9. November 2020, 9:34
Research / knowledge transfer, research projects
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– Heterogeneous catalysts are often used in powder form. After the reaction, this can then sink to the bottom, which greatly simplifies the separation of the catalyst.,, LIKAT / Nordlicht
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At the Leibniz Institute for Catalysis in Rostock, chemists discovered the molecular mechanism of a photocatalyst that uses sunlight to completely break down organic contaminants in wastewater. All over the world, for example, more and more drug residues, such as anti-inflammatory drugs, antibiotics or contraceptives, find their way into processing systems, where they are difficult to remove. Laboratories are increasingly working on photocatalytic solutions, but knowledge of the operating principles is patchy. Researchers working with Angelika Brückner and Jabor Rabeah at LIKAT have therefore, to a certain extent, observed photocatalysts live at work and explained the basics of their functioning.
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This opens up new avenues for the development of photocatalysts for wastewater treatment, as Prof. Dr. Brückner, division manager at LIKAT, says. Colleagues from China approached her with a request for cooperation. In densely populated regions of Asia, organic contamination, for example from stable breakdown products from drugs, is an urgent problem.
The Chinese colleagues had developed new catalysts and needed LIKAT’s expertise for special investigations in order to be able to optimally prepare their catalysts. Brückner and her team are specialists in the field of so-called in-situ spectroscopy. This enables them to follow the function of a catalyst during the chemical reaction (in situ) and to document its molecular mode of action.
Carbonitride instead of titanium oxide
Common photocatalysts, such as titanium dioxide, with the z. B. self-cleaning window panes are coated to break down dirt particles, work most effectively with high-energy UV radiation. However, the UV component in sunlight is only 5 to 8 percent. The Chinese chemists are therefore using a new generation of photocatalysts: carbonitride, which is activated in visible light. It is created through the thermal treatment of melamine, which is also used as a raw material for colorful dishes made of thermoset.
The colleagues in China were able to successfully test their catalyst using various substances that are produced when drugs are broken down and get into the wastewater. The powdery photocatalyst is stirred in the water and does its work as suspended particles. The colleagues tested oxygen and ozone as oxidizing agents. “Ozone turned out to be exceptionally effective,” explains Prof. Brückner. “But its activity fluctuated, and that seemed to depend on the preparation conditions of the catalyst.”
In his dissertation at LIKAT, Jiadong Xiao, a young chemist from Peking University, was supposed to find out why this is so and what the optimal conditions for preparing the catalyst are. This research was carried out under the aegis of Dr. Jabor Rabeah, head of department at LIKAT and supervisor of the doctoral student.
Capture and identify radicals
The measurements showed that a species of radicals is responsible for the actual degradation reaction. Angelika Brückner: “These are extremely reactive molecules that attack and break down the pollutants in the water immediately. And the interaction of sunlight, photocatalyst and ozone promotes this formation of radicals. ”In fact, the radicals were so short-lived that at first it was not possible to identify them even with the modern analysis technology at LIKAT.
For such cases, the chemists use a trick they call spin trap: The radicals are captured with a neutral molecule, which then becomes a radical itself, albeit one that is hardly reactive and therefore “lives” long enough to be analyzed to become. This made it possible to identify the highly effective particles as hydroxyl radicals, molecules that consist of a hydrogen and an oxygen atom. The high effectiveness of the combination of photocatalyst, sunlight and ozone in wastewater treatment can be explained by the extremely rapid formation of an enormous number of reactive radicals.
The result of the research is “a new mechanistic concept” for this type of reaction, as Angelika Brückner says. The researchers assume that the method will soon find its way into practice with this background knowledge.
Scientific contact:
Prof. Dr. Angelika Brückner
[email protected]
doi.org/10.1021/acs.accounts.9b00624
Originalpublikation:
J. Xiao, Y. Xie, J. Rabeah, A. Brückner, H. Cao, Acc. Chem. Res. 2020, 53, 1024-1033.
Further information:
https://www.catalysis.de/forschung/katalytische-in-situ-studien/
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