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Ein hierarchischer Clustermodell-Ansatz zum Verständnis der katalytischen Wasserspaltung an Calcium-Manganoxid-Zentren

Fachliche Zuordnung Physikalische Chemie von Molekülen, Flüssigkeiten und Grenzflächen, Biophysikalische Chemie
Förderung Förderung von 2016 bis 2021
Projektkennung Deutsche Forschungsgemeinschaft (DFG) - Projektnummer 300369766
 
Erstellungsjahr 2022

Zusammenfassung der Projektergebnisse

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 CaMn3O4·MnO cluster embedded in protein ligands inside the thylakoid membrane of plants, algae, and cyanobacteria. In the proposal we did aim to develop a new hierarchical approach to probe fundamental concepts of the water splitting reaction employing isolated, massselected calcium-manganese-oxide clusters as simplified models of the natural catalytic water splitting center. This agenda was successfully realized by gas-phase preparation of the manganese- and calcium-manganese-oxide clusters in a large variety of compositions (and also with additional stabilizing organic ligands) and their systematic investigation by gas-phase reaction kinetics measurements and infrared multi photon dissociation spectroscopy supported by first principles calculations. Major findings include: (1) A surprising composition dependence of the water splitting reaction (each atom counts) with a prominent focus on the CaMn3O4-unit which represents the sole cluster among the tetrameric CanMn4-nO4+ (n=0-4) that is observed to oxidize water via elimination of hydrogen peroxide, thus, emphasizing the importance of the role of the calcium atom in the water oxidation process. (2) The systematic study of size, stoichiometry, dimensionality, and Ca doping dependence of the water oxidation propensities of the clusters in conjunction with density functional simulations reveals that water oxidation to H2O2 proceeds via formation of a terminal oxyl radical followed by oxyl/hydroxy O-O coupling. In particular, the water oxidation capability of the CaMn3O4+ is attributed to a controlled structural fluxionality and a favorable coupling between different spin states that results in the stabilization of the oxyl radical intermediate. This conceptual insight provides guidelines for the rational design of calcium-manganeseoxide based catalysts for the splitting of water. (3) In further investigations different ligand molecules (CO2, carboxylic acids, esters) were employed to structurally stabilize the metal oxide cluster cores and to test their influence on the water oxidation reaction, thus, paving the way to a next step in our hierarchical approach to understand the catalytic water splitting at calcium-manganese-oxide centers.

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