Electroreduction of Sulfur Dioxide in Some Room-Temperature Ionic Liquids

The mechanism of sulfur dioxide reduction at a platinum microelectrode was investigated by cyclic voltammetry in several room-temperature ionic liquids (RTILs) [C 2 mim][NTf 2 ], [C 4 mim][BF 4 ], [C 4 mim][NO 3 ], [C 4 mim][PF 6 ], and [C 6 mim][Cl] where [C 2 mim] is 1-ethyl-3-methylimidazolium, [C 4 mim] is 1-butyl-3-methylimidazolium, [C 6 mim] is 1-hexyl-3-methylimidazolium, and [NTf 2 ] is bis(trifluoromethylsufonyl)imide with special attention paid to [C 4 mim][NO 3 ] because of the well-defined voltammetry, high solubility, and relatively low diffusion coefficient of SO 2 obtained in that ionic liquid. A cathodic peak is observed in all RTILs between −2.0 and −1.0 V versus a silver quasi-reference electrode. In [C 4 mim][NO 3 ], the peak appears at −1.0 V, and potential step chronoamperometry was used to determine that SO 2 has a very high solubility of 3100 (±450) mM and a diffusion coefficient of 5.0 (±0.8) × 10 -10 m 2 s -1 in that ionic liquid. On the reverse wave, up to four anodic peaks are observed at ca. −0.4, −0.3, −0.2, and 0.2 V in [C 4 mim][NO 3 ]. The cathodic wave is assigned to the reduction of SO 2 to its radical anion, SO 2 -• . The peaks at −0.4 and −0.2 V are assigned to the oxidation of unsolvated and solvated SO 2 -•, respectively. The peak appearing at 0.2 V is assigned to the oxidation of either S 2 O 4 2- or S 2 O 4 -• . The activation energy for the reduction of SO 2 in [C 4 mim][NO 3 ] was measured to be 10 (±2) kJ mol -1 using chronoamperometric data at different temperatures. The stabilizing interaction of the solvent with the reduced species SO 2 -• leads to a different mechanism than that observed in conventional aprotic solvents. The high sensitivity of the system to SO 2 also suggests that [C 4 mim][NO 3 ] may be a viable solvent in gas sensing applications.