Kinetics of oxygen transfer reactions on the graphene surface. Part ii. H₂o vs. Co₂

This computational chemistry analysis compares the interactions of H₂O and CO₂ with zigzag graphene edges, and in particular their adsorption and desorption steps. We provide detailed information regarding the geometric and electronic factors that influence both their adsorption and desorption (of H₂ and CO) processes. Density functional theory results are compared with experimental information to offer heretofore unavailable insights into key aspects of the rate-limiting steps, inhibition phenomena and nanoscale differences in these two reactions. Thus, for example, the orientation and rotation of the adsorbing H₂O molecules are elucidated using intrinsic reaction coordinate calculations and molecular orbital analysis, and these results complement recent pulse field gradient NMR measurements and molecular dynamics calculations. Such mechanistic findings reveal the H₂O adsorption process to be a slow rotational phenomenon whereas the low-temperature H₂ desorption is geometrically constrained by the presence of contiguous (di)hydrogenated carbon atoms with or without hydroxyl groups. This surface-assisted desorption mechanism is proposed to be responsible for the formation of hexagonal pits during the graphite-H₂O reaction. Similar insights were obtained for the graphene-CO₂ reaction. Mechanistic schemes are proposed for the desorption products observed in experiments, distinguishing zigzag from armchair sites.