Nanoframes (NFs) made of noble metals have received great attention in recent years due to their remarkable performance in many applications including catalysis, plasmonics, and biomedicine. The highly open structure of NFs is mainly responsible for their enhanced properties and superior performance. In catalysis, for example, nanocatalysts with frame morphologies ensure increased surface area to volume ratio and reduced coordination number for atoms on the surface, thereby maximizing the catalytic activity.

… Over the past two decades, a variety of noble-metal NFs have been actively developed, including those made of Au, Ag, Pt, Pd, Rh, and a combination of them. … However, to the best of our knowledge Ru NFs have never been reported before. It should be noted that Ru nanostructures have found widespread use in many industrially important catalytic reactions such as hydrogenation, ammonia synthesis, and CO oxidation, which motivated us to explore NFs of Ru and investigate their catalytic properties.

—Ye et al.

In an effort to make better catalysts, Xia brought together a team from the Argonne National Laboratory, University of Texas at Dallas, and Temple University. Haihang Ye, Xia's graduate student, is first author on the paper; undergraduate Joseph Vermeylen also contributed to the work.

Ruthenium, a noble metal and fairly rare, has joined an elite group of catalyst metals including gold, platinum and palladium. Since the metals are so rare, there is a strong incentive to reduce how much of a catalyst is used in any given process. Usually researchers control shape and size—the key is more surface area. Nanoframes, which are nanoparticles with open centers, have an advantage with their gazebo-like atomic arrangements.

Since we have the interior space available, it’s another benefit. Because catalytic reactions occur only on the surface of materials, the surface atom arrangement has a great impact on determining the catalytic activity.

—Xiaohu Xia

The crystal structure of a material—i.e.the arrangements of atoms—can be difficult to manipulate. In general, ruthenium nanocrystals adopt the hexagonal close-packed (hcp) structure. But Xia and his team came up with an elegant solution for making ruthenium nanocrystals with another structure: face-centered cubic (fcc) structure.

The process involves two steps, growth and etching. Basically, ruthenium doesn’t naturally grow in a crystal structure that can be made into a nanoframe. Instead, the team grew the nanoframe on a core of palladium, which they later removed.

The first step starts with a palladium seed with an fcc structure, and the researchers used the metal as an atomic template. Ruthenium was directed to preferentially grow on the edges and corners of the palladium octahedron. That way, the newly formed ruthenium simply replicated the fcc structure of the palladium seed. The second step is called etching, which removes the palladium seed, leaving behind the hollowed out ruthenium nanoframe.

Schematic illustration showing the two steps involved in the synthesis of Ru NFs: (1) selective nucleation and growth of Ru on the corners and edges of Pd truncated octahedra, yielding Pd−Ru core− frame octahedra, and (2) formation of Ru octahedral NFs by etching away the Pd cores. Credit: ACS, Ye et al. Click to enlarge.

To ensure the material had catalytic potential, the team ran the ruthenium nanoframes through several diagnostic tests: the reduction of p-nitrophenol by NaBH₄ and the dehydrogenation of ammonia borane. While more data is needed to quantify how well ruthenium holds up against existing metal catalysts, Xia says the results from their experiments are promising.

Improving the material’s stability is the next line of research. Being such a new catalyst, it needs to be fully characterized and have its limits tested. For example, Xia says they will want to know how thick the nanoframe material can grow and still keep the fcc structure and be effective as a catalyst.

Most importantly, once the material has been vetted, researchers will be able to start applying the catalyst to several big challenges. Namely, Xia says that ruthenium nanoframes and other catalysts with unique crystal structures could improve hydrogen fuel production and carbon storage.

Resources

• Haihang Ye, Qingxiao Wang, Massimo Catalano, Ning Lu, Joseph Vermeylen, Moon J. Kim, Yuzi Liu, Yugang Sun, and Xiaohu Xia (2016) “Ru Nanoframes with an fcc Structure and Enhanced Catalytic Properties” Nano Letters doi: 10.1021/acs.nanolett.6b00607