Reaction Mechanism of the Reduction of Ozone on Graphite

Abstract

The kinetics of the ozonation of graphite with different particle sizes (106 μm, G 106 ; 6.20 μm, G 6.2 ) was studied at several temperatures under a flow of O 3 diluted in O 2 . The reaction was first-order with respect to graphite and to the consumption of ozone. X-ray photoelectron spectrum (XPS) showed that the reactions occurring in the solid under steady-state conditions maintain the original stoichiometry, as predicted by the postulated mechanism for SO 2 . The deoxygenation reaction occurred along with the ozonation reaction at 100 °C. The rate of oxygen elimination in the flow system has the same rate-determining kinetic barrier as ozone insertion. Ozonation and deoxygenation reactions are sequentially related. Ozonation occurs with the insertion of O 3 , forming a 1,2,3-trioxolane followed by an oxygen transfer that produces a peroxide valence tautomer in equilibrium with 1,3-dicarbonyl, [peroxide ↔ dicarbonyl], and an oxirene that eliminates atomic oxygen. The decarboxylation reaction was studied at 600 °C from the ozonated G 106 (ΔG ≠ = 83.60 ± 0.08 kcal•mol −1 ). Total decarboxylation at 600 °C matched the number of moles of CO 2 removed and the oxygen content after ozonation, showing that the reduction of ozone on graphite was essentially a clean reduction with no secondary oxidations. When ozonized graphite was heated to 600 °C, only [peroxide ↔ dicarbonyl] species remained in the matrix. The peroxide tautomer isomerized to dioxirane and eliminated CO 2 as a dioxicarbene. Total deoxygenation of decarboxylated graphite G 106 was obtained by pyrolysis. There was residual oxygen that arose from the atomic oxygen eliminated from the oxirene, intercalated in graphite layers, and formed basal epoxy groups. Also, incoming O atoms reacted with the intercalated O atoms to produce O 2 molecules. Thermal annealing deintercalated molecular oxygen (600−900 °C).

Intramolecular Amino-thiolysis Cyclization of Graphene Oxide Modified with Sulfur Dioxide: XPS and Solid-State NMR Studies

“…There was a balance of oxygen between the number of moles of inserted ozone and the moles of the oxygen eliminated from total decarboxylation. Consequently, the activation parameters at 700 °C of the decarboxylation reaction of graphite after SO 2 reduction ( ΔG ‡ = 96.1 kcal·mol –1 ) were the same as the values obtained after ozone reduction ( ΔG ‡ = 95.7 kcal·mol –1 ) …”

“…35 The TBA model was also used successfully for the mechanism of reduction of ozone on graphite (Figure 2). 36 Ozone is isoelectronic with sulfur dioxide and both reactions have the same stoichiometry (eq 1)…”

“…Consequently, the activation parameters at 700 °C of the decarboxylation reaction of graphite after SO 2 reduction (ΔG ‡ = 96.1 kcal• mol −1 ) 31 were the same as the values obtained after ozone reduction (ΔG ‡ = 95.7 kcal•mol −1 ). 36 Sulfur and oxygen transport route out the carbon matrix is initiated from tautomers 3 by extrusion of the element from the thiirene or the oxirene (Figure 2). The nonoxidized sulfur intermediate is an unstable thiirene.…”

“…41 Atomic oxygen from the oxirene, produced during the ozonation of graphite, is eliminated as molecular oxygen or is intercalated in the graphite matrix (Figure 4). 36 Intercalated oxygen can react with new incoming atoms to produce O 2 molecules or can form epoxy groups with adjacent C atoms.…”

Ozone uptake by commercial brake pads and brake pad components: assessing the potential indirect air quality impacts of non-exhaust emissions

“…These multiple reactivity contributions indicate that ozone uptake by brake pads likely occurs via multiple mechanisms, such as addition to double bonds (unsaturated organics), 25,26 decomposition at active sites (hematite; a-Fe 2 O 3 ), 24 reaction at unsaturated sites (graphite), 30,31 and redox reactions (metals). 32,33 Similarly, multiple plausible mechanisms exist for photoenhanced ozone uptake by brake pads, including energy transfer from photoexcited phenolic resin to molecular oxygen, with subsequent reaction of singlet oxygen with ozone, 26,34 and reduction of ozone by photogenerated electrons produced by semiconducting iron oxides.…”