Our study was initiated by a surprising voltammetry result, that showed the presence of lactic acid in water changed the oxidation states accessible by iridium-based catalysts. This is unusual, since electrochemical wastewater treatment typically involves partial oxidation of water to form a hydroxyl radical, which has little to no effect on the first coordination sphere of the catalyst.
In the case of iridium, it turns out that this is not the case - the organic substrate coordinates to the metal center and is oxidized through an inner-sphere electron transfer pathway. Our proposed mechanisms predict an anodic shift in the Ir(IV)/Ir(V) reversible potential if this is the case, which we see in cyclic voltammograms (Figure 1).
To confirm this result, kinetic isotope effect (KIE) studies (Figure 3) show that catalysts that proceed by classical hydroxyl radical decomposition (Pt) exhibit a high H/D KIE, while inner-sphere electron transfer catalysts (Ir) do not.
Product detection (Table 1 and Figure 4) shows that the reaction intermediates vary between catalysts that use hydroxyl radicals to oxidize lactic acid version inner-sphere electron transfer catalysts. These results are critical, since they determine how these catalysts can be integrated into practical systems to treat wastewater.