Determination of CO Adsorption Sites on Gold Clusters Au_<i>n</i>^– (<i>n</i> = 21–25): A Size Region That Bridges the Pyramidal and Core–Shell Structures

We perform a joint photoelectron spectroscopy and theoretical study to investigate CO adsorption sites on midsized gold clusters, Aun– (n = 21–25), a special size region that bridges the highly symmetric pyramidal cluster Au20– (Li et al. Science 2003, 299, 864) and the prevailing core–shell clusters starting from Au26– (Schaefer et al. ACS Nano 2014, 8, 7413). Particular attention is placed on whether the CO binding can significantly change structures of the host clusters in view of the fact that the size-dependent structural change already occurs for bare gold clusters in this size range. A transition from hollow-tubular to fused-planar structures is identified for the AunCO– clusters even though the CO molecule mostly binds to an apex gold atom. The computed CO adsorption energy and HOMO–LUMO gap of the gold clusters suggest that among the five gold clusters the Au23– cluster exhibits the strongest CO binding and thereby could be a good catalytic model system