Metallic Hydrogen in Atomically Precise Gold Nanoclusters

Hydrogen-metal interaction is the foundation of many technologies and processes, but how hydrogen behaves in atomically precise gold nanoclusters remains unknown even though they have been used in hydrogenation catalysis and water splitting. Herein, we investigate how hydrogen interacts with [Au-25(SR)(18)](q) dusters and mono-atom-doped bimetallic [M1Au24(SR)(18)](q) clusters (M = Pt, Pd, Ag, Cu, Hg, or Cd) from first principles. We find that hydrogen behaves as a metal in these clusters and contributes its is electron to the superatomic free-electron count. This opposite behavior compared to that of the hydride in Cu and Ag dusters allows the small hydrogen to interstitially dope the gold dusters and tune their superatomic electronic structure. The doping energetics shows that when an eight-electron superatom is formed after H doping, the binding energy of H is much stronger, while binding of H with an already eight-electron superatom is much weaker. Indeed, frontier orbitals and the HOMO-LUMO gaps of [Au25H1(SR)(18)](0), [Au25H2(SR)(18)](+) [PtAu24H2(SR)(18)](0), [PdAu24H2(SR)(18)](0), [AgAu24H(SR)(18)](0), and [CuAu24H(SR)(18)](0) all have very similar features, because they are all eight-electron superatoms. By calculating the Gibbs free energies of hydrogen adsorption, we predict that PtAu24(SR)(18), PdAu24(SR)(18), and center-doped CuAu24(SR)(18) can be good electrocatalysts for the hydrogen evolution reaction.