Non-ideal stoichiometry and thermochemistry of aqueous iridium oxide nanoparticles in proton-coupled electron...

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 and 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 _x Ir-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.