What the research is about
The gas used for cooking and heating water is made up mainly of methane. If methane could be produced from CO₂ captured from factory emissions or the atmosphere, together with hydrogen made using renewable energy, CO₂ could be reused as a carbon resource.
This process is known as CO₂ methanation, and demonstration projects are already exploring its practical use. However, simply mixing CO₂ and hydrogen does not produce methane efficiently. A catalyst is needed to help the reaction along.
Catalysts combining nickel and cerium oxide have long been studied as promising materials for this purpose. They are relatively inexpensive and can efficiently convert CO₂ into methane. One way to improve their performance is to make the nickel particles smaller, spread them more evenly, and increase the contact between nickel and cerium oxide. Conventional manufacturing methods that achieve these features, however, often involve multiple complex steps, making large-scale production costly and difficult.
To address this challenge, a research team led by Associate Professor Tsuyoshi Nagasawa of Institute of Science Tokyo (Science Tokyo), together with Designated Associate Professor Maki Nakamura, then at Nagoya University's Institutes of Innovation for Future Society, applied flame-assisted spray pyrolysis to this catalyst for the first time.
The researchers dissolved compounds containing nickel and cerium in water, turned the solution into a fine mist, and sprayed it into a flame. Water rapidly evaporated from the tiny droplets, while the remaining compounds were transformed by heat into catalyst particles. Because the particles can be produced continuously in a single flow, without separate drying and heating processes, the method is well suited to future large-scale production.
Why this matters
The resulting catalyst consisted mainly of particles just 10 to 20 nanometers across-thousands of times smaller than the width of a human hair. Catalytic reactions take place mainly on particle surfaces, so smaller particles provide more surface where reactions can occur.
Compared with a catalyst made by a conventional method, the new catalyst had about 15 times the surface area. The proportion of nickel exposed at the particle surface and available for the reaction was also about seven times greater. In other words, even with roughly the same amount of nickel, the new structure allowed reactions to proceed at many more sites.
Closer analysis revealed numerous oxygen vacancies on the catalyst surface-sites where oxygen atoms were missing. These sites help capture CO₂ and initiate the reaction. The catalyst also had more contact points between nickel and cerium oxide, as well as more nickel in a reduced state, which helps split hydrogen into a form that can react. Together, these features allowed both CO₂ and hydrogen to be activated more effectively.
As a result, at 400°C, the catalyst achieved a methane yield of 83%, close to the maximum predicted by thermodynamics. At 300°C, the methane production rate also ranked among the highest reported for similar catalysts, despite the relatively small amount of nickel used.
What's next
Because this method can produce catalyst nanoparticles continuously, it has strong potential for large-scale manufacturing. The researchers will next investigate how flame temperature and other conditions affect particle size and structure. Their goal is to identify manufacturing conditions that produce catalysts capable of maintaining high performance over long periods.
They will also refine the equipment so that the same particle quality can be maintained as production is scaled up. If these efforts succeed, tiny catalysts born in flames could help transform captured CO₂ into fuel that supports everyday life.
Comment from the researcher
Even small differences in temperature and other conditions inside a flame can change the size and function of the particles that form. We want to understand the process taking place inside the flame and use that knowledge to develop technology for creating particles with precisely controlled properties.
In the future, we hope to make high-performance catalysts more easily and in larger quantities, contributing to a society that uses CO₂ as a valuable resource.
Flame-based particle synthesis can also produce many other materials, including those used in fuel cells and batteries, as well as fluorescent materials. We will continue developing this research and exploring its potential across a wide range of applications.
(Tsuyoshi Nagasawa, Associate Professor, Department of Mechanical Engineering, School of Engineering, Institute of Science Tokyo)
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