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Nanoparticles for Improved Catalysts

Modern life relies heavily on the efficient functioning of catalysts. They can improve chemical reactions, are indispensable in many industrial manufacturing processes, and can even contribute to reducing emissions. They are also an integral part in battery design and transportation. It is all the more astonishing, therefore, that the exact mechanisms of a catalyst are still not entirely understood by science.

Now (2021), scientists from University of Illinois Chicago (UIC) and the U.S. Department of Energy’s (DOE) Argonne National Laboratory may have come one step closer to unravelling the mechanisms of catalytic processes. They have conducted a study on the nature of catalysts and found out that some types display higher stability and durability during chemical reactions which often cause quick catalyst degradation. The catalysts used for the study were made of alloy nanoparticles or nanosize particles consisting of multiple metallic elements, such as cobalt, nickel, copper and platinum. These particles are particularly suitable for many processes in the energy sector, such as water-splitting to generate hydrogen in fuel cells, methanol production through CO2 conversion, or more efficient energy production in solar cells. The study investigated these highly stable alloy nanoparticles and tried to analyse their composition during the oxidation process. For this purpose, the scientists embedded them into a silicon nitride membrane and directed different types of gas through a channel over the particles. Using a beam of electrons, the reactions between the particles and the gas were analysed, which showed a low rate of oxidation and the migration of certain metals — iron, cobalt, nickel and copper — to the surfaces of the particles during the process.

For many years, scientists have tried to improve catalytic processes using nanoparticles. In 2012, an experiment tried to assess what influence on the overall catalytic performance characteristics such as finite particle size, particle structure, particle chemical composition and metal-support interactions had. Therefore, they grew thin oxide films on metal single crystals under ultra high vacuum conditions and used these films as supports for metallic nanoparticles. During the experiment, special attention was paid to: the effect of dopants in the oxide support on the growth of metal nanoclusters; the effects of size of metal clusters on the binding energy of gas-phase adsorbates; the role of surface modifiers on catalytic activity; and the structural and compositional changes of the active surface as a result of strong metal-support interaction. They found that doping oxide materials was a promising method to change the morphology and electronic properties of supported metal particles and to start the direct dissociation and reaction of molecules bound to the oxide surface.

In 2020, scientists used operando transmission electron microscopy to show that Pd nanoparticles (NPs) underwent reversible structural and activity changes during heating and cooling in mixed gas environments consisting of O2 and CO. Below 400°C, the NPs took the shape of flat low index facets, which were inactive towards CO oxidation. Above 400°C, the NPs became rounder, and an increase in the conversion of CO to CO2could be witnessed. This behaviour reversed when the temperature was later reduced. This was probably due to adsorbed CO molecules suppressing the activity of Pd NPs at lower temperatures by stabilizing low index facets and reducing the number of active sites.

The advantages of this discovery lie in the many possible applications and benefits for improved catalytic processes, such as energy storage and conversion technologies, including fuel cells, lithium-air batteries, super capacitors and catalyst materials. Nanoparticles could also be used to develop corrosion-resistant and high-temperature materials.

The aim of the research was to understand how fast high-entropy materials react with oxygen and how the chemistry of nanoparticles evolves during such a reaction. The results showed that the high-entropy alloy nanoparticles had higher resistance to oxidation than general metal particles. The next step will be to use the knowledge gained to design better catalysts and test their suitability for different applications.