How do you keep a copper catalyst from losing its oomph? Just add a dusting of platinum, says a new study published in Nature Materials.
A team of researchers, including scientists at the Department of Energy’s SLAC National Accelerator Laboratory, investigated a class of metal nanoparticles used as catalysts in major industrial processes. They found that adding a trace amount of platinum to copper nanoparticles greatly reduced an effect known as “sintering,” which causes these catalysts to degrade over time.
The Problem of ‘Sintering’
At peak performance, metal nanoparticles – which include copper, silver and gold – increase manufacturing throughput by a thousandfold, enabling efficient production of everything from plastics, fertilizer and fuels to medicine manufacturing and fuel cell operation.
But there’s a problem: Over time, the high temperatures needed for these reactions cause atoms to jump from some nanoparticles and glom onto others, an effect known as sintering. Sintering causes the nanoparticles to double or even triple in size, decreasing the active surface area and slowing the whole process down.
Anecdotal reports suggested a possible solution: adding trace amounts – less than one part per hundred – of an alloy material to the metal nanoparticles can sometimes reduce sintering. But it wasn’t clear why this “single-atom alloy” approach worked or how the effect could be applied more broadly.
“It was important to understand what was happening on a fundamental, mechanistic level, so we could apply this understanding to the design of other catalysts, rather than trying to find our way by trial and error," said Simon Bare, distinguished staff scientist at SLAC, co-director of the Stanford Synchrotron Radiation Light Source (SSRL) Chemistry & Catalysis Division.
Seeing Chemistry in Action
Using three distinct experimental approaches, researchers compared regular copper nanoparticle samples to copper containing less than one part per hundred of platinum, measuring how the alloy affected the copper’s reactivity, mobility and particle size over time.
At the University of California, Santa Barbara (UCSB), researchers first ran the samples through an accelerated aging process. The regular copper catalyst decreased to about 16% of its original reactivity, while the platinum-copper alloy maintained about 42% of its power. Then, researchers at Tufts University used scanning tunneling microscopy to image and probe the metal surface. As temperatures rose, atoms in the regular copper sample started hopping around, merging into larger, less effective particles. Meanwhile, atoms within the alloyed sample stayed put.
“We were intrigued by how trace amounts of platinum could have such a large effect on the stability of copper nanoparticles,” said Charles Sykes, author and professor of chemistry and chemical and biological engineering at Tufts.
Researchers then turned to SSRL, using in-situ X-ray absorption microscopy to study changes in nanoparticle size. "The capabilities we have at SSRL allow us to get X-rays in and out while the reaction is occurring, allowing us to follow the chemistry as it's actually happening, under real-world conditions, over many hours,” Bare said. He is the co-founder of Co-ACCESS, a program that supports SSRL users studying catalysis, including the authors of this study.
At SSRL, the regular copper sample grew substantially, from less than 1 nanometer to over 3 nanometers – an approximate 80% loss in active surface area. But the alloyed copper sample stayed under 1 nanometer.
"It was exciting to see how distinct experimental methodologies consistently showed substantially slower copper nanoparticle sintering with the addition of just one atom of platinum per hundred copper atoms,” said Phillip Christopher, author and associate professor of chemical engineering at UCSB.
From Curiosity to Confirmation
Together, the suite of experiments confirmed that this single-atom alloy method does indeed help the copper nanoparticles resist sintering and stay reactive. More importantly, they reveal the fundamental mechanism at play: In these reactions, the platinum atoms anchor themselves to key parts of the nanoparticles – known as edge and kink sites – which prevents copper atoms from detaching and jumping to larger particles.
The results have implications beyond copper nanoparticle catalysts. By understanding exactly how these alloys work, researchers now have a playbook for designing better metal nanoparticle catalysts across the board – helping to improve manufacturing and industrial processes across the country.
“My favorite part of this project is that it started with a curious observation,” said Jordan Finzel, co-lead author and chemical engineering PhD student at UCSB. “We just continued to pull on the thread, hypothesizing new experiments that continued to show the same trend time and again, which convinced us we were observing something real.”
Finzel conducted the experiment at SSRL as part of the DOE Office of Science Graduate Student Research (SCGSR) program. “The project would not have been possible without the SCGSR program, which allowed me to learn the intricacies of X-ray and beamline science from some of the best minds around and enabled us to design and execute these high impact experiments,” Finzel said.
This work was supported in part by the DOE Office of Science and the National Science Foundation. SSRL is an Office of Science user facility. Authors included researchers from University of California, Santa Barbara; Tufts University; SLAC; and University of Oklahoma, Norman.