Nanostructured Ba-Al-oxides prepared by cogel procedure for NOx-storage-reduction catalysts having enhance performances and hydrothermal stability.

The reactivity of Pt/Ba(O)/Alumina catalysts prepared by sol-gel (1% wt. Pt, 15% wt. BaO) in the reaction of NOx storage-reduction (NOxSR) before and after severe hydrothermal treatment (800°C) has been studied as a function of the preparation method (type of catalysis during the sol-gel, nature of the precursors, ageing, calcination temperature). The results are compared with those of a reference sample prepared by wet impregnation method. The large spreading of the catalytic performances, in terms of both NOx storage and resistance to deactivation by hydrothermal treatment, evidences that a complex surface chemistry is present and that the performances are considerably depending on the catalysts nanostructure. It is suggested that Ba doping and/or decoration of alumina nanoparticles increases their thermal stability, but crystallization of Ba-aluminate occurs in the presence of water vapour with a drastic lowering of the NOx storage properties. The analysis of the results also suggests that an amorphous nonstoichiometric BaAl2O4 like surface phase is present together with Ba-carbonate surface species. The two species are active in different range of temperatures and their relative amount depends on the preparation. Their amount also influences the hydrothermal stability of the samples in terms of both activity after this treatment and severity of the deactivation after this treatment. The sol-gel preparation method leads in general to improved resistance to deactivation by hydrothermal treatment and better performances at reaction temperatures above about 400°C with respect to samples prepared by wet impregnation method.