Effect of large cations (La3+ and Ba2+) on the catalytic performance of Mn-substituted hexaaluminates for N2O decomposition

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Abstract

Mn-substituted La-hexaaluminate (LaMnxAl(12−x)O19) and Ba-hexaaluminate (BaMnxAl(12−x)O19) catalysts were prepared using the carbonates route and investigated for high-concentration of N2O decomposition. It was for the first time found that the Ba-hexaaluminate exhibited higher activity than the La-hexaaluminate at a given Mn content, both of which were much more active than Mn/Al2O3 after being subjected to high-temperature (1400 °C) treatment. The catalytic activity varied with the Mn content and attained the best one at x = 1. X-ray diffraction (XRD) characterizations showed that a small amount of Mn (up to x = 1) promoted greatly the formation of phase-pure hexaaluminate, while excess Mn caused formation of catalytically inactive impurity phases, such as LaAlO3, BaAl2O4, Mn3O4, and LaMnO3, which covered partially the active sites and then led to a loss of the activity. UV–visible spectra showed that Mn2+ preferentially enter tetrahedral Al sites at a low Mn content (x = 0.5) for the La-hexaaluminate, which is quite different from the case of Ba-hexaaluminate where Mn3+ can substitute octahedral Al sites even at x = 0.5. Such a difference in the number of catalytically active Mn3+ sites in the octahedral position should be responsible for the higher activity of the Mn-substituted Ba-hexaaluminate.

Introduction

N2O is being considered as a promising green propellant used for small satellite propulsion systems due to its low toxicity compared with traditional hydrazine propellant, as well as its capability for self-pressurizing and compatibility with the common construction materials [1], [2]. The chemistry for N2O as a propellant lies in that the decomposition of N2O into N2 and O2 accompanies with a large amount of heat release and volume expansion, which can be used as propulsion power. Quite different from the abatement of environmental pollutant N2O where the concentration of N2O is only in ppm level [3], [4], [5], [6], the N2O used as the propellant must have a high-concentration, even being a pure chemical, so as to generate the propulsion power as high as possible. However, the decomposition of pure N2O is a highly exothermic reaction (the enthalpy is −82 kJ/mol) and leads to a temperature rise over 1000 °C [1], [2], [7]. In this case, the catalysts used for N2O propellant must have a very good thermal stability. Actually, the high-temperature stability of a material has become a paramount factor to be considered for evaluating its feasibility as the catalyst for N2O propellant.

Hexaaluminates are a class of excellent high-temperature materials thanks to their unique peculiar layered structure consisting of γ-Al2O3 spinel blocks intercalated by mirror planes in which the large cations (Ba, Ca, La and Sr) are locating [8], [9], [10], [11]. More importantly, the aluminium cations in the spinel blocks can be substituted with transition metals, giving rise to redox centers for a variety of catalytic reactions [12], [13]. In our previous work [14], [15], via introduction of Ir into the Fe-substituted BaAl11O19 hexaaluminate we obtained a novel catalyst for N2O propellant decomposition which can initiate the reaction at a low temperature (350 °C) and can maintain the stable performance at a temperature as high as 1200 °C. Nevertheless, the loss of the active component Ir is still inevitable, in particular after many cycles of startup-shutdown. A possible solution to this problem is to develop a two-bed reactor, in which the Ir-hexaaluminate or other more active catalyst constitutes the front bed while the back bed requires a more thermally stable catalyst, just like the dual-bed in CH4 combustion [16], [17]. To this end, we investigated the Mn-substituted hexaaluminates for the catalytic decomposition of high-concentration of N2O in the present work, with a focus on the effect of large cations (Ba2+ and La3+).

Mn-substituted hexaaluminates have been regarded as the most active catalysts for methane combustion reaction [18], [19], [20], [21], [22], [23]. Various studies have been devoted to the effect of large cations (La3+, Ba2+, Sr2+, Mg2+) on the performance of the Mn-substituted hexaaluminates for the catalytic combustion of methane. The size of different cations (the radius of La3+, Ba2+ and Sr2+ is 1.06 Å, 1.35 Å and 1.12 Å, respectively) may influence the structure of hexaaluminate, and then the oxidation state of Mn species. Jang et al. reported that LaMn1Al11O19 was much more active than the BaMn1Al11O19, and they ascribed the activity enhancement to the different oxidation state of Mn ions in the two types of hexaaluminates [24]. Partial substitution of La3+ with Sr2+ (Sr1−xLaxMnAl11O19) could further enhance [25] or diminish [26] the catalytic activity of the Mn-substituted hexaaluminates, while incorporation of Mg2+ into the LaMn1Al11O19 largely increased the catalytic activity by stabilizing the Mn ions at a high oxidation state [27]. Li and Wang prepared Mn-substituted Ba-La-hexaaluminate rod-like nanoparticles and found that Ba0.2La0.8MnAl11O19 catalyst exhibited much higher activity than either BaMnAl11O19 or LaMnAl11O19 [28]. Evidently, the nature of large cations in the Mn-substituted hexaaluminates affect significantly the Mn2+/Mn3+ redox cycle, and then the catalytic performance. However, the influences of large cations have rarely been investigated for N2O decomposition. Recently, Pérez-Ramírez et al. investigated a series of metal-substituted hexaaluminates ABAl11O19 (A = La, Ba, and B = Mn, Fe, Ni) for high-temperature N2O abatement [29], [30]. They claimed that both Fe- and Mn-substituted hexaaluminates were active for this reaction, but the influence of large cations were not investigated in detail. In the present work we found that Ba-hexaaluminate was much more active than the La-hexaaluminate at the fixed Mn content. In order to gain an insight into the effect of large cations, a variety of characterization techniques were employed to reveal the relationship between structure and oxidation state of Mn cations in the two types of hexaaluminates. Finally, a two-bed reactor was designed to evaluate the feasibility for pure N2O propellant decomposition in a thruster.

Section snippets

Catalyst preparation

La-Mn-Al oxide catalysts (LaMnAl11O19, denoted as LMA-t, t indicates calcination temperature) and Ba-Mn-Al oxide catalysts (BaMnAl11O19, denoted as BMA-t) were prepared by coprecipitation with (NH4)2CO3. For example, to prepare LMA-t, 1.08 g of La(NO3)3·6H2O, 0.90 g of Mn(NO3)2 (50%), and 10.31 g of Al(NO3)3·9H2O were dissolved individually in deionized water at 60 °C, and then added into a saturated aqueous solution of (NH4)2CO3 under stirring to form the hexaaluminate precursor precipitate. After

Effect of calcination temperature on the phase composition and catalytic performance

Fig. 2 shows the XRD patterns of LMA-t and BMA-t samples which were obtained by calcination of precursors at different temperatures. For comparison, Mn/Al2O3-t samples were also characterized by XRD. In agreement with the results in literature [27], 500 °C-calcination of the precursors only results in amorphous structure, regardless of large cations (Ba2+ or La3+). In contrast, intense XRD peaks corresponding to Mn2O3 [JCPDS No. 89-4836] are clearly observed together with the broad peaks of γ-Al2

Conclusion

In summary, we found that Mn-substituted hexaaluminates were active for high-concentration of N2O decomposition. Large cations (Ba2+ and La3+) greatly affected the catalytic performances by changing the number of octahedral Mn3+ sites entering into the framework of the hexaaluminates, which is regarded as the active sites for N2O decomposition. The Ba-hexaaluminate with a β-Al2O3 structure has a larger fraction of octahedral Mn3+ than the La-hexaaluminate with a MP structure. As a result, the

Acknowledgments

Financial support was provided by the National Science Foundation of China (NSFC) grants (20773122, 20773124) and the External Cooperation Program of Chinese Academy of Sciences under grant (GJHZ200827).

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