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Hydrogen is one of the simplest and most abundant elements on this planet. It is also an element which can make a valuable contribution to clearing the path to a sustainable energy industry. This has, so far, not been fully achieved, especially when it comes to large-scale production of hydrogen. Accessing hydrogen still is not a simple or clean process. Current methods to gain the element mostly involve fossil fuels as raw material. Sustainable production of hydrogen particularly relies on the use of efficient catalysts which split apart the hydrogen and oxygen in water molecules to gain hydrogen. These catalysts and the suitable combination of elements to create a catalyst, however, are one of the greatest challenges which need to be overcome for efficient hydrogen production.
Now (2023), chemists at the University of Kansas and the U.S. Department of Energy’s (DOE) Brookhaven National Laboratory have carried out research to get a better understanding of the mechanisms of hydrogen catalysts and managed to unravel the entire reaction mechanism for an important class of water-splitting catalysts.

The scientists were researching water-splitting catalysts in general when they noticed something peculiar about one catalyst called a pentamethylcyclopentadienyl rhodium complex, or Cp*Rh complex, which exhibited reactive behaviour in areas where molecules are usually stable. Typically, reactivity is known to occur directly at the metal center, but in this catalyst the ligand scaffold appeared to directly take part in the chemistry. To find out more about the unusual behaviour, they used a specialized research technique called pulse radiolysis. With the help of the power of particle accelerators pulse radiolysis can isolate rapid, hard-to-observe steps within a catalytic cycle. Electrons are accelerated to high velocities. When the electrons pass through the chemical solution, they ionise the solvent molecules and generate charged species that are intercepted by the catalyst molecules, which rapidly alter in structure. Time-resolved spectroscopy tools were used to monitor the chemical reactivity after this rapid change occurred. Spectroscopic studies yielded spectral data, which gave a good idea of a molecule’s structure. By comparing these structures to known structures, the scientists could analyse the physical and electronic changes within the short-lived intermediate products of catalytic reactions. Pulse radiolysis, finally,  enabled the scientists to single out one step and look at it on a very short timescale as it could resolve events at one millionth to one billionth of a second. Through combination of pulse radiolysis and time-resolved spectroscopy with electrochemistry and stopped-flow techniques, the team was able to get a detailed picture of every step of the complex catalytic cycle, including the unusual reactivity occurring at the ligand scaffold.

Finding a clean and reliable catalyst for hydrogen synthesis is one of the key interests of modern science as hydrogen has the potential to play an important part in the energy transition. In 2018, scientists prepared iridium, iridium-nickel and iridium-copper catalysts using incipient wetness impregnation and analysed them in the aqueous-phase reforming of glycerol with the help of La₂O₃ or CeO₂ as supports. The reactions were performed in a fixed bed reactor, feeding a solution of glycerol (10 wt %) in water, at 270°C and 58 bar. All IrNi catalysts had higher activity than Ir and IrCu, and, in general, La₂O₃ catalysts showed a better performance when compared to CeO₂ catalysts. The highest hydrogen production yield was achieved by bimetallic IrNi catalysts.

Image: EDS Mapping of (a) Ir/CeO₂; and (b) Ir/La₂O₃



Source: Francisco Espinosa-Moreno, Putrakumar Balla, Wenjie Shen, Juan C. Chavarria-Hernandez, Miguel Ruiz-Gómez, Saúl Tlecuitl-Beristain/ Ir-Based Bimetallic Catalysts for Hydrogen Production through Glycerol Aqueous-Phase Reforming/ Catalysts 2018, 8(12), 613, 3 December 2018/ doi.org/10.3390/catal8120613/ Open Access This is an Open Access article is distributed under the terms of the Creative Commons Attribution 4.0 International (CC BY 4.0)

In 2023, a novel design of a steam reforming reactor for an efficient biogas conversion to hydrogen was created. The design comprised a radial division of the catalytic insert into individual segments as well as substituting parts of the catalytic material with metallic foam. The segment configuration was improved with the help of a genetic algorithm to maximise the efficiency of the reactor. Changes in the catalytic insert design were found to have great bearing on the thermal conditions inside the reactor and to lead to moderation of the reaction rate. A significant enhancement in the reforming process effectiveness was achieved when decreasing the amount of the catalyst. The outcome of the research showed the capability for acquiring a similar level of biogas conversion with a 41% reduction of the catalytic material applied.

Image: The catalytic insert designs: (a) Reference reactor with homogeneous and continuous catalytic insert, (b) Strategy I-catalytic insert divided in the radial direction (equal width of inlet surface), (c) Strategy II-catalytic insert divided in the radial direction (equal area of inlet surface)



Source: Marcin Pajak, Grzegorz Brus, Shinji Kimijima, Janusz Szmyd/ Enhancing Hydrogen Production from Biogas through Catalyst Rearrangements/ Energies 16(10):4058, May 2023/ DOI:10.3390/en16104058/ Open Access This is an Open Access article is distributed under the terms of the Creative Commons Attribution 4.0 International (CC BY 4.0)

The research provided several potentially beneficial insights: One of the most remarkable features of this catalytic cycle the scientists discovered was that the ligands were directly involved in the catalytic process. Often, this area of the molecule does not partake in any reaction process. The scientists observed reactivity within the ligands that had not yet been shown for this class of compounds before. They discovered that a hydride group, an intermediate product of the reaction, latched itself onto the Cp* ligand, which proved that the Cp* ligand was an active part of the reaction mechanism.

The scientists are convinced that this research will make it significantly easier to design more efficient, stable, and cost-effective catalysts for producing pure hydrogen. They hope that they can make a valuable contribution towards creating a more sustainable energy future.

By the Editorial Board