Nanocrystals get in shape for catalysis

New research in fine tuning the shape and size of nanoparticles could lead to important advances in catalysis, say scientists. Two independent groups of researchers have shown that the catalytic activities of tiny platinum crystals can be greatly enhanced by tightly controlling their shape and size.

Metallic nanoparticles make powerful catalysts due to their high surface area to volume ratios. But activity also depends on shape and, so far, scientists have struggled to create crystals with well-defined surfaces, or facets, below the 100 nanometre boundary.

Now Matthias Ballauff and colleagues at the University of Bayreuth in Germany and the Israel Institute of Technology (Technion) have developed a method for creating faceted platinum crystals in the range of two to five nanometres.¹ In a separate study, a team at the University of California, Riverside, US, and University of Lyon in France, led by Francisco Zaera, made tetrahedral nanocrystals of a similarly small size and applied them to trans-cis isomerisation reactions.²

Ballauff’s team made their nanocrystals, which are stuck to polymer beads, from gold-platinum alloys, using cyanide to leach out the gold. ’It was a good luck discovery,’ says Ballauff. ’If you take the gold atoms out of these nanoalloys, you have these very nice, faceted platinum nanocrystals.’

The discovery, he says, is a good example of how things in the nanoworld may be qualitatively different than in the bulk world - as bulk metals, gold and platinum don’t mix. The team used gold to form their nanoalloys, because it can be easily dissolved, but Ballauf hopes their research will encourage others to start working through the rest of the periodic table, trying out different metal combinations.

Taking sides

Using a model reaction - the reduction of p-nitrophenol - the researchers tested their crystals’ catalytic activity and found they had some of the highest turnover rates ever shown. But Ballauf thinks their nanocrystals could be used to perform more technically challenging organic reactions, such as the coupling reactions that join together two phenyl rings.

’Most previously published methods for the synthesis of defined nanocrystals unfortunately deliver particles which are too large,’ says Edman Tsang, who studies nanocrystals as University of Oxford, UK. ’What’s most impressive is that such a simple leaching method works in a controlled manner without small surfactant or stabiliser molecules, so the surfaces of the nanoparticles remain clean from strong adsorbates. Therefore, this new method could allow the production of supported metal particles as real practical catalysts for chemical conversions in aqueous phase.’

YuYe Tong, an expert in nanocrystal catalysis at Georgetown University in Washington, US, says the key to single crystal catalysis at the nanoscale is controlling the precise orientation of the crystals, so that only one type of facet is exposed. In Ballauf’s case, he notes, two different types of facet - 002 and 111 facets - are exposed. ’The novelty for me is not the shape or the size,’ he says, ’But this kind of heterogeneous catalysis approach where the nanoparticles are on the polymer beads and you can dissolve these in solution.’

Flipping fats

Using a previously reported technique,³ Zaera’s team created nanocrystals that self assemble on a silicon surface from colloidal platinum, resulting in only 111 facets being exposed at the surface. But they went further, showing the crystals could selectively drive the isomerisation of olefins in the trans to cis direction. Their work raises the possibility of using nanoparticles to catalyse a wide range of different isomerisation reactions of hydrocarbons, including reactions important in fuel cells and even in minimising the production of the unhealthy trans fats produced in the food manufacture.

According to Zaera, however, the real aim is to demonstrate the potential of new synthetic approaches to catalysis. ’Platinum is expensive so it is conceivable that the food industry would not pick up our way of making the catalyst. But what we are doing is opening up a new approach in general catalysis; using novel synthetic techniques - nanotechnology and self-assembly - and applying them to make these catalysts in a very specific way,’ he says.

Tong agrees. He says Zaera’s work reflects a push within the field towards a new direction in catalysis. Although, he adds, the researchers aren’t the first to show such control over the shape of their crystals.

’They demonstrate an elegant and useful example of catalysis using nanoparticles,’ says Ballauf. ’There is a lot of literature on the catalytic activity of metallic nanoparticles, but now people like us are going and pursuing more practical applications, which are more interesting for the organic chemists.’

Hayley Birch