Binding energies of CO on gold cluster cations Aun+ (n=1–65): A radiative association kinetics study

Abstract

Room temperature CO adsorption on isolated gold cluster cations is studied over a wide size range $({\mathrm{Au}}_n^{+},1<n<66)$ in a Fourier transform ion cyclotron resonance mass spectrometer. Kinetic data which yield absolute bimolecular rate constants are analyzed by means of a radiative association model to give adsorption energies for the first CO molecule binding to the gold cluster ions. The results show that the binding energies decrease with increasing cluster size from about $1.09\pm 0.1\mathrm{eV}$ $(n=6)$ to below $0.65\pm 0.1\mathrm{eV}$ (for $n>26$ ), with notable exceptions at $n=30$ , 31 and 48, 49 which manifest local binding energy maxima. For the smallest sizes ($3<n<9$ and $n=20$ ), the experimental $\mathrm{C}\mathrm{O}–{\mathrm{Au}}_n^{+}$ binding energies are compared with those found for the most stable isomers by density functional theory based ab initio computations. While the overall size trend can be understood in terms of classical electrostatics, the atom by atom variations are likely due to differently coordinated binding sites available on the gold clusters.