Formation and removal of Ba-carbonates or carboxylates on Pt/BaO/γ-Al2O3 lean NO_x traps

摘要

In addition to feed composition, the NO_x storage capacity (NSC) of a Pt/BaO/γ-Al2O3 lean NO_x trap (LNT) also depends on the chemical state of Ba prior to the start of the lean NO_x exposure phase during which the NSC is measured. This state of Ba is a result of the history of the trap's cyclic exposures to lean and rich conditions and is a function of the feed composition and the time intervals of the exposures. We have systematically investigated the role of adsorbed CO2 on the NO_x sorption process on traps with 20,8 and 4 wt.% Ba. This was accomplished through a sequence of NO_x storage and regeneration cycles with CO2 and H2O in the feed followed by purge in Ar and then by NO_x storage and regeneration without CO2 and H2O at 300 C. The amount of pre-adsorbed CO2 released during NO_x storage upon the introduction of NO2 + O2 correlated well with the amount of NO_x stored on the trap with 20 wt.% Ba but not on the traps with 8 and 4wt.% Ba loadings. These results were further investigated in DRIFTS sequential adsorption experiments for CO2, NO2 +O2, H2 or NO2 + O2, CO2, H2 at 300°C. The DRIFTS experiments showed that, similar to bulk vs. surface nitrate formation on samples with high vs. low Ba loading, CO2 adsorption forms either carbonates or carboxylates on high vs. low Ba loadings, respectively. Regeneration by H2 removed all NO_x adsorbates and any carboxylates, but not the carbonates. Thus, the formation of carbonates slows the NO_x storage process as NO_x must compete with pre-adsorbed carbonates for adsorption sites after regeneration. In addition, comparison between the NO_x breakthrough curves with 7% CO2 in the lean feed and with pre-adsorbed CO2 on the catalyst showed that the NO_x storage before the slip occurs was intimately related to the oxygen spillover from Pt to the surrounding Ba and competition between CO2 and NO_x for adsorption sites. These findings provide insights to further improve the LNT formulations by using optimum Ba loadings and dispersions targeted at increasing the zero NO_x slip time after each fuel rich pulse.