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

自然界在光系统II中实现了太阳能驱动的与富土金属氧化物在析氧络合物中的催化水裂解反应。该复合物的催化中心由嵌入在植物、藻类和蓝藻类囊体膜内的蛋白质配体中的CaMn4O5簇组成。在这个项目中，我们的目标是开发一种新的分层方法来探索水分解反应的基本概念，采用分离的，大量选择的钙-锰-氧化物簇作为天然催化水分解中心的简化模型。这些团簇与水（以及其他相关配体，如同位素标记的水、O2、H2等）的反应将在气相离子阱实验中作为CaxMnyOz团簇的确切大小和组成（即系统的复杂性）的函数进行系统研究。这些研究可以为不同簇解离水和介导H2、O2或H2O2催化形成的能力提供直接的实验证据。动力学实验使详细的反应机制的假设，并与第一性原理计算相结合，允许分子水平的洞察催化反应。此外，选择的团簇系统将通过红外振动光谱和紫外/可见光光解光谱进行研究，以获得对团簇几何结构的实验见解。基于所有获得的结果，设想开发一个概念框架，可以为合理设计用于水裂解的钙-锰-氧化物催化剂提供指导。在这方面，气相实验提供了精确控制和调整重要参数的独特可能性，这些参数决定了催化水分解能力，随着材料的层次复杂性的增加，如催化中心的大小，所涉及的锰原子的氧化态，钙原子的贡献，以及活性配合物的几何结构。

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.