Mechanism of water oxidation catalyzed by Co/M2P-oxides studied by isotope-ratio membrane inlet mass spectrometry

• Co-oxides are promising water oxidation catalysts for artificial photosynthesis devices. Their catalytic mechanism was studied previously both experimentally and theoretically, but there is presently no agreement whether the O-O bond formation occurs via nucleophilic attack or by direct coupling, and whether or not bridging oxygens participate as substrate during O-O bondformation. Here we present time-resolved 18O-labelling isotope-ratio membrane-inlet massspectrometry experiments employing the previously introduced Co/methylenediphosphonate (Co/M2P) system in combination with [Ru(bpy)3]3+ (bpy = 2,2’-bipyridine) as chemical oxidant. Our data demonstrate that for Co/M2P-oxide O-O bond formation occurs between two pre-bound, fast exchanging oxygen species, i.e. likely via direct coupling between two terminal water-derived oxygen ligands. Detailed modeling of the dependence of the O2-isotope ratios on the [Ru(bpy)3]3+ concentration revealed that in the Co/M2P-oxidenanoparticles almost all Co ions are catalytically active (2.35 Co per catalytic site) and that, starting from a ‘resting state‘, 3.5 electrons need to be removed from each catalytic site for thefirst O2 formation. Since it was previously demonstrated that in the resting state most Co ions are in the oxidation state Co(III), we conclude that the coupling mechanism involves at least one Co(IV)-O• radical.

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NATURVETENSKAP  -- Kemi (hsv//swe)

NATURAL SCIENCES  -- Chemical Sciences (hsv//eng)

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