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Van Marum Colloquium: Proton-coupled electron transfer at interfaces: the importance of non-ideal isotherms

Tuesday 14 May 2024
Gorlaeus Building
Einsteinweg 55
2333 CC Leiden


Hydrogen is a ubiquitous component of our environment, of most interfaces, and of many catalytic processes. A hydrogen atom is a proton and an electron, so the thermochemistry of H transfer and coupled 1H+/1e– transfer (PCET) are the same (± a constant). The electrochemical potential of the Volmer reaction (eEVolmer) is thus equivalent to the surface–H bond dissociation free energy (BDFE; top image below). Using both electrochemical and equilibration methods, our laboratory has extended these long-known principles to oxide and other interfaces, including NiO thin film electrodes, colloidal IrO2 and CeO2-x nanoparticles (schematic below), high surface area CoP, and Au nanocrystals. These measurements show that most materials do not have a single surface–H BDFE but rather a range of values. The width of these distributions, fit with a Frumkin isotherm, vary from a few tenths of an eV to > 0.5 eV. By poising the material at different redox levels, rate constants for simple reactions have been measured for surface–H bonds in different portions of the distribution. In some cases, these k’s vary substantially with coverage. In addition, different materials have different values of the Brønsted α (equivalent to the electrochemical symmetry factor β). While the origins of the variations are not yet understood, it is clear that non-idealities and surface coverages are important to catalysis and other processes. The frequent assumptions of ideal behavior of adsorbates (Langmuir isotherms) and α or β ≅ 0.5 can be poor approximations of the real system, especially for nanoscale and/or binary materials.

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