Diverse metals mix it up in novel nanoparticles

In the March 30 cover article of the journal Science, the researchers, from Johns Hopkins and three other universities, reported that their new technique enabled them to combine multiple metals, including those not usually considered capable of mixing. This process has created new, stable nanoparticles with useful applications in the chemical and energy industries, the researchers said.

Many industrial products like fertilizers and plastics are made with the help of catalysts, substances that speed up chemical reactions. Metallic alloy nanoparticles -- particles ranging from about a billionth to 100 billionths of a meter in size -- are a critical ingredient in such processes. Until now, however, only a small set were available because of limitations that arise when combining extremely different metals to form homogeneous alloys, which are combinations of metals. The problem is even more challenging when downsizing to the microscopic nanoscale required for catalytic applications.

The new method uses shock waves to heat the metals to extremely high temperatures -- 2,000 degrees Kelvin (more than 3,140 Fahrenheit) and higher -- at exceptionally rapid rates, both heating and cooling them in the span of milliseconds. The metals are melted together to form small droplets of liquid solutions at the high temperatures, which are then rapidly cooled to form homogeneous nanoparticles. The new materials, known as high-entropy-alloy nanoparticles, are expected to have broad applications as catalysts in industry-relevant chemical reactions, with the potential to improve energy efficiency in the manufacturing process and lower production costs.

"This method enables new combinations of metals that do not exist in nature and do not otherwise go together," said Chao Wang, a Johns Hopkins Whiting School of Engineering assistant professor of chemical and biomolecular engineering and one of the study's co-authors. Wang's research group designed a five-metal catalyst based on these high-entropy-alloy nanoparticles and demonstrated superior catalytic performance for selective oxidation of ammonia to nitrogen oxide, a reaction used by the chemical industry to produce nitric acid, an important chemical in the large-scale production of fertilizers and other products.