Hydrogen-directed Au-H-Au chain networks redefine the active structure of sub-2 nm gold nanoparticles

Under H₂, sub-2 nm Au nanoparticles are highly fluxional and evade static coordination-based descriptions. Using DFT and 300 K ab initio molecular dynamics, we track hydrogenation of icosahedral Au₁₄₇ and truncated-octahedral Au₂₀₁ from ideal adsorption motifs to thermally reconstructed, dynamically sampled structures. Near-monolayer hydrogen acts as a structure-directing reactant, reorganizing the outer shell into contracted Au-H-Au chain networks while sustaining a fluctuating population of adsorbate-induced unusual low-coordination Au sites. Electronic descriptors reconcile hydride-like polarity with metallic character: hydrogen becomes negatively charged whereas gold is positively polarized, yet the density of states remains finite at the Fermi level. ELF connectivity and saddle-point analysis of the hydrogenated gold nanoparticles reveal the emergence of a delocalized, heterogeneous electronic network in the reconstructed state, able to redistribute charge and mediate hydrogen transfer. Calculated IR features at 2000-2160, 1780, and 1630-1680 cm^-1 rationalize debated operando band assignments. In ethene hydrogenation, Au-H-Au chain ensembles weaken the driving force for successive hydrogen additions by disrupting stabilizing bridging motifs, providing a mechanistic basis for the high selectivity of ultrasmall Au catalysts.