Catalytic nanocages, which are tiny, open structures with reactive surfaces that could boost key chemical processes, are notoriously difficult to synthesize. Scientists recently succeeded in a new approach: they built the hollow structures by depositing platinum onto a palladium template and then etching away the template. The result? Tiny cages with catalytic surfaces inside and out.
With well-defined facets and high catalytic activity, these nanocages can be engineered to meet future needs. The hollow nanocages also use less amount of palladium, a scarce noble metal.
Improving the efficiency of catalysts could enhance many chemical reactions used in industry and research laboratories. To improve efficiency, researchers have created nanocages (open nanostructures composed of multiple facets and thus, catalysts with tunable activity and selectivity). However, engineering these structures with specific crystallographic facets was difficult. To improve the catalysts, researchers from Georgia Tech, University of Wisconsin-Madison, Arizona State University, and Oak Ridge National Laboratory developed their own synthesis technique to create hollow, shape-selected nanocages.
The cages can be shaped with a template to create various desired facets, thus solving the major problem with the previous structures. To produce the nanocages, platinum is deposited onto a palladium template at high temperatures. Platinum atoms form a relatively uniform shell around the palladium core, typically around 6 atomic layers thick. An intermixing of palladium occurs in the platinum shell during platinum deposition, causing the formation of palladium channels connecting the surface with the interior of the nanocages.
A palladium-specific etching technique is used to remove the core of the structures, starting with the palladium channels. These channels widen and restructure through the etching process, allowing for the majority of the palladium core to be removed. The structure that remains is the platinum shell with the facets from the template preserved.
Both octahedral and cubic nanocages have been synthesized, each with greater catalytic activity for the oxygen reduction reaction, the fundamental reaction occurring at a fuel cell cathode, than a standard platinum/carbon catalyst. In particular, the octahedral structures have more than five times the oxygen reduction reaction activity of the platinum/carbon catalyst.
The nanocages have catalytic surfaces both inside and outside their structure, which likely is the reason for the increased activity. In addition to the two types of nanocages that have already been synthesized, the nanocages can be engineered to have many different catalytic facets. This versatility should allow the platinum nanocages to selectively catalyze a broad range of chemical reactions.
The syntheses were supported by start-up funds from the Georgia Institute of Technology (to Y.X.). As jointly supervised PhD students from Xiamen University, L.Z. and X.W. were also partially supported by fellowships from the China Scholarship Council. The theoretical modeling work at University of Wisconsin-Madison was supported by the U.S. Department of Energy, Office of Science, Office of Basic Energy Sciences (BES), Chemical Sciences, Geosciences, and Biosciences Division, grant DE-FG02-05ER15731. Calculations were performed at supercomputing centers located at the Environmental Molecular Sciences Laboratory, which is sponsored by the DOE Office of Biological and Environmental Research at the Pacific Northwest National Laboratory; Center for Nanoscale Materials at Argonne National Laboratory, supported by DOE contract DE-AC02-06CH11357; and National Energy Research Scientific Computing Center, supported by DOE contract DE-AC02-05CH11231. Part of the electron microscopy work was performed through a user project supported by the Oak Ridge National Laboratory's Center for Nanophase Materials Sciences, which is a DOE Office of Science User Facility. J.L. gratefully acknowledges the support by Arizona State University and the use of facilities in the John M. Cowley Center for High Resolution Electron Microscopy at Arizona State.
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