Copper pairs up to reduce nitrogen oxides in diesel exhaust

High schoolers aren’t the only ones who pair up and break up frequently. Catalytic species in the systems that clean up engine exhaust do it too, according to a study presented on Tuesday at the American Chemical Society national meeting in Washington, D.C.

The investigation uncovers an unusual mechanism in the catalytic process that rids diesel engine exhaust of smog-causing nitrogen oxides (NO_x). The findings may eventually lead to more effective catalysts.

Diesel-powered engines score high marks for fuel efficiency, which is why nearly all long-haul trucks use them. But they emit various pollutants, including hydrocarbons, CO, and NO_x.

Selective catalytic reduction (SCR) systems—part of the overall exhaust cleanup equipment on diesel vehicles—reduce NO_x to nitrogen and water through a reaction with ammonia. The ammonia comes from an aqueous urea solution, which is carried on board like windshield cleaner. SCR systems catalyze NO_x reduction with the help of chabazite zeolite that has been treated to incorporate copper in its lattice.

These systems come standard on many diesel vehicles, yet details of how they work remain a subject of debate.

A team led by Rajamani Gounder of Purdue University and William F. Schneider of the University of Notre Dame studied some of the details in copper-chabazite SCRs. Using X-ray spectroscopy, kinetic measurements, and quantum calculations, the researchers tracked unusual behavior of the zeolite’s copper ions.

Speaking at a symposium sponsored by the Division of Catalysis Science & Technology, Gounder reported that in the presence of ammonia, copper ions form Cu(NH₃)₂ complexes. The NH₃ moities make the species mobile, allowing them to migrate through openings that interconnect hollow cages in the porous zeolite framework. As the copper species move about, they briefly and reversibly form dimers that are bridged by a pair of oxygen atoms. The dimers facilitate an O₂-mediated Cu(I) to Cu(II) redox step that’s central to reducing NO_x to nitrogen and water (Science 2017, DOI: 10.1126/science.aan5630).

Gounder explained that, in effect, individual copper ions come together and work in tandem to carry out the difficult step of breaking apart oxygen molecules. The copper ions then go back to being isolated after the reaction is complete. He noted that this step might be accelerated by fine-tuning the spatial distribution of copper ions in the zeolite, leading to lower NO_x emissions at cooler operating temperatures than is possible with current SCR systems.

“Exquisite” is how Robert J. Davis described the team’s techniques for exploring this SCR system. Davis, a catalysis specialist at the University of Virginia, remarked that this exciting finding regarding the mobility of copper ions and the dynamic formation of paired copper species may also be relevant to the high selectivity of Cu-treated zeolites used for oxidizing methane to methanol.