Density functional study of the chemisorption of O2 on the armchair surface of graphite

The reaction between O2 and the armchair surface of a model graphite molecule has been studied using density functional calculations at the B3LYP/6-31G(d) level of theory. Both equilibrium and transition state geometries were optimized to provide a fundamental understanding of the energetics and kinetics of the chemisorption, desorption, rearrangement, and migration reactions that contribute to carbon gasification. A small barrier of 18 kJ mol−1 was found for the chemisorption reaction, which is 578 kJ mol−1 exothermic overall, producing a stable quinone. A number of reaction pathways with barriers below 578 kJ mol−1 were characterized. Gasification of carbon occurs as CO, with barriers of 296 and 435 kJ mol−1 for the first and second CO loss, respectively. The stable quinone can also undergo a rearrangement reaction to form two ketene groups, with a barrier of 260 kJ mol−1. If the armchair edge is extended to include an adjacent aromatic ring, the oxide can migrate along the surface. This initially forms a furan-like bridge structure, with a barrier of just 89 kJ mol−1. A further barrier of 383 kJ mol−1 leads to CO desorption from the furan. The furan can also rearrange further with a barrier of 212 kJ mol−1 to form a five-membered lactone, the most stable structure identified on the potential energy surface. Rearrangement and migration reactions, which have not generally been incorporated into carbon gasification models, are shown to be potentially important pathways in carbon oxidation reactions.