Formation and Removal of Ba-Carbonates/Carboxylates On Pt/Ba/Al2O3 Lean NOx Traps

NO_x adsorption and removal on Pt/BaO/Al₂O₃ lean NO_x traps (LNT) has been a major focus of study in the exhaust aftertreatment community.  The practically important effects of CO₂ and H₂O on NO_x adsorption have been studied in our laboratory using NO_x breakthrough curves obtained from flow reactor experiments [1] as well as by other research groups [2]. In our work, we focused on samples with 4, 8 and 20 wt.% Ba to investigate the effects of the state of Ba at the beginning of NO_x adsorption on the amounts of NO_x stored before slip occurs.  Each of our experiments contained a sequence of NO_x capture and regeneration with CO₂ and H₂O in the feed followed by purge in inert and finally NO_x capture and regeneration without CO₂ and H₂O at 300°C. The amount of pre-adsorbed CO₂ released during NO_x capture upon the introduction of NO₂ correlated well with available Ba sites for NO_x storage and the amount of NO_x stored on the monolithic trap with 20Ba but not on the traps with lower Ba loadings. Subsequent DRIFTS experiments in the form of sequential adsorption of CO₂, NO₂ + O₂, H₂ or NO₂ + O₂, CO₂, H₂ at 300°C were completed.  At low loadings of Ba (8Ba and 4Ba) the CO₂ adsorbed mainly in the form of carboxylates, presumably on highly dispersed Ba sites.  On 20Ba samples, where the BaO phase is known to form in large, bulk-like structures, CO₂ was found to adsorb only as a carbonate.  Competitive adsorption between CO₂ and NO_x encouraged replacement of the pre-adsorbed species by the one in the gas phase.  Given enough time, NO_x was able to completely displace all CO₂ and subsequent regeneration with H₂ returned the catalyst to its original clean state, i.e. BaO.  When pre-adsorbed NO_x was partially displaced by carbonates/carboxylates, regeneration by H₂ was able to remove all NO_x adsorbates and any carboxylates, but not the carbonates.  The implications of this adsorbate behavior played out in the following manner.  During regeneration in the flow reactor with H₂, and with CO₂ and H₂O also present, adsorbed nitrates were removed, producing Ba sites available for CO₂ storage.  On the 20Ba sample, CO₂ adsorbed as carbonate and remained on the storage component during regeneration and purge.  On the 8Ba and 4Ba samples, which preferentially formed carboxylates, the instability of the adsorbed carboxylates in H₂ ensured that the majority of available Ba sites remained open (as BaO), thus explaining the loss of correlation between the amount of pre-adsorbed CO₂ and NO_x storage capacity mentioned above.  Furthermore, the low Ba loadings exhibited greater use of total Ba for fast NO_x storage, defined as the amount of Ba used to store NO_x until 1% of inlet NO_x breakthrough occurs in the flow reactor, which were 9, 8 and 4% for the 8Ba, 4Ba and 20Ba, respectively.  The diminished use of Ba on the 20Ba sample is explained by carbonates blocking available Ba sites vicinal to Pt centers which facilitate fast NO_x adsorption by supplying the surrounding Ba with spilled over oxygen.  It has been shown that NO_x will adsorb first on BaO, then Ba(OH)₂ and finally BaCO₃ sites when all sites are present in the catalyst [3] as was the case in our experiments.  Carbonates adsorbed on Ba vicinal to Pt lead to slowed NO_x adsorption kinetics which cause the reduced amount of Ba used for fast NO_x storage on the 20Ba catalysts.  Thus the initial state of the Ba trapping phase becomes greatly important and a properly chosen Ba loading can lead to improved Ba usage.  Different Pt loadings were also investigated, and on the 20Ba samples they primarily affected the carbonate/nitrate exchange through supply of the oxidant for formation of nitrates leading to higher amounts of fast NO_x storage with increased Pt loading. Implications of these chemical findings on NSR catalyst optimization will be discussed.

References

[1] S.S. Chaugule, A. Yezerets, N.W. Currier, F.H. Ribeiro, W.N. Delgass, Catal. Today, In Press, Corrected Proof, 2010, doi: 10.1016/j.cattod.2010.02.024

[2] W.S. Epling, G.C. Campbell, J.E. Parks, Catal Lett, 90 (2003) 45-56.

[3] I. Nova, L. Castoldi, L. Lietti, E. Tronconi, P. Forzatti, Catal. Today, 75 (2002) 431-437.