Imagine a scientist who wants to quickly spread the word about an exciting discovery from their lab. A snappy social media post will likely reach more people than a 10,000-word thesis. In chemistry, just like in communications, sometimes smaller is better. A class of chemicals called single-atom catalysts (SACs) exemplifies this perfectly.
Downsizing catalysts to a single-atom level offers many benefits for industrial-scale chemical reactions, as they are more efficient and can enhance reactions in ways that larger heterogenous catalysts can’t. As A*STAR’s Institute of Sustainability for Chemicals, Energy and Environment (ISCE2) Scientist Shibo Xi explained, “SACs can achieve maximum atom utilisation, exhibit unique selectivity and activity, and be easily separated from the reaction system so that the catalyst is recycled.”
However, SACs struggle with more complex reactions involving multiple reactants. An example is nitrile-azide cycloaddition, a click reaction used in applications such as drug development. The reaction involves a dinuclear process that requires the activation of two or more reactants. Furthermore, such multi-reactant requirements aren’t compatible with SACs—which are single metal atoms dispersed on a solid support—because the individual metal atoms are scattered too far apart.
In response, Xi teamed up with collaborators from the Department of Chemistry at the National University of Singapore to explore ways of drawing metal atoms closer together to facilitate more elaborate chemical reactions. In particular, the team looked closely at how the inter-atomic distance on copper-based SACs influenced its ability to catalyse nitrile-azide cycloaddition.
The researchers first created a spectrum of SACs, from low-loading SACs bearing a smaller ratio of metal atoms to ultrahigh-loading SACs. Their goal was to find the optimal distance between copper atoms that supports multi-reactant reactions.
After extensive testing, they found this inter-atomic distance sweet spot to be around 0.7 nm, which is achieved at a 21-weight percent of copper. The team then verified the structural evolution of such material during catalytic reactions using an advanced solid catalyst-monitoring technique called operando X-ray absorption fine structure (XAFS).
According to Xi, this verification revealed the active catalytic centres in their ultrahigh-loading SACs. “Once a cluster is formed, the Metal-Metal scattering pathway is reflected in the XAS spectrum,” Xi said. “We confirmed that two nearby copper atoms facilitate the dinuclear activation of reactants.”
This is only the beginning for the researchers, who are already looking to expand the use of SACs to include other liquid-phase organic reactions. “The SAC used in this work is based on g-C3N4 which has good photocatalytic activity,” said Xi, adding that the group plans to look into how their ultra-high loading SACs may benefit other light-activated reactions.
The A*STAR-affiliated researcher contributing to this research is from the Institute of Sustainability for Chemicals, Energy and Environment (ISCE2).