Figure 1. This false color scanning electron microscopy image shows tetrahexahedral nanoparticles that are more catalytically active than those available commercially. Source: Northwestern University.
A new method using metal nanoparticles to make catalysts could lead to better fuel cells, among other applications, say researchers at Northwestern University, Evanston, Ill. The technique yields so-called high-index facet nanoparticle catalysts that significantly outperform conventional ones, and also can restore spent catalysts, they add.
The multi-faced gem-shape of the catalysts allow atoms at the surface to become more catalytically active than those available commercially. Basic metal precursors combined with heat and stabilizing trace elements rapidly transform the shape into structures that are highly active. The technique doesn’t use ligands that can compromise catalytic activity.
The method works with five monometallic nanoparticles and a library of bimetallic nanoparticles, spanning seven different metals, including platinum, cobalt and nickel. The platinum catalysts were found to be 20 times faster than a conventional commercial catalyst for formic acid electro-oxidation. An article in the journal Science details the method and its results.
“We not only can prepare commercially desirable catalysts, but we can recycle used fuel cell catalysts into the most active forms… The fact that we can reclaim and reactivate these catalysts made of expensive materials is extremely valuable,” says Chad A. Mirkin, who led the research.
“The deactivated or post-use catalysts can be further transformed into high-index facet ones by alloying/dealloying with foreign metal elements (Sb, Bi, Pb, or Te). There are no limitations on the number of times they can be recycled as long as the catalyst support remains intact after the synthesis,” elaborates Liliang Huang, a member of the research team.
“This type of technology is ready to be scaled up and utilized widely in the catalysis community,” notes Mirkin. “Moreover, …there are no obvious challenges for larger-scale synthesis,” he adds. However, scale up will require some optimization efforts to achieve high yields of the high-index facet catalysts, cautions Huange.
The catalysts also are less susceptible to poisoning than conventional catalysts: “For the high-index facet nanocatalysts synthesized here, their surface is modified by foreign shape-regulating elements (Sb, Bi, Pb, or Te), which are known to be favorable for reducing poisoning from the reaction byproduct, carbon monoxide. Therefore, these catalysts are less easily deactivated than the commercial Pt/C catalyst,” explains Huang.
Future work at the Mirkin lab will focus on identifying the best catalysts for specific reactions of interest by screening megalibraries of multimetallic nanoparticles with both composition and size gradients. This will be coupled with the high-index facet particle synthesis method.
“Many industrial processes which use metal nanoparticles as catalysts will benefit from this strategy. The merit of this work is helping researchers and industry to design and optimize their target catalysts because now they only need to focus on developing protocols to produce size and composition-controlled catalysts of interest without worrying about particle morphology. This strategy can be used further to improve the surface reactivity of the catalysts and make them recyclable. We are looking forward to seeing the commercial application of this strategy.”
