Proton coupled electron transfer from Co 3 O 4 nanoparticles to photogenerated Ru(bpy) 3 3+ : base catalysis and buffer effect

International audienceCo3O4 nanoparticles in the spinel crystalline structure are among the most promising catalyst for water oxidation reaction, displaying remarkable activity under electrochemical and light-assisted conditions. In the presence of Ru(bpy)3 2+ as photosensitizer (bpy = 2,2'bipyridine) and Na2S2O8 as 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)=O, along a proton-coupled electron-transfer (PCET) pathway; (ii) the rate of the process depends on pH, on the nature and concentration of the solution buffer; (iii) borate promotes general base catalysis in the PCET of Co3O4 surface sites; (iv) inhibition of the PCET mechanism is observed at high buffer concentration, due to H3BO3 poisoning of the surface Co sites, resulting in depletion of the O2 evolution activity