A team of researchers from Northwestern University, Evanston, Ill., have discovered a method for controlling the faces exposed by metal nanoparticle catalysts that could improve their performance for a wide range of structure-sensitive catalytic reactions. This could have great impact for chemical companies and catalyst makers because the selectivity and yield of these nanoparticles as catalysts depends on which of their crystal faces are exposed.
Previous attempts to change the shape of the nanoparticles were unsuccessful as the structures reverted back to their original shape.
The team, led by Laurence D. Marks, professor of materials science and engineering at the McCormick School of Engineering and Applied Science, discovered that epitaxy -- the relationship between the position of the atoms in the nanoparticle and atoms on the substrate -- plays a bigger role than expected in determining which faces of a nanoparticle are exposed.
"Instead of trying to engineer the nanoparticles, we've engineered the substrate that the nanoparticle sits on," explains Marks. "That changes what faces are exposed."
Marks notes that current testing shows the nanoparticles appear to be stable enough to survive long-term use as catalysts. More details appear in an article by Marks and his team in a recent issue of Nano Letters.
"The trick is to find the appropriate reaction (which is structure-sensitive), then tune the appropriate metal/substrate combination to improve the performance. I would not be surprised if we find that the effect we talk about is already there in some current metal/substrate catalyst combinations used industrially -- and perhaps by using our concepts this could be designed to work better.
"We believe the concept as a method of designing better catalysts is most promising," Marks says. "This method could be used with a variety of different metal nanoparticles. It's a new strategy, and it could have a very big impact." For instance, Marks has seen a "noticeable improvement in propane oxidation due to the epitaxy for platinum on strontium titanate samples prepared by the same approach." He adds: "I think it fair to say that for some classes of reaction such as combustion (e.g., automotive catalysts) the improvements could be dramatic, and there may well be others."
For industrial production, Marks notes strontium titanate nanocubes substrate could be fairly easy to scale up. "There are discussions in the ALD [atomic layer deposition] community on scaling up the method of depositing the platinum for catalysts and similar systems, or with some developmental work one could probably work out an alternative method of depositing the Pt nanoparticles based upon existing methods which are already known to be industrially scalable," he adds.
The team would welcome chemical companies or catalyst makers cooperating on further development of such catalysts.