CO Adsorption on Noble Metal Clusters: Local Environment Effects

We have used ab initio density functional theory calculations to study the adsorption of CO on clusters of 13 noble metal atoms. The cluster composition ranges from 100% Pt to 100% Au. Because our goal is to study the effect of local environment on CO adsorption, adsorption is only studied on the top atom site. This atom can be either Pt or Au, depending on the cluster considered. Results are analyzed in terms of CO adsorption energy, CO bond stretching frequency, geometry of the CO + cluster system, and HOMO-LUMO gaps. It is found that, as expected, the CO adsorption energy on Pt is > 1 eV more favorable than that on Au and that the cluster composition affects both adsorption energy and stretching frequency. Specifically, when CO adsorbs on Pt, increasing Au content decreases the adsorption energy. In contrast, when CO adsorbs on Au, increasing Pt content increases the adsorption energy. In general, higher adsorption energies lead to lower C-O stretching frequencies. Electronic-structure details (i.e., density of states) are discussed to explain the observed results, toward improving the interpretation of experimental spectroscopic data and possibly designing new catalysts.