Plasma-enhanced catalytic reduction of SO₂: Decoupling plasma-induced surface reaction from plasma-phase reaction

Complex processes take place when coupling nonthermal plasma with heterogenous catalysis, obscuring mechanistic understanding of the plasma–catalysis synergy. Simultaneous homogenous plasma-phase reactions and heterogenous surface catalytic reactions occur in the system, which makes distinguishing the plasma's contributions very challenging. In this work, we perform a careful and rigorous evaluation of surface reactions under plasma conditions. We use SO₂ hydrogenation as a probe reaction to examine plasma-induced surface reactions, desorption, and dissociation of SO₂ adsorbate. A temperature-programmed plasma-induced surface reaction approach is used to decouple plasma-induced surface reactions from plasma-phase reactions. The qualitative and quantitative analyses reveal a new Eley-Rideal reaction between plasma-generated atomic hydrogen in the gas-phase and strongly adsorbed SO₂ over alumina, a reaction that is not thermally feasible. Furthermore, plasma causes partial desorption of weakly adsorbed SO₂ species and enhances the strong chemisorption of SO₂ over supported iron sulfide catalyst. Moreover, plasma facilitates H₂ chemisorption and reaction with iron sulfide, producing sulfur vacancies. This work sheds light on the possible origin of plasma–catalysis synergy and offers fundamental insight about the underlying mechanisms of plasma-assisted SO₂ hydrogenation reaction.