Reactive Site Model of the Reduction of SO₂ on Graphite

Abstract

Computational quantum chemistry calculations were carried out for the reduction of SO2 on graphite to produce elemental sulfur and CO2. Two models of the reactive site of graphite were used and a viable mechanism was proposed for the reaction pathways based on experimental results and known reactions. The SO2 OO approach to the zigzag edge of the model cluster yielded a sulfur-oxidized intermediate 1,3,2-dioxathiolane (1). Sulfur transfer step takes place from 1 to a neighbor benzyne site forming a reduced sulfur intermediate thiirene (2) along with a 1,3-dicarbonyl in equilibrium with the peroxide valence tautomer. The calculated barrier from 1 to thiirene 2 at 900 °C was 39.4, kcal mol–1. The peroxide tautomer isomerizes to a dioxirane intermediate that is eliminated as dioxicarbene to produce CO2. The total free energy of activation for the decarboxylation reaction at 900 °C was in the range 110.8–122.7 kcal mol–1 depending on the model (ΔG ‡ experimental, 114.3 kcal mol–1). The reduced intermediate thiirene 2 decomposes through a transport mechanism where polysulfane species with increasing number of sulfur atoms eleminate elemental S x .

Reaction Mechanism of the Reduction of Ozone on Graphite

“…The first step, that is, chemisorption, is highly exothermic and, consequently, very fast. The rate-determining step of sulfurization is the transfer of the heavy sulfur atom that required Δ G ‡ = 39.4 kcal.mol –1 . We postulated a similar mechanism for ozone, shown in Figure , using 1,2,11,12-tetradehydrogenated-benzo­[α]­anthracene (TBA), as a model for the graphite reactive site, that works as a molecular reactor .…”

“…The rate-determining step of sulfurization is the transfer of the heavy sulfur atom that required Δ G ‡ = 39.4 kcal.mol –1 . We postulated a similar mechanism for ozone, shown in Figure , using 1,2,11,12-tetradehydrogenated-benzo­[α]­anthracene (TBA), as a model for the graphite reactive site, that works as a molecular reactor . The insertion of ozone 1 at 100 °C ( k 1 ) would produce a trioxolane 2 , which, through oxygen transfer, forms an oxirene 3 ( k 2 ) and a [peroxide ↔ dicarbonyl] tautomer.…”

“…The reduction of SO 2 on graphite, at a flow of constant concentration, occurs with the sulfurization of the carbon matrix by the insertion of the reagent SO 2 as dioxathiolane followed by sulfur transfer to form a thiirene along with a peroxide in tautomeric equilibrium with a dicarbonyl (eq 14). 26,55 The rate of insertion of SO 2 reached a plateau when the system entered the steady state. The first step, that is, chemisorption, is highly exothermic and, consequently, very fast.…”

Atomic Observation of Filling Vacancies in Monolayer Transition Metal Sulfides by Chemically Sourced Sulfur Atoms

“…From our atomic-scale observation of the S vacancy healing phenomenon, we report that the S vacancy sites are directly filled by the excess S atoms supplied by the TFSI molecules. Previous theoretical studies suggested that after losing a H + due to its strong acidity, TFSI molecules can dissociate into the ionic compound [CF 3 SO 2 NCF 3 ] − and a reactive SO 2 molecule and also that SO 2 molecules can dissociate into an S atom and CO 2 , where the S atom adsorbs on the graphite surface . On the basis of these previous theoretical predictions, the energy landscape for the S vacancy healing process was explored through DFT calculations.…”

“…Previous theoretical studies suggested that after losing a H + due to its strong acidity, TFSI molecules can dissociate into the ionic compound [CF 3 SO 2 NCF 3 ] − and a reactive SO 2 molecule 48 and also that SO 2 molecules can dissociate into an S atom and CO 2 , where the S atom adsorbs on the graphite surface. 49 On the basis of these previous theoretical predictions, the energy landscape for the S vacancy healing process was explored through DFT calculations. From these calculations, we found that the formation of SO 2 + (CF 3 SO 2 NCF 3 ) is energetically more favorable than the formation of CF 3 SO 2 NSO 2 CF 3 by 0.73 eV, which indicates that the dissociation of an SO 2 molecule from a TFSI anion is plausible (the complete energy landscape is given in Figure S13).…”