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Molecular glue stabilizes single-atom catalysts

The date of: 2022-11-03
viewed: 1
source:acs


By using a molecular “glue” to hold atoms in place, researchers have come up with a method that disperses individual metal atoms across a solid surface and prevents them from coalescing. The advance may lead to practical procedures for making commercial catalysts that maximize use of expensive metals such as platinum.
Many industrial-scale chemical processes are driven by solid catalysts consisting of tiny chunks of a noble metal such as platinum or rhodium dispersed on a solid support—often a metal oxide. Known as heterogeneous catalysts because the catalytic material is solid and the reactants are gases and liquids, these catalysts drive petroleum refining, polymerization chemistry, emissions cleanup, and other processes.
Smaller is better when it comes to supported metal catalysts. That’s because the catalytic hot spots that convert reactants to products reside on the surfaces of the particles. Reactant molecules cannot reach metal atoms in the interior of a metal particle, so those atoms don’t participate in the reaction. In a sense, they’re wasted. To use these precious metals as efficiently as possible, catalyst manufacturers generally disperse the metal as particles in the low-nanometer size range. Doing so maximizes the number of exposed reactive sites, which boosts catalytic activity. It also reduces costs by using less precious metal to achieve the desired level of activity.
The ultimate in efficiency, therefore, would be single-atom catalysts. Recently, scientists have come up with ways to make such catalysts and have shown that they can be used to drive Suzuki reactions and generate hydrogen from methanol and from water.
But single-atom catalysts tend to be unstable, especially when used in high-temperature reactions, because metal atoms don’t like being isolated, which is energetically unfavorable. So the atoms often diffuse across the surface of the support and bond with other metal atoms, growing into many-atom particles through a process known as sintering. Researchers have ways to suppress sintering, but those methods often render catalysts inactive.
To bypass those problems, an international team of researchers came up with a cerium oxide glue that pins metal atoms in place but leaves them catalytically active. Led by Yong Wang of Washington State University; Bruce C. Gates of the University of California, Davis; and Jingyue Liu of Arizona State University, the researchers used an aqueous method to deposit positively charged cerium species on a negatively charged silica support. They heated the product to form isolated cerium oxide islands. Then they used another electrostatic adsorption step to decorate each island, on average, with a single platinum atom (Nature 2022, DOI: 10.1038/s41586-022-05251-6).
To assess stability, the team subjected the catalysts to high-temperature oxidizing and reducing conditions typical of catalytic reactions and found that the metal atoms stay put. Proof-of-concept tests show that the single-atom catalysts actively mediate oxidation of carbon monoxide, a key reaction in engine emissions cleanup. The team explains that because of platinum’s much stronger affinity for cerium oxide than for silica, the atoms have the freedom to move about their island and mediate reactions, but they remain confined to their respective ones.
“This is an impressive example of engineering at the atomic scale,” says E. Charles H. Sykes, a surface scientist at Tufts University. “I expect this approach will be widely investigated as a way to increase the stability of single-site catalysts.”
Sykes notes one drawback. He says the chemistry that the single atom performs will be related to the composition of the nanoglue island, which slightly limits the method’s versatility. “But for commercial applications, stabilizing platinum atoms under both oxidizing and reducing conditions is a big step forward.”



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