Scientists have figured out how to make platinum more affordable as a catalyst: turn it into a liquid at low temperatures.
It has been known for centuries that noble metals like platinum, gold, ruthenium, and palladium are excellent catalysts for chemical reactions because they help break chemical bonds between atoms more efficiently than other metals.
But noble metals are rare and expensive, so large-scale industrial manufacturers typically opt for cheaper, less efficient alternatives like iron. (Iron is used as a catalyst in the mass production of fertilizers, for example.)
The downside of using inferior catalysts is that chemical reactions must be heated to high temperatures, which increases the carbon footprint of many industrial processes.
In a record achievement, researchers from UNSW Sydney and RMIT in Australia have dissolved platinum in liquid gallium, splitting the platinum atoms so that there is more catalytic potential in a smaller amount of platinum.
Platinum normally has a melting temperature of 1700°C (3092 Fahrenheit), which means it is usually a solid when used as a catalyst.
By infusing platinum into a gallium matrix, it adopts the melting point of gallium – a soft, silvery, non-toxic metal that essentially melts at room temperature 29.8°C. A useful characteristic of liquid gallium is that it dissolves metals (like water dissolves salt and sugar) by breaking apart the individual atoms of each molecule.
According to the researchers, the invention has the potential to save energy costs and reduce emissions in industrial manufacturing.
“A range of important chemical reactions could be carried out at a relatively low temperature with the use of a more efficient catalyst like liquid platinum,” lead author and chemical engineer Md. Arifur Rahim of UNSW told ScienceAlert. Sidney.
Since 2011, scientists have been trying to make expensive noble metal catalysts more affordable through a process of “miniaturization”, says Rahim.
When metals are solid, only the atoms on the outside can be used in reactions, so there is a lot of waste. If you break this solid down into smaller and smaller clumps (think nanoparticles), you get a more efficient reaction because more metal atoms can fit into it – lots of hands do light work.
The most efficient and smallest system would make every atom available to do the work of a catalyst.
“When you miniaturize the system, you maximize surface-to-volume ratio and atom-use efficiency, so your overall catalyst consumption decreases over time, which can eventually make your product affordable,” says Rahim. .
“Theoretically, you get the maximum efficiency from this catalytic metal when it’s at the atomic scale, because you can’t go beyond that.”
In single atom catalysts, the bonds that hold the catalyst together are split and each atom is anchored individually in a substance called the matrix.
So Rahim and his colleagues tested gallium as a matrix. Once dissolved in the gallium, they discovered that each platinum atom was separated from all the other platinum atoms, making it a perfect miniature catalyst.
“Once dissolved, the platinum atoms are spatially dispersed in the liquid gallium matrix without atomic clustering (i.e. absence of platinum-platinum bond) which can result in different catalytic reactions with activity of remarkable mass,” the researchers write in their paper.
Platinum is mobile when in a liquid matrix, and much less prone to the problem of coking, where solid catalysts become coated in carbon and must be cleaned before they can be used again.
Gallium is not as cheap as iron. But it can be used over and over again for the same reactions. Indeed, like platinum, gallium does not deactivate or degrade during the reaction.
The process of dissolving platinum in gallium requires a temperature rise to around 400°C for a few hours. But this is a one-time energy investment that avoids further temperature rises later in the chemical manufacturing process, the researchers say.
The team hopes their technique will lead to much cleaner and cheaper products, from fertilizers to green fuel cells.
The study was published in the journal natural chemistry.