Supported Pt-Ba NOx storage-reduction catalysts: Influence of support and Ba loading on stability and storage efficiency of Ba-containing species

A series of supported Pt-Ba NOx storage-reduction catalysts has been prepared by impregnation of corresponding supported 1 wt.% Pt catalysts with Ba(Ac)2 precursor and subsequent calcination at 500 °C. Ceria, zirconia and silica were applied as supports and the Ba loading was varied from 0 to 28 wt.%. The Ba-containing phase of these catalysts was systematically investigated concerning stability and NOx storage efficiency.Temperature programmed reaction-desorption experiments of the catalysts calcined in situ revealed two differently stable barium carbonate phases, low-temperature barium carbonate (LT-BaCO3) and high-temperature barium carbonate (HT-BaCO3). The total amount of barium carbonate was highest in Pt-Ba/ceria and Pt-Ba/zirconia, and significantly smaller in the Pt-Ba/alumina reference catalysts, whereas in Pt-Ba/silica barium carbonate was only detected at the highest Ba loading. The relative amount of LT-BaCO3, which is considered the most efficient Ba-phase for NOx storage, depended on the support material and the Ba loading. Ceria and zirconia as supports afforded catalysts where the LT-BaCO3 was the dominant form of barium carbonate. At low Ba loading (4.5 wt.%), the barium carbonate phase was exclusively made up of LT-BaCO3, while with higher loadings the contribution of HT-BaCO3 increased strongly. NOx storage measurements indicated a correlation between the relative amount of LT-BaCO3 and the NOx storage efficiency. At low Ba loading the efficiency decreased in the following order: ceria > zirconia > alumina > silica, whereas at high Ba loading the storage efficiency of the differently supported catalysts became similar. The different behavior of the Ba-containing phases on the supported Pt-Ba catalysts was traced to the different chemical (basicity) and textural properties of the investigated support materials.