#worldenvironmentday Improving diesel catalysts for cleaner skies

Diesel engines are widely used in heavy transportation industry (ships, lorries, etc), contributing to global pollution and, hence, climate change. Electric solutions for heavy duty vehicles are still not available, so scientists work to improve current systems in order to effectively decrease gas emissions.

The emission of nitrogen oxides from diesel vehicles is a global environmental challenge. Catalytic cleaning systems in transportation remove carbon monoxide, unburnt hydrocarbons and nitrogen oxides (NOx) before emission in the atmosphere. The most modern catalysts for diesel exhausts use copper zeolites, porous crystalline silicon and oxygen structures with some nanometer-sized pores, which reduce well over 90% of the NOx emitted by the engine using ammonia and oxygen. This combination produces nitrogen and water, which are safe for the environment.

The chabazite–type zeolite is a particularly useful zeolite structure for catalysts. In it, copper ions are added to the active sites in the framework, where NOx reduction takes place. Copper runs the reaction to remove NOx inside the zeolite and can present different forms in the catalyst. The zeolite framework avoids the sintering of the copper particles, which would make their surface much smaller and hence, they would be less reactive.

Despite the optimal functioning of the catalyst, sulphur is present in the fuel, it becomes SO2 in combustion and progressively deactivates the catalysts.

Scientists at the ESRF, University of Turin (Italy) and the Danish office of UMICORE, a global leader in clean mobility materials and recycling  used in-situ X-ray absorption spectroscopy on beamline BM23 and X-ray emission spectroscopy on ID26 to find an explanation for the deactivation of NOx commercial catalysts in presence of sulphur. “We exposed the catalyst to conditions to form the species that can occur in the real cycle”, explains Anastasia Molokova, PhD student at the ESRF, UniTO and UMICORE and first author of the publication.

Results showed which copper species are the most sensitive to SO2. “It turns out that the one we found to be the most damaged by SO2 is the one that is most needed for the catalyst to work”, says Molokova.

The team’s next step is to submit the catalyst to operando conditions. “Based on the knowledge we now have, we will reproduce what happens in the catalyst and exhaust in the car”, explains Kirill Lomachenko, scientist at the ESRF and corresponding author of the publication.

“The long-standing collaboration between the ESRF and UMICORE has been very fruitful over the years”, explains Ton Janssens, scientist at UMICORE. “The results have allowed us to get a better picture of what goes on inside the catalyst, and it has allowed us to go beyond the trial-and-error way of functioning”, he concludes.

Reference:

Molokova, A. et al, JACS Au 2022, 2, 4, 787–792. DOI:10.1021/jacsau.2c00053

Text by Montserrat Capellas Espuny

Top image: Air pollution in Shanghai.