A new nanoscale three-metal hybrid catalyst promises to lower the price of cleaning up car exhaust.
Automobile manufacturers have well-established technology for adding an oxygen molecule to the toxic carbon monoxide from vehicles’ tailpipes, so that the output instead consists of CO2. Unfortunately, this active catalytic oxidation does not come cheap — it depends on particles of the precious metal platinum to make the process more efficient.
Not surprisingly, researchers are regularly looking for ways of cutting back on the use of this metal in catalytic converter design. A team in Xiamen, China, recently demonstrated a promising alternative, which was confirmed with the help of two X-ray spectroscopy specialists at Dalhousie University. Associate professor of chemistry Peng Zhang was asked by the Chinese team to examine nanoparticles made up of a unique combination of platinum, iron and nickel. This material appeared to work, but the underlying chemical mechanism remained unclear.
“They were able to obtain very high efficiency for carbon monoxide oxidation, but the atomic structure of each element in the tri-metallic catalyst could not be determined with conventional analysis,” Zhang says. Zhang and his PhD student Paul Duchesne took samples to a synchrotron in Taiwan, where they were able to carry out a detailed X-ray absorption spectroscopic analysis on the workings of this material. “The iron species interacts with the platinum to enhance catalytic activity,” says Zhang. This interaction makes it possible to significantly reduce the amount of platinum. At the same time, the nickel species stabilizes the other two components and protects the interface between them from becoming dehydrated.
The result, published in Science, is a catalytic converter that operated for a full month in a humid setting without any decay. “We found that this tri-metallic particle uses a very nice strategy for improving the efficiency, the quality and the lifetime of the catalyst,” Zhang says.
The key to creating this three-metal hybrid lies in the interfaces between platinum, iron and nickel in the particles. Such intricate combinations have been achieved before but were enhanced in this case through the use of a wet-chemical method for optimizing the interfacial structure between the three components. Zhang acknowledges that this is cutting-edge science, where a thorough understanding of the nanoscale interaction of these elements has not yet been established. The success of this example could serve as a model for other investigators seeking to combine elements to create other types of particles with specific catalytic properties.
As this latest research illustrates, determining the source of those properties can pose a significant challenge for any conventional analysis. It took the unique structural determination capabilities of a synchrotron, which shines powerful, coherent X-ray beams at a sample, to carefully pick apart the behaviour of each component. In principle, Zhang says, this technique could distinguish the interaction of many nanoscale constituents but that would go far beyond the infrastructure and expertise at most laboratories. “Normally people try to combine two elements,” he says. “And with three, it’s already quite complicated.”