Scientists have developed a unique catalyst that enhances its ability to produce hydrogen from ammonia as time goes on, and they have identified atomic-level changes that improve its efficiency.
A research group from the University of Nottingham’s School of Chemistry, in partnership with the University of Birmingham and Cardiff University, has created an innovative material made of tiny ruthenium (Ru) clusters placed on graphitized carbon. These Ru nanoclusters interact with ammonia molecules to help separate ammonia into hydrogen and nitrogen, a vital step in generating green hydrogen. This significant research has been published in Chemical Science, the primary journal of the Royal Society of Chemistry.
Ammonia, due to its high energy density, shows great potential as a zero-carbon energy carrier that could support a sustainable economy in the near future. It is crucial to develop quick and energy-efficient processes to convert ammonia into hydrogen (Hâ‚‚) and nitrogen (Nâ‚‚) as needed. While it’s usual for catalysts to lose effectiveness over time, it is uncommon for a catalyst to become more effective with use. Thus, understanding the atomic-level processes that influence changes in catalyst activity is essential for creating future heterogeneous catalysts.
Dr. Jesum Alves Fernandes, an Associate Professor in the School of Chemistry at the University of Nottingham and co-leader of the research team, explained: “Traditional catalysts are made up of nanoparticles, where most of the atoms are not available for reactions. Our novel method involves starting with single atoms that assemble into clusters of a specific size. Consequently, we can stop these clusters from growing once they reach an area of 2-3 nm², ensuring that the majority of atoms remain on the surface and are thus accessible for chemical reactions. In this research, we applied this technique to grow ruthenium nanoclusters directly from atoms supported by carbon.”
The team used magnetron sputtering to produce a stream of metal atoms to build the catalyst. This method, which requires no solvents or additional reagents, allows for the creation of a clean and highly effective catalyst. Maximizing the surface area of the catalyst ensures the most effective use of scarce elements like ruthenium (Ru).
Dr. Yifan Chen, a Research Fellow at the University of Nottingham’s School of Chemistry, stated: “We were surprised to find that the activity of the Ru nanoclusters on carbon actually improves over time, contrary to the typical deactivation seen in catalysts during use. This fascinating discovery could not be explained using traditional analysis techniques, so we developed a microscopic approach to count the atoms in each nanocluster throughout various reaction stages using scanning transmission electron microscopy. We uncovered a range of small yet significant atomic-level changes.”
The researchers found that ruthenium atoms, which were initially disordered on the carbon surface, rearranged into stable, truncated nano-pyramids with stepped edges. These nano-pyramids remained stable for several hours during the reactions at high temperatures and continuously evolved to optimize the number of active sites, thus improving hydrogen generation from ammonia. This behavior accounts for the catalyst’s unique self-enhancing properties.
Professor Andrei Khlobystov from the School of Chemistry at the University of Nottingham remarked: “This discovery leads to a new approach in catalyst design, highlighting a stable and self-improving system for hydrogen production from ammonia as a green energy source. We believe this breakthrough will significantly advance sustainable energy technologies, aiding the transition to a zero-carbon future.”
This innovation represents a significant step forward in comprehending the atomic-level mechanisms involved in heterogeneous catalysis for hydrogen production. It opens opportunities for the creation of highly effective, stable catalysts that utilize rare metals sustainably by meticulously regulating catalyst structures at the nanoscale.
The University of Nottingham is committed to promoting green and sustainable technologies. Recently, the Zero Carbon Cluster was launched in the East Midlands to fast-track the progress and deployment of innovations in green industries and advanced manufacturing.
This research is supported by the EPSRC Programme Grant ‘Metal atoms on surfaces and interfaces (MASI) for a sustainable future,’ which aims to develop catalyst materials for the conversion of three essential molecules — carbon dioxide, hydrogen, and ammonia — that are crucial for both the economy and the environment. The MASI catalysts are produced in an atom-efficient manner to ensure the sustainable use of chemical elements without depleting rare resources and effectively utilize the earth’s abundant elements, like carbon and base metals.
