Pt-Ba/Al2O3 NSR Catalysts at Different Ba-Loading: Characterization of Morphological, Structural and Surface Properties

Morphological, textural and surface properties of several Pt-Ba/Al2O3 NSR (NOx storage reduction) catalysts at different Ba loading in the range 0-30 wt.% were characterized by means of XRD, HRTEM and FT-IR spectroscopy, using CO, CO2 and CH3CN as probe molecules. On increasing the Ba loading, the Pt dispersion progressively decreased, accounting for sintering and masking of the Pt particles by Ba, while the interaction between Pt and the barium oxide phase increased. Upon few cycles of heating in NO2 and subsequent evacuation (conditioning treatment), BaCO3 initially present evolved to Ba(NO3)2, then decomposed into a well-dispersed nanosized BaO phase. Upon conditioning a slight sintering of Pt is observed. However, the presence of Ba avoided a more stressed sintering, as it occured for Pt/Al2O3 sample. Investigation by CO2 and CH3CN adsorption followed by FT-IR spectroscopy revealed a high heterogeneity at the BaO surface. In particular, upon CO2 adsorption a variety of surface carbonate-like species were formed (mainly bridging and chelating carbonates on Ba sites). The most relevant features upon CH3CN adsorption were the formation of anionic species on strongly basic oxygen ions of Ba2+ O2- pairs and the presence of acetonitrile molecules polarized by highly uncoordinated Ba2+ ions, stable under evacuation at room temperature. The FT-IR characterization with the three test molecules suggests that the best spread of the Ba-phase is obtained for a loading between 16 and 23 wt.%.