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

摘要

A series of supported Pt-Ba NO_x 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 deg 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 NO_x 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-BaCO_3)and high-temperature barium carbonate(HT-BaCO_3).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-BaCO_3,which is considered the most efficient Ba-phase for NO_x storage,depended on the support material and the Ba loading.Ceria and zirconia as supports afforded catalysts where the LT-BaCO_3 was the dominant form of barium carbonate.At low Ba loading(4.5 wt.%),the barium carbonate phase was exclusively made up of LT-BaCO_3,while with higher loadings the contribution of HT-BaCO_3 increased strongly.NO_x storage measurements indicated a correlation between the relative amount of LT-BaCO_3 and the NO_x 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.