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MIT Research Shows Nanoparticle Catalysts Could Reduce Need for Precious Metals

Materials that increase the speed of a chemical reaction without being used up in the process, called catalysts, lie within many technologies, right from vehicle emissions-control systems to high-tech devices like electrolyzers and fuel cells. Unfortunately, catalysts are often costly as they generally contain one or more noble metals, like palladium or platinum, both of which have limited supplies.

A sample of a core-shell nanoparticle made by the researchers is shown in images made using scanning transmission electron microscope (STEM) and energy-dispersive x-ray spectroscopy (EDX). Color images show where the different elements are located in the particle, with the precious metals platinum (Pt) and ruthenium (Ru) concentrated in the shell, and the other constituents, tungsten (W), and titanium (Ti), concentrated in the core. (Courtesy of the researchers)

MIT scientists have discovered a potential end-run around this drawback: a method to obtain the equivalent amount of catalytic activity with as less as one-tenth the quantity of precious metal.

The solution is to utilize an atomically-thin noble metal coating over a small particle made of a highly abundant and cheap material: a type of ceramic referred to as transition metal carbide. As this idea has been under extensive research, no one was able to find a method to make the coating stick to the core material, until now. Additionally, the coated particles surpass conventional catalysts (made entirely of noble metal nanoparticles), providing more longevity and a greater resistance to plenty of unnecessary phenomena that affect conventional noble metal catalysts.

The recent discovery is being published this week in the Science journal, in a research paper by Sean Hunt, MIT doctoral student, postdocs Maria Milina and Christopher Hendon, and Associate Professor Yuriy Román-Leshkov from the Department of Chemical Engineering.

As the surface of catalytic particles is only involved in accelerating a reaction, altering the bulk of the particle with a cheap core could result in severe decrease in noble metal usage without sacrificing its performance.

For a long time, many researchers have been trying to find ways to make stable coatings of noble metals over earth-abundant cores. There has been some success using metallic cores like nickel and cobalt, but the particles are not stable over long periods of time and end up alloying with the noble metal shell.

Yuriy Román-Leshkov, Associate Professor, MIT

Carbides, on the other hand, are corrosion and clustering resistant, and cannot alloy with noble metals, making them perfect core candidates.

However noble metals – which derive their name from their usual unwillingness to participate in any type of chemical activity – do not bond with other materials easily, so forming coatings from them has been an elusive objective. Similarly, transition metal carbides are really difficult to engineer into nanoparticles with controlled properties. This is due to the fact that they require high temperatures to force carbon into the metal lattice, which results in particle clumping and surfaces contaminated with surplus carbon layers.

The major breakthrough, Hunt says, was encapsulating the shell and core material precursors into a model produced from silica.

This keeps them close together during the heat treatment, making them self-assemble into core-shell structures, conveniently solving both challenges at the same time.

Sean Hunt, Doctoral Student, MIT

Using a simple room-temperature acidic treatment, the silica template could then be dissolved.

Apart from reducing the quantity of precious metal required greatly, the process turned out to possess other vital benefits also.

We found that the self-assembly process is very general. The reluctance of noble metals to bind to other materials means we could self-assemble incredibly complex catalytic designs with multiple precious metal elements present in the shell and multiple inexpensive elements present in the carbide core.

Sean Hunt, Doctoral Student, MIT

This permitted the scientists to adjust the properties of the catalysts for various applications.

For example, using a nanoparticle with a carbide core made of tungsten and titanium, a platinum and ruthenium shell coating, they created a very active and stable catalyst for potential applications in direct methanol fuel cells. After putting the catalyst through 10,000 electrochemical cycles, and after similar cycling, this new design still worked 10 times better than traditional nanoparticles.

Another benefit is that these nanoparticles are highly resistant to a difficulty that can affect other types of noble-metal catalysts: “poisoning” of the surface by carbon monoxide.

This molecule can drastically curtail the performance of conventional catalysts by bonding to their surface and blocking further interaction, but on the core-shell catalysts, the carbon monoxide detaches more easily.

Yuriy Román-Leshkov, Associate Professor, MIT

While conventional hydrogen fuel cell catalysts could tolerate only 10 ppm of carbon monoxide, the scientists discovered that their core-shell catalysts could endure up to 1,000 ppm.

Finally, the scientists discovered that at high temperatures the core-shell structure was stable under different types of reaction conditions, even as remaining resistant to particle clumping also.

Whereas in other classes of core-shell nanoparticles the shell dissolves into the core over time, noble metal shells are insoluble in carbide cores. This is just another one of the many benefits that ceramic cores can have in designing active and stable catalysts.

Sean Hunt, Doctoral Student, MIT

Even though work for the translation of the new theory into a commercialized form is still in initial stages, in principle it can make a huge difference to applications like fuel cells, where “it would overcome one of the main limitations that fuel cells are facing right now,” says Román-Leshkov, that is the expenditure and availability of the required precious metals. Actually, with the help of MIT’s Translational Fellows Program, Milina has been working on the commercial features of the technology, identifying the prospective market value, and customers for these new materials.

This is an important discovery regarding the potential applications of core-shell carbide particles coated with precious metal layers. It would significantly reduce the amount of precious metals needed, and it could show better catalytic performance due to the synergistic interactions between the precious metal coating and the carbide core,” he says. “Even though these advantages were predicted from previous studies of thin-film model systems, the current study demonstrates the feasibility of potential commercial applications using core shell structures.

Jingguang Chen, Professor of Chemical Engineering, Columbia University

Ana Alba-Rubio and James Dumesic at the University of Wisconsin at Madison were also part of the research team. The U.S. Department of Energy and the National Science Foundation funded the work.

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