Section of Chemistry

Artificial Photosynthesis

Water oxidation – the oxidative side of Artificial Photosynthesis

The aim of artificial photosynthesis is to capture solar energy and store it in chemical bonds. For this purpose the natural photosynthesis serves as model and an inspiration. Natural photosynthesis uses the energy in sunlight to oxidatively split water to oxygen, protons and electrons. The reducing power of these electrons can be stored in various products to be used by the organism as a fuel when needed.

Artificial photosynthesis aims at mimicking these reactions and store the energy from sunlight in a chemical fuel (a solar fuel). A longstanding goal of artificial photosynthesis has been to develop water oxidation catalysts, based on first row transition metals, which operates at low over potential and can be driven by sunlight. These catalysts will be essential in the process to make a renewable solar fuel.

In order to utilize the water oxidation catalyst in a set-up that uses sunlight and water to produce a fuel the water oxidation catalyst has to be coupled to a reduction catalyst that makes the fuel. If protons (released from the water oxidation) are reduced to form hydrogen we get a high energy fuel that does not require any additional starting materials. If we use other reduction catalysts we can also envisage producing other fuels, for example methanol from CO2 and protons, using the same oxidative half-cell. It can be beneficial to have the complete system divided in two compartments. A two compartment solution allows for easy separation of the resulting products and makes internal tuning of the catalysts possible.

A schematic representation of a complete cell for light driven water oxidation and hydrogen production. To the left is the oxidative half cell that is the main focus of this project. To the right is the reductive, hydrogen producing, half cell.

Our projects

Cobalt-based catalysts for Light-Driven Water Oxidation

This project is in collaboration with Sascha Ott and Stenbjörn Styring.

We have developed a number of oxygen evolving water oxidation catalysts that can be driven by visible light in aqueous solutions at neutral or weakly basic conditions. One of the catalysts is a heterogeneous catalyst based on amorphous cobalt oxide nanoparticles and uses methylenediphosphonate (M2P) as an additional ligand. The ruthenium complex Ru(bpy)32+ is used as the photosensitizer and peroxosulfate (S2O82-) as the sacrificial electron acceptor (Scheme below). The presence of a ligand allows for an anchoring point where photosensitizer and catalyst can be linked together.

Schematic representation of the light-driven water oxidation using cobalt oxide nanoparticles.

We have linked the photosensitizer to the cobalt catalyst by using a modified photosensitizer containing M2P groups, creating a hybrid Ru-Co molecular/heterogeneous material. We are currently modifying the photosensitizer to be able to attach to a TiO2 surface to get a complete photo-anode. 

The Ru-Co material with a molecular photosensitizer linked to a heterogeneous water oxidation catalyst via coordination bonds.

We are also investigating molecular cobalt-based catalysts for water oxidation. We have been studying both mononuclear and dinuclear complexes with polypyridine ligands. The point of interest is the mechanism for water oxidation catalysed by these complexes. To understand more about the mechanism we try to study the reaction under different conditions using various different spectroscopic techniques, e.g. EPR spectroscopy and mass spectrometry.

Two examples of cobalt-based catalysts studied in the group.

The mononuclear cobalt complex is currently being modified to be able to anchor to an electrode surface to facilitate the development of photo-anodes.

References

  1. Shevchenko, D.; Anderlund, M. F.; Thapper, A.; Styring, S. Energy Envir. Sci. 2011, 4, 1284-1287.
  2. Risch, M.; Shevchenko, D.; Anderlund, M. F.; Styring, S.; Heidkamp, J.; Lange, K. M.; Thapper, A.; Zaharieva, I. Int. J. Hydrogen Energy 2012, 37, 8878-8888.
  3. Wang, H. Y.; Liu, J.; Zhu, J.; Styring, S.; Ott, S.; Thapper, A. Phys Chem Chem Phys 2014, 16, 3661-3669.
  4. Wang, H. Y.; Mijangos, E.; Ott, S.; Thapper, A. Angew. Chem. Int. Ed. 2014, 53, 14499-14502.

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Iron-based Catalysts for Light-driven Water Oxidation

Very recently mononuclear iron complexes with tetraazadentate ligands have been reported to be active water oxidation catalysts. Iron is a very attractive metal to use for water oxidation catalysis based on its availability and non-toxicity. The reported catalysts uses a strong chemical oxidant, cerium(IV) and has a relatively large over-potential for water oxidation.

We have used polypyridyl ligands to make mononuclear iron catalysts for water oxidation. Based on a ligand system already reported for a cobalt-based water oxidation catalyst we have identified a mononuclear iron complex that can be driven by light energy using a photosensitizer and electron acceptor as described above for the cobalt catalysts. Interestingly the ligand appears to not be as innocent as originally thought and we have indications that the ligand takes active part in the catalysis by opening a binding site for a water molecule. Together with Marcus Lundberg’s group at Theoretical Chemistry we are investigating this system using DFT calculations.

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Bioinorganic chemistry

Iron-manganese cofactors in dimetal carboxylate proteins – why two different metals instead of one?

Metal containing cofactors are present in the active sites in many proteins. The presence of metal ions gives access to a wide variety of chemical reactions that would otherwise be difficult to carry out. One superfamily of iron containing proteins has two iron ions in the active site, predominately coordinated by carboxylate groups (from glutamate and aspartate side chains) and imidazole groups (from histidine side chains). This active site motif has been called the diiron-carboxylate cofactor, or more recently the dimetal-carboxylate cofactor. The most studied members of this family of dimetal-carboxylate proteins are the bacterial multicomponent monooxygenases (BMMOs), especially soluble methane monooxygenase (sMMO), and ribonucleotide reductase R2 proteins (RR2s).

Recently Fe-Mn varieties of dimetal-carboxylate protein were found. One of these Fe-Mn enzymes, an oxidase from Mycobacterium tuberculosis, is an R2 homologue but closely related to BMMOs and is capable of C-H activation. Another Fe-Mn enzyme is an RR2 from Chlamydia trachomatis that store a radical equivalent as a MnIV-FeIII species. Even a Mn-Mn variety of an RR2 protein has been reported that retains the function of R2 proteins but require additional flavodoxin to generate an oxidant. The suggested mechanisms for the Fe-Mn and Mn-Mn proteins involve formation of high-valent metal ions (FeIII, FeIV, MnIII and MnIV) in important intermediates, similar to the mechanism suggested for sMMO.

Together with groups at Stockholm University, Freie Universität Berlin, and Technische Universität Kaiserslautern we are involved in a project studying the Fe-Mn varieties of dimetal-carboxylate protein. We are especially interested in questions regarding metal specificity. How does the enzyme know which metal to bind, and how can it distinguish between the two? Also, why does the enzyme bind two different metals? What type of chemistry is the heterometallic cofactor able to perform that the homometallic cannot do?

The active site of a Fe-Mn containing R2 homologue from Mycobacterium tuberculosis.

Project

Iron-Manganese Complexes as Biomimetic Models for a New Class of Dimetal Carboxylate Proteins 

This project is in collaboration with Sascha Ott.

In this project we are synthesizing and characterizing heterometallic complexes that contain iron and manganese. We are particularly interested in issues regarding metal specificity and differences in reactivity between heterometallic and homometallic dinuclear complexes. The aim is to study the reactivity of similar complexes with Fe-Fe, Fe-Mn, and Mn-Mn cores in aqueous environment using a combination of electrochemistry and spectroscopy.

A set of complexes with Fe-Fe, Fe-Mn and Mn-Mn cores studied in the group.

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