In chemistry, atoms can typically only influence their immediate vicinity. At TU Vienna, a novel effect with surprising long-range action has been revealed, which can make automotive catalytic converters very effective.
Just like the flavor of a chocolate cake´s icing does not depend on whether it is served on a silver or porcelain plate, the substrate (the so-called support) must not play a key role in chemical reactions on the surface of large precious metal grains. The catalytic grains typically have a diameter covering many thousands of atoms, and the support on which they rest should therefore not impact chemical reactions on the other side far away from the interface - at least this was assumed so far.
Experimental studies conducted at TU Wien brought forth unexpected findings. Chemical processes on palladium grains, which are also used for exhaust gas catalysts, transformed considerably when they were placed on particular support materials - regardless of the fact that the material of the support is virtually inactive in the chemical reaction itself. This novel finding has recently been published in the journal Nature Materials.
Toxic Carbon Monoxide
For vehicles fitted with an internal combustion engine, toxic carbon monoxide (CO) must be changed into carbon dioxide (CO2). This is accomplished by using catalysts comprising platinum or palladium powder.
We have investigated chemical reactions on powder grains, which are often used in industrial catalysis. The precious metal grains have a diameter on the order of 100 micrometers - this is very large by nanotechnology standards, one can almost see them with the naked eye.
Professor Günther Rupprechter, The Institute of Materials Chemistry at TU Wien
When oxygen atoms cover the surface of the powder particles, CO molecules react with them and are converted to CO2, leaving empty areas (holes) in the oxygen layer. These holes must be speedily filled by other oxygen atoms to sustain catalysis. But, this is no longer the scenario when CO molecules enter into these holes instead of oxygen. If this occurs on a large scale, an oxygen layer no longer covers the catalyst surface instead a CO layer covers it, and CO2 cannot be produced anymore. This phenomenon is termed as "carbon monoxide poisoning", it neutralizes the catalyst.
The Support Influences the Entire Grain
Whether this takes place or not is subject to the CO concentration in the exhaust gas supplied to the catalyst. However, as the present experiments reveal, the support material on which the palladium grains are placed is also important.
"If the Palladium grains are placed on a surface of zirconium oxide or magnesium oxide, then poisoning of the catalyst occurs at much higher carbon monoxide concentrations," says Prof. Yuri Suchorski, the study’s first author.
Apparently, this is unexpected for such large palladium grains. Why should the nature of the support have an effect on chemical reactions that occur on the surface of the whole metal grain? Why should the contact line between palladium grain and substrate, which is just a few tenths of a nanometer wide, affect the performance of palladium grains that are hundred thousand times larger?
This issue was finally solved with the aid of the special photoemission electron microscope at the Institute of Materials Chemistry at TU Wien. With this device, the spatial propagation of a catalytic reaction can be observed in real time.
"We can clearly observe that carbon monoxide poisoning always starts at the edge of a grain - exactly where it contacts the support," explains Prof. Yuri Suchorski. "From there, the "carbon monoxide poisoning" spreads like a Tsunami wave over the whole grain."
Carbon Monoxide Attacks Best at the Border
It is mostly for geometrical reasons that the poisoning wave begins exactly there: the oxygen atoms at the border of the grain have fewer adjacent oxygen atoms than those on the inner surface. When free sites form there, it is thus easier for a CO molecule to fill these sites than those sites someplace in the middle of the free surface, where CO would easily react with other oxygen atoms all around. Furthermore, it is not easy for other oxygen atoms to fill empty areas at the border, as oxygen atoms always appear in pairs, as O2 molecules. Thus, to fill an empty site, O2 needs two free sites adjacent to each other, and there is not a lot of room for this at the border.
The borderline where the palladium grain is in direct contact with the support is thus of critical strategic significance - and precisely at this interface the support is able to impact the properties of the metal grain:
"Calculations by our cooperation partners from the University of Barcelona show that the bond between the metal atoms of the grain and the protective oxygen layer is strengthened precisely at the borderline to the support," says Prof. Günther Rupprechter.
The palladium atoms in close contact with the oxidic support can, therefore, bind the oxygen stronger.
One may accept that this does not matter for metal sites located away from the border of the grain, as the support can only energetically impact atoms at the border - and these are just very few than the total number of atoms in the palladium grain. Nevertheless, because carbon monoxide poisoning begins at the border, this effect is of critical strategic significance. The metal-oxide border is actually the "weak point" of the grain - and if this weak point is strengthened (the catalytic properties of metal atoms at the border are positively influenced by the support), the whole micrometer-size catalyst grain is protected from CO poisoning.
Various oxide supports are already used in catalysts, but their exact role during catalysis in terms of CO poisoning has not yet been directly observed. With our methods, the ongoing process and its wave-like long-range effect were directly visualized for the first time, and this opens up promising new routes towards improved catalysts of the future.
Prof. Günther Rupprechter
This research received funding from the Austrian Science Foundation (FWF) within the framework of the Special Research Program (SFB) FOXSI and was also a collaborative effort with the Universitat de Barcelona (Spain).