A hierarchical cluster-model approach to understand the catalytic water splitting at calcium-manganese-oxide centers

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Nature realized the solar energy driven catalytic water splitting reaction with earth abundant metal oxides in the oxygen evolving complex in photosystem II. The catalytic center of this complex consists of a CaMn4O5 cluster embedded in protein ligands inside the thylakoid membrane of plants, algae, and cyanobacteria. In this project we aim to develop a new hierarchical approach to probe fundamental concepts of the water splitting reaction employing isolated, mass-selected calcium-manganese-oxide clusters as simplified models of the natural catalytic water splitting center. The reactions of these clusters with water (and other relevant ligands such as isotopically labeled water, O2, H2, etc.) will be systematically studied in a gas phase ion trap experiment as a function of the exact size and composition of the CaxMnyOz clusters (that is, of the complexity of the system). These investigations can provide direct experimental evidence for the ability of the different clusters to dissociate water and to mediate the catalytic formation of H2, O2, or H2O2. Kinetic experiments enable the postulation of the detailed reaction mechanisms and, in conjunction with first principles calculations, permit molecular level insight into the catalytic reactions. Furthermore, selected cluster systems will be investigated by infrared vibrational spectroscopy and ultraviolet/visible photodissociation spectroscopy in order to obtain experimental insight into the geometrical structure of the clusters. Based on all the obtained results it is envisaged to develop a conceptual framework that can provide guidelines for the rational design of calcium-manganese-oxide based catalysts for the splitting of water. In this respect the gas phase experiment offers the unique possibility to precisely control and tune important parameters which determine the catalytic water splitting abilities with hierarchically increasing complexity of the materials like the size of the catalytic center, the oxidation state of the involved manganese atoms, the contribution of the calcium atoms, and the geometrical structure of the active complex.