Sequential adsorption of multiple CO molecules on Au10− and Au9Zn− triangular clusters: the crucial role of a single atomic impurity

Abstract

In this paper, we find, using DFT calculations, that the patterns of sequential adsorption sites of up to six CO molecules on the planar Au₁₀⁻ and Au₉Zn⁻ clusters differ radically. Thus, CO prefers bridge (top) adsorption sites forming Au₁₀(CO)_n⁻ compounds with n = 1–3 (n = 4–5), but top (bridge) sites occur for Au₉Zn(CO)_n⁻ compounds when n = 1–4 (n = 5–6). These facts are distinguishable in the CO stretching spectra of both types of compounds. Severe distortions and broken Au–Au bonds appear in the morphology of pure compounds after CO adsorption, but only small distortions are seen in the doped compounds. Using the nudged elastic band (NEB) method, we find the reaction paths of CO adsorption for pure n = 3 and doped n = 5 compounds. The estimated reaction barrier of the pure compound is much higher than that of the doped one. We also find important differences in the electronic properties of these compounds as functions of the size n: adsorption energy, bond lengths, angles, Bader atomic charges, and HOMO–LUMO gaps. We argue that all these differences are due to a larger charge reorganization in the doped than in the pure gold compounds, because of the Zn atom's lower electronegativity than that of gold and carbon. Comparison with previous results for multiple adsorption of NO molecules sheds light on the characteristics of Au–CO and Au–NO bonds.