The End to Dirty Diesels
December 05 / January 06
Volume 3 Number 6
At Los Alamos National Laboratory, group leader Kevin Ott has designed a catalytic converter for diesel engines. This is the story about how Ott and his team did it.
Gasoline automobiles in the United States have catalytic converters that clean exhaust by converting nitrogen oxides (NOx) into harmless nitrogen gas. Diesel engines, in contrast, do not have effective catalytic converters. This means that diesel engines pollute the air with NOx and contribute to both urban smog and acid rain.
It is unfortunate that diesel engines pollute so much, because diesel engines are much more efficient than gasoline engines. Diesel engines get about 30 percent better fuel mileage than gasoline engines. Most of this increased efficiency is due to diesel engines' near-complete burning of the fuel they are fed. Ott believes his "NOx HyCat" catalytic converter will allow the U.S. to switch from inefficient gasoline vehicles to more efficient diesel cars, trucks and SUVs.
Ott and his team have worked for several years with the big three auto companies on the NOx HyCat project. The first phase of design involved understanding just why normal catalytic converters work well with gasoline engines but fail to clean the exhaust from diesel engines. Ott's team found that the key difference was the ratio of different NOx components in exhaust. NOx contains both nitric oxide (NO) and nitrogen dioxide (NO2). Ott's team found that these two gasses had to be equal for catalytic converters to work efficiently. If there is too much NO and not enough NO2, normal catalytic converters fail to clean the exhaust.
In gasoline engines, a carefully calibrated amount of oxygen is mixed with the fuel. Too much oxygen can cause premature ignition ("engine knock"). This carefully calibrated amount of oxygen releases a near-optimal 1:1 ratio of NO and NO2 in gasoline exhaust. This ideal NOx ratio explains why catalytic converters work so well in gasoline engines.
In contrast, exhaust from diesel engines may contain nine times as much NO as NO2. The reason for the high NO concentration can be traced directly back to just what makes a diesel engine so fuel efficient. Instead of carefully balancing oxygen and fuel, diesel engines deliberately add in "too much" oxygen. By adding in an overabundance of oxygen, diesel engines seek to combust every molecule of fuel. Thus, diesel engines are very efficient, but the overabundance of oxygen leads to high NO concentrations. In light of Ott's findings, it was clear why conventional catalytic converters do not work with diesel engines.
Having earned this understanding of NOx catalysis, Ott and his team were left with the challenge of finding a new catalyst that would convert NO into NO2. If the team could find such a catalyst, then a hybrid catalyst containing the new catalyst plus the normal catalyst should be able to scrub the NOx from diesel exhaust. There were several properties that had to be combined in a final catalyst. First, it had to have strong potential to oxidize NO. Second, the catalyst should not contain toxic metals. Third, the catalyst should work over a wide temperature range.
Ott describes the search for the new catalyst like this: "We had a long history of work in oxidation catalysis so were pretty familiar with metal oxide chemistry. We began looking at certain obvious candidates that were potent NO to NO2 converters." Ott's team also relied on a property of crystals to find combination crystal catalysts that would be most able to convert NO into NO2. "It's often observed that solid solutions of oxides have unusual properties," said Ott. "It turns out in catalysts that one of the properties is the ability to move oxygen atoms throughout the structure. This allows these oxygen atoms to be potent oxidation reagents. We began by looking at potent oxidants and selected ones that formed solid solutions with one another. We ended up with a solid solution that was a cerium-oxide catalyst with a little bit of manganese oxide. Neither part is good enough by itself. The catalyst was the breakthrough."
The new hybrid catalyst—€”dubbed NOx HyCat—€”cleans between 83 and 98 percent of NOx at low temperatures (where the pollution is most problematic). At higher temperatures, NOx HyCat is between 96 and 98 percent efficient. NOx HyCat also has several other important properties. Unlike competing solutions, the NOx HyCat system does not consume additional fuel, so the NOx HyCat system retains diesel's inherent efficiency. NOx HyCat's sulfur tolerance is excellent and the system can deal with the moisture released by diesel engines.
Ott hopes that NOx HyCat will allow the U.S. to follow Europe's lead in adopting diesel automobiles. According to Ott, about 60 percent of all vehicles sold in Europe run on diesel. These new diesel cars already include pollution controls that remove the particular matter—€”the sooty smoke we associate with older-generation diesel vehicles—€”from the exhaust. With NOx HyCat also in place, Ott believes that diesel vehicles will now be able to comply with clean air standards in the U.S. (soot traps do not remove NOx). "We intend to reap the potential benefit of the more fuel-efficient diesel engine," said Ott, "and at the same time minimize the environmental impact."
For more information about NOx HyCat commercialization, contact Eric Canuteson at LANL: 505-667-9595, canuteson@lanl.gov
Jeffrey Stewart is a business development executive, technology transfer, at Los Alamos National Laboratory.
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