Ab initio calculations for the adsorption of small molecules on metal oxide surfaces. I. Cluster calculations for carbon monoxide CO on nickel oxide NiO(100)

Abstract

Quantum chemical ab initio calculations for the adsorption of CO on the NiO(100) surface have been performed at different levels of accuracy: self-consistent field (SCF), complete active space self-consistent field, and coupled electron pair approximation. Basis sets of double zeta and triple zeta + polarization (TZP) quality have been used. The NiO(100) surface is represented by a cluster containing one Ni2+ ion and the five adjacent O2− ions. The charge neutrality of the cluster and the saturation of the dangling bonds is achieved by adding eight protons, which gives the total composition Ni(H2O)3(OH)2. Alternatively, the Ni2+(O2−)5 unit is embedded in a lattice of point charges which correctly represent the half-infinite ionic crystal. In the most favorable configuration, CO is adsorbed linearly in the on-top position on the Ni2+ ion, with the C atom pointing toward the surface. The binding energies at the SCF level are rather small, only 0.08 eV and 0.03 eV for CO and OC bound to the cluster (TZP basis set, counterpoise correction for the basis set superposition error included). For the two configurations the equilibrium Ni–C and Ni–O distances are 5.40 and 5.50 a0, respectively. Electron correlation does not change these values markedly. Estimating the errors in our calculation we arrive at binding energies of 0.10±0.05 and 0.05±0.05 eV, respectively, for CO and OC on NiO(100). This is in agreement with the experimental estimates. The bonding is attributed predominantly to electrostatic and inductive forces. No genuine ‘‘chemical’’ bond (overlap, charge transfer) exists between CO and the NiO(100) surface, i.e., CO is only physisorbed to this ionic surface. The harmonic vibration frequencies for the Ni–C stretching and the Ni–C–O bending vibrations are estimated to 52 and 139 cm−1, respectively.