Platinum Nanoparticle During Electrochemical Hydrogen Evolution: Adsorbate Distribution, Active Reaction Species, and Size Effect

For small Pt nanoparticles (NPs), catalytic activity is, as observed, adversely affected by size in the 1–3 nm range. We elucidate, via first-principles-based thermodynamics, the operation H* distribution and cyclic voltammetry (CV) during the hydrogen evolution reaction (HER) across the electrochemical potential, including the underpotential region (U ≤ 0) that is difficult to assess in experiment. We consider multiple adsorption sites on a 1 nm Pt NP model and show that the characteristic CV peaks from different H* species correspond well to experiment. We next quantify the activity contribution from each H* species to explain the adverse effect of size. From the resolved CV peaks at the standard hydrogen electrode potential (U = 0), we first deduce that the active species for the HER are the partially covered (100)-facet bridge sites and the (111)-facet hollow sites. Upon evaluation of the reaction barriers at operation H* distribution and microkinetic modeling of the exchange current, we find that the nearest-neighbor (100)-facet bridge site pairs have the lowest activation energy and contribute to ∼75% of the NP activity. Edge bridge sites (fully covered by H*) per se are not active; however, they react with neighboring (100)-facet H* to account for ∼18% of the activity, whereas (111)-facet hollow sites contribute little. Extrapolating the relative contributions to larger NPs in which the ratio of facet-to-edge sites increases, we show that the adverse size effect of Pt NP HER activity kicks in for sizes below 2 nm.