Structures, energetics, and reaction barriers for CH_x bound to the nickel (111) surface

To provide a basis for understanding and improving such reactive processes on nickel surfaces as the catalytic steam reforming of hydrocarbons, the decomposition of hydrocarbons at fuel cell anodes, and the growth of carbon nanotubes, we report quantum mechanics calculations (PBE flavor of density functional theory, DFT) of the structures, binding energies, and reaction barriers for all CH_x species on the Ni(111) surface using periodically infinite slabs. We find that all CH_x species prefer binding to μ3 (3-fold) sites leading to bond energies ranging from 42.7 kcal/mol for CH₃ to 148.9 kcal/mol for CH (the number of Ni-C bonds is not well-defined). We find reaction barriers of 18.3 kcal/mol for CH _3,ad → CH_2,ad + H_ad (with ΔE ) +1.3 kcal/ mol), 8.2 kcal/mol for CH_2,ad → CH_ad + H _ad (with ΔE ) -10.2 kcal/mol) and 32.3 kcal/mol for CH _ad → C_ad + H_ad (with ΔE ) 11.6 kcal/mol). Thus, CH_ad is the stable form of CH_x on the surface. These results are in good agreement with the experimental data for the thermodynamic stability of small hydrocarbon species following dissociation of methane on Ni(111) and with the intermediates isolated during the reverse methanation process.