Surface Precursors and Reaction Mechanisms for the Thermal Reduction of Graphene Basal Surfaces Oxidized by Atomic Oxygen

The reduction of graphene oxide surfaces yielding molecular CO/CO(2) is studied from first principles using density functional theory. We find that this reaction can proceed exothermically only from surface precursors containing more oxygen atoms than strictly needed to produce CO/CO(2) in the gas phase. The calculations show that the lowest-energy configurations of multiple O adsorbates do not involve clustering of epoxy groups (the stable form of O adatoms on graphitic surfaces) but always contain lactone groups either in lactone-ether or in ether-lactone-ether form. We identify these lowest-energy structures as the main reaction precursors. The O adatoms near the lactone group catalyze its gasification to CO/CO(2) by reducing the activation energy from above 1.8 eV (from an isolated lactone) to below 0.6 eV (from a lactone-ether). In addition, the residual O adatoms left behind after the lactone gasification minimize the energy of the graphitic products by saturating the dangling bonds of the resulting defective surface. By analyzing defect-free as well as defective surfaces, we identify a common set of concerted reaction mechanisms in which the formation of the gas products and the saturation of the newly formed C vacancies happen simultaneously. The calculated activation energies are in good agreement with the available experimental data.