New Cerium Oxide Nanotubes May Enhance Emission-Control Systems
28 March 2006
Researchers at Brookhaven National Laboratory have created and are investigating the properties of nanotubes made of cerium oxide. These cerium oxide nanotubes have potential applications as catalysts in vehicle emission-control systems and fuel cells.
In a catalytic converter, ceria acts as a buffer, absorbing or releasing oxygen, depending on the conditions of the engine, to maintain the catalyst in its optimum operating condition.
To synthesize cerium oxide nanotubes, the team allowed the compounds cerium nitrate and ammonia hydroxide to react chemically. Initially, this reaction forms nanostructures—such as rods and sheets—made of the intermediate product cerium hydroxide.
The intermediate product is then quickly cooled to zero degrees Celsius, which freezes those structures into place. By letting the chemical reaction proceed over a long period of time, a process called aging, the hydrogen is eventually removed from the intermediate product and a large quantity of the desired end product—cerium oxide nanotubes—is formed.
Wei-Qiang Han, the lead scientist on the project, explained the synthesis method today at the American Chemical Society National Meeting in Atlanta, Georgia.
Han also discussed his group’s recent study on how cerium oxide nanotubes release oxygen ions when immersed in a low-oxygen environment, a process that is critical to the nanotubes’ effectiveness as catalysts.
We’re interested in studying oxygen-atom vacancies in cerium oxide nanotubes because, when combined with their other surface features, these vacancies may make them more functional and effective in the applications mentioned.
—Wei-Qiang Han
Earlier work by the same Brookhaven team found that doping ceria nanoparticles with zirconium increases the number of oxygen vacancies—places for oxygen uptake and release.
The Office of Basic Energy Sciences within the US Department of Energy’s Office of Science funded this research.
With lead and sulphur removed from the fuel and, affordable aftertreatment technologies for HC, CO, NOx (at lambda=1) and now PM available, the primary challenge remaining is reducing the NOx produced in fuel-efficient lean burn (lambda>1) engines. This includes diesels, certain gasoline direct injection and certain natural gas engines.
Unfortunately, a three-way catalyst can only reduce NOx if the gas is poor in O2 and sufficient quantities of HC and/or CO are present. This is the case only in a very narrow window of 0.98 < lambda < 1.02, often referred to as lambda=1.
In addition to strategies for reducing NOx production (high fraction of chilled external AGR, volume ignition at low load), two aftertreatment technologies are available: NOx store catalysts and SCR, i.e. (indirect) ammonia injection.
Cerium nanotubes could represent a third option: by adsorbing the excess oxygen, it would allow a three-way catalyst to be active most of the time. At some point, it would have to be heated up enough for the oxygen to desorb during a brief purge process. The downside is that any hot gas containing very high levels of oxygen is quite aggressive, reducing the life expectancy of the catalyst. A relief conduit used only during purging might be neccessary, at the expense of brief spikes in HC and CO tailpipe emissions.
Posted by: Rafael Seidl | 29 March 2006 at 09:16 AM