Keywords: cooperations (139) BESSY II (269) HZB own research (99)

Science Highlight    31.07.2018

Insight into catalysis through novel study of X-ray absorption spectroscopy

Manganese compounds also play a role as catalysts in photosynthesis.
Copyright: HZB

An international team has made a breakthrough at BESSY II. For the first time, they succeeded in investigating electronic states of a transition metal in detail and drawing reliable conclusions on their catalytic effect from the data. These results are helpful for the development of future applications of catalytic transition-metal systems. The work has now been published in Chemical Science, the Open Access journal of the Royal Society of Chemistry.

Many important processes in nature depend on catalysts, which are atoms or molecules that facilitate a reaction, but emerge from it themselves unchanged. One example is photosynthesis in plants, which is only possible with the help of a protein complex comprising four manganese atom sites at its centre. Redox reactions, as they are referred to, often play a pivotal role in these types of processes. The reactants are reduced through uptake of electrons, or oxidized through their release. Catalytic redox processes in nature and industry often only succeed thanks to suitable catalysts, where transition metals supply an important function.

Soft x-rays at BESSY II

These transition metals and in particular their redox or oxidation state can be examined particularly well using soft X-rays, because electronic states can be precisely measured using X-ray spectroscopy. In what is known as L-edge absorption spectroscopy, electrons from the 2p shell of the transition metal are excited so that they occupy free d-orbitals. An energy difference can be determined from the X-ray absorption spectrum that reflects the oxidation state of the molecule or the catalyst in a known way. However, exactly where the electrons are absorbed or released by the catalyst during a redox reaction, i.e. exactly how the charge density in the catalyst varies with oxidation state, was previously difficult to verify. This was mainly due to the lack of reliable methods for the theoretical description of charge densities in catalyst molecules in ground and excited states, and to the difficulty in obtaining reliable experimental data. If the transition metals are located in larger complex organic molecule complexes, as they typically are for real redox catalysts, their study becomes extremely difficult because the X-rays lead to damage in the sample.

Sample in solution examined in different oxidation states

Now for the first time, an international team from the Helmholtz-Zentrum Berlin, Uppsala University (Sweden), Lawrence Berkeley National Laboratory in Berkeley (USA), Manchester University (Great Britain), and the SLAC National Accelerator Laboratory at Stanford University (USA) has succeeded in studying manganese atoms in different oxidation states – i.e. during different stages of oxidation – in various compounds through in operando measurements at BESSY II. To accomplish this, Philippe Wernet and his team introduced the samples into various solvents, examined jets of these liquids using X-rays, and compared their data against novel calculations from Marcus Lundberg's group at Uppsala University. “We succeeded in determining how – and above all why – the X-ray absorption spectra shift with the oxidation states”, says theoretician Marcus Lundberg. PhD students Markus Kubin (HZB) with his experimental expertise and Meiyuan Guo (Uppsala University) with his theoretical expertise reflect the interdisciplinary approach of the study and they contributed equally as first authors of the paper.

Breakthrough through a combination of theory and experiment

“We combined a novel experimental setup with quantum chemical calculations. In our opinion, we have achieved a breakthrough in the understanding of organometallic catalysts”, says Wernet. “For the first time, we were able to empirically test and validate calculations for oxidation and reduction that do not take place locally on the metal, but instead on the entire molecule.” “These findings are a cornerstone for future work in more complex systems, like the tetra manganese cluster in photosynthesis. They will facilitate new understanding of redox processes for the manganese catalyst in the Photosystem II protein complex”, says Junko Yano, Senior Scientist of Molecular Biophysics and Integrated Bioimaging Division (MBIB) and the Joint Center for Artificial Photosynthesis (JCAP) at Lawrence Berkeley National Laboratory, who is conducting detailed research of photosynthesis.

Published in Chemical Science (2018): Probing the oxidation state of transition metal complexes: a case study on how charge and spin densities determine Mn L-edge X-ray absorption energies; Markus Kubin,  Meiyuan Guo,  Thomas Kroll, Heike Löchel,  Erik Källman,  Michael L. Baker,  Rolf Mitzner,  Sheraz Gul,  Jan Kern,  Alexander Föhlisch, Alexei Erko, Uwe Bergmann,  Vittal Yachandra, Junko Yano,  Marcus Lundberg and  Philippe Wernet;

 

arö


           



You might also be interested in
  • <p>HZB-Teams are exploring and developing new technologies for perovskite based solar cells in the innovation lab HySPRINT.</p>NEWS      16.05.2019

    LAUNCH OF EPKI: European Perovskite Initiative for the development of Perovskite based solar technology

    Perovskite based solar cells have made tremendous progress over the last decade achieving lab-scale efficiencies of 24.2% early 2019 in single-junction architecture and up to 28% in tandem (perovskite associated with crystalline silicon), turning it into the fastest-advancing solar technology to date. With the HySPRINT project and the recruitment of highly talented young scientists, Helmholtz-Zentrum Berlin has built up a considerable research capacity in the field of perovskite materials in recent years and is participating in the European Perovskite Initiative EPKI that has now been launched. [...]


  • <p>Experiments at the femtoslicing facility of BESSY II revealed the ultrafast angular momentum flow from Gd and Fe spins to the lattice via orbital moment during demagnetization of GdFe alloy.</p>SCIENCE HIGHLIGHT      10.05.2019

    Laser-driven Spin Dynamics in Ferrimagnets: How does the Angular Momentum flow?

    When exposed to intense laser pulses, the magnetization of a material can be manipulated very fast. Fundamentally, magnetization is connected to the angular momentum of the electrons in the material. A team of researchers led by scientists from the Max Born Institute for Nonlinear Optics and Short Pulse Spectroscopy (MBI) has now been able to follow the flow of angular momentum during ultrafast optical demagnetization in a ferrimagnetic iron-gadolinium alloy at the femtoslicing facility of BESSY II. Their results are helpful to understand the fundamental processes and their speed limits. The study is published in Physical Review Letters. [...]


  • <p>A green laser pulse initially excites the electrons in the Cu<sub>2</sub>O; just fractions of a second later, a second laser pulse (UV light) probes the energy of the excited electron.</p>SCIENCE HIGHLIGHT      09.05.2019

    Copper oxide photocathodes: laser experiment reveals location of efficiency loss

    Solar cells and photocathodes made of copper oxide might in theory attain high efficiencies for solar energy conversion. In practice, however, large losses occur. Now a team at the HZB has been able to use a sophisticated femtosecond laser experiment to determine where these losses take place: not so much at the interfaces, but instead far more in the interior of the crystalline material. These results provide indications on how to improve copper oxide and other metal oxides for applications as energy materials. [...]




Newsletter