Investigation of La promoted Pd/BEA Performance for Passive NOx adsorption processes
Lean-burn combustion engines have been used to improve fuel efficiency by reducing fuel consumption. However, these engines face challenges in lean reduction of NOx for exhaust after-treatment systems. Therefore, two common techniques are applied for lean-burn gasoline engines and diesel engines to reduce emitted NOx, which includes selective catalytic reduction (SCR) and lean NOx traps (LNT). However, both techniques have limitations at exhaust temperatures below 200℃ which can be problematic for them to achieve ultra-low NOx emissions. Thus, Passive NOx adsorber (PNA) have been suggested for preventing cold-start NOx emissions. In this method, NOx is trapped at low temperatures onto the adsorbent and is later released at exhaust temperatures higher than 200℃. The released NOx can be reduced by the following SCR or LNT unit at higher temperatures. One of the well-known adsorbents in PNA, has been Pd on zeolites due to its high NOx storage capacity. One important aspect is that the NOx desorption should not occur before the light-off temperature has been reched in the SCR or LNT. In this work, Ce, Zr and La have been used as promoters for Pd/BEA. Among these materials, La promotion was shown to efficiently increase the NOx desorption temperature above 200℃ compared with the reference material. Furthermore, the effect of water and hydrogen pretreatment was studied for La-Pd/BEA and the reference material Pd/BEA. The results indicated that, the presence of water blocked the adsorption of weakly adsorbed NO on the samples which shifted the desorption temperatures to higher values (>200℃). On the other hand, hydrogen pre-treatment had a negative effect on NOx trap capacity and desorption temperature. Interestingly, the La promoted sample was less sensitive to H2 pre-treatment. Characterization methods such as TPO and XPS, indicated that the La promoted sample consisted of more stable Pd oxides which needed higher temperatures to desorb O2. DRIFTS analysis showed that NO was adsorbed in the form of more strongly bound species (nitrates) in greater quantities with La promotion compared with the reference sample. In addition, the effect of multiple cycles of NO TPD experiments were studied with elevated levels of CO in the inlet gas, to compare the performance and stability of the La promoted and reference materials. The La promoted adsorbent showed stable behavior in NOx desorption beyond a 5th cycle while the NOx storage increased at around 160℃ with each cycle. Whereas, a stronger degrading behavior was observed for the Pd/BEA sample where NOx storage and desorption decreased with each TPD cycle. The same pattern was observed in DRIFTS analysis with the types and relative quantities of NOx adsorbed species for each material. O2-TPD analysis indicated more Pd clusters formed on the unpromoted material compared with the promoted one with the presence of CO during multiple NO TPD experiments. This caused a degradation in NOx release from the unpromoted material due to its decreasing content of ion-exchange Pd which is considered the main NOx adsorption site at low temperatures.