57Fe MOSSBAUER AND XRD STUDIES OF THE BaFexAl12-xO19 HEXAALUMINATE CATALYSTS FOR N2O AS A PROPELLANT

Nitrous oxide (N2O) is being regarded as a promising green propellant for space propulsion due to its extremely low toxicity compared to traditional hydrazine propellant. Recently, we reported Ir and Fe-substituted hexaaluminates were highly active and stable catalysts for N2O decomposition [1, 2]. In the present work, we made a detailed study on the BaFexAl12-xO19 hexaaluminate catalysts, with focus on the role played by the framework Fe in the N2O decomposition. Figure 1 shows the N2O decomposition activity of BaFexAl12-xO19 catalysts (x=0, 1, 2, 3) as a function of reaction temperature. BaAl12O19 which does not contain Fe was almost inactive over the whole investigated temperature range, indicating Fe is the active component. When x increased from 1 to 2, the activity increased significantly. Figure 2 illustrates the XRD patterns of the BaFexAl12-xO19 catalysts. BaFexAl12-xO19 tends to form monophasic materials (Ba-β-Al2O3 or MP phase) in wide composition range [3]. When x=0 and 1, the characteristic diffraction peaks of Ba-β-Al2O3 were observed. For the Fe-containing sample, the patterns were shifted to low 2θvalues comparing with BaAl12O19, indicating that the substitution of Fe ions for Al ions took place. Moreover, with an increase in the Fe content, the MP phase was detected. 57Fe Mössbauer spectra of BaFexAl12-xO19 (x=1, 2, 3) at room temperature are shown in the Figure 3. When x=1, the spectrum was fitted by two doublets xx’ and yy’ with close values of quadrupole splitting (QS) and different values of isomer shift (IS). When x=2 and 3, the spectra were fitted by three doublets xx’, yy’ and zz’, since a new doublet absorption with larger QS appeared in them. The obtained isomer shifts are small and fall in the range expected for high-spin ferric ions in oxide environments. The two doublets xx’ and yy’ could be assigned to high-spin Fe3+ ions in octahedral coordination (O) and in tetrahedral coordination (T), respectively. The two peaks observed in the H2-TPR for BaFeAl11O19 sample is in agreement with the Mössbauer assignment. The new doublet zz’ could be assigned to high-spin Fe3+ in the trigonal bipyramid site, which is in the mirror plane of hexaaluminate. Similar QS has been observed in BaFe12O19, which shows a MP-type phase. The framework Fe species were highly active for N2O decomposition. Specially, Fe3+ ions located in the mirror plane are attributed to the most reactive Fe species