Platinum is a precious metal, rivaling gold in price per ounce. The high cost of platinum, however, has limited its use in fuel cells, even though platinum is the most efficient electrocatalyst for these devices.
To address this problem, scientists have been studying core-shell electrocatalyst nanoparticles, which consist of a core made of one material coated with another material. That structure accommodates the placement of expensive platinum atoms only on the shell, where they can be accessed easily during the catalytic reaction.
Cu K edge XANES spectra with schematic of Cu local environment from EXAFS showing the transition of the Cu from solvated species during underpotential deposition on Au nanoparticles.
Radoslov Adzic and colleagues at Brookhaven National Laboratory pioneered one method of making such catalysts. It involves displacement of a monolayer of copper, in a reaction in which metallic copper is oxidized by platinum ions in solution to leave metallic platinum on the surface of core catalyst nanoparticles. Much of the understanding of this reaction comes from studying the deposition process on large smooth crystalline gold surfaces. Little was known, however, about differences that may be present on the surface of a practical catalyst.
In work done by Andrea Russell’s group at the University of Southampton, UK, x-ray absorption spectroscopy on beamline X23A at Brookhaven’s National Synchrotron Light Source (NSLS) was used to study the deposition process of copper on gold nanoparticles to investigate initial shell formation. This is the first step in preparing a platinum shell/gold core catalyst. The team’s results were published October 2011 in the Journal of the American Chemical Society.
Russell’s team found that the copper shell coating the gold core was incomplete. During deposition, the gold core remained relatively unchanged, but the local structure of the copper went from a hydrated ion in solution to a more metallic atom surrounded by gold. The amount of gold surrounding the copper showed that it first deposited into defect sites on the nanoparticle’s surface and then started forming clusters of copper around the initial sites. This is not the smooth shell predicted by single crystal studies.
Being able to characterize the structure and coverage of the copper shell has improved the understanding of how deposition occurs on nanoparticles. The researchers expect that this knowledge will lead to better control of shell formation and, eventually, cheaper and more active core-shell catalysts.
The high x-ray flux of NSLS allows the local electronic and geometric structure of nanoparticles to be probed in situ. The next step in this research is to follow the displacement of the copper shell with platinum and/or palladium to observe the final core-shell structure formation and learn how this process responds to changes in potential.
Russell’s research is supported by the Engineering and Physical Sciences Research Council in the U.K. and by Johnson Matthey, an international global specialty chemicals company headquartered in London. NSLS is supported by the U.S. Department of Energy.