Electronic Processes Observed During Catalysis with Novel X-Ray Spectroscopy Method

Electronic processes
Electronic processes
Also in photosynthesis manganese compounds play a role as catalysts. Picture: HZB

An international team has achieved a breakthrough at BESSY II. For the first time, they were able to study electronic processes on a transition metal in detail and draw reliable conclusions from the measured data on their catalytic effect. Their results are useful for the systematic development of catalytic systems with transition metals in their centers for future applications. The work is now published in Chemical Science, the Open Access Journal of the Royal Society of Chemistry.

Many important processes in nature require catalysts: atoms or molecules that allow the desired reaction, but which are themselves unaltered. An example is the photosynthesis in plants, which is only possible with the help of a protein complex, in the center of which four manganese atoms sit. Often, in such processes so-called redox reactions play a crucial role, in which the reactants exchange electrons and thereby reduced (electron uptake) or oxidized (electron donation) are. Catalytic redox processes in nature or in technology often succeed only thanks to suitable catalysts in which transition metals play an important role.

Soft X-ray from BESSY II

Such transition metals and in particular their redox or oxidation state can be examined particularly well with soft X-ray light. In so-called L-edge absorption spectroscopy, electrons from the 2p shell of the transition metal are excited to occupy free d orbitals at short notice. The X-ray absorption spectrum can be used to determine an energy difference which is known to reflect the oxidation state of the molecule or of the catalyst. Where exactly in the catalyst during a redox reaction, however, the electrons are absorbed or released, how exactly the charge density in the catalyst changes with a change in its oxidation state, was previously largely unknown. This was mainly because that reliable methods for theoretical descriptions of the charge densities in catalyst molecules were missing and that reliable experimental data are difficult to obtain. If the transition metals are present in larger, organic molecular complexes, as is typical of functioning redox catalysts, the investigation becomes extremely difficult, since the X-ray radiation immediately leads to damage in the sample.

Sample tested in solution in different oxidation states

For the first time ever, an international team from HZB, Uppsala University (Sweden), the Lawrence Berkeley National Laboratory at Berkeley (USA), Manchester University (UK) and the SLAC National Accelerator Laboratory at Stanford (USA) are taking measurements on BESSY II succeeded in investigating manganese atoms in different compounds and oxidation states in operando – that is, during different oxidation states. The researchers led by Philippe Wernet brought the samples in different solvents, examined the liquid jet in X-rays and compared the measured data with novel calculations from the group of Marcus Lundberg from Uppsala University. “We were able to determine how, and above all, why the X-ray absorption spectra shift with the oxidation states,” as the theorist Marcus Lundberg. The two doctoral students Markus Kubin (HZB) with his experimetnellen and Meiyuan Guo (Uppsala University) with his theoretical expertise reflect the interdisciplinary approach of the study and contributed in equal parts as first authors of the study.

Breakthrough through combination of theory and experiment

“We have combined a novel experimental set-up with quantum-chemical model calculations and thereby, we believe, achieved a breakthrough in the understanding of organometallic catalysts” says Wernet: “For the first time, we were able to compute oxidation or reduction calculations not locally on the metal but on These findings are an important cornerstone for future work in photosynthesis: “They will enable a novel understanding of the redox processes in the manganese catalyst of the photosystem II protein complex,” says Junko Yano, who intensively researching photosynthesis.

Source : Helmholtz-Zentrum Berlin für Materialien und Energie