New approach reversibly configures single and heteronuclear dual-atom catalysts on MoS₂ substrate

Single-atom catalysts (SACs) are materials consisting of individual metal atoms dispersed on a substrate (i.e., supporting surface). Recent studies have highlighted the promise of these catalysts for the efficient conversion and storage of energy, particularly when deployed in fuel cells and water electrolyzers.

Despite their advantages, including good selectivity (i.e., ability to produce specific products), lower cost and tunable reactivity, SACs typically exhibit some limitations. Most notably, substrates used in the past can only host a restricted number of SACs, which adversely impacts their collective catalytic performance.

Moreover, when too many SACs are loaded on a substrate, they can form clusters, which can also reduce a material’s catalytic efficiency. Recent studies have thus been trying to identify materials that can host more SACs, without prompting the formation of these clusters, and yet allowing the metal atoms to interact with individual atoms nearby to form dual-atom catalysts (DACs), pairs of metal atoms that can boost catalytic performance.

In a paper published in Nature Nanotechnology, researchers at the National University of Singapore showed that the two-dimensional (2D) material MoS₂ (molybdenum disulfide) could be a promising substrate for the deployment of SACs and DACs.

Specifically, they found that the selective removal of sulfur atoms using a technique known as electrochemical desulfurization enabled vacancy-rich domains in a metallic phase of MoS₂, known as 1T’-MoS₂, which in turn enables the loading of a high number of metal atoms on the substrate.

“There has been intense research in single atom catalysts (SAC) but the loading amount is low on most substrates, resulting in low catalytic efficiency,” Kian Ping Loh, senior author of the paper, told Phys.org.

“We have thus been looking for a material that can support high loading of SACs. Catalytic efficiency depends on loading. However, if we load too high a density, aggregation of the SACs can occur to form clusters, which reduces the catalytic efficiency.

“We set out to find a substrate that can immobilize the single atoms and prevent aggregation, even if some limited diffusion between the next nearest neighbor SAC is allowed to enable the formation of dual atom single atom catalysts (DAC).”

DACs are pairs of different metal atoms that synergistically interact with each other, boosting catalysis in some reactions. These pairs of atoms have been found to be particularly effective in driving hydrogenation reactions, chemical reactions that entail the addition of hydrogen atoms to molecules via the saturation of their multiple bonds.

In their paper, Loh and his colleagues introduced a new approach to facilitate the loading of more SACs on MoS₂ while also facilitating the formation of DACs. Their proposed approach relies on the use of electrochemical desulfurization, the selective removal of sulfur atoms from a material using an applied electrical current.

“We generated sulfur and molybdenum vacancies in MoS₂ using electrochemical techniques,” explained Loh.

“These vacancies allow foreign single atoms to be loaded with a concentration that is related to vacancy concentration. We then used operando X-ray absorption Near Edge fine structure at the Australian Synchrotron Beamline to probe the coordination environments of copper and platinum single atoms.

“This was complemented by high-resolution scanning transmission electron microscopy studies of the single atom catalysts on vacancy-enriched MoS₂.”

In their experiments, the researchers probed the catalytic material they realized using X-ray absorption spectroscopy, a technique to track changes in a material’s atomic structure.

Remarkably, they found that their approach successfully enabled the reversible shift from SAC to DAC configurations on their 1T’-MoS₂ substrate via the application of an electrical field.

“The most notable achievement of our study was the reversible generation of heterogeneous dual atom catalysts on demand (i.e., driven by an electric field),” said Loh.

“The reversible bonding and unbonding of the two different metal atoms in DACs is linked to the protonation and deprotonation of sulfur atoms on the MoS₂ matrix. In our future work, we plan to look for more examples of single atom A and B that can combine dynamically to form dual atom catalyst under an electric field.”

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