Isopotential Electron Titration: Hydrogen Metal-Adsorbate Charge Transfer

The extent of charge transfer between an adsorbate and thermocatalytic surface plays a key role in determining binding energy and catalytic activity, but direct and quantitative measures have remained elusive. Here, we report the method of isopotential electron titration (IET), an approach that directly measures charge transfer between adsorbates and catalytic surfaces. Charge transfer between Pt and adsorbed hydrogen adatoms was investigated using a catalytic condenser architecture, where the Pt surface was separated from a p-type silicon layer by a hafnia dielectric film. By forcing the Pt and Si layers into isopotential conditions, excess charge resulting from transfer between adsorbate and Pt surface was titrated through an external circuit. Hydrogen atoms donated electrons to Pt upon adsorption, adopting a partial positive charge on the surface, which was quantitatively reversed upon their thermal desorption from the surface. Across a temperature range of 125-200 °C and hydrogen partial pressures of 0.005 - 1 atm, consistent with a surface hydrogen fractional coverage of 80-100%, the charge transferred to Pt by an adsorbed H atom was measured to be 1.89 ± 0.05 mmol e-mol Pt^-1. Bader charge analysis of the extent of charge transfer was comparable to experimental measurements, with a calculated net donation of 4 mmol e^- mol-Pt^-1 by adsorbed hydrogen. The ability to experimentally quantify surface charge transfer events provides an electronic-based approach to understand and characterize catalytic surfaces, the adsorbed moieties residing on them, and the chemical reactions they accelerate.