New catalyst could help diesels meet NOx deadlines

A new catalyst could help car manufacturers meet the US Environmental Protection Agency’s deadline to eliminate 95 percent of nitrogen-oxide from diesel engine exhausts by 2007.

A new catalyst could help auto makers meet the U.S. Environmental Protection Agency’s deadline to eliminate 95 percent of nitrogen-oxide from diesel engine exhausts by 2007, while saving energy.

Developed by the US Department of Energy’s Argonne National Laboratory, the new catalyst is one of a family of related catalysts that also shows promise for reducing NOx emissions from industrial sources, such as coal-fired power plants and furnaces at chemical plants and refineries.

Nitrogen oxides, collectively called ‘NOx’, contribute to smog, acid rain and global climate change.

‘For diesel engines, we envision manufacturers placing ceramic catalytic reactors in the exhaust pipes, where they will convert NOx emissions into nitrogen,’ said inventor Chris Marshall of Argonne’s Chemical Engineering Division. Nitrogen is a harmless gas that makes up more than 80 percent of the Earth’s atmosphere.

‘Our most promising catalyst for diesel engines,’ Marshall said, ‘is Cu-ZSM-5 with an external coating of cerium oxide.’ Cu-ZSM-5 is a zeolite, a common catalyst used in petroleum refining, doped with copper.

Marshall said that those working previously with Cu-ZSM-5 and similar catalysts found that they performed poorly at removing NOx from diesel exhaust. They require temperatures higher than normal exhaust temperatures and do not work well in the presence of water vapour, which is almost always found in engine exhausts.

‘Our new cerium oxide additive,’ he 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 vapour than without it. With a lean fuel-air mixture, it removes as much as 95 to 100 percent of NOx emissions.’

Argonne’s new catalysts also avoid the problems associated with ammonia, which competing catalysts generate.

‘The current standard is ammonia-selective catalytic reduction, using urea as the source,’ Marshall said, ‘but ammonia is toxic, and unless all of it is converted during the process, whatever remains is released to the atmosphere. While some European diesel manufacturers are taking the urea approach, US diesel manufacturers are looking for alternatives.’

Since a system using the new catalyst would not require an on-board urea storage tank, the new catalyst is considered safer and more energy-efficient.

‘We’re also looking at other elements that may work better than cerium under certain engine conditions,’ said Marshall. ‘Zirconia shows good promise.’

Initial research on the cerium-oxide catalyst was funded by the US Department of Energy’s Office of Energy Efficiency and Renewable Energy. The catalyst was developed for chemical plant emissions under a joint research agreement with BP. Research plans call for expanded work aimed at both diesel and natural gas engines and coal-fired power plants.

A patent application has been filed on the new catalyst and it is expected to be available for licensing.