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 H2, O2, 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 Ce4+/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 Ce6 node, the Ce3+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.