Pinpointing Intermediates

The once-familiar scene of long lines of diesel trucks belching black smoke along roadways has largely faded as a result of demanding environmental legislation and the technology it has compelled. A good example is a recently commercialized copper-doped zeolite-based catalyst system that scrubs nitrogen oxides (NO_x) from diesel exhaust, reducing the pollutant to trace levels.

That system may soon do an even better cleanup job now that researchers have identified copper-nitrosyl complexes as its key catalytic reaction intermediates (Angew. Chem. Int. Ed. 2013, DOI: 10.1002/anie.201303498). The finding deepens understanding of a commercially important chemical process. It also demonstrates a mechanistic connection between solid-state (heterogeneous) and biological catalysis by showing that the same type of intermediate plays a role in enzyme chemistry.

Many of today’s diesel trucks rid engine exhaust of NO_x by reducing it with ammonia to nitrogen and water in the presence of a copper-doped chabazite zeolite catalyst, Cu-SSZ-13. Although this selective catalytic reduction technology, which generates ammonia from an on-board supply of aqueous urea, is well established commercially, basic science questions remain unanswered.

Analysis by nuclear magnetic resonance and vibrational spectroscopy have removed a key question from that list. János Szanyi and Charles H. F. Peden of Pacific Northwest National Laboratory; Ja Hun Kwak of Ulsan National Institute of Science & Technology, in South Korea; and coworkers have determined that the key intermediates in that reaction are Cu⁺–NO⁺ complexes bonded in a side-on fashion (NO bonds to Cu through N). In addition to determining geometries and oxidation states, the team combined the findings with results of earlier kinetic studies and deduced the reaction mechanism and catalytic cycle that govern selective catalytic reduction of NO_x on Cu-SSZ-13.

According to Szanyi, that kind of information may be helpful for tailoring the catalyst to enhance its low-temperature performance, an improvement that can help future cooler-running engines meet emission standards.

Surprisingly, the team found that the same types of side-on copper-nitrosyl complexes had been identified roughly 10 years ago as key reaction intermediates in a completely unrelated system: bacterial metabolic processes involving nitrite reductase enzymes.

“This is an interesting example of the kind of similarity between heterogeneous and enzyme catalysis that has long been overlooked,” comments University of California, Berkeley, chemistry professor Gabor A. Somorjai. He notes that identifying such similarities could benefit all areas of catalysis. “It will be interesting to see if there are many more examples that strengthen the correlation between these traditionally separate fields,” he adds.