Fe Promoted NO x Storage Materials: Structural Properties and NO x Uptake
Fe promoted NO x storage materials were synthesized in the form of FeO x /BaO/Al2O3 ternary oxides with varying BaO (8 and 20 wt %) and Fe (5 and 10 wt %) contents. Synthesized NO x storage materials were investigated via TEM, EELS, BET, FTIR, TPD, XRD, XPS, and Raman spectroscopy, and the results w...
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Veröffentlicht in: | Journal of physical chemistry. C 2010-01, Vol.114 (1), p.357-369 |
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Zusammenfassung: | Fe promoted NO x storage materials were synthesized in the form of FeO x /BaO/Al2O3 ternary oxides with varying BaO (8 and 20 wt %) and Fe (5 and 10 wt %) contents. Synthesized NO x storage materials were investigated via TEM, EELS, BET, FTIR, TPD, XRD, XPS, and Raman spectroscopy, and the results were compared with the conventional BaO/Al2O3 NO x storage system. Our results suggest that the introduction of Fe in the BaO/Al2O3 system leads to the formation of additional NO x storage sites which store NO x mostly in the form of bidentate nitrates. NO2 adsorption experiments at 323 K via FTIR indicate that, particularly in the early stages of the NO x uptake, the NO x storage mechanism is significantly altered in the presence of Fe sites where a set of new surface nitrosyl and nitrite groups were detected on the Fe sites and the surface oxidation of nitrites to nitrates is significantly hindered with respect to the BaO/Al2O3 system. Evidence for the existence of both Fe3+ as well as reduced Fe2+/(3−x)+ sites on the freshly pretreated materials was detected via EELS, FTIR, Raman, and XRD experiments. The influence of the Fe sites on the structural properties of the synthesized materials was also studied by performing ex situ annealing protocols within 323−1273 K followed by XRD and Raman experiments where the temperature dependent changes in the morphology and the composition of the surface domains were analyzed in detail. On the basis of the TPD data, it was found that the relative stability of the stored NO x species is influenced by the morphology of the Ba and Fe containing NO x -storage domains. The relative stabilities of the investigated NO x species were found to increase in the following order: N2O3/NO+ < nitrates on γ-Al2O3 < surface nitrates on BaO < bidentate nitrates on FeO x sites < bulk nitrates on BaO. |
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ISSN: | 1932-7447 1932-7455 |
DOI: | 10.1021/jp907982q |