The reduction of SO 2 on activated carbon was studied in the range of 600-700 C in a differential reactor under steady-state conditions and under chemically controlled kinetics. Initial rates of carbon conversion and gas reagent were calculated from the mass balance of the gaseous products. The kinetics was first-order with respect to carbon and first-order with respect to the partial pressure of SO 2 . The activation parameters were ÁH 6 ¼ ¼ 21.5 kcal mol À1 and ÁS 6 ¼ ¼ À211 cal mol À1 K À1 . The activated carbon was ca. 10 5 times more reactive than graphite, and determined by the enthalpy of activation. The main reaction products were CO 2 and sulfur. CO and COS were produced from consecutive reactions of the primary products. During the pre-steady state, the sulfur content of the carbon increased to a plateau where the reaction reached the steady state condition. This sulfur was shown to be chemically bound to the carbon matrix and represents the stable reactive intermediates of the reduction of SO 2 . The XPS spectrum of the residual carbon C(S) showed two forms of sulfur bound to carbon: non-oxidized sulfur (sulfide and/or disulfide) and oxidized sulfur (sulfone, sulfoxide, sulfenate, sulfinate). The sulfur intermediates C(S) reacted with SO 2 at the same rate as pure activated carbon and with CO 2 to produce SO 2 by the reverse reaction. The reaction of C(S) with CO produced COS.

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The reduction of SO 2 on carbons proceeds through reactive intermediates bound to the carbon matrix, which were postulated to be 1,2-oxathiene 2-oxide (or sultine), and 1,3,2-dioxathiolane that decomposes to produce an episulfide and CO 2 . The reactivity of these intermediates was studied in this work through several reactions, using XPS and NMR spectra to postulate their mechanisms. When modified activated carbon obtained after reaction with SO 2 at 630 °C was heated at 900 °C, it was observed that the changes of the XPS spectrum resulted from the forward reaction of decomposition of the oxidized intermediate with S-transfer to produce the episulfide and CO 2 and the reverse reaction with expulsion of SO 2 . Strong bases hydrolyzed the dioxathiolane intermediate and the episulfide. The thiolysis, aminolysis, and reaction of alkyl halides with modified activated carbon occurred with the insertion of the organic moiety in the carbon matrix. Laser photolysis at 266 nm in t-butanol showed insertion of t-butoxide on the matrix. Consistent mechanisms for these reactions were postulated. These results provide additional evidence on the mechanism of reduction of SO 2 on carbons and the chemical nature of the intermediates, offering a new method to modify the physical and chemical properties of a carbon matrix by functionalization with an organic moiety.