A research team from the University of Nottingham's School of Chemistry, in collaboration with the University of Birmingham and Cardiff University, has created a catalyst for hydrogen generation from ammonia that becomes more active with time. The novel material consists of nanosized ruthenium (Ru) clusters anchored on graphitized carbon.

These Ru nanoclusters react with ammonia molecules, catalyzing splitting ammonia into hydrogen and nitrogen. An open-access paper on this research is published in the RSC journal Chemical Science.

Chen et al.

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.

—Dr Jesum Alves Fernandes, co-leader of the research team

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 such as 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.

—Dr Yifan Chen, lead author

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 behavior explains the unique self-improving characteristics of the catalyst.

The 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 work was funded by the EPSRC Programme Grant ‘Metal atoms on surfaces and interfaces (MASI) for sustainable future’, 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.

Resources

• Yifan Chen et al. (2025) Evolution of amorphous ruthenium nanoclusters into stepped truncated nano-pyramids on graphitic surfaces boosts hydrogen production from ammonia, Chem. Sci., doi: 10.1039/D4SC06382A