Structure and NOx uptakr properties of Fe-Ba/Al2O3 as a model NOx storage material

Abstract

The composition-effect of iron (5 and 10 wt. % Fe) on the nature of the NOx species and NOx storage properties of (8 and 20 wt. %) BaO/γ-Al2O3 was investigated. Nitrate-loaded samples, which were synthesized by incipient-wetness impregnation with nitrate precursors, were further treated at elevated temperatures (323 K-1273 K) in order to monitor the thermally induced structural changes. In the first part of this study, diffraction (X-ray Diffraction, XRD), BET (Brunauer, Emmett, and Teller) surface area measurement, spectroscopy (Raman and X-Ray Photoelectron Spectroscopy, XPS) and microscopy (Transmission Electron Microscopy, TEM and Electron Energy Loss Spectroscopy, EELS) techniques were used for investigating the thermally induced structural changes on the sample surfaces. In the second part of the text, NO2 (g) adsorption experiments were performed on the NOx-free samples. FTIR (Fourier Transform Infrared) spectroscopy technique was used to monitor the NOx species stored on the samples. To illustrate the desorption behavior of the adsorbed NOx on 8Ba/Al sample, Temperature Programmed Desorption (TPD) technique was also exploited. For the as prepared (nitrated) samples, the surface distribution and the thermal stability of the deposited Ba-nitrates were found to depend strongly on the interaction with the Fe-containing domains and the γ-Al2O3 support material. It was observed that deposited nitrates have a different thermal stability on the Fe/Ba/Al samples in comparison to the Fe-free Ba/Al samples. Besides, XRD data revealed that BaAl2O4 formation at elevated temperatures was diminished to a certain extent in the presence of Fe. Moreover, the presence of Fe in the form of α- Fe2O3 in the Fe/Al and BaFe/Al systems depressed the γ-Al2O3 → α-Al2O3 phase transformation temperature and favored the corundum formation above 1073 K. Relative surface dispersions of the Fe- and Ba-species on the 10Fe/20Ba/Al sample were also analyzed via TEM and EELS where the dispersion of barium species were found to be relatively higher than that of iron. FTIR experiments revealed that NO2 (g) adsorption at 323 K leads to the formation of nitrites for all of the samples at the initial introduction of NO2 (g). In addition, iron containing samples indicate nitrosyl formation as well. With further doses of NO2 (g), nitrite bands were converted into nitrate signals. NO2 (g) adsorption on 5(10)Fe/8(20)Ba/Al system resulted in the accentuation of the surface/bidentate nitrates. Temperature dependent FTIR experiments showed that ionic (bulk) nitrates were thermally more stable than the surface nitrates in 8(20)Ba/Al and 5(10)Fe/8(20)Ba/Al systems. TPD profile for the 8Ba/Al sample was also found to be in line with the FTIR results, indicating that the high-temperature decomposition of bulk nitrates were in the form of NO (g) and O2 (g) while the surface nitrates decomposed at lower temperatures and mostly as NO2 (g). In the presence of Fe (5 and 10 wt %) thermal stability of the nitrates were found to decrease.