Scientists have created a catalyst for hydrogen generation from ammonia that becomes more active with time, and by counting atoms revealed changes that boost the catalyst's performance.
A research team from the University of Nottingham's School of Chemistry, in collaboration with the University of Birmingham and Cardiff University, has developed a novel material consisting of nanosized ruthenium (Ru) clusters anchored on graphitized carbon. These Ru nanoclusters react with ammonia molecules, catalysing splitting ammonia into hydrogen and nitrogen—an essential step toward green hydrogen production. This groundbreaking research is published in Chemical Science, the flagship journal of the Royal Society of Chemistry (Link to be added).
With its high volumetric energy density, ammonia holds promise as a zero-carbon energy carrier that could drive a sustainable new economy in the near future. Finding fast and energy-efficient methods to crack ammonia into hydrogen (H₂) and nitrogen (N₂) on demand is essential. While catalyst deactivation is common, it is rare for a catalyst to become more active with use. Therefore, understanding the atomic-level mechanisms behind changes in the catalyst activity is critical for designing the next generation of heterogeneous catalysts.
Traditional catalysts consist of nanoparticles, with most atoms inaccessible for reactions. Our approach starts with individual atoms that self-assemble into clusters of a desired size. Therefore, we can halt the growth of the clusters when their footprints reach 2-3 nm-squared, ensuring that the majority of atoms remain on the surface and available for chemical reactions. In this work, we harnessed this approach to grow ruthenium nanoclusters from atoms directly in a carbon support.
The researchers employed magnetron sputtering to generate a flux of metal atoms for constructing the catalyst. This solvent- and reagent-free technique enables the fabrication of a clean, highly active catalyst. By maximizing the catalyst's surface area, this method ensures the most efficient use of rare elements like ruthenium (Ru).
We were surprised to discover that the activity of Ru nanoclusters on carbon actually increases over time, which defies deactivation processes typically taking place for catalysts during their usage. This exciting finding cannot be explained through traditional analysis methods, and so we developed a microscopy approach to count the atoms in each nanocluster of the catalyst through different stages of the reaction using scanning transmission electron microscopy. We revealed a series of subtle yet significant atomic-level transformations.
Researchers discovered that ruthenium atoms initially disordered on the carbon surface rearrange into truncated nano-pyramids with stepped edges. The nano-pyramids demonstrate remarkable stability over several hours during the reaction at high temperatures. They continuously evolve to maximize the density of active sites, thereby enhancing hydrogen production from ammonia. This behaviour explains the unique self-improving characteristics of the catalyst.
This discovery sets a new direction in catalyst design by showcasing a stable, self-improving system for hydrogen generation from ammonia as a green energy source. We anticipate this breakthrough will contribute significantly to sustainable energy technologies, supporting the transition to a zero-carbon future.
This invention marks a major advancement in understanding the atomistic mechanisms of heterogeneous catalysis for hydrogen production. It paves the way for developing highly active, stable catalysts that use rare metals sustainably by precisely controlling catalyst structures at the nanoscale.
The University of Nottingham is dedicated to championing green and sustainable technologies. The Zero Carbon Clusterhas been recently launched in the East Midlands to accelerate the development and deployment of innovation in green industries and advanced manufacturing.
This work is funded by the EPSRC Programme Grant 'Metal atoms on surfaces and interfaces (MASI) for sustainable future' www.masi.ac.uk which is set to develop catalyst materials for the conversion of three key molecules - carbon dioxide, hydrogen and ammonia – crucially important for economy and environment. MASI catalysts are made in an atom-efficient way to ensure sustainable use of chemical elements without depleting supplies of rare elements and making most of the earth's abundant elements, such as carbon and base metals.
