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

In addition to feed composition, the NOx storage capacity (NSC) of a Pt/BaO/γ-Al2O3 lean NOx trap (LNT) also depends on the chemical state of Ba prior to the start of the lean NOx 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 NOx sorption process on traps with 20, 8 and 4 wt.% Ba. This was accomplished through a sequence of NOx storage and regeneration cycles with CO2 and H2O in the feed followed by purge in Ar and then by NOx storage and regeneration without CO2 and H2O at 300 °C. The amount of pre-adsorbed CO2 released during NOx storage upon the introduction of NO2 + O2 correlated well with the amount of NOx stored on the trap with 20 wt.% Ba but not on the traps with 8 and 4 wt.% 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 NOx adsorbates and any carboxylates, but not the carbonates. Thus, the formation of carbonates slows the NOx storage process as NOx must compete with pre-adsorbed carbonates for adsorption sites after regeneration. In addition, comparison between the NOx breakthrough curves with 7% CO2 in the lean feed and with pre-adsorbed CO2 on the catalyst showed that the NOx storage before the slip occurs was intimately related to the oxygen spillover from Pt to the surrounding Ba and competition between CO2 and NOx 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 NOx slip time after each fuel rich pulse.

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► CO2 can store either as carbonates or carboxylates depending on the Ba loading.
► Carbonates are more stable than carboxylates in a reducing atmosphere.
► Nitrates replace all carbonates and carboxylates in absence of CO2 in the gas phase.
► In CO2, carbonates/carboxylates could not be fully replaced by NOx.
► In fast NOx storage with CO2, nitrates compete with carbonates/carboxylates.