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

Abstract

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 protoncoupled electron transfer (PCET) reagents are used as a model system. Nanoceria redox changes, or hydrogen loadings, and overall reaction stoichiometries were followed by both 1 H 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.

Oxygen-Atom Defect Formation in Polyoxovanadate Clusters via Proton-Coupled Electron Transfer

“…This trend mirrors observations by Mayer and co-workers, wherein the relative proportion of Ce III and Ce IV centers in colloidal cerium oxide nanoparticles directs the reactivity of H-atom donors, with more oxidized particles featuring improved H-atom abstraction properties over their reduced analogues ( vide infra ). 46 …”

Proton-Coupled Defects Impact O–H Bond Dissociation Free Energies on Metal Oxide Surfaces

“…8,16−18 On the surface of metal oxides, however, the BDFEs of O−H bonds often depend on the oxidation states of the metal and oxygen surface ions. 8,18 Understanding the interplay between proton-coupled defects (i.e., electronic defects due to adsorbed H) 19−23 and PCET thermochemistry has broad implications for metal oxide catalysis and electrocatalysis.…”

Visible-Light-Driven, Iridium-Catalyzed Hydrogen Atom Transfer: Mechanistic Studies, Identification of Intermediates, and Catalyst Improvements

“…Of particular interest was to evaluate whether 3 Ant* can undergo successful hydrogenation with a HAT donor with similar or stronger E–H BDFEs compared to that of the Ir–H bond of Ir2 (61 kcal/mol, Figure 3 a). Hydroquinone ( H 2 Q ) was selected as a representative hydrogen atom donor because of its relatively low BDFEs (66 kcal/mol) 65 and negligible absorption of visible light. fac - Ir(ppy) 3 (ppy = 2-phenylpyridinato) was chosen as a triplet photosensitizer because of its efficient ISC to the triplet manifolds and high triplet energy ( E 00 = 53.6 kcal/mol) 66 suited for efficient triplet–triplet EnT with Ant .…”

“…Control reactions clearly demonstrated that the iridium photosensitizer was necessary for the observed reactivity. The poor reaction efficiency was attributed to a stronger O–H BDFE of H 2 Q (first O–H BDFE of 80 kcal/mol, 2 and average BDFE of 66 kcal/mol 65 ) than that of Ir2 (61 kcal/mol) and inevitable photodimerization of Ant that is insoluble, decreasing the effective concentration of Ant . 67 Of note, anthracene dimer formation was also observed under prolonged irradiation (24 h) with Ir2 , further confirming the mechanistic relevance.…”