Density functional study of the reaction of O2 with a single site on the zigzag edge of graphene

The reaction between molecular oxygen and an isolated zigzag graphene edge has been studied using density functional theory at the B3LYP/6-31G(d) level of theory. The initial reaction forms a peroxide, ΔH = −135 kJ mol−1. If the graphene edge is pre-oxidised, the dangling peroxy atom can (Ea = 91 kJ mol−1) migrate across contiguous ketone groups until finding another vacant site and stabilizing as a ketone. However, if no further vacant sites are available, the peroxy oxygen has a number of other possibilities open to it, including desorption of an oxygen atom (Ea = 140 kJ mol−1), migration via the basal plane to form a lactone (Ea = 147 kJ mol−1), and direct interaction with an adjacent oxide to form the lactone or a carbonate (Ea = 146 kJ mol−1). The combination of thermal energy and the heat released in the initial formation of the peroxy adduct is likely to be sufficient to overcome these secondary barriers at modest temperatures.Transfer of the dangling peroxy O to the basal plane produces an epoxide that is mobile on the basal surface (Ea = 40–80 kJ mol−1) but that is transferred back to the edge upon coming into proximity of either a vacant edge site or ketone. The instability of the edge epoxide structure implies that it cannot play a significant role in carbon gasification through promoting the reactivity of ketones, contrary to earlier suggestions.The desorption of an oxygen atom creates a very active species capable of reacting with basal or edge sites as well as with oxygen complexes. The reaction of ketone + O has been reported to yield a five-membered ring + CO2, leading to an overall stoichiometry which is consistent with the observed oxyreactivity of carbon surface oxides identified in isotopic labelling studies in which one O atom is gasified and the other forms a new surface oxide.