A) Mass and (B) specific activities of the catalysts at 0.9 VRHE. (C) Mass activities (at 0.9 VRHE) and (D) specific ECSAs of the catalysts before and after accelerated durability test. The color scheme applies to all panels. Source: Zhang et al. Lei Zhang et al. Science 2015; 349:412-416 Click to enlarge.

In catalytic applications, only the surface layers of platinum contribute to the chemical reaction, leading researchers to develop new structures designed to maximize the amount of platinum exposed to reactants.

One strategy to increase the utilization efficiency (UE) of platinum group metals (PGMs) is to increase the proportion of atoms exposed on the surface (the dispersion) by reducing particle size. For example, the UE of platinum (Pt) atoms can be increased from 9.5 to 26% by reducing the edge length of a Pt cube from 11.7 to 3.9 nm. Despite the extensive use of this strategy, it has been difficult to optimize the specific activity of such small nanocrystals (NCs) by engineering their surface structure through facet-controlled synthesis. Such NCs also tend to sinter (form larger particles), detach from the support, or both during operation.

An alternative strategy is to use nanoframes—open nanostructures comprising multiple ridges as thin as a few nanometers. Each ridge of a nanoframe can be considered as a linear aggregate of NCs. Essentially, all the PGMs can be prepared as nanoframes by using methods that involve the selective removal of a sacrificial component: for example, the more reactive metal in alloyed NCs or the NC serving as a template for the site-selected deposition of the PGM. … this method still faces challenges in selecting the exposed crystal facet with which to control their catalytic activity and selectivity.

A different strategy for increasing the UE of a PGM is to assemble the metal atoms into nanosheets. For such a system consisting of four atomic layers, the UE could in principle reach 50%, but the use of PGM nanosheets as catalysts encounters several drawbacks: (i) the top and bottom surfaces of a sheet must be capped by ligands; (ii) the metal atoms can only assume a hexagonal lattice, corresponding to one type of facet only; and (iii) it is challenging to deposit and expose individual nanosheets on a catalytic support. An alternative to this strategy is to deposit the PGM conformally as sub-nanometer-thick shells of only a few atomic layers on the surfaces of NC templates made of another metal.

—Zhang et al.

The Georgia Tech-led team took that last approach. Their technique uses a solution-based method to grow platinum layers on palladium nanocrystal templates. The palladium is then etched away to leave behind nanocages approximately 20 nanometers in diameter, with between three and six atom-thin layers of platinum. When conducted appropriately, the researchers said, the facets presented on the surface of the template can be well preserved during the Pt coating and Pd etching processes so as to engineer the activity and/or selectivity of the catalyst.

In the work described in the paper, the researchers demonstrated the concept by coating the surfaces of Pd nanoscale cubes and octahedra with four atomic layers of Pt, followed by selective removal of the Pd templates. (The shape, cubic or octahedral, controls the surface structure, thus engineering the catalytic activity.)

We can get the catalytic activity we need by using only a small fraction of the platinum that had been required before. We have made hollow nanocages of platinum with walls as thin as a few atomic layers because we don’t want to waste any material in the bulk that does not contribute to the catalytic activity.

We can control the process so well that we have layer-by-layer deposition, creating one layer, two layers or three layers of platinum. We can also control the arrangement of atoms on the surface so their catalytic activity can be engineered to fit different types of reactions.

—Younan Xia, a professor in the Wallace H. Coulter Department of Biomedical Engineering at Georgia Tech and Emory University

Hollow platinum structures have been made before, but not with walls this thin, Xia said. Earlier work produced shells with wall thicknesses of approximately five nanometers. The new process can produce shell walls less than one nanometer thick. With both the inner layer and outer layer of the porous nanocages contributing to the catalytic activity, the new structures can use up to two-thirds of the platinum atoms in an ultra-thin three-layer shell. Some palladium remains mixed with the platinum in the structures.

Durability testing showed that Pt octahedral nanocages delivered the best performance, with the ORR mass activity only reduced by 36% after 10,000 cycles, still showing 3.4-fold enhancement relative to the pristine Pt/C. The ECSAs of the cubic and octahedral nanocages only dropped by 13 and 6% after 5000 cycles and by 32 and 23% after 10,000 cycles, respectively.

Contributing to the experimental work done at Georgia Tech, researchers at Arizona State University and Oak Ridge National Laboratory used their specialized microscopy facilities to map the nanocage structures. Researchers at the University of Wisconsin-Madison modeled the system to help understand etching of palladium from the core while preserving the platinum shell.

Researchers have explored alternatives to platinum, but none of the alternatives so far has provided the equivalent amount of catalytic activity in such a small mass, Xia noted.

Other authors in the paper include Professor Manos Mavrikakis and researchers Luke Roling and Jeffrey Herron from the University of Wisconsin-Madison, Miaofang Chi from Oak Ridge National Laboratory, Professor Jingyue Liu from Arizona State University, Professor Zhaoxiong Xie from Xiamen University, and Lei Zhang, Xue Wang, Sang-Il Choi, Madeleine Vara and Jinho Park, from Georgia Tech.

Resources

• Lei Zhang, Luke T. Roling, Xue Wang, Madeline Vara, Miaofang Chi, Jingyue Liu, Sang-Il Choi, Jinho Park, Jeffrey A. Herron, Zhaoxiong Xie, Manos Mavrikakis, and Younan Xia (2015) “Platinum-based nanocages with subnanometer-thick walls and well-defined, controllable facets” Science 349 (6246), 412-416 doi: 10.1126/science.aab0801