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Abstract: In this paper the reactivity of NO x stored at 350°C onto Pt–Ba/Al2O3 LNT catalyst in the reduction by H2, CO and CO/H2O mixture is investigated by means of transient experiments and complementary FT-IR analyses. Hydrogen is found to be the best reductant, being characterized by the highest NO x removal efficiency in the range 150–350°C. It is proposed that the reduction by hydrogen proceeds according to a dual-steps mechanism in which NO x are first reduced to ammonia, which in turn further reacts with other nitrate species leading selectively to N2. Conversely, when CO is used as reducing agent under dry conditions only a part of the initially stored NO x is removed as N2, the other fraction being reduced to adsorbed cyanates/isocyanates species. It is proposed that the cyanate/isocyanate ad-species lead to the formation of nitrogen by oxidation involving other nitrate species; this reaction is self-poisoned by CO. In the presence of water, CO reduced nitrates into cyanates/isocyanates ad-species that were readily hydrolyzed to ammonia; this species is hence precursor in N2 formation upon reaction with residual nitrate species. Alternatively, stored nitrates are reduced by H2 formed via the water gas shift (WGS) reaction. [Copyright &y& Elsevier]

A series of Pt–Ba/Al2O3 NOx storage-reduction (NSR) catalysts having different Ba loadings (in the range 0–30%, w/w) have been prepared, characterized and tested in the NOx adsorption-reduction under realistic operating conditions. Accordingly, the role of the Ba loading on the storage of NOx and on their subsequent reduction has been addressed.The results indicated that the presence of Ba negatively affects both the catalyst morphological properties and the Pt dispersion in the samples with Ba loading higher than 10%, while a strong increase of the NOx adsorption capacity is observed. This effect has been associated to the involvement in the NOx storage process of Pt–Ba neighboring species. The analysis of the storage phase also showed that the highest the Ba content, the lowest the NO2 concentration at catalyst saturation: this effect has been related to the decrease in the Pt content of the catalyst following Ba addition to the samples and/or to a possible masking of Pt sites when the Ba coverage approaches and exceeds the monolayer capacity of the support.Concerning the reduction phase, the data pointed out that for all the investigated catalysts the stored NOx species are readily reduced upon H2 admission: nevertheless, the reduction process resulted very complex and it can lead to nitrogen and/or ammonia depending on the formulation and properties of the catalysts. [Copyright &y& Elsevier]

Abstract: The storage-reduction of NO x over Pt–Ba/Al2O3 samples is analyzed in this work. It is pointed out that the storage of NO in the presence of excess oxygen proceeds through two parallel routes: (i) the “nitrate route” where NO is oxidized to NO2 on Pt sites followed by NO2 disproportionation on Ba sites to form nitrates with evolution of gaseous NO; (ii) the “nitrite route”, where NO is oxidized on Pt sites and directly stored on Ba neighbouring sites in the form of nitrites that are then oxidized to nitrates. The latter route possibly dominates at high Ba loadings. Concerning the reduction of the stored NO x , the bulk of data indicates that the process under isothermal conditions is not initiated by the release of NO x in the gas phase but occurs through a Pt-catalysed reduction of adsorbed NO x species. This route requires the co-presence of Pt and Ba on the same support. It is speculated that H2 is activated on Pt sites and spills on the alumina support towards nitrate ad-species where it operates the reduction, although alternative explanations are possible. [Copyright &y& Elsevier]

Abstract: The storage of NO2 and NO/O2 in the presence and in the absence of CO2 was investigated at 350°C on Pt-Ba/Al2O3 LNT catalysts and on the reference Ba/Al2O3 system, by coupling in situ FT-IR spectroscopy and transient response method (TRM). Experiments were performed by admitting NO2/CO2 or NO/O2/CO2 mixture (CO2/NO x ratios in the range 1–7) or, in alternative, by admitting NO2 or NO/O2 on catalysts previously saturated by flowing CO2. In the case of NO2/CO2 adsorption, it was found that NO2 was efficiently stored as nitrate species according to a disproportion reaction, which occurs with evolution of NO, as in the absence of CO2. In the case of NO/O2/CO2 adsorption it was confirmed that two pathways operated simultaneously as in the absence of CO2: the “nitrite route”, involving the initial formation of surface nitrite and their subsequent evolution to nitrates, and the “nitrate route”, involving NO oxidation to NO2 over Pt and its subsequent adsorption on Ba in the form of nitrates. The presence of CO2 inhibits to some extent the “nitrite route”, whereas the “nitrate route” proceeds as in the absence of CO2. [Copyright &y& Elsevier]