Unfolding secrets of catalysts
To construct catalysts that can produce fuels from CO2 innumerable times, we need to learn much more about how catalysis works. Irene Groot is conducting groundbreaking research into catalysis at the atomic level.
Zooming in on catalysis
If industry is to produce clean fuels via CO2 conversion in a way that is sustainable and cost-effective, it needs catalysts that offer the optimum performance. These catalysts must be made from a material on which the chemical reaction yields as much product as possible, and on which this reaction can take place a vast number of times. ‘Industry has been using catalysts for more than 100 years, of course, but in fact, they were found on the basis of trial and error,’ says associate professor Irene Groot. She is using highly refined new technology to research what exactly happens at the atomic level during catalysis. She is also replicating the environmental conditions that would be found in a factory: high temperatures and high pressure.
The textbooks say that during heterogeneous catalysis the surface on which the reaction takes place does not change. Groot’s research, however, confirms highly remarkable recent insights that contradict this assertion. ‘A catalyst is alive,’ she concludes, partly on the basis of her own observations. ‘During and also just after the chemical reaction, the structure of the surface changes dramatically.’
A probe tip consisting of an atom
Groot’s research group makes discoveries of this kind by means of scanning tunnelling microscopy used under reaction conditions. This unique technique was developed in Leiden by her predecessor Joost Frenken. He decided to scan the surface of a catalyst with a microscope with a probe tip consisting of a single atom – that is to say, incredibly thin – while a reaction was taking place. This last point is particularly important: until that time, catalysis had often been investigated by looking at the state before and the state after the reaction, but never during the reaction itself.
Another feature of Groot’s research is that she replicates factory conditions during the reaction and the scan: high pressure and high temperatures. ‘We do so by placing the probe in a very small reactor; the rest of the microscope is situated around that reactor,’ explains Groot. ‘In the reactor, we can introduce gases and heat up the catalytic surface, which is also located in the reactor. This allows us to see clearly how the surface changes under the influence of the gases and high temperatures. We also use mass spectrometry to measure the reactor’s emissions, which provides information about what products are made during the reaction on the surface.’
The differences revealed by this technique compared with catalysis under laboratory conditions are huge. Groot sees completely different structures arising on the catalytic surface under high pressure from under low pressure. ‘This is what we call the “pressure gap”. It indicates that a catalyst is alive; the catalyst isn’t “dead”, as people had assumed for a very long time. It plays an active part in the chemical reaction.’ Groot has also seen that after the reaction itself, the catalyst’s structure continues to change as the temperature and pressure decrease.
Leiden’s scanning tunnelling microscope is being further developed and marketed by a spin-off company, Leiden Probe Microscopy.
Developing new techniques
In addition to researching catalysis, Groot’s group also focuses on developing new techniques for studying catalytic surfaces. She was recently awarded a prestigious research grant to develop a technique that will clarify the atomic structure of the catalyst (where is each atom located?) and at the same time provide chemical information (which atoms are located on the surface?). This information will be collected while the reaction takes place. ‘We want to simultaneously look at the surface and determine which elements in the periodic system can be seen. This is theoretically possible, if you combine a scanning tunnelling microscope with X-rays. The microscope probe allows you to see the atoms, but you don’t know what their chemical composition is. When you radiate the catalytic surface with X-rays, you “knock” the electrons out of the atoms. For each chemical element, we know the specific X-ray wavelength at which this happens. The electrons that you knock out of the atoms with the X-rays are then captured with the microscope probe. The microscope probe thus acts as a kind of antenna, which receives chemical information about that one atom over which it is positioned. By scanning the surface with the probe while you also radiate it with X-rays, you can look at each atom to see where it is and which atom it is.
‘We started developing this new X-ray and probe technique two years ago, and now have a “proof of principle”. But this is only at room temperature and in an inert gas, that is to say, a gas that doesn’t react chemically with the catalyst. Our dream is that this grant will enable us to develop this technique so that it can be used for gases that do react with each other and at high temperatures.’
While Groot continues her research, she already has partnerships with companies such as Shell, DSM and Sabic, which are making use of her knowledge and findings.