Insights into thermal reduction of the oxidized graphite from the electro-oxidation processing of nuclear graphite matrix

Abstract

Electro-oxidation disintegration is a potential technology pathway to detach graphite waste from high-level radioactive wastes in the spent fuel reprocessing of high temperature gas cooled reactors (HTGR). But this electrochemical treatment can introduce oxygen functional groups into the graphite lattice. Further disposal will require the development of safe and cost-effective methods to achieve the reduction and deoxygenation of the oxidized graphite wastes. In general, oxygen species can be removed from oxidized graphite directly in an easy-to-operate way by thermal annealing. In this paper, the thermal reduction process of oxidized graphite from the electro-oxidation processing of graphite matrix from simulative element of HTGR was researched. The thermodynamic state and kinetic process of the deoxygenation reaction of electrolytic graphite oxide (EGO) have also been investigated for better understanding of the reduction mechanism. According to thermal analysis results, the oxygen functional groups were removed mostly between 160 °C and 250 °C, and the main weight loss of EGO during thermal annealing was caused by evolved volatile gases (CO₂, CO and H₂O). The activation energy of deoxygenation reaction for EGO is calculated to be 115.6 kJ mol⁻¹ (1.20 eV per atom). And the persistent residual oxygen species on reduced EGO (REGO) is determined to be from two parts, i.e. the stubborn built-in oxygen species of pristine nuclear graphite and the newly formed C–O moieties on the basal plane of graphite sheet during thermal annealing process. In addition, an attempt was also made to elucidate how the thermal treatment acted on the electronic structure of EGO. Meanwhile, the mechanism of dissociation of oxygen species (–OH, C–O–C, CO, –COOH) on the thermal reduction of oxidized graphite has been suggested in this paper.