Mechanistic Investigation of the Reduction of NOx over Pt- and Rh-Based LNT Catalysts

Abstract

Abstract:The influence of the noble metals (Pt vs. Rh) on the NO x storage reduction performances of lean NO x trap catalysts is here investigated by transient micro-reactor flow experiments. The study indicates a different behavior during the storage in that the Rh-based catalyst showed higher storage capacity at high temperature as compared to the Pt-containing sample, while the opposite is seen at low temperatures. It is suggested that the higher storage capacity of the Rh-containing sample at high temperature is related to the higher dispersion of Rh as compared to Pt, while the lower storage capacity of Rh-Ba/Al 2 O 3 at low temperature is related to its poor oxidizing properties. The noble metals also affect the catalyst behavior upon reduction of the stored NO x , by decreasing the threshold temperature for the reduction of the stored NO x . The Pt-based catalyst promotes the reduction of the adsorbed NO x at lower temperatures if compared to the Rh-containing sample, due to its superior reducibility. However, Rh-based material shows higher reactivity in the NH 3 decomposition significantly enhancing N 2 selectivity. Moreover, formation of small amounts of N 2 O is observed on both Pt-and Rh-based catalyst samples only during the reduction of highly reactive NO x stored at 150˝C, where NO x is likely in the form of nitrites.

Composition/Performance Evaluation of Lean NOx Trap Catalysts for Coupling with SCR Technology

“…First, in the standard operation condition (lean phase) of the engine, NO x from the raw, oxygen‐rich emissions is stored in the catalyst [9,10,11] . After a period of 60–120 s, [12,13,14] depending on the raw emission composition and the driving profile, the storage is filled and needs regeneration. This is achieved by fuel‐rich combustion with excessive H 2 , CO and hydrocarbons in an O 2 ‐deficient exhaust gas atmosphere.…”

“…This is achieved by fuel‐rich combustion with excessive H 2 , CO and hydrocarbons in an O 2 ‐deficient exhaust gas atmosphere. After a reduction period of normally 3–10 s, the catalyst is regenerated and the engine switched back to lean‐burn operation [12,13,15] …”

Hybrid Technology for DeNOxing by LNT-SCR System for Efficient Diesel Emission Control: Influence of Operation Parameters in H2O + CO2 Atmosphere

“…At present, NO x Storage and Reduction (NSR) technology is the most efficient in the removal of nitrogen oxides from diesel exhaust gases. However, it is not enough to achieve zero emissions at the outlet of the vehicles, due to the formation of an NH 3 slip that is strongly dependent on the catalyst formulation and the operation conditions [2][3][4][5]. The Selective Catalytic Reduction (SCR) process, which is based on the mineralization of NO x to N 2 and water with ammonia [6][7][8][9], requires the injection of an external reducing agent.…”

Improved NOx Storage/Release Properties of Ceria-Based Lean NOx Trap Compositions with MnOx Modification

“…The stored ad-NO x species are desorbed and reduced to nitrogen on catalytically active noble metals in subsequent short rich periods at elevated temperatures [7]. Most common lean NO x trap catalysts contain a high surface-area material like γ-Al 2 O 3 , mixtures of BaO and CeO 2 and noble metals, typically Pt or Pd and Rh [8,9].…”