A team led by Andy Beale (University College London) in collaboration with the Research Complex at Harwell, Ghent University and Igor Sazanovich (CLF), made an insightful discovery about industrial catalysts using the CLF's Ultra Laser last year. Today, Nature Materials published their paper.
Zeolites are industrial catalysts and are very effective at converting methanol for the production of chemicals and fuels, especially to gasoline, aromatics or olefins. The use of zeolites as catalysts in industry are a rapidly growing global phenomenon due to the fact they help to replace the use of fossil fuels. However, in order to improve efficiency and reduce cost, far more needs to be understood about why the zeolite's performance declines with time. The stability of the catalyst performance is perhaps the major challenge for the widespread use of this technology, specifically the fast formation of carbon deposits during operation severely compromises process efficacy both due to this short effective lifespan and the cleaning process required for reactivation.
Despite much research on zeolites over the past 40 years, their reaction mechanisms are so complicated that there is a great deal left to be understood. Spectroscopy techniques such as Raman spectroscopy may aid research into zeolites, yet due to obstructing fluorescence that occurs during the process this has never been successfully implemented.
However, by applying a technique called Kerr Gate Raman, which was first developed at the CLF in 1999, the team were able to see what really goes on in industrial catalysts when they deactivate. This discovery could potentially lead to future improvement in catalyst efficiency.
The Raman Kerr Gate:
Prof. Mike Towrie, who helped collaborated on was a CLF collaborator on this experiment, said, “Kerr Gate Raman is remarkable because it makes it possible to detect tiny Raman signals from zeolite reactions that would otherwise be lost in a sea of fluorescence noise. The Ultra Laser Facility at the CLF allowed us to achieve a signal quality approximately 100X better than if we'd used standard Raman."
Raman spectroscopy involves detecting light from a laser beam that is scattered from an object; a tiny proportion of this scattered light interacts with molecules in the object causing the scattered light to change colour. This colour change is like a unique “Raman fingerprint" that can be used to identify what molecules in the object are made of.
However, the issue with zeolites is that they emit a large amount of fluorescence during the interaction. This completely overwhelms the tiny Raman signals and makes it practically impossible to detect the useful Raman fingerprint.
The team used the Ultra facility to apply a Kerr Gate modification to Raman Spectroscopy. This allowed them to take advantage of the different properties of fluorescence and Raman light, where Raman scattering happens instantly while fluorescence happens tens of picoseconds to nanoseconds after interacting with the object. This means that by using a picosecond pulse length laser for the Raman scattering it is possible to “gate out" the unwanted fluorescence after letting the Raman light come through.
The “Gating" in Kerr Gating relies on some very interesting physics to work. An intense laser beam operating in the near infrared triggers a liquid material called carbon disulphide to align its molecules briefly, making it birefringent (meaning that any light passing through changes polarisation). This lasts for about 3-4 picoseconds, after which the molecules return to normal and lose their birefringence.
When the picosecond laser beam hits the object, the Raman light scatters in a picosecond burst with the fluorescence light coming just after. Lenses collect the Raman and fluorescence light and it is then vertically polarised using a polariser before arriving at the Kerr Gate (the carbon disulphide). Due to the gate's birefringence, the Raman lights polarisation rotates to the horizontal plane. However, by when the fluorescence arrives at the gate, the birefringence has been lost, leaving the fluorescence vertically polarised.
Both types of light then in turn reach a crossed polariser that only allows the horizontally polarised Raman light through and blocks the vertically polarised fluorescence, leaving scientists with a clear Raman signal to read.
Using this technique, the team could see around 100X improvement in the signal quality and discovered something novel about the way the zeolites deactivated.
The science: For years, it was commonly thought that the formation of polycyclic aromatics on the surface of the zeolites was responsible for their deactivation. However, using a combination of Kerr gate Raman Spectroscopy and some insightful computational modelling, the team realised that what actually deactivates them is a species called polyenes, which form just before they react further to form polycyclic aromatics.
This was a completely new and unexpected finding. It is with this knowledge the team has proposed a design criteria for zeolites with ideal characteristics in order for them to last far longer.
There are not only financial implications in doing this, but environmental. There are huge energy costs associated with having to reactivate catalysts frequently, so the longer they last for the more energy efficient they are. With this new knowledge, the team plans to continue their investigation into the improvement of industrial catalysts.
Prof Mike Towrie said, "This work has been exciting because it demonstrates the potential of Kerr Gate Raman to help answer many more questions in catalysis and in other areas such as battery science."
Read the paper here.