NOx Removal by LNT-SCR Dual-layer Catalysts

The increasingly strict emission standards have driven the progress of NOx storage and reduction (NSR) technology. NOx is stored in a lean NOx trap (LNT) catalyst during fuel-lean mode and reduced to N2 during fuel-rich mode.

First, we investigated the impact of ceria on NSR in an Pt/Ce LNT catalyst. The physisorbed oxygen over the ceria-containing LNT catalyst led to a spatio-temporal temperature rise in the monolith upstream after the cyclic introduction of H2/CO to a pre-oxidized catalyst. The stored oxygen over ceria enhanced NO storage by in-situ NO2 formation, while it competed with NO2 for storage sites. During the NOx reduction over the Pt/Ceria, the Pt surface purgation was the first step and the oxygen reduction preceded the NOx reduction.

Second, we studied the NSR by dual-layer catalysts consisting of a selective catalytic reduction (SCR) catalyst layer on top of a LNT catalyst. During periodic switching between lean and rich feeds, the LNT layer reduced NOx to N2 and NH3. The SCR layer trapped the latter leading to additional NOx reduction.

The dual-layer catalysts exhibited high N2 selectivity and low NH3 selectivity over the temperature range of 150-400 ºC. The NOx conversion was incomplete due to undesired NH3 oxidation. The dual-layer catalyst has a higher NOx conversion and N2 selectivity than the LNT catalyst when H2O and CO2 were present in the feed.

Ceria was used to adjust the dual-layer catalyst performance. The ceria addition increased NOx storage capacity, promoted hydrothermal durability and mitigated CO poisoning. However, ceria decreased the high-temperature NOx conversion by promoting NH3 oxidation. Ceria zoning led to the highest NOx reduction for both low- and high- temperatures due to the beneficial interaction of ceria and H2.

The impact of catalyst design and operation strategy was evaluated. The low-temperature NOx conversion of an aged dual-layer catalyst was increased by a high SCR catalyst loading. The ratio of lean to rich feed duration and the total cycle time were optimized to improve the NOx conversion. The results suggest the dual-layer catalyst could be used to reduce precious metal loading and improve the fuel economy.