Pt-Ba/alumina NOx storage-reduction catalysts: Effect of Ba-loading on build-up, stability and reactivity of Ba-containing phases

A series of Pt-Ba/Al₂O₃ catalysts with Ba-loadings in the range 4.5–28 wt.% has been prepared by wet impregnation of Pt/Al₂O₃ with barium acetate (Ba(Ac)₂) as Ba precursor. The build-up and thermal stability of the deposited Ba-containing species was followed by means of XRD and thermogravimetry (TG) combined with mass spectroscopy (MS). Samples were characterized before and after thermal treatment (calcination). The study showed that the thermal stability of the Ba-containing phases depends on their interaction with the alumina support and the presence of dispersed platinum. In calcined catalysts, three different Ba-containing species could be distinguished based on their crystallinity and thermal stability. The relative concentration of these species varied with the Ba-loading. The first layer of Ba-containing species, corresponding to saturation of the alumina surface with Ba(Ac)₂, contained up to 12.5 wt.% of Ba in the form of amorphous BaO. Increasing the Ba-loading further resulted in 5–6 wt.% of Ba in the form of amorphous carbonates with relatively low thermal stability (LT-BaCO₃). At Ba-loadings higher than about 16 wt.%, crystalline barium carbonate became discernible which exhibited remarkably higher thermal stability (HT-BaCO₃). NO_x storage tests accomplished by exposing the catalysts to pulses of NO in oxygen containing carrier gas at 300 °C indicated that from all characterized Ba-containing phases, LT-BaCO₃ possesses the highest reactivity for NO_x storage, i.e. LT-BaCO₃ is transformed most rapidly to Ba(NO₃)₂.