The Quest for the Low-Active Surface
In surface science, you may not only want to know how to get a good surface, but also how to get a bad one (so you can avoid it).
Catalysts are needed for purification of exhaust gas
Automotive engines release a number of pollutants which might be harmful to humans, animals, and the environment. One such pollutant is carbon monoxide (CO), which is colourless and odourless, but acutely toxic. However, it can be transformed into carbon dioxide (CO2), which is less toxic. For this to happen, the CO must be oxidized. Oxygen molecules (O2) react with CO molecules and form CO2 molecules, according to the following reaction:
2 CO + O2 –> 2 CO2
However, this reaction is difficult to have in the gas phase. But with a catalyst, it can be achieved more easily. A catalyst is a substance that enables a chemical reaction without being consumed itself. Catalysts can thus be used to enable desired reactions that do not easily occur without their help. The image below shows an example of a catalyst. The polluted air passes through the grid. The walls of the grid are covered with catalytically active nanoparticles where the reactants can adsorb, and the formed product then desorbs from the surface of the nanoparticles. The air that comes out through the grid of the catalyst has then been purified from the toxic substances.
For CO oxidation, a catalytically active metal can be used as a catalyst. The CO molecules and O2 molecules adsorb on the catalyst surface, then the O2 dissociates into atomic oxygen (O). The O atoms react with the CO molecules, forming CO2 that then desorbs from the catalyst surface. Some examples of catalytic metals that can oxidize CO to CO2 are palladium (Pd), platinum (Pt), and rhodium (Rh). It is also possible to have alloys as catalysts.
But sometimes, things go wrong
Unfortunately, the catalyst surface may become oxygen poisoned, meaning that oxygen adsorbs strongly to the atoms in the surface layer so that no adsorbates can bond at the surface. Thus, no reaction can occur. Oxygen poisoning renders the catalyst low-active, which of course is a problem. To avoid this, one needs to understand how it works and which surface configurations (i.e. how the surface is built atomically) that are high- and low-active, respectively.
Who is the guilty one?
It has been shown that metallic Pd and thin surface oxides of PdO are high-active, but if the oxide grows too thick, it becomes low-active. So what happens when the oxide grows thicker? Why does it become less catalytically active? A possible explanation to this is that the oxide film exposes different surface configurations depending on its thickness.
The thin PdO film exposes a surface configuration called (101), then the film is called PdO(101). The (101) is called the Miller index of the surface and it indicates how the atoms in the surface are positioned relative to each other. This surface has undersaturated Pd atoms, meaning these atoms are bonding to fewer neighbours than four, so they have “dangling bonds” and thus can bond to adsorbates such as CO and O2 molecules which can react and form CO2.
When the oxide films grows thicker, another surface configuration called (100) may be exposed instead of (101). According to theoretical calculations, PdO(100) should be the most stable surface configuration, so it is likely to form when the oxide film grows thick enough to lose registry with the substrate (see Figure 4 in this article for a graphical explanation). Opposite to PdO(101), the PdO(100) surface does not have undersaturated Pd atoms; all Pd atoms bond to four neighbours and cannot bond to adsorbates. This makes the surface in question low-active.
The quest for the low-active surface
If we know how the low-active surface forms and how oxygen poisoning works on the atomic scale, we can prevent the catalyst from losing its catalytic functions. We have done different experiments trying to decide if the oxide film really does expose the (100) surface when it grows thick enough. Experiments with low-energy electron diffraction (LEED) and transmission surface diffraction (TSD) indicate that a PdO(100) film is formed when the oxide grows thick. However, our experiments with high-energy surface X-ray diffraction (HESXRD) did not confirm that this is the case.
To summarize, we need more studies to fully understand how PdO(100) forms and how it affects the catalytic activity. The quest goes on…