Catalyst knocks out a smog maker

Unique system could help truckers meet new emissions standards

A novel type of catalytic converter could be available within a year, just in time to help some trucks meet new standards limiting emissions of smog-generating nitrogen oxides.

NEW USE FOR OLD CATALYST Chris Marshall, foreground, of Argonne National Lab holds a container of a new material that could help heavy-duty trucks spew much less nitrogen oxide, a major component of smog. In the background are team members who are preparing to test the material. Argonne National Lab

ZEOLITE CANDY A close-up of the new de-NOx catalyst shows the sharp edges of the zeolite crystals (left), full of tiny, barely visible holes. The cerium-oxide coating applied to protect the zeolite’s aluminum scaffolding from degradation by hot water vapor gives the surface of the treated microparticles a sort of fluffy, cotton-candy texture (right). Argonne National Lab

By next year, heavy-duty diesel trucks will be permitted to spew no more than 10 percent of the amounts of NOx permitted in 1990. Currently a liquid-urea–based catalytic converter system is available to clean up NOx, but it’s messy and cumbersome. Within a year, however, a no-muss alternative could be ready to rumble down the interstates: a catalytic converter that uses onboard diesel fuel as its additive, not urea.

Both catalyst systems convert nasty NO x into nitrogen gas, the primary constituent of air. The standard catalytic converters in gasoline-fueled vehicles won’t work with diesel engines, though. These converters require that the exhaust contain fairly equal ratios of oxygen to nitrogen or oxygen to hydrocarbons, but in diesel exhaust, oxygen dominates.

The problem: Converting NOx to nitrogen requires a steady diet of electrons in addition to the catalyst — and oxygen tries to rob those electrons. So Chris Marshall’s team at Argonne National Laboratory, outside Chicago, hunted down an alternative to the traditional catalysts, one that could operate in the presence of the bonus oxygen. The team settled on a copper-zeolite material that had been around since the 1990s, but needed considerable tweaking to handle diesel exhaust.

It worked only in “bone dry” environments, Marshall explained, and water vapor is a primary constituent of diesel exhaust. The catalyst also worked best at around 450° Celsius. Diesel exhaust typically runs closer to 340°C, and during idling, exhaust temperatures can fall far lower still.

So Marshall’s team jacketed each powdery catalyst microparticle with a thin coating of cerium oxide before bonding the microparticles onto a chunk of porcelain. That chunk is riddled with millimeter-diameter holes through which a vehicle’s exhaust flows. The copper-zeolite fosters the reaction between the NOx and electrons from the diesel fuel to create the nitrogen gas. The coating protects the catalyst so that it can operate in the presence of water. The coating also lowers the optimal operating temperature, putting it smack in the middle of the diesel exhaust temperature range.

The electron donor for this system is diesel fuel, a tiny bit of which is vaporized and steadily pumped into the exhaust, upstream of the catalyst. For the other technology, expected to find use in 2010 trucks, urea is the electron donor.

In preliminary tests, Argonne’s recently patented catalyst-based technology matched urea in terms of NOx breakdown at peak operating temperatures — and beat urea at lower temps. Furthermore unlike the urea systems, the Argonne catalytic converter is a one-time installation that never needs replacement.

And the kicker: Once the new catalyst hits the commercial marketplace — perhaps later this year — it should cost truck manufacturers only about two-thirds as much as urea systems, says Robert Firebaugh. He heads Integrated Fuels Technology, outside Seattle — the company that licensed Argonne’s de-NOx catalyst for commercialization.

“We are currently designing a system for stationary platforms,” Firebaugh says, like the diesel engines used in heavy industry. “We are also testing it with two of the largest light-truck manufacturers, and with the [Los Angeles] Port Authority for ships and so forth.”

In the 11-million-vehicle U.S. truck market, there’s a strong aversion to urea systems, he says. Current plans are to put a five-gallon tank of urea on board a truck, but the tank would have to be refilled at every other refueling. And because urea can freeze up, trucks using it would have to carry an auxiliary heater for winter. In fact, “Right now, you won’t find urea anywhere in the country available for trucking. So there’s a major infrastructure problem,” Firebaugh says.

Asked about how engine makers are viewing the technology changes on the horizon this year, Joe Suchecki of the Chicago-based Engine Manufacturers Association said: “I’m not familiar with the Argonne one … but there were a number of options that manufacturers looked at for 2010.” Except for two systems, the urea-based system and an exhaust-gas recirculation system, diesel-engine makers decided the alternatives “were either not mature enough to be reliable or ended up looking too expensive in terms of fuel economy.” The result: all but one of the engine manufacturers have decided to go with the urea system.

In the future, Suchecki said, “all of the manufacturers will review the technology that’s available and the cost analysis, and will obviously provide the best product they can.” But he conceded that “there are concerns” among truckers and their companies about how cumbersome the new urea-based technologies might be.

Urea systems are expected to add about 5 percent to the effective cost of fuel, Firebaugh says, while fuel penalties associated with the new system that he is working to commercialize “are 1 percent or less.” He acknowledges that his team is “a little late to the game,” so this technology will likely miss 2010 model-year vehicles. But unlike competing anti-NOx technologies, he notes, “ours can be retrofitted onto older trucks.”

Janet Raloff is the Editor, Digital of Science News Explores, a daily online magazine for middle school students. She started at Science News in 1977 as the environment and policy writer, specializing in toxicology. To her never-ending surprise, her daughter became a toxicologist.

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