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The electrochemical oxidation and combustion of carbon. Turnbull, John Douglas Shand

Abstract

The oxidation and combustion of graphite in a lead-borosilicate slag of the composition PbO.SiO₂.0.1Na₂B₄O₇ were studied with and without an applied potential. The reaction was followed by the analysis of the desorbed anode gas. The CO/CO₂ ratio of the anode gas in any one experiment was found to increase linearly with time in all cases. This increase was thought to be related to the decreasing oxygen-ion concentration. The CO/CO₂ ratio was found to increase with temperature and to decrease with increasing current density. The apparent difference in activation energies for the desorption of CO and CO₂ without an applied potential was found to be 32 ± 6 kilocalories. This is considerably higher than the values (8 to 17 kilocalories) reported for the gaseous reaction. A theoretical explanation for the increased production of CO₂ in chemical and electrochemical combustion over that observed in gaseous combustion was advanced. This explanation extended the desorption model of Long and Sykes to the slag-graphite reaction. This extended model explains why the CO/CO₂ ratio increases with time in chemical combustion, but does not predict this observed effect for the electrochemical reaction. The rate of oxygen removal from the slag during the chemical reaction was calculated from the rate of evolution and the composition of the desorbed gas. The activation energy is 26 ± 5 kilocalories. Absolute reaction rate calculations were made for rate-controlling steps of immobile adsorption, mobile adsorption, chemical reaction, and desorption. The calculated rates were at least a factor of 10⁵ different from the observed rate.

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