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Morphological, textural, and surface properties of a NSR (NO x storage reduction) Pt-K/Al 2 O 3 model catalyst (Pt 1 wt %; K 5.4 wt %) were characterized by means of XRD, HRTEM, and FT-IR spectroscopy. Thin crystalline K-containing layers, in the form of cubic K 2 O and monoclinic K 2 CO 3 and very small roundish Pt particles with a mean diameter of 1.5 nm, have been observed. Monoclinic K 2 CO 3 disappears, and a certain degree of Pt sintering occurs (d Pt ≈ 3.4 nm) after use. However, the presence of potassium limits the Pt sintering which occurs on the Pt/Al 2 O 3 reference sample (Pt 1 wt %). FT-IR spectra of CO adsorbed at RT, compared with those recorded for Pt/Al 2 O 3 , revealed a marked interaction between the Pt and K phases that is much higher than the interaction between the Pt and Ba phases observed for the classic Pt-Ba/Al 2 O 3 catalyst. CO 2 adsorption at RT indicated a high heterogeneity of the K phase, evidenced by the formation of a variety of surface-carbonate-like species (mainly bridging carbonates on K sites). Minor amounts of nitrites and nitrates were formed at RT under NO admission, while the uptake was sensibly higher under NO/O 2 or NO 2 admission; nitrites (mono-and bidentate) and nitrates (ionic and bidentates) were formed in different amounts, both relative and absolute, and the nitrate to nitrite ratio increased in parallel with the NO/O 2 ratio. Also, at each contact time, the amount of the stored NO x species increased upon increasing the NO/O 2 ratio.

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, highresolution 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 2pairs 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 %.

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