Ab initio studies of CN adsorbed on Ni(111)

Abstract

The adsorption of cyanide (CN) on Ni(111) is treated using an ab initio embedding theory. The Ni(111) surface is modeled as a three-layer, 28-atom cluster with the Ni atoms fixed at bulk lattice sites. The present calculations show that CN is able to bind to the surface either via the carbon, or nitrogen, or in a side-on geometry with very small differences in total energy (≊0.1 eV). Adsorption energies at threefold, bridge and atop sites are comparable, with the fcc threefold site more favorable over other adsorption sites by ≊0.1 eV. At the fcc threefold site, adsorption energies and C–N stretching frequencies are 5.0 eV and 2150 cm−1 for the η1-cyanide-N, 4.9 eV and 1970 cm−1 for the η1-cyanide-C, and 4.9 eV and 1840 cm−1 for the η2-cyanide-C,N, respectively. Dipole moment calculations show that the bonding of CN to the Ni surface is largely ionic, while η2-cyanide-C,N has more covalent character. Calculated energy barriers in going from η1-cyanide-C to η2-cyanide-C,N, and from η2-cyanide-C,N to η1-cyanide-N are around 0.1 eV. Thus although CN is strongly bound to the surface (at ≊5 eV), within an energy range of ≊0.2 eV, the molecule is free to rotate to other geometries. During this rotation there are large changes in the dipole moment.