Kerr-Gated Raman Spectroscopy is Being Used to Study Catalysis Reactions for Investigating Renewable Energy Sources

Recent collaborative research demonstrates the use of Kerr-gated Raman spectroscopy to study catalysts and catalytic reactions for investigating renewable energy sources. The approach overcomes fluorescence interference problems normally encountered using Raman spectroscopy of catalysts and can facilitate the development of renewable energy sources, such as producing gasoline from methanol.

This research, published in the journal Nature Materials (1), was performed through a collaboration between the Central Laser Facility (CLF) of the Science and Technology Facilities Council (STFC), University College London, Ghent University, and the Catalysis Hub, The Catalysis Hub is a United Kingdom consortium aimed at leading the world in catalytic science.

Catalysis is an extremely important field of study in chemistry and involves the materials and processes of increasing the rate of a chemical reactions by the use of catalysts. Catalysts are the materials used to increase reaction rates, by lowering activation energies for reactions, and can be repeatedly used while not being consumed in the catalyzed reaction. Different classification types of catalysts include electrocatalysts, organocatalysts, photocatalysts, enzymes, and biocatalysts. Catalytic reactions occur in nature as well as in synthetic chemical reactions, such as in the production of petroleum products (alkylation, catalytic cracking, catalytic converters); bulk chemical production (catalytic oxidation, hydrogenation, and hydroformylation); fine chemical production (Heck reaction, and Friedel–Crafts reactions); and in food processing (hydrogenation and biocatalysis).

Natural and synthetic zeolite crystals (Figure 1) are essential catalysts, and represent the most-used catalysts by weight in the world. Zeolites are crystalline alumino-silicate, porous materials with regular molecule-sized nanopores—having a specific physical structure of channels and cavities, which affect their catalytic properties and efficiencies (their adsorption, diffusion, and reaction properties). Understanding zeolite structure and reaction properties is an important area of research in the field of catalysis.

Figure 1: An electron microscope image of the sort of catalyst crystals under study at CLF. (Image courtesy of Prof Andrew Beale et al. University College London).

Understanding the detailed structure and function of zeolites in chemical reactions has been an analytical challenge. The pores of zeolites may be contaminated with heavy hydrocarbons over time that get trapped within them—as contamination occurs over time the catalytic activity will decrease and eventually cease. When the zeolite is deactivated it must be replaced or reactivate using an expensive process. This entire activation and deactivation process is of keen interest to catalysis researchers.

One of the most common techniques used to investigate zeolite structure and catalysis is Raman spectroscopy. However, the fine structural information that might be available using the Raman technique is often obscured due to background fluorescence. CLF’s expertise in Kerr-gated Raman spectroscopy is providing the means for detailed spectra to be obtained without the fluorescence background problems.

Kerr-gated Raman spectroscopy uses the Kerr gate optical effect to remove most of the fluorescence background from the Raman signal when using Raman spectroscopy. The Kerr gate effect functions by plane polarizing the combined Raman scattered signal and fluorescence signal (resulting from a 1 ps laser pulse) by using a plane polarizer. Then by passing that combined signal through a 45^o tilting CS₂ containing cell of 2 mm pathlength (Kerr medium), the resulting signals are separated into a plane polarized fluorescence signal and a perpendicularly polarized Raman signal. By then passing the signals through a cross polarizer, the Raman scattering will pass (is transmitted) to the detector, while the fluorescence signal is blocked. Therefore the fine Raman spectral structure may be observed relatively free from the fluorescence background. The Kerr gate effect takes advantage of the time delay between the instantaneous Raman scattering signal and the delayed background fluorescence that occurs a few picoseconds or nanoseconds later.

“Kerr-gated 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, said Professor Mike Towrie of CLF “The Ultra Laser Facility at the CLF allowed us to achieve a signal quality approximately 100 times better than if we’d used standard Raman. For me, this work has been exciting because it demonstrates the potential of Kerr-gated Raman to help answer many more questions in catalysis and in other areas such as battery science.”

Kerr-gated Raman spectroscopy is being applied to gain understanding of the potential for catalytic reaction chemistry to produce gasoline (mostly octane) from simple hydrocarbons, such as methanol. Better understanding of this reaction could result in producing gasoline from biomass fermentation as a renewable energy source—reducing both fuel production costs and environmental impact. Methanol is considered a clean energy option that can be produced from many renewable sources. A methanol molecule has only half the energy density of gasoline and, in the 1970s, the chemistry was developed to convert methanol into gasoline using a catalytic reaction involving zeolites. The size of the zeolite pores determines the type of hydrocarbon produced by the catalytic reaction—small pores produce light hydrocarbons—medium pores produce gasoline.

Using Kerr-gated Raman spectroscopy the CLF team were able to track all the stages of the reaction, in increments of three to four picoseconds, and observe a conversion of methanol to octane as longer chains of hydrocarbons were being formed. This allowed detailed observations of the entire reaction process. The Raman technique also provides information of the activation and deactivation of the zeolite catalysts. This rapid analysis technique has provided insight into the formation of polyenes as the cause of deactivation of the zeolite catalysts—a new and unexpected discovery. “I think that this will change the way people think about how these materials deactivate because, as soon as you know who the culprits are, you are able to design a strategy, said team leader Professor Andy Beale of University College London. He said, “There is a huge energy cost, as well as time cost, associated with reactivating catalysts frequently, so anything that can keep catalysts doing their job for longer will save money, energy and reduce the environmental impact resulting in greener fuels. The study could be significant for industry especially as methanol can be produced from biomass stocks meaning that gasoline may not have to come from fossil fuel stocks in the future.”

Reference
(1) I. Lezcano-Gonzalez, E. Campbell, A. E. J. Hoffman, M. Bocus, I. V. Sazanovich, M. Towrie, M. Agote-Aran, E. K. Gibson, A. Greenaway, K. De Wispelaere, V. Van Speybroeck, and A. M. Beale, “Insight into the effects of confined hydrocarbon species on the lifetime of methanol conversion catalysts,” Nature Materials 19, 1081–1087 (2020). (https://doi.org/10.1038/s41563-020-0800-y)