Exhaust gas purification performance "surpasses rhodium"
High-endurance multi-element nano alloy catalyst developed by Kyoto University, Shinshu University, and others
Toward building a base for composite materials

High-performance materials are discovered by mixing several elements that traditionally do not form alloys together, at the atomic level. A research group comprising Professor Hiroshi Kitagawa and Program-Specific Assistant Professor Kohei Kusada of the Graduate School of Science, Kyoto University, Professor Michihisa Koyama of the Center for Research Initiative for Supra-Materials, Interdisciplinary Cluster for Cutting Edge Research, Shinshu University, Professor Katsutoshi Nagaoka of the Graduate School of Engineering, Nagoya University, Professor Syo Matsumura of the Faculty of Engineering, Kyushu University, and others, succeeded in developing a high-endurance multi-element nano alloy catalyst that surpasses rhodium, which boasts the highest performance for automobile exhaust gas purification. The results were published in Advanced Materials magazine. "We have succeeded in synthesizing approximately 40 varieties of metal atoms in our laboratory, and hope to utilize this technology to create a base for composite materials. Furthermore, where catalysts are concerned, we are scheduled to pursue large-scale joint research with corporations," explains Kitagawa.

Kitagawa: "Our goal is to assemble 10,000 items of data."

Regulations governing automobile exhaust gas emissions are growing stricter every year, and the price of rhodium, which is capable of efficiently purifying nitrogen oxides (NOx), also continues to climb year by year. As of March 9, the price of rhodium ore was 102,057 yen/gram, or 15 times the price of gold.

The research group previously succeeded in forming a solid solution by mixing palladium and ruthenium, which are more abundantly available than rhodium, at the atomic level for the first time, and achieved a similar NOx purifying performance. However, this alloy is a combination of metals that do not combine inherently. The group found that the alloy's solid solution will gradually end up breaking down and deteriorating when subjected to high temperatures.

In light of this the group applied the concept of high-entropy alloys (HEAs). When alloys comprise five or more elements and the ratio of each element is between 5% and 35%, the affinity between the metal atoms is low. The more elements there are the more the entropy increases, making the solid solution structure stable at high temperatures. Because HEAs display high specific strength, fracture toughness, high temperature strength, heat stability, corrosion resistance and other properties, compared to typical alloys, they are the subject of research and development, chiefly as structural materials.

In the most recent research, the group searched for highly active, high-endurance catalysts by using a non-equilibrium chemical reduction method to combine a palladium-ruthenium alloy with a third element. More specifically, three varieties of metal ion in a nonequilibrium state were reduced instantaneously and simultaneously, by gradually atomizing a solution containing each metal ion to a sufficiently-heated reducing agent solution. A protective agent was used to suppress the process by which each atom generated as a result of the reduction agglutinates within the solution, and nano-sized alloy particles were synthesized.

When the various alloys that were synthesized were made to react to a mixed gas that simulated exhaust gas, while being evaluated using synchrotron radiation powder X-ray diffraction, an atomic-resolution scanning electron microscope and a hard x-ray photoelectron spectroscopy, a palladium-ruthenium-iridium alloy was found to offer higher activity and higher endurance than a palladium-ruthenium alloy or rhodium. When the alloy's properties were confirmed in a first principle calculation using a supercomputer, the group found that as a result of the increased entropy the alloy was stable in a high-temperature region, and that, in the case of these three elements, they lowered the stable temperature of the solid solution structure by as much as 900°C compared to the two elements.

In partnership with FURUYA METAL Co., Ltd., the research group has developed a technology for quantity production of the nano alloy it developed, and has introduced a pilot of a flow-type synthesizing machine. It is successfully producing a one-nanometer-class nano alloy stably and in volume. Additionally, the group is pursuing endurance tests and other trials with a number of automobile and motorcycle makers on actual machines toward making the alloy commercially-viable.

Furthermore, from April, the research group will embark on large-scale joint research with an automobile maker that will install high-throughput evaluation equipment with a value of around 200 million yen at Kyoto University. The research group is also scheduled to carry out large-scale joint research with a large chemical maker on process catalysts relating to petroleum plants.

The research group says that the synthesizing method and HEA theory that it used this time is applicable to metal elements (typical metals and transition metals). Kitagawa says that, "If there are around three types, it is possible to synthesize them using intuition- and experience-based guesswork, but when it comes to the 40 varieties that can be synthesized, there are too many combinations for us to handle. Going forward we are considering synthesizing and assessing a large number of combinations to build a database. According to experts, if there are around 10,000 data items, a certain level of things will become possible, so for the time being our target is to build 10,000 data items. We want to use cross appointments to have information specialists and others join us in building a base for developing new materials."

Support from JST programs for Kitagawa's group

In the U.S. and Europe, significant investments are being made in building bases for materials development and in data-driven materials development. Ramping up Japan's research and development is thus a pressing issue. In light of that, on the 12th March, the Ministry of Education, Culture, Sports, Science and Technology announced its strategy targets for FY2021. One of those targets is "the cultivation of spaces for seeking out unexplored multi-element/compound/quasi-stable materials, with the element strategy as a base," and as specific research examples, it cites the clarification of mechanisms for function expression in the formation of multi-elements and compounds, and the establishment of technologies for discovering and processing materials. The group that centers on Kitagawa also looks likely to receive assistance from the JST Strategic Basic Research Programs (CREST, ERATO, PRESTO).