Mechanism of the desulfurization route of the reduction of SO2 on carbons. Dimerization of Disulfur and Tetrasulfur

The desulfurization of carbons modified with SO₂ was studied as a dispersion in boiling cyclohexane (81°C) using activated carbon (mAC) and graphene oxide (mGO), modified by SO₂. The steady-state species in the carbon matrix after the catalytic reduction of SO₂ was considered a trisulfane. For mAC, there was a burst of a sulfur species identified as S₂ by UV spectrum with a maximum at 217 nm (ε_M at 217 nm = 2.56 × 10³) that showed a second-order decay of absorbance with $k_{S_2}$ = 47.29 M^-1·sec^-1 ( ${\mathrm{\Delta G}}_{354}^{‡}$= 18.1 kcal·mol^-1). The product was postulated to be S₄. No other consecutive reaction was observed because of the possible adsorption of S₄ in the carbon matrix. The desulfurization of mGO was shown by XPS and the kinetics were a second-order decay up to 16 min ( $k_{S_2}=$ 18.41 M^-1·sec^-1; ${\mathrm{\Delta G}}_{354}^{‡}$= 18.8 kcal·mol^-1) followed by a second-order increase of absorbance with $k_{S_4}$ = 3.84 M^-1·sec^-1 ( ${\mathrm{\Delta G}}_{354}^{‡}$= 19.9), where the product showed a double maximum at 260-285 nm typical of S₈. These results are consistent with a mechanism of consecutive thermodynamically favorable dimerizations of S₂ and S₄ and with the desulfurization mechanism that has been previously postulated.