Pt-Ba/alumina NOx storage-reduction catalysts: Influence of Ba loading on NOx storage behavior

The NO_x storage behavior of a series of Pt-Ba/Al₂O₃ catalysts, prepared by wet impregnation of Pt/Al₂O₃ with Ba(Ac)₂, has been investigated. The catalysts with Ba loadings in the range 4.5–28 wt.% were calcined at 500 °C in air and subsequently exposed to NO pulses in 5 vol.% O₂/He atmosphere. Catalysts were characterized by means of thermogravimetry (TG) combined with mass spectroscopy (MS) and XRD before and after exposure to NO pulses. Characterization of the calcined catalysts corroborated the existence of three Ba-containing phases which are discernible based on their different thermal stability: BaO, LT-BaCO₃ and HT-BaCO₃. Characterization after NO_x exposure showed that the different Ba-containing phases present in the catalysts possess different reactivity for barium nitrate formation, depending on their interfacial contact. The different Ba(NO₃)₂ species produced upon NO_x exposure could be distinguished based on their thermal stability. The study revealed that during the NO_x storage process a new thermally instable BaCO₃ phase formed by reaction of evolved CO₂ with active BaO. The fraction of Ba-containing species that were active in NO_x storage depended on the Ba loading, showing a maximum at a Ba loading of about 17 wt.%. Lower and higher Ba loading resulted in a significant loss of the overall efficiency of the Ba-containing species in the storage process. The loss in efficiency observed at higher loading is attributed to the lower reactivity of the HT-BaCO₃, which becomes dominant at higher loading, and the increased mass transfer resistance.