Potential energy curves along the diffusion paths of NO on the small-sized Cu(100) model cluster

Abstract

Hybrid density functional theory (DFT) calculations have been carried out for the nitric oxide NO molecule on the Cu model cluster in order to shed light on the diffusion mechanism of the NO molecule on the Cu(100) cluster model surface. The metal surface was represented approximately by a finite metal cluster Cu 9. Three binding sites, ‘two-fold site’, ‘four-fold site’, and ‘on-top site’, were considered in the present study. In two-fold site, NO binds to two Cu atoms in the shorter Cu–Cu bond of the surface, whereas NO in four-fold site was bound in the longer Cu–Cu bond and interacts with four Cu atoms. The binding energies of NO were larger in the order, on-top<two-fold<four-fold sites. Also, it was found that on-top site is transition state for the diffusion of NO. The activation energy along the diffusion path from two-fold site to next neighboring two-fold site via on-top site was calculated to be 4.8 kcal/mol, while the diffusion was a symmetry-allowed adiabatic process. On the other hand, the barrier height for the diffusion path from four-fold site to four-fold site was as high as 8.90 kcal/mol, and the diffusion needs non-adiabatic process (symmetry-forbidden). The mechanism of the adsorption of NO on Cu was discussed on the basis of theoretical results.