Single-atom catalysts show great promise on the surface

Catalysts make our cars more environmentally friendly and they are indispensable for the chemical industry. This is because they make certain chemical reactions possible – such as the conversion of carbon monoxide into carbon dioxide in car exhaust gases – that would otherwise happen very slowly or not at all.

Now, surface physicists at the Vienna University of Technology (TU Wien) in Austria have found that incorporating individual metal atoms into a surface in the right way allows their chemical behavior to be adapted, making new, better catalysts possible. They have reported promising results with iridium atoms in a paper in Angewandte Chemie.

Solid catalysts containing platinum are used to convert car exhaust gases. The gases come into contact with the metal surface, where they react together.

"Only the outermost layer of metal atoms can play a role in this process. The gas can never reach the atoms inside the metal, so they are basically wasted," explains Gareth Parkinson from the Institute of Applied Physics at TU Wien. It therefore makes sense to construct the catalyst not as a single large block of metal, but in the form of fine granules, as this makes the number of exposed active atoms as high as possible. Since many important catalyst materials (such as platinum, gold or palladium) are very expensive, cost is also a major issue.

For years, scientists have tried to formulate catalysts as finer and finer particles. In the best-case scenario, the catalyst would be made up of individual catalyst atoms, which would all be active in just the right way, but this is easier said than done. "When metal atoms are deposited on a metal oxide surface, they usually have a very strong tendency to clump together and form nanoparticles," explained Parkinson.

Instead of attaching the active metal atoms to a surface, it is also possible to incorporate them into a molecule with cleverly selected neighboring atoms. The molecules and reactants are then dissolved in a liquid, and the chemical reactions happen there.

Both approaches have advantages and disadvantages. Solid metal catalysts have a higher throughput, and can be run in continuous operation. With liquid catalysts, on the other hand, it is easier to tailor the molecules as required, but more difficult to separate the product from the catalyst after the reaction.

Parkinson's team at TU Wien is working to combine the advantages of both approaches. "For years, we have been working on processing metal oxide surfaces in a controlled manner and imaging them under the microscope," says Parkinson. "Thanks to this experience, we are now one of a few laboratories in the world that can incorporate metal atoms into a solid surface in a well-defined way.”

In much the same way that liquid catalyst molecules are designed, it has now become possible to choose the neighboring atoms in the surface that would be most favorable from a chemical point of view. This means that, using special surface-physics tricks, iridium atoms can now be incorporated into a solid matrix on a special iron oxide surface, producing a catalyst that can convert carbon monoxide into carbon dioxide.

"Single atom catalysis is a new, extremely promising field of research," says Parkinson. "There have already been exciting measurements with such catalysts, but so far it was not really known why they worked so well. Now, for the first time, we have full control over the atomic properties of the surface and can clearly prove this by means of images from the electron microscope."

This story is adapted from material from TU Wien, with editorial changes made by Materials Today. The views expressed in this article do not necessarily represent those of Elsevier. Link to original source.