DFT and kinetics study of O/O2 mixtures reacting over a graphite (0001) basal surface

Abstract

Spin-polarised density functional calculations were used to investigate the interaction of atomic and molecular oxygen on the basal graphite surface at several atomic coverages. Two carbon layers were enough to represent the graphite surface. Oxygen atoms bind mainly over C-C bridge sites forming an epoxide-like structure with a two carbon puckering and with adsorption energies in the 0.95-1.28 eV range, depending on the atomic coverage. Molecular oxygen only shows a very weak physisorption. Atomic adsorption and diffusion along with atomic recombination via Eley-Rideal and Langmuir-Hinshelwood mechanisms were studied. All surfaces processes were activated with energy barriers that decreased for lower atomic coverages. Relaxation effects were non-negligible. A microkinetic model with six elementary surface processes was proposed to see the overall behavior of several initial O/O2 mixtures flowing over a graphite surface at 300-1000 K. Thermal rate constants were derived from Density Functional Theory (DFT) data and standard Transition State Theory. A very low steady-state atomic coverage ( < 0.5 %) was predicted and also very low atomic recombination coefficients were observed (gamma_O < 5x10-4). The Eley-Rideal together with the adsorption and desorption processes were much more important than the Langmuir-Hinshelwood reaction.