Scientists have figured out how to make platinum more accessible as a catalyst: turning it into a low-temperature liquid.
It has been known for centuries that noble metals such as platinum, gold, ruthenium and palladium are excellent catalysts for chemical reactions because they help to break the chemical bonds between atoms more efficiently than other metals.
But noble metals are rare and expensive, so large-scale industrial manufacturers often opt for cheaper, less effective alternatives, such as iron. (Iron is used as a catalyst in the mass production of fertilizers, for example.)
The disadvantage of using lower quality 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 dissolved platinum in liquid gallium, splitting the platinum atoms so that there was more catalytic potential in a smaller amount of platinum.
Platinum typically has a melting temperature of 1,700 °C (3,092 Fahrenheit), meaning 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 basically melts at room temperature of 29.8°C. A useful feature of liquid gallium is that it dissolves metals (as water dissolves salt and sugar) by separating the individual atoms in each molecule.
The invention has the potential to save energy costs and reduce emissions in industrial manufacturing, the researchers say.
“A number of important chemical reactions can be carried out at relatively low temperatures using a more efficient catalyst such as liquid platinum,” said lead author and chemical engineer Md. Arifur Rahim of UNSW Sydney told ScienceAlert.
Scientists have been trying to make expensive noble metal catalysts more affordable through a “miniaturization” process since 2011, explains Rahim.
When metals are solid, only the atoms on the outside can be used in the reactions, so there’s a lot of waste. If you break this solid into smaller and smaller clumps (think nanoparticles), you get a more efficient reaction as more metal atoms can clump together – lots of hands do the light work.
The smallest and most efficient system would make each individual atom available to do the work of a catalyst.
“When you miniaturize the system, you’re maximizing the surface-to-volume ratio and the atom utilization efficiency so your overall catalyst consumption is lower over time, and that can make your product affordable,” says Rahim.
“Theoretically, you get the maximum efficiency of this catalytic metal when it’s on 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 individually anchored in a substance called a matrix.
So Rahim and his colleagues tested gallium as their matrix. Once dissolved in gallium, they found that each platinum atom was separated from every other platinum atom, making it a perfect miniature catalyst.
“When dissolved, platinum atoms are spatially dispersed in the liquid gallium matrix with no atomic grouping (i.e., the absence of platinum-platinum bond) which can drive different catalytic reactions with remarkable mass activity,” the researchers write in their paper. .
Platinum is mobile when in a liquid matrix and much less prone to the coking problem where solid catalysts become covered in carbon and need to be cleaned before being reused.
Gallium is not as cheap as iron. But it can be used over and over again for the same reactions. This is because, like platinum, gallium is not deactivated or degraded during the reaction.
The process of dissolving platinum in gallium requires increasing the temperature to about 400°C for a few hours. But it’s a one-time energy investment that saves more temperature rises later during 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 Chemistry of Nature.