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Reported here are reactions of aqueous colloidal IrO_x nanoparticles (NPs) with proton-coupled electron transfer (PCET) and oxygen-atom transfer (OAT) organic reagents, determining the reaction stoichiometries and thermochemistry. IrO_x NPs have attracted much attention for their high electrocatalytic activity, but understanding of their fundamental reaction chemistry is limited. This IrO_x NP model system is simple, with UV-vis titrations demonstrating reversible interconversion between predominantly Ir^IV and predominantly Ir^III NPs. This simplicity allows studies that reveal their complex non-idealities. The NP redox chemistry has a “super-Nernstian” stoichiometry of ∼1.3H⁺ per 1e⁻ transferred during both PCET and OAT reactions, as measured with electrochemistry andmore » chemical methods. Spectroelectrochemistry revealed a broad distribution of surface IrO_x–H bond dissociation free energies (BDFEs), becoming weaker as more H is added. Such variation in binding strengths—a non-ideal binding isotherm—is common for surface adsorbates. For IrO_x, the variation of BDFE(IrO–H)s is fit well to a Frumkin isotherm with a width of 6.5 kcal mol⁻¹. For OAT from the reactive oxygen atoms of IrO_x NPs, bracketing experiments gave 93 ± 24 kcal mol⁻¹ for the average BDFE(O_xIr–O), with a predicted spread much larger than that for the BDFE(IrO–H). Taken together, the results show the importance of non-ideal stoichiometry and thermochemistry for IrO_x NPs, and they open a path to more complete models to understand catalytic redox reactions at such surfaces.« less

Redox transformations at metal oxide (MO_x)/solution interfaces are broadly important, and oxygen atom transfer (OAT) is one of the simplest and most fundamental examples of such reactivity. OAT is a two-electron transfer process, well-known in gas/solid reactions and catalysis. However, OAT is rarely directly observed at oxide/water interfaces, whose redox reactions are typically proposed to occur in one-electron steps. Reported here are stoichiometric OAT reactions of organic molecules with aqueous colloidal titanium dioxide and iridium oxide nanoparticles (TiO₂ and IrO_x NPs). Me₂SO (DMSO) oxidizes reduced TiO₂ NPs with the formation of Me₂S, and IrO_x NPs transfer O atoms to amore » water-soluble phosphine and a thioether. The reaction stoichiometries were established and the chemical mechanisms were probed using typical solution spectroscopic techniques, exploiting the high surface areas and transparency of the colloids. Furthermore, these OAT reactions, including a catalytic example, utilize the ability of the individual NPs to accumulate many electrons and/or holes. Observing OAT reactions of two different materials, in opposite directions, is a step toward harnessing oxide nanoparticles for valuable multi-electron and multi-hole transformations.« less

Added electrons and holes in semiconducting (nano)materials typically occupy “trap states,” which often determine their photophysical properties and chemical reactivity. However, trap states are usually ill-defined, with few insights into their stoichiometry or structure. Our laboratory previously reported that aqueous colloidal TiO₂ nanoparticles prepared from TiCl₄ + H₂O have two classes of electron trap states, termed Blue and Red. Herein, we show that the formation of Red from oxidized TiO₂ requires 1e^– + 1H⁺, while Blue requires 1e^– + 2H⁺. The two states are in a protic equilibrium, Blue ⇌ Red + H⁺, with K_eq = 2.65 mM. The Bluemore » states in the TiO₂ NPs behave just like a soluble molecular acid with this K_eq as their K_a, as supported by solvent isotope studies. Because the trap states have different compositions, their population and depopulation occur with the making and breaking of chemical bonds and not (as commonly assumed) just by the movement of electrons. In addition, the direct observation of a 2H⁺/1e^– trap state contradicts the emerging H atom transfer (1H⁺/1e^–) paradigm for oxide/solution interfaces. Finally, this work emphasizes the importance of chemical stoichiometries, not just electronic energies, in understanding and directing the reactivity at solid/solution interfaces.« less