Catalyst Knocks out a Smog Maker

March 2, 2009

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.

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 NOx 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. 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 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.

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.