The thermodynamics of hydrogen atom adsorption on metal oxides are a fundamental parameter in defining their band energies and their (electro-/photo-)catalytic properties. Recent experimental and computational findings have eluded the proton-coupled electron transfer (PCET) thermochemistry of redox active metal/metal oxides to be dependent on the surface coverage of H-atoms. The studies to be presented probe how the average and the distribution of O-H bond dissociation free energies (BDFEs) of nickel oxide are affected by the surrounding media. For instance, pH/pKa-dependent electrochemical measurements of nickel oxide revealed that the average BDFEs of O-H bonds involved in its redox reactions are independent whether the surrounding solvent is an aqueous or an acetonitrile-based electrolyte, and what buffer was used in acetonitrile. Spectroelectrochemical measurements probing the H-atom adsorption/desorption on the electrode surface suggest either an interaction between the adsorbates or distribution of O-H BDFEs, so that Frumkin isotherms more accurately model the surface population of adsorbed H-atom rather than the commonly assumed Langmuir isotherm. Remarkably, the Frumkin interaction parameters, 6±1 kcal/mol or 0.26 eV, were also medium independent. Thus, the observed distribution in O-H BDFE is an inherent property of the nickel oxide electrode film. Implications of the observed BDFE distribution on its photo- or electrocatalytic properties will be discussed.

Hydrogen-atom binding to nickel oxide electrodes and its catalytic implications

ACS Spring 2022

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