Water Oxidation Catalysis & Artificial Photosynthesis

One of the most important reactions on earth is the photosynthesis, as it converts the energy-poor, abundant chemicals H₂O and CO₂ into energy-rich carbohydrates using the energy of sunlight. Most of the energy we use has its origin from photosynthesis (natural oil, gas and coal resources). The primary process is the capture of sunlight in photosystem II used for the oxidation of H₂O to O₂. The generated electrons are used to produce NADPH/H⁺ that is needed in the dark reaction for the reduction of CO₂. The electron transfer produces a proton gradient which is enhanced by the protons formed by the water oxidation. This proton gradient is used by ATP synthase to produce ATP which is also needed for CO₂ reduction in the dark reaction.

While nature uses a highly sophisticated photosynthetic machine to separate the electrons and protons generated in the water oxidation, this reaction would produce H₂ without this charge separation. Thus, to understand and to mimic the water oxidation by use of sunlight for the production of H₂ and O₂ is an attractive goal in energy research.

The water oxidation in photosystem II occurs at the oxygen evolving center (OEC), which is comprised of a tetranuclear manganese cluster.The absorption of sunlight results in a highly oxidized form of the cluster that is capable of oxidizing two water molecules to O₂ .

We are developing new molecular water oxidation catalysts. The main intention is to combine the water oxidation catalyst with a light harvesting unit to establish artificial photosynthesis. In this respect, we are currently investigating ruthenium and manganese complexes, but also iron, cobalt, nickel, and copper complexes.