Kinetics and mechanisms in flow systems: reduction of SO2 on carbons

Surface-science studies of some reactions have allowed postulating the mechanisms using a variety of techniques. The surface chemistry of Group IV elements has been particularly studied recently because Si, Ge, and C are semiconductors, and the surface organic chemistry to functionalize these elements has seen immense progress in recent years. In this review article we describe the methodology used to study the mechanism of a solidgas reaction, the reduction of SO2 on carbons, and the necessary conditions that allowed the measurement of the solid conversion and product distribution when the kinetics was chemically controlled and the reaction had reached the steady state. The reaction was first-order with respect to SO2 and with respect to carbon and the stoichiometry was determined from the product distribution. The reaction proceeds through two thermally stable reaction intermediates bound to the surface, a six-membered ring 1,3,2-dioxathiolane and a five-membered ring 1,2-oxathietene 2-oxide or ?-sultine, both species being in equilibrium. The reaction was shown to be reversible. The primary product is an episulfide that, after a series of consecutive reactions of atomic sulfur, is able to free sulfur. The reactivity of the intermediates and the episulfide was studied for several reactions and consistent mechanisms were postulated from the X-ray photoelectron spectroscopy and solid state nuclear magnetic resonance spectra. Aminolysis and the reaction with alkyl halides resulted on the insertion of the alkyl moiety in the episulfide primary product, while the thiolysis and photolytic alcoholysis resulted in the insertion of the alkyl moiety in the oxidized intermediates. Copyright (C) 2012 John Wiley & Sons, Ltd.