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Mechanism of water oxidation catalyzed by Co/M2P-oxides studied by isotope-ratio membrane inlet mass spectrometry
Umeå University, Faculty of Science and Technology, Department of Chemistry.
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(English)Manuscript (preprint) (Other academic)
Abstract [en]

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.

National Category
Chemical Sciences
URN: urn:nbn:se:umu:diva-86302OAI: diva2:698448
Available from: 2014-02-21 Created: 2014-02-21 Last updated: 2014-02-27Bibliographically approved
In thesis
1. Water splitting in natural and artificial photosynthetic systems
Open this publication in new window or tab >>Water splitting in natural and artificial photosynthetic systems
2014 (English)Doctoral thesis, comprehensive summary (Other academic)
Abstract [en]

Photosynthesis is the unique biological process that converts carbon dioxide into organic compounds, for example sugars, using the energy of sunlight. Thereby solar energy is converted into chemical energy. Nearly all life depends on this reaction, either directly, or indirectly as the ultimate source of their food. Oxygenic photosynthesis occurs in plants, algae and cyanobacteria. This process created the present level of oxygen in the atmosphere, which allowed the formation of higher life, since respiration allows extracting up to 15-times more energy from organic matter than anaerobic fermentation. Oxygenic photosynthesis uses as substrate for the ubiquitous water. The light-induced oxidation of water to molecular oxygen (O2), catalyzed by the Mn4CaO5 cluster associated with the photosystem II (PS II) complex, is thus one of the most important and wide spread chemical processes occurring in the biosphere. Understanding the mechanism of water-oxidation by the Mn4CaO5 cluster is one of today’s great challenges in science. It is believed that one can extract basic principles of catalyst design from the natural system that than can be applied to artificial systems. Such systems can be used in the future for the generation of fuel from sunlight.

In this thesis the light-induced production of molecular oxygen and carbon dioxide (CO2) by PSII was observed by membrane-inlet mass spectrometry. By analyzing this observation is shown that CO2 not only is the substrate in photosynthesis for the production of sugars, but that it also regulates the efficiency of the initial steps of the electron transport chain of oxygenic photosynthesis by acting, in form of HCO3-, as acceptor for protons produced during water-splitting. This finding concludes the 50-years old search for the function of CO2/HCO3 in photosynthetic water oxidation.

For understanding the mechanism of water oxidation it is crucial to resolve the structures of all oxidation states, including transient once, of the Mn4CaO5 cluster. With this application in mind a new illumination setup was developed and characterized that allowed to bring the Mn4CaO5 cluster of PSII microcrystals into known oxidation states while they flow through a narrow capillary. The optimized illumination conditions were employed at the X-ray free electron laser at the Linac Coherent Light Source (LCLS) to obtain simultaneous x-ray diffraction (XRD) and x-ray emission spectroscopy (XES) at room temperature. This two methods probe the overall protein structure and the electronic structure of the Mn4CaO5 cluster, respectively. Data are presented from both the dark state (S1) and the first illuminated state (S2) of PS II. This approach opens new directions for studying structural changes during the catalytic cycle of the Mn4CaO5 cluster, and for resolving the mechanism of O-O bond formation.

In two other projects the mechanism of molecular oxygen formation by artificial water oxidation catalysts containing inexpensive and abundant elements were studied. Oxygen evolution catalyzed by calcium manganese and manganese only oxides was studied in 18O-enriched water. It was concluded that molecular oxygen is formed by entirely different pathways depending on what chemical oxidant was used.  Only strong non-oxygen donating oxidants were found to support ‘true’ water-oxidation. For cobalt oxides a study was designed to understand the mechanistic details of how the O-O bond forms. The data demonstrate that O-O bond formation occurs by direct coupling between two terminal water-derived ligands. Moreover, by detailed theoretical modelling of the data the number of cobalt atoms per catalytic site was derived.

Place, publisher, year, edition, pages
Umeå: Umeå University, 2014. 96 p.
Water splitting, photosystem II, artificial catalyst, MIMS, inorganic carbon
National Category
Chemical Sciences Biophysics
urn:nbn:se:umu:diva-86363 (URN)978-91-7459-800-1 (ISBN)
Public defence
2014-03-21, KB3B3, stora hörsalen, KBC-huset, Umeå, 13:00 (English)
Available from: 2014-02-28 Created: 2014-02-24 Last updated: 2014-02-27Bibliographically approved

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