Non-Ideal Stoichiometry and Thermochemistry of Aqueous Iridium Oxide Nanoparticles in Proton-Coupled Electron Transfer and Oxygen-Atom Transfer

Abstract

Reported here are reactions of aqueous colloidal IrOx nanoparticles (NPs) with proton-coupled electron transfer (PCET) and oxygen-atom transfer (OAT) organic reagents, determining the reaction stoichiometries and thermochemistry. IrOx have attracted much attention for their high electrocatalytic activity, but understanding of their fundamental reaction chemistry is limited. This IrOx NP model system is simple, with UV-vis titrations demonstrating reversible interconversion between predominantly IrIV and predominantly IrIII NPs. This simplicity allows studies that reveal their complex non-idealities. The NP redox chemistry has a “super-Nernstian” stoichiometry of ~1.3 H+ per 1 e- transferred during both PCET and OAT reactions, as measured with electrochemistry and chemical methods. Spectro-electrochemistry revealed a broad distribution of surface IrOx-H bond dissociation free energies (BDFE), becoming weaker as more H is added. Such variation in binding strengths—a non-ideal binding isotherm–is common for surface adsorbates. For IrOx, 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 IrOx NPs, bracketing experiments gave 93 ± 24 kcal mol-1 for the average BDFE(OxIr–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 IrOx NPs, and they open a path to more complete models to understand catalytic redox reactions at such surfaces.