Carriers for catalyst particles: bystanders or active players after all?

Theoretical and Computational Chemistry Institute (IQTC)
  • Found weak spot of catalysts used to remove toxic car exhaust gases
  • Inert supports to catalyst converters have a paradoxically non-neutral effect
  • The finding will help improve and extend lifespan of catalysts used to convert car exhaust gases

 

A new catalytic effect

A catalytic effect has been discovered that may contribute to enhance the effectiveness of catalytic converters in cars. The expectable impact: carbon monoxide (CO) emission cuts, and increased durability of these car components. A team of researchers from the Institute of Theoretical and Computational Chemistry (IQTC) of the University of Barcelona and the Vienna University of Technology contributed to this breakthrough.

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Catalytic converters are used to transform toxic exhaust gases into less harmful substances.

The work, published in the journal Nature Materials, had its experimental part conducted by the team of Prof. Günther Rupprechter, from Vienna UT. The data were analysed using computational modelling in Barcelona by the group of ICREA Professor Konstantin Neyman from the IQTC. Results showed that chemical processes in particles used as catalysts for automobile exhaust gases change significantly if they are placed on oxide supports. These supports, that are expected to be inert and not active in the chemical reaction, would hence be playing a role nonetheless, and affecting the catalyst, consisting in palladium microcrystalline particles.

The same as food should not vary depending on the material of the dish it is served on, in chemical reactions catalysed by metal particles, the substrate (i.e. the inert support) should not play a role in the outcome, neither on the conservation of the particle. Catalytic particles, with a diameter often spanning thousands of atoms, are thus not expected (neither desired) to be affected by chemical reactions happening outside the reaction interface; that is, away from the location where the actually catalysed reaction is taking place.

 

The poisoning of catalysts by carbon monoxide

Vehicles using a combustion engine use a catalytic converter to convert toxic exhaust substances into more innocuous compounds. Different types of catalysts are used in cars, but in all of them one of the main reactions involved is the conversion of the toxic gas carbon monoxide (CO) into carbon dioxide (CO2), which, while still a pollutant, it is far less toxic.

For this chemical transformation to take place, the catalyst surface first becomes covered with oxygen molecules. This is due to a phenomenon called adsorption, by which the atoms of these molecules reversibly interact with the surface of the catalyst, forming a thin layer around the catalyst surface.

While adsorbed on the catalyst particle surface, the atoms of the oxygen molecules become more reactive. As result, they become more readily available for converting CO to CO2. It is at the oxygen-covered surface that the interaction of the atoms of oxygen with CO takes place, leading the reaction to take place effectively and turning CO into CO2. Alas, under certain circumstances, a disruption of this process may occur.

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By the phenomenon of adsorption, a thin layer of molecules or atoms is formed around the adsorbent surface.

During the normal operation of these catalysts, the surface is covered by a layer of oxygen molecules. However, after a given oxygen atom reacts with a CO molecule, empty spaces appear on the oxygen layer that covered the catalyst particles. In order to sustain catalysis, these oxygen “gaps” on the catalyst need to be rapidly filled by other oxygen atoms. In the presence of significant amounts of CO molecules, this takes yet another turn: when CO is abundant enough, the empty sites can be occupied by CO instead of by oxygen.

When the latter process happens at a large enough scale, it results in the catalyst surface being covered by a CO layer, rather than with O2. As a result, CO2 can not be formed and the catalyst does not fulfil its function anymore. In such cases, “one speaks of a deactivated or ‘poisoned by carbon monoxide’ state of the catalyst”, says Professor Neyman.

 

An inert support... not just a bystander, nonetheless

The poisoning of a catalyst, and the rate of poisoning depend on how high is the CO concentration of exhaust gases. In fact, results showed that the material of the substrate or physical support where palladium catalyst grains placed is crucial. For instance, “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”, says Professor Yuri Suchorski, first author of the article.

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If large enough concentrations of CO are present in exhaust gases, the catalyst can more readily become "poisoned".

Questions arise as a result of this finding. For instance, why should the nature of the expectedly inert support affect the chemical reactions that take place on the surface of the catalyst particle? Moreover, why does the interface between the palladium catalyst particle and the support influence the behaviour of palladium particles deposited on them? Such questions were addressed by the researchers of this work.

Using a photoemission electron microscope, researchers can control the propagation of a chemical reaction in real-time. With this instrument, the authors learned that carbon dioxide poisoning always starts at the edge of a grain, at the contact place with the inert support. It is from there that carbon monoxide poisoning quickly expands over the whole particle, as happens with decay in a deteriorating apple.

 

Carbon monoxide attacks at the edge

The poisoning of catalyst particles starts where it does essentially due to geometrical reasons. To start with, oxygen atoms at the edge of the particle have fewer neighbouring oxygen atoms to replace them. In addition, when “oxygen gaps” appear at the edges, a CO molecule can fill the gap more easily than in the middle surface.

It is proven that the support modifies the properties of the catalyst metal particle: “according to our calculations, the bonds between the metal atoms of the particle and the adsorbed oxygen layer are strengthened precisely at the borderline to the support”, notes Professor Neyman. Stressing the effect of the support, he concludes, “palladium atoms in intimate contact with an oxide support can bind oxygen atoms stronger”.

As carbon monoxide starts its “poisoning” at the edge, this effect is crucial: the edge of the metallic oxide is the weak spot of the particle. If this is circumvented the particle of the catalyst is hence protected from CO poisoning, its long-term effectiveness in removing toxic exhaust gases enhanced, and its lifespan, largely extended.

 

Image credits

Car in the nature in Autumn picture is in the public domain and was downloaded from Pexels.

Adsorption phenomenon picture was is in the public domain and was downloaded from Simple Wikipedia.

Automobile exhaust gas was downloaded from Wikipedia and licensed via an Attribution-ShareAlike 3.0 Unported (CC BY-SA 3.0) license.

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