NOx storage and reduction properties of model manganese-based lean NOx trap catalysts

• Mn-containing catalysts exhibited higher activity for NO oxidation than Pd/Ba/Al.
• Pd/Mn/Ba/Al showed significantly improved NOx conversion under cycling conditions.
• Mn greatly lessened the inhibiting effects of H2O and CO2 on NOx conversion.

In order to study the role of manganese in LNT catalysis, model Pd/Mn/Ba/Al, Pd/Mn/Al and Pd/Ba/Al catalysts were prepared and characterized. Both Mn-containing catalysts exhibited higher activity for NO oxidation to NO2 than the Pd/Ba/Al reference catalyst, although NOx-TPD experiments showed that adsorbed NOx species were weakly bound on Pd/Mn/Al. Consequently, the presence of Ba was essential for high storage capacity. Compared to the Pd/Ba/Al reference, Pd/Mn/Ba/Al showed significantly improved NOx conversion under lean-rich cycling conditions as a consequence of its increased activity for NO oxidation and hence, superior NOx storage efficiency. In addition, the presence of Mn greatly lessened the inhibiting effects of H2O and CO2 on cycle-averaged NOx conversion, this being due to the facile decomposition of manganese carbonate at low temperatures as evidenced by DRIFTS. Significantly, the Pd/Mn/Ba/Al catalyst displayed comparable activity to a traditional LNT catalyst of the Pt/Ba/Al type, showing the promising prospect of such new type of LNT catalysts.

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