Proton coupled electron transfer from Co3O4nanoparticles to photogenerated Ru(bpy)33+: Base catalysis and buffer effect

Co3O4 nanoparticles in the spinel crystalline structure are among the most promising catalysts for the water oxidation reaction, displaying remarkable activity under electrochemical and light-assisted conditions. In the presence of Ru(bpy)32+ as a photosensitizer (bpy = 2,2′-bipyridine) and Na2S2O8 as an electron acceptor, 5 ± 1 nm size Co3O4 nanoparticles show a slow primary electron transfer (ET) to photogenerated Ru(III), occurring in a timescale of tens of milliseconds. We demonstrate herein that (i) photo-oxidation of Co3O4 NPs by Ru(III) involves transformation of surface Co(III)–OH sites to formal Co(IV)[double bond, length as m-dash]O, along a proton-coupled electron-transfer (PCET) pathway; (ii) the rate of the process depends on pH, and on the nature and concentration of the buffer; (iii) borate promotes general base catalysis of the PCET; and (iv) inhibition of the PCET is observed at high buffer concentration, due to H3BO3 poisoning of the surface Co sites, resulting in depletion of the O2 evolution activity.