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Our research is focused on synthetic inorganic chemistry with the aim to prepare components for artificial, biomimetic and light-driven water-splitting systems. Our main activities deal with the process of water-oxidation catalysed by manganese compounds (equation 1 below), but we address hydrogen evolution, the second half-reaction of water splitting, as well.

eq1_2.jpg

The water-oxidation reaction (1) is an essential process which has to be mastered for the conversion of renewable energy into fuels or bulk chemicals

   

With our projects we firstly aim to prepare model compounds for a better understanding of the individual reactions within the amazing biological photosynthetic reaction machinery. Secondly, mankind becomes more and more aware of the dangerous consequences of a potential climate change on earth caused by the extensive combustion of fossil fuels. In consequence, researchers have turned their attention to the development of alternative methods to generate the large amounts of energy needed to meet global demand without the use of oil, natural gas or coal.

 

Artificial photosynthesis, the transformation of solar energy into chemical energy by man-made devices, has gained increasing attention in this context in the last years – and the Kurz group has been a part of this research community since 2005. If successful, the products of an artificial photosynthetic device would be either “solar fuels” like H2 or MeOH which could be oxidised by atmospheric oxygen to generate energy (equation 2). Alternatively, other useful bulk chemicals like NH3 could be generated using energy from the sun.

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Figure 1. Components of a biomimetic water splitting system to generate hydrogen as a solar fuel.

 

To master a reaction scheme as shown in eq. 2 in a bio-inspired way, a number of components have to be developed, each “specialised” in different tasks similar to the biological photosynthetic process. Figure 1 shows a schematic representation of the interaction of the “actors” for a complete water-splitting system, namely: catalytic sites for water oxidation / reduction (Catox / Catred), photosensitizers for both half reactions (P1 / P2) and redox relays (R) to couple the two half-reactions. There are many possibility to vary and combine these components (it’s a a great playing field for chemists!), but most currently pursued research in the field of artificial photosynthesis somehow follows the general concept of Figure 1, which was already proposed in the 1970s in a visionary paper by Kirch, Lehn and Sauvage in Strasbourg.
 
In the context outlined by Figure 1, our group addresses three different major topics:
 
a) Synthesis and characterisation of manganese coordination compounds

The biological catalytic unit for water oxidation, the oxygen-evolving-complex (OEC) of Photosystem II, contains four manganese and one calcium ion. These metal centres are interconnected via bridging oxido- ligands (µ-O2-). Carboxylate and aromatic nitrogen ligands complete the manganese coordination sphere. To mimic at least a section of the OEC’s structure in form of molecular compounds, we synthesize and study dinuclear manganese complexes which contain μ-oxido dimanganese units - an example is shown in Figure 2, left. We then investigate such compounds with a special focus on their redox chemistry in the presence of water, using a variety of methods like electrochemistry, UVVis-spectroelectrochemistry or EPR spectroscopy.

As a “spin-off” topic, we have started to investigate manganese carbonyl compounds (synthetic precursors for multinuclear Mn-compounds) concerning their light-triggered CO-releasing properties. Such “photoCORMs” are studied worldwide for possible pharmaceutical applications.
 
 
fig2, left.jpgfig2, middle.jpgfig2, right.jpg
 
 
Figure 2. Structure of a synthetic Mn2-complex (left), its electrochemical behaviour studied by cyclic voltammetry (middle) and the “classic” Mn2III,IV- EPR spectrum recorded for the electrochemically generated oxidised form of the compound (right).

b) Water- oxidation catalysis by manganese oxides
Despite year-long efforts, a true, well-functioning, homogeneous water-oxidation catalyst has so far not been found among manganese complexes like the one shown in Fig. 2. However, we obtained very promising systems for manganese-based water-oxidation-catalysis once we shifted our strategy from molecular manganese complexes to solid state manganese oxide materials.
 
 
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 Figure 3. Model of the structure of a layered, highly amorphous calcium manganese oxide from the birnessite mineral family (left, with Ca in green, Mn in purple and O in red); oxygen evolution curves for different alkaline earth metal birnessites in water-oxidation catalysis using Ce4+ as chemical oxidant (right).
 
We found that especially the synthesis of “biomimetic oxides” containing (like the OEC!) both calcium and manganese yielded robust and efficient catalysts for reaction (1). Unlike the investigations of multinuclear manganese complexes, the study of such heterogeneous CaMnOx catalysts is not at all routine in this field, so we are constantly have to develop new methods here to understand and optimize these very promising catalytic systems. Additionally, the immobilization of such oxides on electrodes is a further area where we carry out research. Here, it is our aim to contribute new approaches for the manufacturing of anodes for electrochemical water oxidation, which are key components for water electrolysers or integrated photoelectrochemical devices like “artificial leafes”.

 
c) Photosensitisers for light-driven electron transfer
Besides the catalysts, light-absorbing compounds are key components of all systems for artificial photosynthesis. While most research groups use either ruthenium complexes or semiconductor materials as pigments (P1 or P2 in Fig. 1), we try to follow a more bio-inspired approach. Synthesising and characterising metalloporphyrins of p-block metals, we have been able to drive electron-transfer reactions by tin(IV)-porphyrins which act as “artificial chlorophylls” in these reactions (Fig. 4). The intensely-coloured dyes show intriguing photochemical reaction pathways under catalytic conditions and they are thus both interesting for applications in solar energy conversion and good model compounds for P680/P700, the biological light-absorbing units of the photosynthetic process.
 
 
fig4, left.jpg fig4, middle.jpgfig4, right_1.jpgfig4, right_2.jpg
 
Figure 4. A water-soluble tin(IV)metalloporphyrin used for light-driven electron transfer (left); reaction sequence for photocatalytic H2-production using the porphyrin as photosensitiser (PS) (middle); during catalysis, the porphyrin is reduced from the purple start compound to its green chlorin form.(right)
 

 

Funding and collaborators

Firstly, we profit from the excellent research environment at the Institute for Inorganic and Analytical Chemistry of the Albert-Ludwigs-University Freiburg. In addition, our research is generously supported by the DFG, in particular via the DFG Priority Program SPP1613 “Fuels Produced Regeneratively Through Light-Driven Water Splitting” and the BMBF within the research cluster MANGAN.

In the last years, we had the pleasure to work on and publish about joint research projects carried out together with the following research groups (in alphabetical order):

- Roger Alberto, Universität Zürich, Switzerland.

- Bernd Clement, Christian-Albrechts-Universität Kiel, Germany.

- Holger Dau, Freie Universität Berlin, Germany.

- Sven Kerzenmacher, Albert-Ludwigs-Universität Freiburg, Germany.

- Johannes Messinger, Uppsala Universitet, Sweden.

- Suzanne Mohney, Pennsylvania State University, United States.

- Birgül Zümreoğlu-Karan, Hacettepe Üniversitesi, Ankara, Turkey.

 
 

 

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