Competing, coverage-dependent decomposition pathways for C₂H _y species on nickel (111)

Competing, coverage-dependent pathways for ethane (CH₃CH ₃) decomposition on Ni(111) are proposed on the basis of quantum mechanics (QM) calculations, performed by using the PBE flavor of density functional theory (DFT), for all C₂H_y species adsorbed to a periodically infinite Ni(111) surface. For CH₂CH₃, CHCH₃, and CCH₃, we find that the surface C is tetrahedral in each case, with the surface C forming bonds to one, two, or three Ni atoms with bond energies scaling nearly linearly (E_bond = 32.5, 82.7, and 130.8 kcal/mol, respectively). In each of the remaining six C₂H _y species, both C atoms are able to form bonds to the surface. Three of these (CH₂CH₂, CHCH₂, and CCH₂) adsorb most favorably at a fcc-top site with the methylene C located at an on-top site and the other C at an adjacent fcc site. The bond energies for these species are E_bond = 19.7, 63.2, and 93.6 kcal/mol, respectively. The remaining species (CHCH, CCH, and C₂) all prefer binding at fcc-hcp sites, where the C atoms sit in a pair of adjacent fcc and hcp sites, with binding energies of E_bond = 57.7, 120.4, and 162.8 kcal/mol, respectively. We find that CHCH_ad is the most stable surface species (δH_eth = -18.6), and an important intermediate along the lowest-energy decomposition pathway for ethane on Ni(111). The second most stable species, CCH₃, is a close competitor (δH_eth = -18.2 kcal/mol), lying along an alternative decomposition pathway that is preferred for high-surface-coverage conditions. The existence of these competing, low- and high-coverage decomposition pathways is consistent with the experiments. The QM results reported here were used as training data in the development of the ReaxFF reactive force field describing hydrocarbon reactions on nickel surfaces [Mueller, J. E.; van Duin, A: C. T.; Goddard, W. A. J. Phys. Chem. C 2010, 114, 4939-4949]. This has enabled Reactive dynamics studying the chemisorption and decomposition of systems far too complex for quantum mechanics. Thus we reported recently, the chemisorption and decomposition of six different hydrocarbon species on a Ni₄₆₈ nanoparticle catalysts using this ReaxFF description [Mueller, J. E.; van Duin, A: C. T.; Goddard, W. A. J. Phys. Chem. C 2010, 114, 5675-5685].