PNNL

Abstract

The effects of thermal aging and H2O treatment on the physicochemical properties of a BaO/Al2O3 model catalyst were investigated by means of XRD, BET, TEM/EDX and NO2 TPD. Thermal aging at 1000 °C for 10 hrs resulted in conversion of dispersed BaCO3 into low surface area crystalline BaAl2O4. It was found that H2O treatment on a BaO/Al2O3 sample at room temperature transformed not only the BaAl2O4, but also the dispersed BaCO3 into highly crystalline BaCO3 segregated from the Al2O3 support, as evidenced in TEM/EDX and XRD analysis. The sample containing dispersed BaCO3 in the initial phase segregated more severely than the BaAl2O4 containing one, with the Ba in the BaAl2O4 matrix exhibiting higher resistance towards segregation. Contacting the BaO/Al2O3 sample with liquid water over a prolong period of time leads to an increase in crystallinity of the segregated BaCO3. These phenomena imply that special care must be taken during catalyst synthesis and during realistic operation of Pt/BaO/Al2O3 NOx trap catalysts since both processes involve potential exposure of the material with liquid H2O. Based on the results, a model to explain the behavior of Ba containing species upon thermal aging and H2O treatment is proposed.