Free Energies of Proton-Coupled Electron Transfer Reagents and Their Applications.

We present an update and revision to our 2010 review on the topic of proton-coupled electron transfer (PCET) reagent thermochemistry. Over the past decade, the data and thermochemical formalisms presented in that review have been of value to multiple fields. Concurrently, there have been advances in the thermochemical cycles and experimental methods used to measure these values. This Review (i) summarizes those advancements, (ii) corrects systematic errors in our prior review that shifted many of the absolute values in the tabulated data, (iii) provides updated tables of thermochemical values, and (iv) discusses new conclusions and opportunities from the assembled data and associated techniques. We advocate for updated thermochemical cycles that provide greater clarity and reduce experimental barriers to the calculation and measurement of Gibbs free energies for the conversion of X to XHn in PCET reactions. In particular, we demonstrate the utility and generality of reporting potentials of hydrogenation, E°(V vs H2), in almost any solvent and how these values are connected to more widely reported bond dissociation free energies (BDFEs). The tabulated data demonstrate that E°(V vs H2) and BDFEs are generally insensitive to the nature of the solvent and, in some cases, even to the phase (gas versus solution). This Review also presents introductions to several emerging fields in PCET thermochemistry to give readers windows into the diversity of research being performed. Some of the next frontiers in this rapidly growing field are coordination-induced bond weakening, PCET in novel solvent environments, and reactions at material interfaces.

Coverage-Dependent Rate-Driving Force Relationships: Hydrogen Transfer from Cerium Oxide Nanoparticle Colloids.

Rate-driving force relationships, known as Brønsted-Evans-Polanyi (BEP) relations, are central to many methods for predicting the performance of heterogeneous catalysts and electrocatalysts. Methods such as Tafel plots and "volcano" analyses often assume that the effect of adsorbate coverage on reaction rates across different materials is constant and known. Here, we use UV-visible spectroscopy to test these assumptions by measuring rates of net hydrogen atom transfer from colloidal cerium oxide nanoparticles (nanoceria) to organic reagents at varying surface CeO-H bond strengths and surface coverages. The resulting rate constants follow a linear BEP relationship, Δlog(k) = αΔlog(Keq), across two sizes of nanoceria, two organic reagents, and a ∼10 kcal mol-1 range of CeO-H bond strengths. Interestingly, the Brønsted slope is only 0.2, demonstrating that the rate constants are far less sensitive to CeO-H bond strength than would commonly be assumed for a heterogeneous nanomaterial. Furthermore, we observe a Brønsted slope >1 when altering the reaction driving force via the organic reagent bond strength instead of that of CeO-H. The implications of these Brønsted slopes for either concerted or stepwise mechanisms are discussed. To our knowledge, these are the first solution-phase measurements of BEP relationships for hydrogen coverage on a (nano)material.

Nanoparticle O-H Bond Dissociation Free Energies from Equilibrium Measurements of Cerium Oxide Colloids.

A novel equilibrium strategy for measuring the hydrogen atom affinity of colloidal metal oxide nanoparticles is presented. Reactions between oleate-capped cerium oxide nanoparticle colloids (nanoceria) and organic proton-coupled electron transfer (PCET) reagents are used as a model system. Nanoceria redox changes, or hydrogen loadings, and overall reaction stoichiometries were followed by both 1H NMR and X-ray absorption near-edge spectroscopies. These investigations revealed that, in many cases, reactions between nanoceria and PCET reagents reach equilibrium states with good mass balance. Each equilibrium state is a direct measure of the bond strength, or bond dissociation free energy (BDFE), between nanoceria and hydrogen. Further studies, including those with larger nanoceria, indicated that the relevant bond is a surface O-H. Thus, we have measured surface O-H BDFEs for nanoceria-the first experimental BDFEs for any nanoscale metal oxide. Remarkably, the measured CeO-H BDFEs span 13 kcal mol-1 (0.56 eV) with changes in the average redox state of the nanoceria colloid. Possible chemical models for this strong dependence are discussed. We propose that the tunability of ceria BDFEs may be important in explaining its effectiveness in catalysis. More generally, metal oxide BDFEs have been used as predictors of catalyst efficacy that, traditionally, have only been accessible by computational methods. These results provide important experimental benchmarks for metal oxide BDFEs and demonstrate that the concepts of molecular bond strength thermochemistry can be applied to nanoscale materials.

Determining Proton-Coupled Standard Potentials and X-H Bond Dissociation Free Energies in Nonaqueous Solvents Using Open-Circuit Potential Measurements.

Proton-coupled electron transfer (PCET) reactions are increasingly being studied in nonaqueous conditions, where the thermochemistry of PCET substrates is largely unknown. Herein, we report a method to obtain electrochemical standard potentials and calculate the corresponding bond dissociation free energies (BDFEs) of stable PCET reagents in nonaqueous solvents, using open-circuit potential (OCP) measurements. With this method, we measure PCET thermochemistry in acetonitrile and tetrahydrofuran for substrates with O-H and N-H bonds that undergo 1e-/1H+ and 2e-/2H+ redox processes. We also report corrected thermochemical values for the 1/2H2(g)/H•1M and H+/H• (CG) couples in several organic solvents. For 2e-/2H+ couples, OCP measurements provide the multielectron/multiproton standard potential and the average of the two X-H BDFEs. In contrast to traditional approaches for calculating BDFEs from electrochemical measurements, the OCP method directly measures the overall PCET reaction thermodynamics and avoids the need for a pKa scale in the solvent of interest. Consequently, the OCP approach yields more accurate thermochemical values and should be general to any solvent mixture compatible with electrochemical measurements. The longer time scale of OCP measurements enables accurate thermochemical measurements for redox couples with irreversible or distorted electrochemical responses by cyclic voltammetry, provided the PCET reaction is chemically reversible. Recommendations for successful OCP measurements and limitations of the approach are discussed, including the current inability to measure processes involving C-H bonds. As a straightforward and robust technique to determine nonaqueous PCET thermochemistry, these OCP measurements will be broadly valuable, with applications ranging from fundamental reactivity studies to device development.

Redox Reactivity of Colloidal Nanoceria and Use of Optical Spectra as an In Situ Monitor of Ce Oxidation States.

Nanoscale cerium oxide is of increasing interest in catalysis, biomedicine, renewable energy, and many other fields. Its versatility derives from the ability to form nonstoichiometric oxides that include both Ce3+ and Ce4+ ions. This work describes oxidation and reduction reactivity of colloidal cerium oxide nanocrystals, termed nanoceria, under very mild solution conditions. For instance, the as-prepared nanoceria oxidizes hydroquinone to benzoquinone, with reduction of some of the Ce4+ ions. Highly reduced nanoceria, prepared by UV irradiation in the presence of ethanol, oxidize hydroquinone back to benzoquinone. This and related reactivity allow tuning of the average cerium oxidation state in the nanocrystals without changes in size or other properties. The amounts of Ce3+ and Ce4+ in the nanoceria were determined both by X-ray absorption spectroscopy and from the stoichiometry of the reactions, measured using 1H NMR spectroscopy. The results demonstrate, for the first time, that the optical absorbance of nanoceria is linearly related to the percent Ce3+ in the sample. The decrease in absorption (blue-shift of the band edge) is due to increasing amounts of Ce3+, not to a quantum confinement effect. These findings demonstrate the facile solution reactivity of nanoceria and establish UV-visible spectroscopy as a powerful new tool for in situ determination of Ce oxidation states in ceria nanomaterials.

Inverse kinetic isotope effects in the oxygen reduction reaction at platinum single crystals.

Although the oxygen reduction reaction (ORR) involves multiple proton-coupled electron transfer processes, early studies reported the absence of kinetic isotope effects (KIEs) on polycrystalline platinum, probably due to the use of unpurified D2O. Here we developed a methodology to prepare ultra-pure D2O, which is indispensable for reliably investigating extremely surface-sensitive platinum single crystals. We find that Pt(111) exhibits much higher ORR activity in D2O than in H2O, with potential-dependent inverse KIEs of ~0.5, whereas Pt(100) and Pt(110) exhibit potential-independent inverse KIEs of ~0.8. Such inverse KIEs are closely correlated to the lower *OD coverage and weakened *OD binding strength relative to *OH, which, based on theoretical calculations, are attributed to the differences in their zero-point energies. This study suggests that the competing adsorption between *OH/*OD and *O2 probably plays an important role in the ORR rate-determining steps that involve a chemical step preceding an electrochemical step (CE mechanism).