Argonne-developed Cerium-Oxide Catalyst Helps Eliminate NOx from Diesel Exhaust

Researchers in Argonne's Chemical Engineering Division have developed a catalyst that could help diesel truck manufacturers eliminate harmful nitrogen-oxide (NOx) emissions from diesel exhausts. The technology appears so promising that several 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 that the technology could be available commercially within two to three years.

Argonne post-doctoral associate Sundar Krishnan (left), and researchers Steve Ciatti (center) and Chris Marshall (foreground, right) working with the equipment that will be used to engine-test the Cu-ZSM-5 catalyst. Marshall is holding a beaker of the catalyst material.

"For diesel engines, we envision manufacturers placing ceramic catalytic reactors in the exhaust pipes, where they will convert NOx emissions into nitrogen," said Marshall. Nitrogen, or N2, is a harmless gas that makes up 79 percent of the Earth's atmosphere. NOx emissions are well-known contributors to smog, acid rain, and global warming, yet they are among the most difficult pollutants to eliminate from diesel exhaust, and many technologies that reduce NOx cause increases in undesirable particulate emissions.

"Our most promising catalyst for diesel engines is a cerium-oxide catalyst," Marshall said. "It 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. 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 vapor, which is almost always found in engine exhausts.

Using Argonne's Advanced Photon Source to analyze the structure and performance of various catalysts, Marshall's group developed an additive that allows Cu-ZSM-5 and similar catalysts to overcome these difficulties. "Our new cerium-oxide additive," Marshall said, "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% to 100% 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, thus requiring no additional fuel tanks.

"Another type of technology is ammonia-selective catalytic reduction, using a material called urea as the ammonia source," Marshall said. "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 onboard 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 sulfur, which is present in most commercial diesel fuel. Since the Argonne-developed catalyst contains no platinum, it is less prone to the negative effects of fuel-borne sulfur.

Marshall says the Argonne catalyst has been tested and performed well with a number of diesel and diesel-type fuels, including standard diesel, synthetic diesel, biodiesel, and JP8, which is a jet fuel preferred by the military. The next step is to subject the catalyst to engine testing, which will occur soon at Argonne's Diesel Engine Test Facility. Marshall expects these tests will show that, in addition to its other advantages, the Argonne catalyst has a greater life expectancy than other catalysts currently on the market.

October 2, 2007