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 degrees 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 S-2 by UV spectrum with a maximum at 217 nm (epsilon(M) at 217 nm = 2.56 x 10(3)) that showed a second-order decay of absorbance with k(s2) = 47.29 M-1.sec(-1) (Delta G(354)double dagger= 18.1 kcal.mol(-1)). The product was postulated to be S-4. No other consecutive reaction was observed because of the possible adsorption of S-4 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(s2) = 18.41 M-1.sec(-1); Delta G(354)double dagger = 18.8 kcal.mol(-1)) followed by a second-order increase of absorbance with k(s4) = 3.84 M-1.sec(-1) (Delta G(354)double dagger= 19.9), where the product showed a double maximum at 260-285 nm typical of S-8. These results are consistent with a mechanism of consecutive thermodynamically favorable dimerizations of S-2 and S-4 and with the desulfurization mechanism that has been previously postulated.