Wenjie Shen

Altering the shape of catalyst particles can boost their activity.

Chemical catalysts that mop up toxic pollutants created by vehicle use and other human activities have made a difference to the environment, but there is still ample room for improvement. For example, the metal oxides currently used to catalyse the oxidation of carbon monoxide (CO) to carbon dioxide (CO₂) are only active at temperatures much higher than ambient. Wenjie Shen, a physical chemist at the Chinese Academy of Sciences' Dalian Institute of Chemical Physics, has found that changing the shape of one metal-oxide catalyst renders it practical for use at low temperatures. The discovery may lead to ways of producing more efficient catalysts for a broad range of reactions.

Researchers have known since the 1970s that tricobalt tetraoxide (Co₃O₄) can catalyse CO oxidation at low temperatures, but only under artificially dry conditions. The catalyst works by binding CO at its active sites, which contain Co³⁺ cations, so that CO can react with nearby oxygen molecules. But, explains Shen, “at low temperatures, the active sites eventually become occupied by water molecules and can no longer bind CO”. This severely limits most practical applications of this catalyst.

Most metal-oxide particles are spherical, with their active sites located in 'dents' on the sphere's surface. “The exposed active sites are limited in these particles,” says Shen, who several years ago had the idea that changing the particles' shape might increase the number of active sites. “I had been following the material sciences field. There were many reports of metal and metal-oxide particles designed with different shapes that had altered properties,” he explains. “I thought that the same concept could be applied to the design of solid catalysts.

Shen and his colleagues tested a number of strategies for synthesizing Co₃O₄ particles under various conditions in the hope of building different shapes. They eventually produced tiny rods, 5–15-nanometres wide by 200–300-nanometres long. “We immediately tested these in CO oxidation,” says Shen. “Right away we were surprised by the results.”

In contrast to the spherical particles, which perform poorly at low temperatures, the nanorods could convert 100% of CO in a flowing stream of gas. The rods' activity lasted for 6 hours at very low temperatures — down to −77 °C. Importantly, the nanorods maintained their high level of activity and stability at temperatures up to 400 °C, as well as in gases containing a lot of water and CO₂ — conditions that mimic the exhausts of cars (see page 746).

Excited by the results, Shen and his co-workers examined why the nanorods were so active. Detailed transmission electron microscope images revealed that the nanorod shape exposes one specific crystal plane that makes up about 41% of the total surface area of the particle and consists primarily of Co³⁺ active sites.

The work not only paves the way for designing the next generation of catalysts for reducing CO in the air but, more generally, demonstrates that particle shape is a crucial parameter to consider in the design of high-efficiency metal-oxide catalysts. “This discovery is very exciting. Until now, researchers emphasized the importance of particle size in making catalysts more effective,” says Shen. “But now we know that morphology should also be considered. By controlling the morphology you can quantitatively design and enrich the active sites.”