NOX-Free Exhaust

Of the six air pollutant emissions that the EPA has tracked since the 1970s—carbon monoxide, lead, particulate matter, sulfur dioxide, volatile organic compounds, and nitrogen oxides (NOX)—all have decreased significantly except for NOX.

Formed when fuel is burned at high temperatures, and naturally occurring as well, NOX gases are highly reactive. They contribute to smog, to acid rain, to global warming, even to oxygen depletion in water, which reduces fish and shellfish populations. (NOX  is one of the largest sources of nitrogen pollution in the Chesapeake Bay.)

According to the EPA, motor vehicles produce more than half of anthropogenic (that is, human-produced) NOX emissions in the U.S. Prompted by federal regulations, researchers have for decades been looking for ways to reduce those emissions. “Getting the right catalyst to get the reaction 2NO — N2 + 02 is the Holy Grail of catalysis,” says chemical engineering professor Fabio Ribeiro (shown above with two of his graduate students). “All you need is the right catalyst, and you’d be richer than Bill Gates.”

Researchers have yet to crack that specific problem, but Ribeiro and his colleagues in Purdue’s Center for Catalyst Design are working on catalytic processes for NOX abatement in lean-burn engines, engines that mix excess air with fuel. A lean-burn gasoline engine consumes about 20 percent less fuel than the gasoline engines currently used and thus emits less of the greenhouse gas CO2. Working with Cummins, the engine manufacturer and a leader in catalyst technology innovations, Ribeiro’s group is studying the application of one particular catalytic process—NOx storage-reduction (NSR), originally created for gasoline engines—to diesel engines.

“It’s fundamental research,” says Ribeiro. “We’re performing experimental and modeling efforts on NSR catalysts to understand what’s going on using a multitude of techniques, and we’ve proposed a reaction mechanism that can explain some of the kinetics we’ve observed for part of the process.”

The lean-burn engine is still under development in the automotive industry, but its promise—fuel efficiency without sacrificing engine power—will burn brighter as engineers learn more about NOX and about how catalysts will perform.