(401a) 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.