¹⁹⁵Pt NMR of Platinum Electrocatalysts:  Friedel−Heine Invariance and Correlations between Platinum Knight Shifts, Healing Length, and Adsorbate Electronegativity

The influence of a series of different ligands (H, O, S, CN -, CO, and Ru), electrochemically adsorbed onto carbon-supported nanoscale Pt particles, has been investigated by 195 Pt NMR, in an electrochemical environment. The NMR Knight shifts from atoms in the interior of the particles are invariant to surface ligand adsorption, as expected by the Friedel−Heine invariance theorem. However, the shifts of the surface and subsurface atoms vary over a ∼11000 ppm range for the adsorbates investigated. The Knight shifts of the surface and subsurface platinum atoms, obtained by using an NMR layer-model simulation, linearly map the electronegativity of the adsorbate. These observations permit the derivation of a correlation between the “healing length” of the Pt metal, a parameter indicating how fast the bulk metal properties are recovered when moving in from the surface, and the electronegativity of the adsorbate, providing a new probe of electrocatalyst and heterogeneous catalyst surface electronic structure.