Three-way catalysts: Cool way to make catalytic converters

02/29/2016

Nanorods with high oxygen storing capacities at moderate temperatures have been synthesized using a simple reaction

A high-resolution transmission electron microscopy image showing fine cerium oxide nanorods fabricated by a mild-temperature method.
A high-resolution transmission electron microscopy image showing fine cerium oxide nanorods fabricated by a mild-temperature method.

© 2016 Naoki Asao and Koji Nakayama

A new, mild-temperature method for producing cerium oxide nanorods has been developed by AIMR researchers. The nanorods show an excellent oxygen storage capacity at temperatures below 200 degrees Celsius, making them promising for use as catalysts to control harmful emissions from vehicles1.

Catalytic converters in cars use so-called three-way catalysts to convert noxious pollutants such as carbon monoxide and nitrous oxides into more benign compounds. Cerium oxide is an attractive material for such catalysts due to its high oxygen storage capacity ― a critical parameter for three-way catalysts.

However, most methods for fabricating nanostructures require high temperatures, which induce crystal growth and thereby have the detrimental effect of reducing the surface area of the nanostructures ― another crucial parameter for catalysts. This has made it challenging to produce cerium oxide nanostructures that possess high oxygen storage capacities at temperatures below about 400 degrees Celsius.

Now, a team of five researchers from the AIMR at Tohoku University led by Naoki Asao and Koji Nakayama has devised a mild-temperature method for producing nanorods of cerium oxide that exhibit an excellent oxygen-storage capacity at moderate temperatures.

The nanorods are about 5 to 7 nanometers in diameter as measured by high-resolution transmission electron microscopy (see image). The cool reaction conditions made it possible to produce such fine structures.

The team adopted a method that they had previously developed to produce nanowires of sodium titanate (see earlier highlight). It basically involves corrosion of ribbons of cerium–aluminum alloys in an alkaline medium; in this reaction, aluminum is leached, whereas cerium is oxidized. Importantly, this reaction occurs under mild conditions.

“This technique is really different from previous methods. Its key aspect is the mild fabrication conditions, which make it possible to fabricate fine structures,” says Asao. “Based on our work with titanate nanowires, we had a feeling that the corrosion-based method would result in some unexpected properties. But the findings far exceeded our expectations.”

Asao is very excited about the potential of this new method. “We believe that this research will have a significant impact on the automobile industry,” he says. In addition, the researchers anticipate that the nanorods could be used in other applications, including fuels cells, ultraviolet blockers, solar cells and sensors.

By optimizing the reaction conditions and varying the composition of the cerium–aluminum mother alloys, the scientists will seek to further enhance the oxygen storage capacity and other properties of the nanorods to make them suitable for practical applications.

References

  1. Ishikawa, Y., Takeda, M., Tsukimoto, S., Nakayama, K. S. & Asao, N. Cerium oxide nanorods with unprecedented low-temperature oxygen storage capacity. Advanced Materials 28, 1467−1471 (2016). | article

This research highlight has been approved by the author of the original article and all empirical data contained within has been provided by said author.