Scientists from the University of Barcelona and the Vienna University of Technology have unearthed a new catalytic effect that could render automotive catalytic converters more effective and can reduce the emission of carbon monoxide (CO).
The outcomes of this research have been reported recently in the Nature Materials journal and were acquired through the experiments performed by the team of Professor Günther Rupprechter, from TU Vienna (Austria). The outcomes were understood with the help of the computational modeling carried out by the team headed by ICREA Professor Konstantin Neyman, from the Institute of Theoretical and Computational Chemistry of the University of Barcelona (IQTCUB).
In contrast to what was believed to date, the experiments performed by the group demonstrate significant variations in the chemical processes that occur in palladium microcrystalline particles used as catalysts for automobile exhaust gases, upon being placed on oxide supports — which are inactive during the chemical reaction.
The icing on a chocolate cake should taste the same irrespective of whether it is served on a silver or porcelain plate. Likewise, in the case of chemical reactions that take place on large metal particle surfaces, the substrate (or the support) should not have a major role. Often, the diameter of the catalytic particles spans thousands of atoms, and thus far, it was considered that the support on which they are placed does not have an impact on the chemical reactions that occur far from the interface.
Toxic carbon monoxide poisoning
In vehicles with combustion engines, toxic CO is transformed into carbon dioxide (CO2) by the catalyzers containing platinum or palladium. In this process, oxygen atoms cover the surface of the catalyst particles, with which CO molecules react and are transformed into CO2, making empty sites in the oxygen layer. For catalysis to be sustained, these sites must be quickly filled by other oxygen atoms.
Yet, this is not what happens when these empty sites are filled by CO molecules, and not oxygen molecules. In case this takes place on large scale, the surface of the catalyst is no more covered by an oxygen layer but by a CO layer, and thus, it is no longer possible to form CO2.
One speaks of a deactivated or ‘poisoned by carbon monoxide’ state of the catalyst.
The support influences the entire particle
This condition occurs based on the concentration of CO in the exhaust gas supplied to the catalyst. Yet, the experiments validated the material in which palladium grains have been supported to be significant. “If palladium particles are placed on a surface of zirconium oxide or magnesium oxide, then poisoning of the catalyst happens at a higher concentration of carbon monoxide,” stated Professor Yuri Suchorski, first author of the article.
The reason for the impact of the nature of the support on the chemical reactions that occur on the surface of the entire metal particle and for the effect of the interface between the support and the palladium particle on the behavior of palladium particles, which are a thousand times larger, is still obscure. This can be found out through experiments conducted in the Institute of Materials Chemistry of the TU Vienna and the computational quantum modeling performed at the University of Barcelona.
The scientists employed an exclusive photoemission electron microscope for controlling the propagation of a chemical reaction in real time. They were able to observe the way CO2 poisoning always began at the edge of a grain that was in contact with the support. Following this, the CO poisoning propagated over the entire particle, similar to a tsunami wave.
Carbon monoxide attacks best at the edge of the particle
There are geometrical reasons for this poisoning to begin at the edge: oxygen atoms located at the particle’s edge have fewer neighboring oxygen atoms than those located within the catalyst.
Once the free sites are opened at the edges, a CO molecule can fill them easily when compared to the free sites in the middle, where CO can easily react with other atoms. However, it is hard for the oxygen atoms to easily fill these voids at the edge as they go in pairs, similar to O2 molecules. Hence, to fill one of these free sites, O2 requires two free spaces — one adjacent to the other.
Therefore, the borderline at which the palladium particle gets into direct contact with the support is highly significant: support alters the characteristics of the metal particle.
According to our calculations, the bonds between the metal atoms of the particle and the absorbed oxygen layer are strengthened precisely at the borderline to the support, palladium atoms in intimate contact with oxide support can bind oxygen atoms stronger.
This could be considered irrelevant for the metallic sites located far away from the edge of the particle since the support has an effect only on the edge atoms — and these are only a few when compared to the total number of atoms of a palladium particle. Yet, as CO starts its “poisoning” at the edge, this effect turns out to be very important. The weak part of the particle is the edge of the metallic oxide; if strengthened, the catalyst particle can be protected from the CO poisoning.