Expensive platinum needs cheap metal oxides for catalysis

Platinum is a great catalyst and can be used for many different applications. It's expensive stuff, though, so tiny platinum nanoparticles sitting on cheap metal oxide materials are often used to convert harmful carbon monoxide into carbon dioxide.

Using scanning tunneling microscopes, scientists at TU Vienna have now been able to image the catalytic behavior of platinum nanoparticles sitting on iron oxide, which has allowed them to understand the process at an atomic scale. Surprisingly, the chemical reactions do not take place on the platinum nanoparticles themselves, rather it is the interplay between platinum particles and the iron oxide surface that makes the reaction so efficient. This work is reported in Angewandte Chemie.

The tiny nanoparticles used for catalysis often consist of only a few platinum atoms. They promote oxidation by keeping target molecules in place and bringing them into contact with oxygen. In this way, carbon monoxide (CO) can be turned into carbon dioxide (CO₂), and hydrogen gas (H₂) can be oxidized into water (H₂O). These reactions do still take place without platinum, but they can occur at much lower temperatures in the presence of platinum particles.

"We used to believe that these chemical reactions occur right on top of the platinum particles, but our pictures clearly show that the iron oxide really does the job", says Gareth Parkinson. For years he has been studying the behavior of tiny particles on metal oxide surfaces together with Ulrike Diebold (both TU Vienna). Now, the scientists have been able to show that the oxygen needed for the chemical reactions does not originate from the surrounding atmosphere but from the iron oxide below.

The iron oxide (Fe₃O₄) on which the platinum particles rest possesses remarkable properties. It has a regular crystal structure and each atom has a well-defined position, but the iron atoms are still relatively free to travel through the material. When the platinum nanoparticles catch molecules from the surrounding gas, whether carbon monoxide or hydrogen, and combine them with oxygen atoms from the iron oxide surface, this leaves a surplus of iron atoms. These iron atoms then migrate deep into the material, leaving a hole in the surface that can clearly be seen in the images taken with the scanning tunneling microscope.

This process can even trigger a chain reaction. When the chemical reaction at the platinum nanoparticle creates a hole in the iron oxide surface, atoms right at the edge of the hole are only bound weakly to the rest of the material. As a consequence, the next chemical reaction can occur much more easily at these edges. The platinum nanoparticle is shifted slightly and is ready for the next step. "In the end, we can see long trenches on the surface, left behind by a single platinum nanoparticle," says Diebold.

The opposite phenomenon occurs when platinum and iron oxide are exposed to an oxygen atmosphere. The platinum particles break up the oxygen molecules (O₂) into single atoms, which can then be integrated into the surface. As iron atoms travel to the surface from within the material, they form iron oxide islands, right next to the platinum nanoparticles. Instead of holes, many small islands grow on the surface.

For years, the team at TU Vienna has been working hard to lay the necessary groundwork for this kind of research. In many important steps, the surface science team perfected ways to handle metal oxides and tiny particles. In recent years, they have presented important new findings about the structure of metal oxides, the mobility of atoms on their surface and their chemical properties. Based on this experience, it has become possible to reveal the chemical processes of platinum catalysis and explain them in detail.

This new knowledge can now be used to create even better catalysts. For instance, the team propose that pre-treating platinum catalysts with hydrogen should increase their efficiency. The atomic trenches created that way should keep the platinum nanoparticles from clustering, as this clustering decreases their reactivity.

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