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 ¹⁹⁵Pt 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.