Al 2 O 3 catalysts has been investigated by IR spectroscopy and temperature-programmed desorption. Upon NO interaction, small amounts of nitrites, nitrates, and hyponitrite species were formed on the Ba-containing samples. The NO x storage capacity of the catalysts was highly enhanced upon adsorption of NO/O 2 mixtures and further upon NO 2 admission. Upon adsorption of NO/O 2 on Pt/Al 2 O 3 sample nitrites, nitrates and NO 2 δ+ species were mainly formed, showing a moderate thermal stability. Barium markedly increased the amount and stability of the stored NO x species, which were bidentate and monodentate nitrites and, in minor amounts, nitrates. Nitrites were removed below 750 K and/or transformed into ionic Ba nitrates, stable up to 800-900 K. Upon NO 2 adsorption, huge amounts of nitrates, but no nitrites, were formed on all the samples. Also in this case, Ba increased the amount and stability of the stored NO x species. The nature and the amounts of the stored NO x species formed upon adsorption of NO, NO/O 2 , or NO 2 were similar on Ba/Al 2 O 3 and Pt-Ba/Al 2 O 3 catalysts, whereas Pt slightly decreased their thermal stability. Bulky Ba nitrate was formed during the thermal desorption of NO 2 (and to a less extent of NO/O 2 ), inducing an extensive decomposition of the Ba carbonate or oxycarbonate phase which was detected on the calcined samples.

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Nowadays, severe regulations are in force in the industrialized countries to limit the emission of pollutants from the exhausts of passenger cars and trucks. Today, post-treatment catalytic technologies are necessary to meet the current most strict or the forthcoming emission limits. [1] In traditional gasoline-fueled stoichiometric engines, threeway catalysts are used for the abatement of NO x , unburned hydrocarbons, and CO. However, the need to reduce fuel consumption and the corresponding CO 2 emissions has led in the past to an impressive spread in the market of lean-burn engines, such as diesel or direct-injection (DI) gasoline engines. These engines operate in the presence of excess oxygen, in which the current three-way catalysts, optimized for exhausts that fluctuate around oxygen-free conditions, do not ensure acceptable emission levels of NO x .The NH 3 selective catalytic reduction (SCR) and the NO x storage-reduction (NSR) or lean NO x trap systems are at present the top contenders for reducing NO x concentration in the exhaust from diesel and lean-burn gasoline direct-injection vehicles. Although the NH 3 -SCR technology, which accomplishes NO x reduction by injecting urea (a precursor of NH 3 ) in the exhaust gases and requires an onboard urea tank, is preferred for heavy-duty vehicles and minivans, lean NO x traps have been specifically developed for small engines.[1] This technology is based on the use of a suitable catalytic material that consists of an alumina carrier on which alkali-and/or alkali-earth metal compounds (e.g., K and Ba) and noble metals (e.g., Pt) are deposited. These catalysts operate under cyclic conditions, which alternate long periods in the presence of excess oxygen during which NO x species are stored on the alkali-and/or alkali-earth metal compounds with short rich phases during which the adsorbed NO x species are reduced to nitrogen on Pt. [2] Although the NSR technology is presently being used on a commercial scale, an agreement and understanding of the mechanistic aspects of the storage of NO x species and of their reduction is still lacking.[3] Regarding the storage phase, many authors suggest that NO is at first oxidized by Pt to NO 2 , which is then stored onto the alkali-or alkali-earth metal compound (e.g., BaO) in the form of nitrates according to the overall reactions in Equations (1) and (2):in which O = is a lattice oxygen and NO 3 À is an adsorbed nitrate species. In this pathway, hereafter called the nitrate route, Pt and the alkali-or alkali-earth metal compounds catalyze the NO oxidation and the NO 2 storage reactions, respectively. Also, the reaction in Equation (2) in which NO 2 À is an adsorbed nitrite species. On the basis of spectroscopic evidence, both nitrite and nitrate ad-species have been simultaneously observed at the preliminary stage of storage from NO 2 ; nitrites eventually oxidize to nitrates, and so only nitrates are detected on the catalyst surface after a long storage period. [5,6] Notably, according to the suggested pathway for the nitr...

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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 %.