-lived nature. In this study, a DC plasma discharge was applied to sulfuric acid solutions, and activation of sodium persulfate (Na₂S₂O₈) to generate •SO₄⁻ and investigate their contribution to carbon sulfonation was employed. The effects of discharge polarity and treatment duration on sulfonation efficiency and catalytic performance were evaluated. In addition, we constructed a similar in-situ absorption diagnostic setup. A broadband lamp and a high signal-to-noise camera were used for in-situ absorption imaging to visualize •SO₄⁻ distribution in the liquid phase.