What the research is about
When we cook, the same ingredients can produce very different results depending on the temperature and cooking time. Chemical reactions work in a similar way: changing the conditions can change what is produced and how much of it is made. This is where catalysts come in. Catalysts help chemical reactions proceed without being used up overall.
In recent years, researchers have been exploring ways to use catalysts to convert carbon dioxide (CO₂) into useful chemicals such as formic acid. One approach uses copper sulfide (CuS) as a catalyst and electricity to drive the conversion of CO₂.
The electrical conditions applied to the catalyst can affect the movement of electrons and the chemical reactions that take place. One important factor is the electric potential. Previous studies have shown that switching the potential back and forth between positive and negative values, rather than keeping it constant, can make it easier to selectively produce certain chemicals.
But why should simply changing how electricity is applied change what the reaction produces?
To find out, a research team led by Associate Professor Akira Yamaguchi at Institute of Science Tokyo (Science Tokyo) focused on what happens to copper sulfide while the reaction is actually taking place.
Why this matters
What they found was intriguing: the catalyst did not remain in its original form while it worked. It changed along with the applied potential.
When the potential was shifted to the negative side, some of the sulfur left the copper sulfide, exposing metallic copper at the surface. When the potential was then shifted to the positive side, copper oxide containing oxygen formed. Switching back to a negative potential returned the surface to metallic copper. These transformations occurred repeatedly during the reaction.
The researchers also found that sulfur and oxygen did more than simply change the form of the catalyst. They played important roles in the reaction itself. Sulfur helped reactions involving intermediate species formed from CO₂, while oxygen promoted reactions that bond carbon atoms together.
In other words, what the reaction produces depends not only on which catalyst is used, but also on how that catalyst changes while it is working.
What’s next
CO₂ can be converted into a variety of useful chemicals, including formic acid and ethylene. A better understanding of how catalysts transform during reactions, and how to control those changes, could make it possible to produce desired chemicals more selectively. Rather than treating a catalyst as something that remains unchanged, researchers may be able to take advantage of the transformations it undergoes during a reaction. This approach could open new possibilities for designing catalysts that convert CO₂ into useful chemicals.
Comment from the researcher
To improve a catalyst’s activity, we need to ask both “How can we make it more active?” and, when we observe high activity, “Why did it become more active?” In this study, a range of measurements helped us reveal how the catalyst changes during the reaction and the roles played by sulfur and oxygen. By uncovering these mechanisms one by one and making use of the material’s distinctive properties, we hope to design catalysts that can more efficiently produce the chemicals we need from CO₂.
(Akira Yamaguchi, Associate Professor, School of Materials and Chemical Technology, Institute of Science Tokyo)
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