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Cerium-Oxide-Coated Catalyst Could Knock Out NOx by 2010?
Nitrogen oxides (NOx), contributors to smog, acid rain, and global warming, are notoriously stubborn pollutants to remove from diesel exhaust. Existing technologies are aimed at reducing them to nitrogen, the harmless gas that makes up 80 percent of the Earth’s atmosphere. These technologies are chasing the 0.2-limit in time to meet the 2010 EPA deadline, but many of them come with considerable downsides. Some that reduce NOx also increase particulate emissions. Those that use ammonia-selective catalytic reduction, with urea as the ammonia source, risk releasing toxic ammonia to the atmosphere if less than 100 percent of the ammonia is converted in the process. NOx traps are another option for U.S. automakers, but these platinum-based systems work well as long as they do not come into contact with sulfur. Sulfur, which acts to degrade the platinum, is present in commercial diesel fuel. The catalyst that would eventually be developed by Marshall’s group, Cu-ZSM-5 (a zeolite compound doped with copper), had been studied before with poor results for removing NOx from diesel. This and other similar catalysts did not work well in the presence of water vapor, a persistent component of engine exhausts, and they required higher than normal exhaust temperatures. The group developed an additive, cerium oxide, which allows Cu-ZSM-5 to work at normal exhaust temperatures (325° to 350° C); and, with the additive, the catalyst is actually more effective in the presence of water vapor than without it. When exposed to a lean fuel:air mixture, the cerium-oxide-coated CU-ZSM-5 can remove as much as 100 percent of NOx emissions. Marshall foresees manufacturers installing ceramic catalytic converters in exhaust pipes to carry out the conversion of NOx to nitrogen. This approach, as opposed to the urea-based approach being chosen by some European companies, forestalls the need for an onboard urea tank. The Argonne group’s catalyst will be safer and more energy-efficient because it will use the onboard diesel fuel as the reductant. The Argonne researchers have tested the catalyst with a number of diesel and diesel-type fuels—standard diesel, synthetic diesel, biodiesel, and JP8, a jet fuel preferred by the military—and it has performed well. Marshall did note, however, that the reductant must be one with at least one carbon-carbon bond, eliminating the possibility of using it with methane or hydrogen. It is the goal of the team to perform more testing to show that the cerium-oxide-coated catalyst will offer an additional advantage in that it may have a greater life expectancy than others that are already available. |
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A promising cerium-oxide-coated catalyst developed at the Argonne National Laboratory is now patented and may be commercially available in two to three years, at approximately the same time as the 2010 EPA deadline for the reduction of diesel NOx emissions to 0.2 grams per brake-horsepower hour. In fact, Argonne researcher Christopher Marshall, a developer of the technology, says that the catalyst may be capable of removing as much as 100 percent of NOx emissions.