Catalyst for hydrogen from ammonia gets more active over time

This is the claim of a research team from Nottingham University's School of Chemistry that has developed a novel material consisting of nanosized ruthenium (Ru) clusters attached to graphitised carbon. Developed in collaboration with Birmingham and Cardiff Universities, the Ru nanoclusters react with ammonia molecules, splitting them into hydrogen and nitrogen. The team’s findings are detailed in Chemical Science.

With high volumetric energy density, ammonia is said to hold promise as a zero-carbon energy carrier, so 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, so understanding the atomic-level mechanisms behind changes in the catalyst activity is critical for designing the next generation of heterogeneous catalysts.

In a statement, Dr Jesum Alves Fernandes, an Associate Professor in Nottingham’s School of Chemistry, and co-leader of the research team, said: “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 2-3nm-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.”

To construct the catalysts the researchers employed magnetron sputtering to generate a flux of metal atoms. According to Nottingham University, this solvent- and reagent-free technique enables the fabrication of a clean, highly active catalyst. By maximising the catalyst's surface area, this method ensures the most efficient use of rare elements like ruthenium (Ru).

Dr. Yifan Chen, a Research Fellow at Nottingham University's School of Chemistry, said: “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.”

The researchers found that ruthenium atoms initially disordered on the carbon surface rearrange into truncated nano-pyramids with stepped edges. The nano-pyramids are said to demonstrate remarkable stability over several hours during the reaction at high temperatures, continuously evolving to maximise the density of active sites, thereby enhancing hydrogen production from ammonia.

Professor Andrei Khlobystov, from the School of Chemistry at Nottingham University, said: “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 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 carbon dioxide, hydrogen and ammonia.