 |
 |
||||||
[May 2007]
New Ceramic Catalyst Helps Eliminate NOx From Diesel Exhaust
A catalyst developed by researchers at Argonne National Laboratory (Argonne, IL, USA) could help diesel truck manufacturers eliminate harmful nitrogen oxide emissions from diesel exhausts.
The technology – which has a patent pending – appears so promising that multiple large and small companies have expressed interest in licensing it and working with Argonne researchers to scale up the technology and bring it to market. Argonne researcher Christopher Marshall, one of the technology's developers, believes there could be a commercially available product within two to three years.
Nitrogen oxides – collectively called NOx – contribute to smog, acid rain and global warming, yet they are among the most difficult pollutants to eliminate from diesel exhaust. For example, many technologies that reduce NOx result in increases in undesirable particulate emissions.
"For diesel engines, we envision manufacturers placing ceramic catalytic reactors in the exhaust pipes, where they will convert NOx emissions into nitrogen," said Christopher Marshall, who works in Argonne's Chemical Engineering Division. Nitrogen, or N2, is a harmless gas that makes up more than 80% of the Earth's atmosphere.
"Our most promising catalyst for diesel engines," Mr Marshall said, "is Cu-ZSM-5 with an external coating of cerium oxide." Cu-ZSM-5 is a zeolite with copper ions attached within its micropore structure. Zeolites are common catalysts in the petroleum industry.
Those working previously with Cu-ZSM-5 and similar catalysts, he said, found that they performed poorly at removing NOx from diesel exhaust. They require temperatures higher than normal diesel exhaust temperatures and don't work well in the presence of water vapour, which is almost always found in engine exhausts.
With the help of the Advanced Photon Source at Argonne to analyse the structure and performance of various catalysts, Marshall's group at Argonne developed an additive that allows Cu-ZSM-5 and similar catalysts to overcome these difficulties.
"Our new cerium-oxide additive," said Mr Marshall, "is the breakthrough that makes it work. When it's combined with Cu-ZSM-5, the resulting catalyst works at normal exhaust temperatures and is actually more effective with water vapor than without it. With a lean fuel-air mixture, it removes as much as 95-100 percent of NOx emissions."
Argonne's new catalyst also avoids the problems associated with ammonia, which competing catalysts use as the reductant. The Argonne catalyst uses the diesel fuel that is already on board thereby requiring no additional tankage.
"Another type of technology is ammonia-selective catalytic reduction, using a material called urea as the ammonia source," said Mr Marshall. "Ammonia is toxic, and unless all of it is converted during the process, whatever remains could be released to the atmosphere. While some European diesel manufacturers are taking the urea approach, U.S. diesel manufacturers are looking for alternatives." Since a system using the new catalyst would not require an on-board urea storage tank and uses the onboard diesel fuel as the reductive material, the new catalyst is considered safer and more energy-efficient.
Another alternative for U.S. manufacturers is the use of NOx traps. These are platinum based systems that work well if they don't come into contact with sulphur, which is present in most commercial diesel fuel. Since the Argonne-developed catalyst contains no platinum, it is degraded far less by the fuel borne sulphur.
Initial research on the cerium oxide catalyst was funded by the U.S. Department of Energy's Office of Energy Efficiency and Renewable Energy. The catalyst was developed for chemical plant emissions under a joint research agreement with BP. Research plans call for expanded work aimed at both diesel and natural gas engines and coal fired power plants.
www.anl.gov
ENDS