Iron, Support Material Boost CO₂ Conversion

One such method is chemical looping, which converts CO₂ into carbon monoxide (CO). When iron comes into contact with CO₂, it takes oxygen from the CO₂, converting it into CO. The iron then becomes iron oxide. By removing the oxygen with hydrogen, the iron can be returned to its original state and used again. Chemical looping converts CO₂ by repeatedly oxidizing and reducing iron.

The CO produced through this process can be used to make gasoline, diesel, and various other chemical products. In particular, when the process uses hydrogen produced by water electrolysis powered by renewable energy sources such as solar and wind power, the resulting CO could serve as a feedstock for e-fuels. These synthetic fuels are expected to help address climate change.

For iron to react efficiently, as much of it as possible must remain exposed to CO₂. Researchers therefore disperse tiny iron particles over another material. Known as a support material, it has traditionally been regarded as a supporting player whose main job is to keep the iron particles stable.

Previous studies, however, suggested that support materials may do more than simply hold the iron in place: they may actively assist the reaction. Exactly how they speed up the reaction between iron and CO₂ remained unclear.

To find out whether different support materials affect the reaction rate, the team compared several materials with different properties. One proved especially effective: calcium titanate (CaTiO₃) in which some of the titanium atoms were replaced with iron. Both electrons and oxide ions can move easily through this material.

When used as the support, this material enabled CO₂ to react with iron faster than any of the other materials tested. The reaction rate constant was approximately three times higher than when a material that mainly transports oxygen was used. In a separate test using a different type of reactor, the process produced almost exclusively CO from the CO₂ that reacted.

Further analysis revealed that electrons and oxygen move where the iron and the support material meet, helping transfer oxygen from CO₂ to the iron. The support material, once thought to do little more than hold the iron in place, was actually working together with it to drive the reaction forward.

Until now, support materials have mainly been selected for their ability to keep iron stable. This study showed that allowing electrons and oxygen to move easily through the support is also important for accelerating the reaction. Using this property as a guide, researchers may be able to identify support materials that bring out more of iron's potential. This could lead to more efficient technologies for turning CO₂ into fuels and chemical feedstocks.

The ultimate goal of this research is to establish technologies that enable carbon dioxide to be used repeatedly as a resource. Iron is a familiar material, but it can also be used to convert carbon dioxide into carbon monoxide.

Even with the same iron, changing what supports it can change how it behaves. That is precisely what makes this research interesting. The support does not simply hold the iron in place-it also helps transfer oxygen. To improve a material's performance, we do not always have to change the main component itself. We can also create an environment that allows it to perform more efficiently.

This study was conducted in collaboration with Arufa Shiota and other researchers at the Advanced Technology R&D Center of Mitsubishi Electric Corporation. Working with industry will be essential to bring this technology into real-world use. I look forward to sharing with our research partners the moment when it begins to benefit society.

(Junichiro Otomo, Professor, School of Environment and Society, Institute of Science Tokyo)

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