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/Al 2 O 3 NO x storage reduction (NSR) catalysts at different Ba loading in the range 0−30 wt % were characterized by means of X-ray diffraction, high-resolution transmission electron microscopy, and Fourier-tranform infrared (FT-IR) spectroscopy using CO, CO 2, and CH 3 CN as probe molecules. Upon increasing the Ba loading, the Pt exposure progressively decreased, accounting for sintering and masking of the Pt particles by Ba, whereas the interaction between Pt and the barium oxide phase increased. After a few cycles of heating in NO 2 and subsequent evacuation (conditioning treatment), BaCO 3 initially present evolved to Ba(NO 3 ) 2 and 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 occurred for the Pt/Al 2 O 3 sample. Investigation by CO 2 and CH 3 CN adsorption followed by FT-IR spectroscopy revealed a high heterogeneity at the BaO surface. In particular, upon CO 2 adsorption a variety of surface carbonate-like species were formed (mainly bridging and chelating carbonates on Ba sites). The most relevant features upon CH 3 CN adsorption were the formation of anionic species on strongly basic oxygen ions of Ba 2+ O 2- pairs and the presence of acetonitrile molecules polarized by highly uncoordinated Ba 2+ 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 %.