Reaction Mechanism of the Reduction of Ozone on Graphite.

The kinetics of the ozonation of graphite with different particle sizes (106 μm, G106; 6.20 μm, G6.2) was studied at several temperatures under a flow of O3 diluted in O2. 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 SO2. 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 O3, 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 G106 (ΔG≠ = 83.60 ± 0.08 kcal·mol-1). Total decarboxylation at 600 °C matched the number of moles of CO2 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 CO2 as a dioxicarbene. Total deoxygenation of decarboxylated graphite G106 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 O2 molecules. Thermal annealing deintercalated molecular oxygen (600-900 °C).