Platinum got plenty of attention over the weekend as the marker of the Queen’s 70th Jubilee. Its rarity is what makes it an extremely expensive metal, but platinum has some very valuable chemical properties too.
Platinum is a powerful catalyst – it can enhance a huge variety of chemical reactions, from making pharmaceuticals to capturing CO2 from the air. But it’s typically too expensive to use widely and it often demands very high temperatures to work.
A team of Australian researchers has figured out a way to use a very tiny amount of platinum as a catalyst – at close to room temperature. This discovery could eventually make platinum an affordable way to improve carbon capture, green hydrogen electrolysis, ammonia production for fertilisers, and a host of other industrial processes.
The trick, described in a paper in Nature Chemistry, revolves around mixing platinum with another, less expensive metal – gallium – and turning it into a liquid.
Lead author Dr Md Arifur Rahim, a postdoctoral researcher at the University of New South Wales’ School of Chemical Engineering, says that the group has been working on liquid gallium “for a very long time”.
“Gallium is interesting, because it’s a liquid close to room temperature, just like mercury,” says Rahim. Gallium melts at 30 °C – in the palm of your hand.
“Being liquid, we can consider these as a solvent.”
This means that, like water dissolving salt, gallium can dissolve other metals that are usually solid at much higher temperatures. Platinum, for instance, usually has a melting point of 1700°C, but it can be dissolved into gallium after being heated at 300°C for a couple of hours.
And once it’s in this liquid format, the platinum becomes a very efficient catalyst.
In fact, a mixture where only 0.0001% of the atoms were platinum was capable of catalysing several different types of test reactions the researchers tried. This catalyst was over 1000 times more efficient than a conventional solid, 10% platinum, catalyst and it worked between 40°C and 70°C which are very low temperatures on an industrial scale.
The researchers’ computational modelling suggests one possible reason for this: it’s to do with the way the platinum atoms are dispersed in the gallium mixture.
“The platinum never aggregates, and the [platinum] atoms don’t form a cluster,” says Rahim.
Instead, the platinum atoms provoke the gallium atoms completely surrounding them into working like catalysts, too.
“The platinum also activates the gallium toward catalysis,” summarises Rahim.
Co-author Dr Andrew Christofferson, a researcher at RMIT says: “The magic is happening on the gallium under the influence of platinum.
“But without the platinum there, it doesn’t happen. This is completely different from any other catalysis anyone has shown, that I’m aware of. And this is something that can only have been shown through the modelling.”
As well as being more efficient, this substance is more practical than its solid counterparts. Liquid catalysts are generally easier to refresh and use for longer in chemical reactions.
Rahim says that the team is now examining whether gallium has this effect on other noble metals (things like silver, gold and ruthenium) – all expensive and powerful catalysts in their own right.
“We see that there are some similarities, but still I can’t [yet] say that there is a rationalisation we can make,” says Rahim.