This work attains significance because, to date, chemists have struggled to create nanoparticles combining platinum and gold-which act as super-efficient catalysts-in an industrially useful form.
Catalysts are compounds that speed up or slow down chemical reactions without being consumed by them. The chemical and drug industries spend billions of dollars each year for catalysts that are needed to process drugs and other high-value chemicals.
"There are some industrial reactions where drugmakers have no choice but to use platinum and palladium catalysts, but the majority of these are homogenous, which means they mix readily with reactants and are very difficult to remove," said lead researcher Eugene Zubarev, associate professor in chemistry at Rice.
"Because these heavy metals are toxic, they must be completely removed from the drug after its synthesis is completed. However, the removal of homogeneous catalysts is very time-consuming and expensive, which creates a big problem for pharmaceutical companies," Zubarev added.
Zubarev and Rice graduate student Bishnu Khanal revealed that they wanted to make a heterogeneous platinum catalyst that was soluble enough for industrial use, but that could also be easily removed.
They already knew from previous studies that combining platinum with gold in tiny nanoparticles could enhance the platinum's catalytic effect. Thus, they started with tiny rods of pure gold. and coated them with a layer of platinum so thin that it left the gold exposed in some places.
Having confirmed the structure of the gold-platinum nanorods, the researchers then set out to find a way to make them soluble in organic solvents that are favoured by industry.
Building on Zubarev's previous work in making soluble gold nanorods, they found a way to attach hair-like molecules of polystyrene to the surface of the gold-platinum rods.
Zubarev and Khanal found the coated particles were easy to remove from solution with a conventional centrifuge, and that the polystyrene shells made them completely soluble in organic solvents and dramatically enhanced their catalytic selectivity.
"The selectivity of the coated gold and platinum nanorods will be very attractive to industry. For example, we found they had nearly 100 percent catalytic selectivity for the hydrogenation of terminal olefins," Zubarev said.
The researchers are using similar methods to produce gold-palladium catalysts in a follow-up study. Palladium is another high-demand catalyst.
"The early indications are very promising," he said.
A research article on this work has been published in the German scientific journal Angewandte Chemie International Edition.