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Chemical oscillations in palladium nanoparticles could pave way for recycling precious metal catalysts

Researchers at the University of Nottingham's School of Chemistry used transmission electron microscopy (TEM) to observe the complete lifecycle of palladium nanoparticles in a liquid environment, from nucleation through growth to dissolution, with the entire cycle repeating multiple times. This study is published in .

One of the most important applications of metal nanoparticles is in catalysis, which forms a backbone of chemical industries.

Dr. Jesum Alves Fernandes, an expert in the field, said, "The mechanisms of catalysis involving palladium have been hotly debated for many years, particularly as the distinction between homogeneous (in solution) and heterogeneous (on the surface of nanoparticles) catalysts becomes blurred at the nanoscale.

"The discovery that palladium nanoparticles can switch between these two modes can help us to develop new efficient catalysts for net-zero reactions, such as carbon dioxide reduction and ammonia synthesis. Additionally, this knowledge could help in the recycling and reuse of critical metals like palladium, whose global supplies are rapidly decreasing."

The laws of thermodynamics cause chemical reactions, including those involving nanoparticles, to proceed in one direction. While oscillating chemical reactions are uncommon in manmade processes, they frequently occur in living organisms that exist away from thermodynamic equilibrium.

Understanding these chemical oscillations could help us unravel some of nature's mysteries, including transitions from chaos to order, emergent behaviors, patterns in animal coats, and even the origins of life on Earth.

Professor Andrei Khlobystov leads the research group at the University of Nottingham that focuses on imaging chemical reactions of individual molecules and atoms, in real time, and direct space.

He says, "We set out to study the formation of palladium nanoparticles in a liquid and were happy to observe the nanoparticles forming directly during TEM observation. These nanoparticles emerged from the palladium salt solution, growing larger and more structured over time.

"To our astonishment, once the nanoparticles reached a size of about 5 nanometers, they began to dissolve back into the solution, disappearing completely, only to undergo re-growth again."

The nanoparticles create a complex branching pattern in a liquid pool, pulsating cyclically as they grow and dissolve. However, when the reaction is carried out in a droplet of solution contained within a carbon nanotube—serving as a miniature test tube—the lifecycle of the nanoparticles can be observed at atomic resolution.

The carbon nanotube slows down the process, allowing for detailed observation of the early stages of nucleation, growth, and dissolution. This reveals a disk-like shape with crystal facets, suggesting interactions of the nanoparticles with the solvent molecules.

Dr. Will Cull, a Research Fellow at the School of Chemistry, University of Nottingham, said, "The key to understanding this unexpected phenomenon lies in recognizing that electron microscopy is a powerful imaging technique that can also alter the material being observed.

"This approach is often used to carve structures with the electron beam, but in this case, the energy of the electron beam is harnessed to break carbon-hydrogen bonds and displace valence electrons from the bromide anions in the solvent. As a result, chemical reactions are triggered while we image our sample."

Dr. Rhys Lodge, who conducted the measurements, explains, "We believe that the chemical reactions involving the solvent, activated by the electron beam, drive the reduction of palladium ions to palladium metal, as well as the oxidation of palladium metal back to palladium ions. Due to the competition between these two processes, the nanoparticles continuously grow and shrink, oscillating chemically between these two states."