Hydrogen Atom Binding Energy of Structurally Well-Defined Cerium Oxide Nodes at the Metal–Organic Framework–Liquid Interfaces

Abstract

Redox-active metal oxides are prevalent in the fields of thermal, photo-, and electrocatalysis. Thermodynamics of proton-coupled electron transfer (PCET) reactions at their surfaces are critical, as they scale with their activity as a catalyst. The free energy of H atom binding on the catalyst surface is employed as a catalytic descriptor for reactions of H 2, O 2, and many others. The structural heterogeneity and ambiguity of surface sites have largely precluded structural understanding of the exact redox-active sites, challenging chemists to design the catalyst structure down to the atomic level. Here, we report electrochemically determined stoichiometry and thermodynamics of PCET reactions of the cerium-based metal–organic framework (MOF), Ce-MOF-808. Cyclic voltammograms (CVs) of the MOF-deposited electrodes in aqueous buffers at various pHs revealed a Faradaic couple that can be ascribed to Ce 4+/3+ redox. Plotting the half-wave potential ( E 1/2 ) against the electrolyte pH resulted in a Pourbaix diagram with a slope of 65 ± 9 mV/pH, suggesting a 1H + /1e – stoichiometry. Using the thermochemical analogy between 1H + /1e – and one H atom (H•), the H atom binding energy on the hexanuclear Ce 6 node, the Ce 3+ O–H bond dissociation free energy (BDFE), was calculated to be 78 ± 2 kcal mol –1 . In-silico calculations quantitatively corroborated our BDFE measurements. Furthermore, multiple proton topologies were computationally elucidated to exhibit BDFEs similar to the experimental values, agreeing with the wide Faradaic features of all CVs, implicating that the system has a substantial BDFE distribution. To the best of our understanding, this is the first thermochemical measurement of H atom binding at the MOF-liquid interface. Implications of the presented thermochemical measurements for catalysis using metal oxides and MOFs are discussed.

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