Date of Award
Doctor of Philosophy (PhD)
From the surface of a platinized electrode in a hydrogen fuel cell to the oxygen-evolving complex in photosystem II, the binding and transfer of hydrogen is central to many important chemical transformations in our world. The core thermochemical and kinetic concepts which connect these proton-coupled electron transfers across a continuum of compound sizes, including small molecules, nanoparticles, and bulk materials, are explored. In Chapter 1, an introduction to the thermochemical basis for this thesis is presented, along with the underlying connections it enables for studying hydrogen transfer across thermochemical and electrochemical reactions, as well as gas and solution phase reactivity.Methods for measuring the thermochemistry of hydrogen transfer in solution and at solution-solid interfaces has been critical to developing these connections. In Chapter 2, an accessible potentiometric technique for measuring molecular potentials of hydrogenation in nonaqueous media is presented. This methodology provides a roadmap to measuring the thermochemistry of hydrogen transfer for molecules under ill-defined solvent conditions. The resulting thermochemical values are then exploited in Chapter 3 to measure the hydrogen transfer thermochemistry of cerium oxide nanoparticles (nanoceria). Experiments demonstrate that reactions of molecular PCET reagents, whose hydrogen transfer thermochemistry is known, and colloidal nanoceria reach equilibrium states. These equilibrium states provide the first measures of surface O–H bond dissociation free energies bond dissociation free energies for a metal oxide nanoparticle. Remarkably, bond dissociation free energies are shown to vary by 13 kcal mol-1 with changes in the average redox state of nanoceria. This range of bond strengths is significant for understanding the application of cerium oxide as a non-innocent catalyst support, and more broadly, kinetic analyses of other binary materials. The thermochemical basis developed in Chapter 1–3 is essential to studying the kinetics of the same processes. In Chapter 4, the rates of reactions between ceria at different levels of reduction and substituted arylhydrazyl reagents are measured. These rate constants and the nanoceria bond dissociation free energies determined in Chapter 3 are combined to determine Brønsted-Evans Polanyi relationships for the hydrogen transfer reactivity of nanoceria. These are, to the best of our knowledge, the first experimental Brønsted-Evans Polanyi relationship for hydrogen transfer at the solution-solid interface. Analogies to the molecular literature and implications for heterogeneous catalyst design are discussed. Studies of reaction mechanism require an arsenal of tools. In Chapter 5, deuterium oxide solvent isotope effects are revisited as a mechanistic tool for the study of hydrogen transfer at electrocatalytic interfaces. These studies are enabled by the development of a novel methodology for preparing ultrapure deuterated electrolytes. The robustness of this procedure is shown through voltammetry of highly sensitive single crystal facets of platinum, which also provide measurements of the equilibrium solvent isotope effects on hydrogen and hydroxide adsorption. The product solvent isotope effect for the hydrogen evolution reaction at polycrystalline gold electrodes is then explored through studies with a homebuilt differential electrochemical mass spectrometer. Novel analysis is presented to determine that the product isotope effect is significantly larger than previous measures of the kinetic isotope effect. Implications for the mechanism of the hydrogen evolution reaction at gold electrodes are discussed.
Agarwal, Rishi, "Determination and Application of Hydrogen Transfer Thermochemistry: Studies of Molecules, Nanoparticles, and Metallic Electrodes" (2021). Yale Graduate School of Arts and Sciences Dissertations. 290.