Single Atom Alloy
Figure 1. Single platinum atoms on copper nanoparticles split hydrogen into atoms, enabling efficient and selective hydrogenation of butadiene. Source: Tufts University.
Copper, while a relatively cheap metal, is not nearly as catalytically powerful as platinum, notes Charles Sykes, professor of chemistry at Tufts. “We were excited to find that the platinum metal dissolved in copper, just like sugar in hot coffee, all the way down to single atoms. We call such materials single atom alloys,” he adds.
“We found that even at temperatures as low as -300°F these platinum atoms were capable of splitting hydrogen molecules into atoms, indicating that the platinum atoms would be very good at activating hydrogen for a chemical reaction,” notes Sykes.
Collaborating with Maria Flytzani-Stephanopoulos, a professor at the university’s School of Engineering, the researchers determined butadiene most suitable for industrial applications.
Flytzani-Stephanopoulos’s team synthesized small quantities of realistic catalysts, such as platinum-copper single atom alloy nanoparticles supported on an alumina substrate, and tested them under industrial pressure and temperatures. The catalysts achieved both 100% conversion and more than 95% selectivity to butenes. The stability of the catalyst also is very good, she notes. “To our delight, these catalysts worked very well and their performance was steady for many days.”
The researchers also discovered that using more platinum results in a less-efficient reaction, because clusters of platinum atoms have less selectivity compared to individual ones. An article in Nature Communications has more details on the experiment.
The next step is to try nickel/copper alloys. “O-H and C-H activation are important reaction steps for both hydrocarbon and alcohol reactions, and industrially Ni is a key element whose reactivity actually needs to be scaled back to avoid catalyst poisoning. Single atoms alloys offer great potential here,” explains Sykes.
At this time, the team has no plans to try the catalysts on pilot-plant scale. Sykes admits making such catalysts on an industrial scale does pose challenges, namely, long-term stability. “We are currently working and testing this as it is somewhat unknown,” he adds.
Other challenges to address include how to apply the approach more broadly beyond hydrogenation reactions, such as oxidations, coupling reactions, etc. “…We believe this approach is also applicable to other precious metals if added as minority components in copper,” notes Flytzani-Stephanopoulos.
“In this study, we took a fundamental approach to understanding the atomic scale structure and properties of single atom alloy surfaces and then applied this knowledge to develop a working catalyst. …We are now ready to compare the stability of these single atom alloy catalysts to single atom catalysts supported on various oxide or carbon surfaces. This may give us very useful criteria for industrial catalyst design,” she adds.
“…Catalyst research groups around the world are beginning to work in this new area we pioneered. This will drive the whole field forward and hopefully results will eventually be applied in industrial catalyst design. In fact, some industrial catalysts may already be benefiting from single-atom alloy architecture, but purely by accident,” Sykes concludes.
