Scientists from King Abdullah University of Science and Technology (KAUST) expect to eliminate smog with a novel exhaust catalyst concept.
The best combination of metals to remove nitrogen oxides from vehicle emissions
Scientists conducted a systematic study that resolves a debate on the best combination of metals to catalytically clean nitrogen oxides (NOx ) from vehicle emissions. Smog-producing chemicals could be almost eradicated from the tailpipes of diesel cars and vans, utilising a novel exhaust catalyst concept developed by KAUST researchers.
This discovery was made after scientists systematically studied multiple catalyst compositions. This allowed researchers to detect and develop the perfect atomic recipe to catalytically remove NOx from vehicle emissions.
Current-generation NOX catalysts versus novel exhaust catalyst concept
To meet the growing demand for engine exhaust catalysts, scientists have developed high-efficiency engine designs alongside tightening vehicle emission regulations.
Current-generation NOx catalysts for small diesel engines reach their optimum performance above 200°C. However, it is now required for catalysts to operate at lower temperatures. Such catalysts must quickly remove NOx after a cold start and then partner with new low-temperature combustion engines.
Therefore, to develop this new generation of improved NOx catalysts, the automotive emissions control company, Umicore, partnered with a research team from KAUST’s Catalysis Centre to optimise the catalyst design.
“We investigated materials that are based on manganese due to their good performance and low cost,” explained Javier Ruiz-Martínez, leader of the study.
Manganese-based NOx catalysts have typically utilised cerium as a dopant, even though there was no consensus on cerium’s role in NOx removal. “The best way to develop new catalysts is by first understanding how those materials work,” added Ruiz-Martínez.
Thus, the team produced a series of catalysts to determine the best solution, incorporating varying amounts of cerium.
Correlations between catalytic activity and the amount of cerium and manganese
The team first established methods to produce each catalyst with a homogenous nanostructure to enable a comparison between them. “After making sure that the catalyst materials were as we designed, we looked for correlations between catalytic activity and the amount of cerium and manganese,” said Ruiz-Martínez.
After accounting for differences in catalyst surface area, the team demonstrated that the presence of cerium lowered the catalytic activity of the manganese atoms.
In past studies, cerium has been noted to boost catalytic NOX removal, however, scientists observed that this apparent positive impact vanished once the force on the catalysts surface area had been considered.
Although, cerium did have one notable advantage in suppressing an undesired side-reaction of producing N2O. As N2O formation likely requires the participation of two neighbouring manganese sites, it is believed that the addition of cerium may dilute the number of surface manganese sites and suppress the reaction.
“Our findings show that the design of more active catalyst materials requires the maximisation of manganese atoms on the catalyst surface and that these manganese atoms be atomically spaced to avoid N2O formation,” Ruiz-Martínez concluded. “We are now designing catalysts exposing manganese atomically dispersed on the surface, and the results are extremely promising.”
Gevers, L.E., Enakonda, L.R., Shahid, A., Ould-Chikh, Silva, C.I.Q., Paalanen, P.P., Aguilar-Tapia, A., Hazemann, J-L., Hedhili, M.N., Wen, F. &Ruiz-Martínez, J. Unravelling the structure and role of Mn and Ce for NOx reduction in application-relevant catalysts. Nature Communications.