Mechanisms of Solid–Gas Reactions: Reduction of Air Pollutants on Carbons

Abstract

Ozone is a strong oxidizer and sulfur dioxide is a precursor to acid rain, both are air pollutants that can damage the respiratory tissues of animals and plants making them hazardous to the environment. They are isoelectronic valence O = X = O (X = S, O) molecules and their mechanism of reduction on carbons is similar. The solid–gas kinetics were studied in a flow system with a tubular reactor under differential and steady state conditions. Initial rates and product distribution were used to determine the stoichiometry of the reaction. The reduction of XO2 on carbons proceeds through a common primary mechanism with oxidized and reduced intermediates. The reactivity of the intermediates that were inserted on carbons (graphite, activated carbon, graphene oxide) modified by SO2 is selective with respect to aminolysis and thiolysis. A theoretical study of the chemisorption of SO2 on dehydrogenated pyrene as graphite active site model showed that at 900 °C the chemisorption occurs mainly on the diradical zigzag edge. Computational quantum chemistry calculations were carried out for the reduction of SO2 on graphite to produce elemental sulfur and CO2 using tetradehydrogenated-benzo[α]anthracene (TBA) as model. The mechanisms of the decarboxylation and sulfur transport steps were postulated. Ozonation of graphite showed that the 1,2,3-trioxolane decomposes to an oxyrene, eliminating O2. Both reactions, the SO2 and O3 with graphite, have the same experimental free energy of activation for the decarboxylation reaction. The results show that for SO2 the desulfurization has a much lower energetic demand than the decarboxylation route raising the important possibility of using the reaction of reduction of SO2 on carbons to reduce the acid rain, producing elemental sulfur as the main product, without increasing the greenhouse effect due to the formation of CO2.