UPTON, N.Y. — Chemists at the U.S. Department of Energy’s (DOE) Brookhaven National Laboratory, Stony Brook University (SBU), and their collaborators have uncovered new details of the reversible assembly and disassembly of a platinum catalyst. The new understanding may offer clues to the catalyst’s stability and recyclability. The work, described in a paper just published in the journal Nanoscale, reveals how single platinum atoms on a cerium oxide support aggregate under reaction conditions to form active catalytic nanoparticles — and then, surprisingly, fragment once the reaction is stopped.
Fragmentation may sound shattering, but the scientists say it could be a plus.
“Such reversible fragmentation of a platinum nanocatalyst on cerium oxide could be potentially useful for controlling the catalyst’s long-term stability,” said Anatoly Frenkel, a chemist at Brookhaven Lab and professor at SBU who led the research.
When the platinum atoms return to their starting positions, they can be used again to remake active catalytic particles. Plus, the post-reaction fragmentation makes those active particles much less likely to fuse together irreversibly, which is a common mechanism that ultimately deactivates many nanoparticle catalysts.
“Part of the definition of a catalyst is that it helps disassemble and reassemble reacting molecules to form new products,” Frenkel noted. “But it was shocking to see a catalyst that also assembles and disassembles itself in the process.”
The paper describes how the scientists observed the nanoparticles forming as single platinum atoms aggregated on the cerium oxide surface at 572 degrees Fahrenheit (300 degrees Celsius) — the temperature of the reaction they were studying.
“After the reaction, we expected that these nanoparticles would stabilize once back at room temperature in whatever particle size they reached when they were activated,” Frenkel said. “But what we observed was a reverse process. The particles began fragmenting into single atoms again.”
The team had a hypothesis to explain what they were seeing, which was confirmed by thermodynamic calculations performed by theory colleagues at Chungnam National University in Korea. Carbon monoxide, one of the products of the reaction — often considered a “poison” for catalysts — was actively tearing the nanoparticles apart.
“Carbon monoxide molecules have a very strong repulsive interaction when they are next to each other,” Frenkel explained. During the “reverse water gas shift” reaction, which converts carbon dioxide (CO2) and hydrogen (H2) into carbon monoxide (CO) and water (H2O) at high temperatures, the CO typically leaves the catalyst surface as a gas. But once the heat is turned off, the CO molecules bind strongly to the platinum atoms of the catalyst. This brings the CO molecules closer to each other as the system cools down and their numbers rise.
Read more on BNL website
Image: Scientists have shown that platinum atoms (gold spheres) on cerium oxide (red and silver/black surface) can assemble into active nanocatalysts under reaction conditions and then disassemble when cooled down before reuse.
Credit: Valerie Lentz/Brookhaven National Laboratory