Mechanism of the passive NOx adsorption process on zeolite-supported Pd in controlling cold-start emissions

Doktorsavhandling, 2022

Passive NOx adsorbers (PNA) have been used for preventing NOx emissions being released during a cold start. This method involves trapping NOx on an adsorbent at low temperatures and releasing them later, when the exhaust temperature exceeds 200℃. One of the well-known adsorbents used in PNA is Pd on zeolites, due to its high storage capacity of NOx.
In this work, La was found to promote Pd/BEA by increasing the desorption temperature of NOx above 200℃ compared to the reference material. DRIFTS analysis provided formation of more stable NOx species being formed during adsorption within La-modified Pd/BEA, thereby improving thermal stability. In addition, La-promoted Pd/BEA, unlike non-promoted Pd/BEA, showed increased performance stability after multiple cycles of NO-TPD experiments with elevated levels of CO in the inlet gas. Different characterization techniques, indicated the formation of less agglomerated Pd clusters on the zeolite framework in the La modified sample after multiple cycles, which led to a more stable performance. A study of the deactivation mechanism of CO on Pd/SSZ-13 indicated that the agglomeration mechanism of Pd consists of two sintering modes: Ostwald ripening in the first cycles of NO-TPD is followed by particle migration in the later cycles. On the other hand, low concentrations of CO shifted the NOx release peaks towards more favourable temperature ranges in Pd/SSZ-13. XPS analysis was used to investigate the positive effects of low CO levels and showed that the Pd species were reduced by CO adsorption which, in turn, increased the NOx binding strength.
Moreover, the presence of phosphorous on Pd/SSZ-13 caused a reduction in the efficiency of the adsorbent: this deactivation was more severe when the phosphorous content was increased. The formation of three main phosphorous species were observed: phosphorus pentoxide (P2O5), which causes physical deactivation of Pd/SSZ-13, and metaphosphate (PO3-) and phosphate (PO43-), both of which cause chemical deactivation. Different characterization techniques revealed the interaction of P with Pd and the zeolite framework that caused this chemical deactivation: it was found that the BET surface area and pore volume of Pd/SSZ-13 were reduced due to physical blockage caused by phosphorous pentoxide species.
Keywords: Passive NOx adsorber, Pd/zeolite adsorber, La promotion, thermal stability, multiple NO-TPD CO effect and degradation, Phosphorous poisoning