After systematically studying multiple catalyst compositions, researchers at KAUST and Umicore have clarified the role of cerium (Ce) in Mn-based oxide catalysts for the selective catalytic reduction (SCR) of NOx with NH3 at temperatures below 200 °C. An open-access paper on the study appears in Nature Communications.
Recent developments in high-efficiency engine design, alongside tightening vehicle emission regulations, demand improved engine exhaust catalysts. Current-generation NOx catalysts for small diesel engines perform optimally above 200 ˚C. Catalysts that operate at lower temperatures are now required. Such catalysts need to quickly remove NOx after a cold start and partner with new low-temperature combustion engines.
In this respect, manganese-containing mixed metal oxides exhibit excellent catalytic activity in the NH3-SCR reaction operating at temperatures below 200 °C, and therefore is of particular interest as a potential low-temperature component in NH3-SCR.
Typically, Mn-based catalysts are prepared by impregnation or homogeneous precipitation methods with other metal oxides, such as Ti and Ce oxides, that act as support, dopants, or promoters. During the last decades, the role of the different components on the catalytic activity and selectivity have been debated extensively. Mn catalytic activity originates from its excellent redox ability at low temperatures. The importance of specific surface area, dispersion, and oxidation state of the different Mn oxides have been highlighted. TiO2 is considered a metal oxide support providing optimal dispersion of Mn active species, surface area, thermal stability, and Lewis acid sites to adsorb NH3. For Ce and other transition metal, there is no clear consensus on their role on the catalytic reaction. The promotional effect is often explained by an improvement of the catalytic redox cycles by intimate contact of the active Mn oxides and the promotors. Among the transition metals, Ce is widely used and probably one of the most promising promotors.
In binary MnCe systems, the addition of Ce was reported to improve the conversion levels compared to individual Mn oxides. This promotional effect is generally explained by an enhancement of the redox functionality, which is proven by the easier reduction of Ce and/or Mn during temperature-programmed reduction experiments. Baiker et al. also postulated that binary MnCe oxides have a higher adsorption of NO and NH3, which promotes catalytic activity. In ternary MnCeTi oxides, the improvement of activity by Ce is also frequently explained by an increase of the Mn redox properties. In contrast, other studies suggest that the MnCe electronic interaction decrease the activity of Mn oxide species for NO conversion by a reduction of the Mn4+/Mn3+ ratio. On the basis of the measured surface areas, binary MnCe and tertiary MnCeTi systems show better textural properties when Ce is added, but this is rarely discussed as a main promoting effect.
To resolve this unsettled dilemma, we studied the structure and catalytic performance of Mn, Ce, and Ti mixed-oxide catalysts with a wide range of metal-oxide compositions.—Gevers et al.
The team synthesized 30 catalysts with different Mn, Ce, and Ti compositions, using established methods to produce each catalyst with a homogenous nanostructure to enable a comparison between them. After accounting for differences in catalyst surface area, the team showed that the presence of cerium lowered the catalytic activity of the manganese atoms.
In previous studies where cerium had appeared to boost catalytic NOx removal, cerium’s apparent positive effect disappeared once the team had factored in its impact on catalyst surface area. However, the cerium did have one benefit: suppressing an undesired side-reaction producing N2O. As N2O formation likely requires the participation of two neighboring manganese sites, the addition of cerium may dilute the number of surface manganese sites and so suppress the reaction.
Our findings show that the design of more active catalyst materials requires the maximization of manganese atoms on the catalyst surface and that these manganese atoms be atomically spaced to avoid N2O formation. We are now designing catalysts exposing manganese atomically dispersed on the surface, and the results are extremely promising.—Javier Ruiz-Martínez
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. (2022) “Unraveling the structure and role of Mn and Ce for NOx reduction in application-relevant catalysts.” Nature Communications 13, 2960 doi: 10.1038/s41467-022-30679-9