Hydrogen evolution descriptors of 55-atom PtNi nanoclusters and interaction with graphite.

Density functional simulations have been performed for Ptn Ni55-n clusters (n = 0,12,20,28,42,55) to investigate their catalytic properties for the hydrogen evolution reaction (HER). Starting from the icosahedral Pt12 Ni43 , hydrogen adsorption energetics and electronic d-band descriptors indicate HER activity comparable to that of pure Pt55 (distorted reduced core structure). The PtNi clusters accommodate a large number of adsorbed hydrogen before reaching a saturated coverage, corresponding to 3-4 H atoms per icosahedron facet (in total ∼70-80). The differential adsorption free energies are well within the window of |∆GH| < 0.1 eV which is considered optimal for HER. The electronic descriptors show similarities with the platinum d-band, although the uncovered PtNi clusters are magnetic. Increasing hydrogen coverage suppresses magnetism and depletes electron density, resulting in expansion of the PtNi clusters. For a single H atom, the adsorption free energy varies between -0.32 (Pt12 Ni43 ) and -0.59 eV (Pt55 ). The most stable adsorption site is Pt-Pt bridge for Pt-rich compositions and a hollow site surrounded by three Ni for Pt-poor compositions. A hydrogen molecule dissociates spontaneously on the Pt-rich clusters. The above HER activity predictions can be extended to PtNi on carbon support as the interaction with a graphite model structure (w/o vacancy defect) results in minor changes in the cluster properties only. The cluster-surface interaction is the strongest for Pt55 due to its large facing facet and associated van der Waals forces.