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 [Au₂₅(SR)₁₈]^<i>q</i> clusters and mono-atom-doped bimetallic [M₁Au₂₄(SR)₁₈]^<i>q</i> 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 1s electron to the superatomic free-electron count. This opposite behavior compared to that of the hydride in Cu and Ag clusters allows the small hydrogen to interstitially dope the gold clusters 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