The unique structure of "nanoporous metals": Key to developing innovative catalyst materials

FUJITA Takeshi

Specialized field: Material Science, Electron Microscopy

Atoms are the smallest constituents of materials and matter. Professor Takeshi Fujita aims to fabricate nanostructures from all sorts of materials, and create new functions, by using an electron microscope enabling detailed observation of these atoms. Within the diverse domain of nanostructures, nanoporous metals stand out as a subject of interdisciplinary research cutting across specialized fields. These metals are "sponge-like bodies" in which nanosize pores are connected at random inside the material. With the high electrical conductivity typical of metals and increased surface area from pores, nanoporous metals have enhanced catalytic activity, raising expectations as a catalyst material to replace conventional nanoparticles. Prof. Fujita and his team have established a "dealloying" technique for easier fabrication of nanoporous metals. By using this technique, they are developing innovative catalyst materials.

Development of nanoporous catalysts to reduce environmental impact

In developing the concept of "dealloying," a technique for producing nanoporous metals, Prof. Fujita and his collaborators were inspired by the phenomenon whereby specific elements selectively elute from an alloy due to corrosion. The technique is very simple and involves simply alloying metals that are stable and metals that are unstable to corrosion, and then performing selective corrosion with an electrolytic solution. Prof. Fujita: "No special experimental equipment is needed to fabricate nanoporous metals through dealloying, so the technique is very simple and has great potential."

Among the diverse functions of nanoporous metals, catalyst applications are particularly promising. In previous nanoparticle catalysts, multiple nanoparticle metals were arranged on the surface of an oxide. Only the vicinity of the nanoparticle was active, and other regions were not effectively used. A nanoporous catalyst, on the other hand, has a structure in which metal is complexly entangled with oxides, and thus not only does the active region greatly expand, the active time of the chemical reaction can be maintained for a long time because it has high durability.

Prof. Fujita is convinced that "nanoporous catalysts with high catalytic activity can be used in a wide variety of chemical reaction systems." Using their characteristics, he has developed catalysts to "more effectively use methane and carbon dioxide, greenhouse gases that have an adverse effect of the global environment." The dry reforming of methane (DRM) reaction is attracting attention as a method for manufacturing valuable synthesis gas from methane and carbon dioxide. However, a significant carbon emitting reaction occurs at temperatures less than 600°, and this causes catalyst deactivation and degradation of reaction equipment. Therefore, temperatures of 800° or higher are needed, and practical application has not been achieved. Prof. Fujita and colleagues set out to drive stable, long-term DRM at lower temperatures, and they successfully fabricated a "rooted catalyst" with braid-like entanglement of metallic nickel and oxide yttrium due to dealloying. Results so far indicate that this is an industrially adaptable catalyst, and practical application looks realistic.

Prof. Fujita has also begun developing a revolutionary new catalyst system which allows DRM to be driven at room temperature by a high voltage applied to the nanoporous catalyst (a conductor) and causing corona discharge^*2. One recent trend is to optimize chemical reactions for industrial purposes by leveraging external fields, such as light or an electric field, as the driving force to control the reaction path and speed. With conventional nanoparticle catalysts, however, the reaction only proceeds near the nanoparticles dispersed on the surface of an oxide, so extremely low coupling efficiency between the catalyst and external field is an issue. To solve this problem, Prof. Fujita and colleagues turned to nanoporous catalysts because they can accumulate electric fields on their surface. The idea was to bring the electric field of the catalyst surface closer to the gas which generates the external field through corona discharge, thereby improving the coupling efficiency with the external field, and activating the catalytic reaction. Experiments with the developed equipment confirmed that the catalyst accelerates the reaction through corona discharge.

Other innovative results have been accomplished, promising substantial reductions in environmental impact. One notable achievement involves applying acid etching and heat treatment to impart electrical conductivity to a metal-organic framework (MOF) initially impermeable to electricity. This transformation successfully converts it into an exceptional catalyst for water electrolysis.

*2) Phenomenon which occurs due to the non-uniform electric field when a high voltage is applied to a local point like a sharp electrode

Striving for a "universal catalyst"

Another major theme Prof. Fujita is exploring is the development of "super multi-element catalysts" combining multiple elements.

He is pushing forward with this research, driven by the idea that "combining many different elements may lead to additional effects, and the creation of catalysts with exotic functions." Thus far, he has successfully fabricated catalysts incorporating 12 elements, but he is eager to "incorporate even more elements." The high number of incorporated elements is not the only notable point about this research. The ability to fabricate the catalysts with a simple method based on dealloying is also highly significant. The procedure is very straightforward: just fabricate an alloy of the pertinent elements and aluminum, cool and form into a ribbon, and then submerge in an alkaline solution, where the aluminum is selectively corroded. Prof. Fujita: "It's been said that crafting an alloy with 10 or more elements is hard, yet we achieved it with an extremely simple method. In conducting research, I think it's important to set aside preconceived ideas."

Behind this success was an idea based on "reversal." A bottom-up technique was used to make previous multi-element catalysts, building up elements one by one. Prof. Fujita and his colleagues had the opposite idea. They used a "top-down" technique of first fabricating a composite alloy of multiple elements and then optimizing the end product by removing those that are unneeded. This overturned the received wisdom regarding catalyst design techniques, and led to dramatic progress.

The final objective of Prof. Fujita's research in this area is a universal catalyst that works in all reactions.

"By placing numerous elements in the reaction field, and endowing the material with complexity (entropy) comparable to that of life, perhaps we can develop a catalyst which deforms on its own into forms adapted to any reaction, and can withstand diverse reactions. We are working hard with that possibility in mind."

The method of choosing elements is also important for fabricating multi-element catalysts. On that front, Prof. Fujita is planning to merge catalyst science and data science, and develop a system for finding optimal combinations of elements. In March 2021, the university installed a state-of-the-art electron microscope equipped with outstanding features like high resolution and wide-ranging analytic capabilities. "This equipment will likely drive a quantum leap in the progress of our research," says Prof. Fujita with conviction.

Date of posting: February, 2024