Graphitic oxygenation reactions

The first indications of surface oxide formation were obtained in the course of combustion studies. Bonnetain et al. (141) and Bonnetain (142) studied the kinetics of the graphite-oxygen reaction and concluded that oxygen was intermediately bonded to the periphery of the carbon layers. 

For a particular gas-carbon reaction. Equation (39), with one reservation, leads to the conclusion that under identical reaction conditions (i.e., Cg, Dfree, and S are constant), the rate of reaction in Zone III is independent of the type of carbon reacted. The reservation is that in the carbon-oxygen reaction, the nature of the carbon may affect the CO-CO2 ratio leaving the surface and hence the reaction rate per unit of oxygen diffusing to the surface. Unfortunately, little data are available on reactivities of different carbons where the reaction has been conducted completely in Zorn III. Day (2Ii) reports that the reaction rates of petroleum coke, graphitized lampblack, and graphitized anthracite rods agree within 12 % at a temperature of 1827° and at a constant gas velocity. 

The reactivity of the molecular fullerene solid resembles the expected pattern for a homogeneous material. Only a small prereactivity at 700 K indicates that a fullcrcne-oxygen complex [12] is formed as an intermediate stoichiometric compound [15, 105], At 723 K the formation of this compound and the complete oxidation are in a steady state [12, 106, 107] with the consequence of a stable rate of oxidation which is nearly independent of the bum-off of the fullerene solid. This solid transforms prior to oxidation into a disordered polymeric material. The process is an example of the alternative reaction scenario sketched above for the graphite oxidation reaction. The simultaneous oxidation of many individual fullerene molecules. leaving behind open cages with radical centers, is the reason for the polymerization. 

Magne, P. and Duval, X., Comparaison des effets catalytiques de divers metaux dans les reactions graphite-oxygene et graphite-protoxyde d azote, Bull.Soc.Chim.France A5, 1593-7 (1971). 

For a given reaction, AG° and AH° would need to be calculated from standard formation values (graphite, oxygen, and carbon dioxide) first, before plugging into the equation. Also, AS° would need to be calculated from standard entropy values. 

The classical anode material was graphite. It is slowly oxidized to carbon dioxide, due to the formation of small amounts of oxygen (reaction 3), and the anode loses material. Thus, electrode gap and ohmic voltage losses increase, or anodes have to be mechanically readjusted. 

Figure 5.6. Diagram indicating how the rate of the graphite-molecular oxygen reaction changes with increasing reaction temperature, 1300-2300 K (Lewis (1970)).

Marsh H, O Hair TE, Wynne-Jones Lord, The carbon-atomic oxygen reaction - surface-oxide formation on para-crystalline carbon and graphite. Carbon 1969 7(5) 555-558. 

The oxygen contribution from these reactions is dependent on the nature of the anode material and the pH of the medium. The current efficiency for oxygen is generally 1—3% using commercial metal anodes. If graphite anodes are used, another overall reaction leading to inefficiency is the oxidation of... 

Any sihcate that forms thermally and chemically stable residual compounds as its oxygen content is reduced provides a suitable source of siUcon for this reaction. A typical process consists of alternating aluminum, siUca, and graphite plates separated by 2—4-cm thick graphite spacers stacked in a graphite-lined alumina tube and heated to 1400°C for 12 h in a nitrogen atmosphere. After cooling for approximately 6 h the fibers are removed. 

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