GOLD CATALYST
Figure 1. Oxygen atoms form wiggly lines on the iron-oxide surface, allowing single gold atoms to fall into place permanently.
Source: Vienna University of Technology.Gold atoms typically cluster on a surface, forming nanoparticles. The researchers found that using a special iron-oxide (Fe3O4) surface locked the single gold atoms in place, thus requiring less of the precious metal.
"Currently, industrial catalysts utilize precious metal nanoparticles supported on oxide surfaces. Such a nanoparticle contains hundreds or thousands of atoms. If one could have a similarly (if not more) efficient catalyst in which each active site was just one atom, cost savings of the order 100 times could be achieved. It is also worth mentioning that Fe3O4 is an extremely abundant and cheap material itself," says Gareth Parkinson, who along with Professor Ulrike Diebold, led the research at the Institute for Applied Physics at TU Vienna.
A slight deformation of the iron-oxide crystal structure, in which oxygen atoms in the topmost layer are bent into wiggly lines that allow the atoms to become closer to each other, keeps the gold atoms attached permanently (Figure 1). More details can be found in a recent article in Physics.
The team now is determining whether palladium and platinum afford similar single atom stability; the work should take a few months to complete. "Since we understand the electronic structure of the substrate to be responsible for the behavior, we expect the effect to be universal among such metals. Initial signs for Pd are encouraging in this regard," says Parkinson.
"We are particularly interested in investigating any differences in the thermal stability for different metals, since this might help us further understand the underlying physics involved," he adds.
While the single gold atoms are stable up to 400° C on the Fe3O4(001) surface, chemical stability remains a challenge.
"The crucial next step is to assess…whether the Au atoms are stable at high gas pressures. Such experiments require quite specialist equipment in which we can transfer samples produced in UHV [ultra high vacuum] into a high-pressure cell for analysis," says Parkinson. "To this end, we are collaborating with fellow researchers at the TU Vienna (Prof. Rupprechter), and also Maxlab in Sweden (Prof. Schnadt) to assess the Au/Fe3O4 system with high pressure IRAS [infrared reflection absoprtion spectoscopy] and high pressure XPS [X-ray photoelectron spectroscopy], respectively," he notes.The team also plans to run laboratory trials to determine whether single atoms can be used as a catalyst for CO oxidation and the water-gas-shift reaction. The team is using simple probe molecules such as CO, CO2, H2O and H2 to study the behavior of the system at the atomic scale.
"The experiments are already underway; I got back from the Maxlab synchrotron in Sweden a few days ago after a very interesting beamtime. Once we establish the atoms are stable at high pressures, we can begin to look into their catalytic properties," says Parkinson.
