Characterization of Fe Substitution into La-Hexaaluminate Systems

Article pubs.acs.org/JPCC

Characterization of Fe Substitution into La-Hexaaluminate Systems and the Effect on N2O Catalytic Decomposition Yan Zhang,†,‡ Xiaodong Wang,*,† Yanyan Zhu,§ Baolin Hou,† Xiaofeng Yang,† Xin Liu,† Junhu Wang,† Jun Li,∥ and Tao Zhang† †

State Key Laboratory of Catalysis, Dalian Institute of Chemical Physics, Chinese Academy of Sciences, Dalian 116023, P. R. China University of Chinese Academy of Sciences, Beijing 100049, P. R. China § College of Chemical Engineering, Northwest University, Xi’an 710069, P. R. China ∥ Department of Chemistry, Tsinghua University, Beijing 100084, P.R. China ‡

S Supporting Information *

ABSTRACT: Compared to Ba-hexaaluminates with β-Al2O3 structure, La-hexaaluminates with magnetoplumbite (MP) structure have more substituted Al sites in the preferentially exposed mirror plane, which should favor N2O decomposition. In this regard, LaFexAl12−xO19 catalysts with x = 1 and 5 were herein prepared using the coprecipitation method. 57Fe Mössbauer spectroscopy and X-ray diffraction structure refinements revealed that Fe3+ ions originating in Fe2O3 species mainly entered tetrahedral Al(2) and trigonal bipyramidal Al(5) sites. Meanwhile, Fe3+ ions in octahedral sites of perovskite-type LaFeO3 intermediates preferentially accommodated in octahedral Al(3) sites in the mirror plane of La-hexaaluminates. Correlation of normalized rates (moles of N2O per hour per square meter) of the catalysts at 500 °C with the occupancy of Fe ions in different Al crystallographic sites of the MP phase indicated that Fe3+ ions in the Al(3) and Al(5) sites were highly active for N2O decomposition. In contrast, Fe2+ derived from a Fe3+ → Fe2+ reduction under high-temperature calcination played a negative role.

1. INTRODUCTION Micropropulsion systems, aiming at highly accurate stationkeeping and attitude control for small satellites, is extensively applied in microspacecrafts.1,2 Energetic compounds known as propellant are used for propulsion purposes. A conventional monopropellant used in microthrusters is hydrazine.3 However, the high toxicity of hydrazine increases the hazard and causes high storage and handling costs. It is thus desirable to develop a less toxic propellant as a substitute for hydrazine. Nitrous oxide (N2O) is a compound which has been recognized as a greenhouse gas and ozone depletion agent, and anthropogenic emissions are being rigorously regulated.4,5 Different from the N2O emissions as an environmental pollutant, high-concentration N2O (vol % = 30−100%) is considered to be a promising green propellant. This can be attributed to the nontoxicity and low-cost of N2O compared to the traditional hydrazine, as well as self-pressurization, good compatibility, and system simplicity.1,6 The great potency of nitrous oxide for providing micropropulsion is based on the chemical decomposition into nitrogen and oxygen which can generate large amounts of heat (ΔH0r ≈ −82.5 kJ/mol). However, in view of the extreme operating conditions of N2O decomposition applicable in the propulsion systems (T > 1000 °C, Ea ≈ 250 kJ/mol), catalytic materials with high resistance to thermal shock are required to lower the initiation temperature and accelerate the decomposition rate.7 Among the investigated systems, hexaaluminates have been considered as suitable © 2014 American Chemical Society

catalysts for high-temperature catalytic applications, such as methane catalytic combustion8−12 and high-temperature N2O decomposition,13−19 because of their remarkable sintering resistance and thermal stability. Machida et al.8,20 and Groppi et al.11,21 related this property to the peculiar layered structure of the material that consists of alternate γ-Al2O3 spinel blocks and large cation (e.g., Ba, La, Sr)-containing mirror planes (Figure 1). Considering the anisotropic crystal growth of this layered material, a limited ability of rearrangement of the stacks occurs, which causes a preferential exposure of the mirror planes at the surface.11,22 Moreover, oxygen within the mirror plane is less tightly bound than that in the rigid spinel block, making it a preferential diffusion route of oxide ions,8,23−26 which should favor the oxygen-involved catalytic reaction. Depending on the charge and radius of the large cations in the mirror plane, two structures corresponding to β-Al2O3 and magnetoplumbite (MP) are obtained. For example, Bahexaaluminates (hexaaluminates with Ba as the mirror plane cation) are generally considered to have the β-Al2O3 structure, and La-hexaaluminates have the magnetoplumbite structure. More importantly, because of the close similarity of ionic radii, transition-metal ions can be partially or completely substituted for Al ions. Arai and co-workers8 first introduced transition Received: October 15, 2013 Revised: December 13, 2013 Published: January 3, 2014 1999

dx.doi.org/10.1021/jp410207d | J. Phys. Chem. C 2014, 118, 1999−2010

The Journal of Physical Chemistry C

Article

formation of nonsubstituted hexaaluminates have been proposed. For Ba-β-Al2O3, Groppi et al.37 revealed two routes involving solid-state reactions between γ-Al2O3 and dispersed barium compounds, or γ-Al2O3 and BaAl2O4. On the basis of this reacting mechanism, our recent work18 investigated Fesubstituted Ba-hexaaluminates (BaFexAl12−xO19). It confirmed that Fe ions in the intermediate BaAl2O4 mainly entered Al(2) sites in the spinel block, while Fe ions originating from oxidic entities preferentially entered the mirror plane. For the La-MP phase, however, only one route was reported via the reaction between the scattered, uncrystal La species and γ-Al2O3.12,38 Considering these different formation routes, which probably play a key role in determining the incorporation of the introduced ions in the final materials, one would expect a distinct substituting mechanism of transition metals in Lahexaaluminates. Laville et al.39 and Park et al.40 observed that Mg2+ entered the structure of La-hexaaluminate in tetrahedral sites. For Mn-substituted samples, Groppi et al.12 reported that Mn preferentially entered as Mn(II) in tetrahedral holes and as Mn(III) in octahedral sites and further incorporation of Mg into the LaMnAl11O19 resulted in the displacement of Mn ions from tetrahedral to octahedral position, which largely increased the catalytic activity. Structural and spectroscopic investigations of LaFeAl11O19 were performed by Tronc et al.,41 and the results showed that Fe2+ ions were localized in the tetrahedral sites of the spinel block. By air annealing, Fe2+ was partially converted into Fe3+ and a significant amount of Fe3+ ions appeared in trigonal bipyramidal sites. However, up to now, no details were executed on the substituting mechanism of transition-metal ions from amorphous compounds into Lahexaaluminates, which is of crucial importance for the subsequent utilization of the resulting materials in catalysis. In our present work, Fe-substituted La-hexaaluminates were prepared based on the same coprecipatition route previously used for M-substituted Ba-β-Al2O3.18,19 A combination of characterizing techniques (X-ray diffraction (XRD), 57Fe Mössbauer spectroscopy, extended X-ray absorption fine structure (EXAFS), Rietveld refinement, and theoretical computation) was systematically applied to monitor the evolution of the chemical state and localization of Fe ions during thermal treatment (700−1400 °C). The reactivity of the obtained hexaaluminate catalysts have been investigated in high-concentration N2O decomposition. The aim of this study was to correlate the catalytic performance with different Fe crystallographic sites in the substituted La-hexaaluminate materials.

Figure 1. The structure of MP-type La-hexaaluminate. Numbers in parentheses refer to the different Al sites. Al(1), Al(3), and Al(4), octahedral sites; Al(2), tetrahedral site; Al(5), trigonal bipyramidal site (distorted tetrahedral site: one of the two pseudotetrahedral sites of the bipyramid).

metals in the lattice of hexaaluminate and found that the combustion activity was enhanced compared to the unsubstituted sample. Since then, a series of metal-substituted hexaaluminates AMxAl12−xO19 (A = La, Ba, Ca, Sr, etc. and M = Cr, Co, Mn, Fe, and Ni, etc.) have been investigated,17,18,21,23,27−32 and most of the studies focus on transition-metal-substituted Ba-hexaaluminates. Kikuchi et al.29,30 reported that framework Ni in Ba hexaaluminates possessed excellent activities for CH4 and C3H8 partial oxidation. Gardner et al.23 further revealed that Ni ions in the Ba-hexaaluminate structure preferentially substituted the tetrahedrally coordinated Al3+ near the mirror plane. Moreover, Artizzu-Duart et al.31 and Astier et al.32 directly related the methane combustion catalytic activity of Mn-based catalysts to the most reactive Mn3+ species located in an Al interstitial site near the mirror plane in the hexaaluminate structure. Our group18 recently reported that Fe3+ ions in Al(5) sites in the mirror plane of β-Al2O3 and Al(3) sites in the mirror plane of MP phase exhibited remarkable performance for highconcentration N2O decomposition. These results strongly suggest that the substituted metal ions, which are inlaid in the mirror plane of hexaaluminate structure, should be highly active for catalytic activity. Compared to Ba-hexaaluminates, La-hexaaluminates have been found to have superior resistance to poisoning by carbonates and sulfur compounds and possess higher catalytic activity.33−35 Groppi et al.36 attributed this different combustion activity to the effect of the mirror plane, that is, the different oxidation state of the cation, and the crystal composition of magnetoplumbite and β-Al2O3 which have different distribution and coordination of ions in the mirror plane. In Ba-β-Al2O3, per unit formula there are only two tetrahedral (Al(3)) sites in the mirror plane; whereas in the LaMP structure, two octahedral (Al(3)) and one trigonal bipyramidal (Al(5)) sites appear. This implies that there are more potential active sites in the MP structure, which could responsible for promoting the catalytic activity. Other than the crystal structure of the layered materials (β-Al2O3 and MP), it is also worth noting that different reaction routes for the

2. EXPERIMENTAL SECTION 2.1. Catalyst Synthesis. Fe-substituted La-MP samples with nominal composition LaFexAl12−xO19 (x = 0, 1, and 5) were synthesized by a coprecipitation method proposed by Groppi et al.42 Stoichiometric nitrate solutions of lanthanum, iron, and aluminum were used as starting materials, and saturated ammonium carbonate solution as a precipitating agent. The preparation procedure has been detailed elsewhere.18 To trace thermal evolution of the systems, the obtained hexaaluminate precursors were calcined at 700, 900, 1000, 1100, 1200, 1300, and 1400 °C (heating rate 1 °C/min, held at each step for 4 h). Chemical analysis was performed using inductively coupled plasma (ICP) for all the samples. Calculations indicated that the Fe/Al atom ratios were quantitative and very close to the nominal ratios (1:11 or 5:7), whereas a slight loss in La was observed during calcination 2000



Article

the gas. The products were monitored online with an Agilent 6890N GC. Peak areas of the gas (N2O, N2, and O2) were calibrated by standard gas mixtures with different molar ratios, and a mass balance could be reached. N2O conversion was then calculated based on the balance of nitrogen species. To exclude experimentally the mass and heat transport limitations for reported rates in the text, Weisz−Prater criterion53−55 was used, and the calculations detailed in the Supporting Information confirmed negligible transfer effect.

with respect to the initial composition. This nonstoichiometry of La may have resulted from cationic and anionic vacancies. In the following description, LaFexAl12−xO19 samples are abbreviated as LA-t when x = 0, LF1A-t when x = 1, and LF5A-t when x = 5 (t indicates calcination temperature). 2.2. Catalyst Characterization. XRD patterns of the calcined samples were measured on a PANalytical X’Pert-Pro powder X-ray diffractometer (radius, 240.0 mm; X-ray tube, PW 3373/10 Cu; operating voltage, 40 kV; current, 40 mA) with Cu Kα radiation (intended wavelength type, Kα1; Kα2/ Kα1 intensity ratio, 0.50) in the angle interval 10−80° (2θ), with a step size of 0.0334 o (2θ) and a counting time of 19.685 s per step. The incident beam passed through a 0.04 rad Soller slit, a 10 mm fixed mask, a 1° antiscatter slit, and a 1/2° divergence slit. International Center for Diffraction Data (ICDD) PDF database was used for phase identification. Crystalline size was estimated by the Scherrer equation. The crystal structure and atom crystallographic sites of the final La-MP hexaaluminates were refined using FULLPROF program with references (ICSD entries 38398, 38371, 203003, and 35174) as the baseline patterns. For this purpose, spectra were collected several times for the same sample with a step size of 0.0167° (2θ) and a counting time of 29.846 s per step. Further details for the Rietveld refinement process were reported in our previous contributions.19 Mössbauer measurements were performed to determine Fe distributions in the calcined La−Fe−Al samples by using an electrodynamic spectrometer with a 57Co/Rh γ-ray source. All the spectra were obtained at room temperature and fitted using the MossWinn fitting package. Theoretical calculations were performed at the level of relativistic density functional theory (DFT) using the Vienna ab initio simulation package (VASP).43−46 The core and valence electrons were represented by the projector augmented wave (PAW) method and plane-wave basis functions with a kinetic energy cutoff of 400 eV.47,48 The generalized gradient approximation (GGA) with the Perdew−Burke−Ernzerhof (PBE) exchange−correlation functional49 was used in the calculations. A Monkhorst−Pack 3 × 3 × 1 grid was used to sample the surface Brillouin zone.50 Ground-state atomic geometries were obtained by minimizing the forces on the atoms to below 0.02 eV/Å. Because of the strong d-electron correlation effects for Fe, the calculations were carried out with the DFT+U method, using the formalism suggested by Dudarev et al.51 The parameters were set at U = 4 eV and J = 1 eV according to previous reports.52 The bulk cell of MP with initial a = b = 5.56 Å and c = 22.07 Å was used, and the dimensions of the cell were all allowed to relax during the geometry optimizations. Surface area has been measured by nitrogen adsorption using the Brunauer−Emmett−Teller (BET) method (Micromeritics ASAP 2010 instrument). A JSM 6360-LV scanning electron microscope (SEM) was employed for morphological analysis. Catalytic activity of La-hexaaluminate catalysts was determined by temperature-programmed reaction of N2O in a range of 400−800 °C with a heating rate of 5 °C/min. Experimental conditions were as follows: catalyst weight, 100 mg (20−40 mesh); pretreatment, Ar at 300 °C for 0.5 h; reactant stream, 30 vol % N2O in Ar as balance; total flow rate, 50 mL/min. A 5 mm bed containing pure quartz sand was placed upstream in the reactor (i.d. = 7 mm) to ensure uniformity of the inlet mixture gas. Another 3−5 mm quartz sand bed was fixed downstream to support the catalyst and avoid backmixing of

3. RESULTS AND DISCUSSION 3.1. Thermal Evolution Characterization. XRD Phase Analysis and Morphology. For unsubstituted LA samples (Figure 2a), calcination at 700 °C resulted in an amorphous phase. Upon calcination at 900−1000 °C, smooth modulations attributed to γ-Al2O3 [JCPDS 10-425] were barely detected, parts of which were transformed into LaAlO3 at 1100 °C. On increasing the temperature to 1200−1400 °C, the final MP-type LaAl12O19 [JCPDS 1-77-311] phase formed while those of LaAlO3 were still well-crystallized in these samples. In the case of LF1A (Figure 2b), 700−1000 °C calcination resulted in phase composition similar to that detected in LA samples. No evidence of iron oxide was identified in all these samples, indicating the high dispersion of Fe species on Lamodified γ-Al2O3. Starting from 1100 °C, the monophasic MP phase was obtained and was paralleled by the consumption of γ-Al2O3. However, no LaAlO3 reflections were detected. Compared with the nonsubstituted LA sample (1200 °C), incorporation of Fe led to the formation of hexaaluminate at lower temperature (1100 °C). This was likely due to a similar mechanism that has been reported for Mn-substituted Lahexaaluminate,12 i.e., increasing of the mobility of large cations in the γ-Al2O3 spinel blocks. This easier formation of the LaMP phase that essentially competed with LaAlO3 segregation should be responsible for the disappearance of LaAlO3 in the precursors. The calculated cell parameters (a = b = 5.5724− 5.5926 Å, c = 22.046−22.081 Å) of the hexaaluminate phase in LF1A-t (t = 1100−1400 °C) were higher than those of the unsubstituted LA-1400 sample (a = 5.5681 Å, c = 21.895 Å). This indicated that the dispersed Fe species on La-modified γAl2O3 incorporated into the MP-hexaaluminate lattice by replacing smaller Al3+ ions. For LF5A-700 (Figure 2c), different from the amorphous structure observed in LA-700 and LF1A-700, several peaks were evident that can be associated with traces of α-Fe2O3 phase [JCPDS 1-84-0310], in line with the increasing Fe content. Differences in phase composition became markedly evident after calcination at 900 °C. In LF5A-900, besides the sharper αFe2O3 diffraction peaks due to sintering of particles, reflections of LaFeO3 [JCPDS 1-75-439] at 2θ = 22.8, 32.5, 40.1, 46.7, and 58.0° were also observed, resulting from the thermal reaction of parts of La compounds and α-Fe2O3. Accordingly, the existence of two different Fe species (α-Fe2O3 and LaFeO3) has been demonstrated in the precursors, which was apparently related to the Fe concentration. After calcination at 1000 °C, incipient formation of MP-type hexaaluminate was observed, which was below the onset temperature for LF1A (1100 °C) and LA (1200 °C). In LF5A-1100, no changes of phase composition were detected and only a higher crystallization of the MP and LaFeO3 phases was observed, accompanied by the consumption of α-Fe2O3. After calcination at higher temperatures (1200−1400 °C), the reflections of a well-crystallized MP phase became the only features in the XRD spectra, whereas α2001



Article

be attributed to the stabilizing effect of Fe, which lowered the structural disorder in La-hexaaluminates. Surface area measurements were performed on the LA-t, LF1A-t, and LF5A-t samples, and the results were listed in Table 1. For unsubstituted samples, the amorphous LA-700 Table 1. Specific Surface Areas of LA-t, LF1A-t, and LF5A-t Samples surface areaa (m2/g) t (°C)

LA

LF1A

LF5A

700 900 1000 1100 1200 1300 1400

231 95 57 41 23 21 12

275 90 48 25 22 14 9

131 45 25 18 8 5 5

Samples were evacuated first at 110 °C for 3 h and then at 350 °C for 5 h prior to their analysis and measured at −196 °C.

a

exhibited high surface area (231 m2/g). As the calcination temperature increased, the values of surface area decreased accompanied by the phase transformation. Up to 1200−1400 °C, the surface area values markedly decreased to 12−23 m2/g in line with the formation of the MP phase. In contrast, the introduction of Fe ions in LF1A and LF5A samples promoted phase evolutions in the entire investigated temperature range, which further accelerated the sintering of particles and led to the decrease of surface areas. SEM images of LA, LF1A, and LF5A calcined at 1400 °C are compiled in Figure 3, showing the hexagonal-shaped particles typical of hexaaluminate structure over all samples. As shown in Figure 3, the observed plates at the surface should reasonably correspond to the preferentially exposed mirror plane.11,22 Moreover, the increased Fe doping resulted in a larger particle size, which was in agreement with Groppi’s report that the sintering resistance of the final material weakened on increasing the Fe loading.56 However, even after calcination at very high temperature (1400 °C), large surface areas (5−12 m2/g) were still retained, indicating the excellent high-temperature stability of La-hexaaluminate compounds. 57 Fe Mössbauer Spectroscopy. Mössbauer measurements were performed to collect further indications of the chemical and structural environment of Fe species in Fe-containing systems during the heat treatment process. Figure 4 shows the Mössbauer spectra (MS) of samples LF1A-t and LF5A-t obtained in the magnetic range, and Table 2 lists the corresponding parameters. For LF1A-700 (Figure 4a), a broadened quadrupole splitting doublet in the spectrum was promptly ascribed to highly dispersed α-Fe2O3,18,57 which was consistent with the XRD results (Figure 2a). Similar assignments were evidently valid in LF1A-900 and LF1A-1000 because of the close parameters of MS doublets (Table 2). For LF1A-1100 and LF1A-1200, the experimental MS was represented by a superposition of two quadruple doublets with close isomer shift (IS) values and significantly different quadrupole splitting (QS) values. The analysis of MS parameters (Table 2) and XRD phase compositions in this temperature (Figure 2b) showed that all doublets were characteristic of Fe3+ ions in tetrahedral coordination18,41,58 in the La-MP structure. Besides the tetrahedral Al(2) sites, Al(5) in La-hexaaluminate was split

Figure 2. X-ray diffraction patterns at room temperature of (a) LA-t, (b) LF1A-t, and (c) LF5A-t samples.

Fe2O3 and LaFeO3 phases were no longer detected. Therefore, it can be unequivocally concluded that α-Fe2O3 and LaFeO3 were reagents in the thermal evolution leading to the formation of La-MP. As compared to LF1A-t (t = 1100−1400 °C, a = b = 5.5724−5.5926 Å), the enlargement of cell parameters for MP phase in LF5A-t (t = 1100−1400 °C, a = b = 5.6251−5.7021 Å) confirmed more Fe ions in the hexaaluminate structure. Moreover, different from the broadening of MP diffraction peaks in LA samples, clearly evident MP reflections in Fecontaining samples (LF1A and LF5A) were observed. This can 2002



Article

Figure 4. 57Fe Mössbauer spectra of (a) LF1A-t and (b) LF5A-t samples at RT.

(XANES) results (Supporting Information). The amount of Fe2+ ions present in the structure was very small (5−6%), indicating the limited extent of the Fe3+ → Fe2+ conversion at higher calcination temperatures. It is noteworthy that when the calcination temperature was elevated from 1200 to 1400 °C, the amount of Fe3+ ions in Al(2) sites decreased progressively, accompanied by the increase of those in Al(5) sites of the MP phase. This implied the rearrangement of Fe3+ ions from Al(2) sites in the spinel block to Al(5) sites in the preferentially exposed mirror plane in the crystalline network. Because of the positive net rate of desorption of surface oxygen at high temperatures,56 Fe3+ ions located in the Al(5) sites, with somewhat reduced asymmetry, were easily suffered with a progressive thermal Fe3+ → Fe2+ reduction. In the case of LF5A-700 (Figure 4b, Table 2), the only doublet was again assigned to α-Fe2O3 with a superparamagnetic state, which is probably due to its small crystalline size (about 5 nm) or the dissolution of hematite in alumina.65,66 For LF5A-900, a magnetic hyperfine field distribution (49%) associated with increased α-Fe2O3 crystallites was detected in the spectrum, resulting from the increasing Fe concentration (x

Figure 3. Scanning electron micrographics of (a) LA-1400, (b) LF1A1400, and (c) LF5A-1400 samples.

from the theoretical five-coordinated position 2b (0, 0, 0.25) into 4e (0, 0, 0.24) sites with 50% occupation for each, so as to have a distorted tetrahedral environment59−61 (Figure 1). Because a more highly distorted coordination polyhedron was expected to show larger QS values,41,57,62 the doublets with QS = 0.79−0.85 mm/s and QS = 2.20−2.33 mm/s were attributed to the resonance absorption of Fe3+ ions in the symmetric tetrahedral Al(2) sites and distorted trigonal bipyramidal Al(5) sites, respectively. This result revealed that Fe3+ ions in the highly dispersed α-Fe2O3 migrated into the structure of Lahexaaluminate in Al(2) sites within the spinel block and Al(5) sites in the mirror plane. In particular, when the calcination temperature was increased, a new doublet subspectrum with IS = 0.99−1.07 mm/s typical of the Fe2+ ion41,63,64 was observed in LF1A-1300 and LF1A-1400 samples (Table 2). This was also supported by the X-ray absorption near-edge spectroscopy 2003



Article

Table 2. Hyperfine Parametersa and Iron Site Assignmentb in LF1A-t and LF5A-t Samples at RT samples LF1A-700 LF1A-900 LF1A-1000 LF1A-1100 LF1A-1200 LF1A-1300

LF1A-1400

LF5A-700 LF5A-900

LF5A-1000

LF5A-1100

LF5A-1200

LF5A-1400

IS (mm/s) 0.30 0.32 0.29 0.29 0.29 0.28 0.20 0.28 0.18 1.07 0.26 0.24 0.99 0.31 0.32 0.35 0.32 0.31 0.34 0.32 0.27 0.35 0.31 0.27 0.28 0.22 0.29 0.38 0.21 0.26 0.38

± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ±

0.01 0.004 0.01 0.01 0.04 0.01 0.02 0.01 0.01 0.03 0.01 0.01 0.02 0.004 0.01 0.01 0.05 0.01 0.003 0.005 0.02 0.004 0.01 0.01 0.01 0.02 0.004 0.003 0.01 0.003 0.01

QS (mm/s)

H (KOe)

± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ±

− − − − − − − − − − − − − − − 489.4 ± 0.8 − − 502.7 ± 0.2 − − 512.7 ± 0.3 − − − − − − − − −

1.21 1.15 1.08 0.85 2.20 0.79 2.33 0.71 2.42 1.08 0.60 2.48 0.52 0.96 0.93 −0.19 0.61 0.96 −0.21 0.61 0.76 −0.20 0.62 0.73 2.16 0.54 2.21 0.62 0.56 2.29 0.62

0.02 0.01 0.01 0.04 0.09 0.01 0.04 0.01 0.02 0.06 0.01 0.02 0.03 0.01 0.02 0.02 0.01 0.02 0.01 0.03 0.06 0.01 0.03 0.04 0.01 0.004 0.01 0.02 0.003 0.01 0.002

A (%) 100 100 100 88 12 80 20 71 24 5 65 29 6 100 43 49 8 17 59 14 10 35 27 28 10 31 15 54 31 15 54

± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ±

2 1 2 2 1 2 2 2 2 1 3 3 1 1 1 2 1 1 1 1 2 1 1 1 1 2 1 3 2 1 3

site assignment Fe3+ in α-Fe2O3 Fe3+ in α-Fe2O3 Fe3+ in α-Fe2O3 Fe3+(Th) in Al(2) of MP Fe3+(Tr) in Al(5) of MP Fe3+(Th) in Al(2) of MP Fe3+(Tr) in Al(5) of MP Fe3+(Th) in Al(2) of MP Fe3+(Tr) in Al(5) of MP Fe2+(Th) in Al(2) of MP Fe3+(Th) in Al(2) of MP Fe3+(Tr) in Al(5) of MP Fe2+(Th) in Al(2) of MP Fe3+ in α-Fe2O3 Fe3+ in α-Fe2O3 Fe3+(Oh) in LaFeO3 Fe3+ in α-Fe2O3 Fe3+(Oh) in LaFeO3 Fe3+ in MP Fe3+ in α-Fe2O3 Fe3+(Oh) in LaFeO3 Fe3+(Th) in Al(2) of MP Fe3+(Tr) in Al(5) of MP Fe3+(Th) in Al(2) of MP Fe3+(Tr) in Al(5) of MP Fe3+(Oh) in MP Fe3+(Th) in Al(2) of MP Fe3+(Tr) in Al(5) of MP Fe3+(Oh) in MP

IS, isomer shift relative to metallic iron; QS, quadrupole splitting; H, hyperfine magnetic field; A, relative area of the MS. bTh, tetrahedral; Tr, trigonal bipyramidal (distorted tetrahedral, one of the two pseudotetrahedral sites of the bipyramid); Oh, octahedral.

a

modifications in the MS of LF5A samples can be noted. Evidenced by the XRD analysis (Figure 2c), the monophasic MP was present in the samples; meanwhile, the α-Fe2O3 and LaFeO3 phases have totally transformed since the MS associated with those oxides completely disappeared. Considering the similarity of Fe distribution of MP phase in LF5A-t (t = 1200−1400 °C) samples (Figure 4, Table 2), only the detailed data for LF5A-1400 are discussed here for brevity. In the spectrum of LF5A-1400, two doublets (31% and 15%) corresponding to Fe3+ in Al(2) and Al(5) sites were observed. In addition, our fit also verified the occurrence of a high-spin octahedrally coordinated Fe3+ ions, the parameters of which were close to the values of Fe3+ ions in octahedral environments dissolved in magnetoplumbite structure.18,58 On the basis of Mössbauer results (Table 2), it is conceivable that Fe3+ ions in octahedral sites in the final MP-hexaaluminate of LF5A samples should originate from the intermediate perovskite-type LaFeO3 and the α-Fe2O3 oxidic entities. Compared to LF1A-t (t = 1300−1400 °C), no evidence of Fe2+ has been found in LF5A samples after high-temperature calcination (1200−1400 °C). This might be due to the relatively small fraction of Fe ions that participated in the reduction phenomenon when considering the total Fe concentration in LF5A, i.e., it is below the sensitivity limits of the characterization techniques. This can be further confirmed

= 5). In addition, a broadened doublet (8%) was also observed in the fitted spectrum. The hyperfine parameters of this component (IS = 0.32 mm/s, QS = 0.61 mm/s, Table 2) were very close to those of the spectra (IS = 0.30−0.37 mm/s, QS = 0.53−0.62 mm/s) corresponding to Fe3+ ions in LaFeO3 perovskite-type oxides with other metals (such as Mg or Ti) incorporated.67 Therefore, in accordance with our XRD results (Figure 2c), this new doublet was attributed to high-spin octahedral Fe3+ in perovskite-type LaFeO3 with the dissolution of Al3+ in the structure. Further elevating the calcination temperature gave rise to the resonance absorption of Fe3+ ions in the newly formed hexaaluminate phase in LF5A-1000 and LF5A-1100, which was paralleled by the increase of relative area (A) of LaFeO3 (8% at 900 °C versus 27% at 1100 °C) and the decrease of α-Fe2O3 (from 92% at 900 °C to 35% at 1100 °C). By applying a mathematical approach to the spectrum of MP phase in LF5A-1100, two quadruple splitting doublets were discerned (Figure 4b, Table 2). Like that of LF1A-1200, one doublet (28%) was assigned to tetrahedral Fe3+ ions in the symmetric Al(2) sites, and the other doublet (10%) corresponded to Fe3+ ions in the distorted Al(5) sites in the MP phase. This result has now undoubtedly corroborated that Fe3+ ions that originated from α-Fe2O3 in the precursors transformed into LaFeO3 on one side and into Al(2) and Al(5) sites in La-MP hexaaluminate structure on the other side. On increasing the temperature to 1200−1400 °C, drastic 2004



Article

be pointed out that deviations from the ideal structure in LA1400 sample were systematically found (Table 4, Figure 1), such as partial off-center shift of La (2d → 6h), displacement of Al(5) (2b → 4e), and oxygen atoms (6h → 12j for O(1), 12k → 24l for O(2)), leading to the vacancies of atoms (La, Al(5)) in the mirror plane. In fact, such a distorted structure was obtained to balance the cation charge (La3+), which is larger than the ideal valence (+2.4) of the large cation for a MP structure.59,68 In contrast to the nonsubstituted matrix, a more stoichiometric MP phase was observed in LF1A (La0.88Fe1.00Al11.00O19) and LF5A (La0.88Fe4.96Al7.04O19). Moreover, the atomic content of Fe in the Al sites increased with Fe loading, indicating more Fe ions were introduced into the MP structure, which was further confirmed by the evidently increased a parameter (Table 3). Table 4 reports the atomic site occupancy in the MP phase. For Fe-containing La-hexaaluminates, the splitting of La and Al(5) persisted as was found in LA-1400, while the oxygen migration no longer occurred, indicating the improved longrange ordering. In particular, with more Fe in the MP phase, the decreased occupancies of La(1) (2d) accompanied by increased La(2) (6h) were observed. This displacement of La3+ ions is related to the substitution of Al3+ (2p6) by the more polarizable Fe3+ (3d5),41 which weakened the Coulombic repulsions with the neighboring La3+ ions and promoted the off-center shift of La (2d → 6h). For the crystallographic sites of Fe in the MP phase, Rietveld analysis for LF1A-1400 showed that only tetrahedral Al(2) and distorted tetrahedral Al(5) sites were occupied by iron and that octahedral sites (Al(1), Al(3), and Al(4)) were free of Fe species (Table 4). This tetrahedral substitution of iron was in line with the Fe state in the precursors. On the basis of the Xray (Figure 2) and Mössbauer (Table 2) results, Fe species initially existed as α-Fe2O3 dispersed on La-modified γ-Al2O3. According to the literature,69 there is sharing of vertices, edges, and faces of FeO6 coordination groups in the α-Fe2O3 structure, and the arrangement of Fe3+ ions around O2− approximates most closely a regular tetrahedron. This evidence provided support for the observed preferential occupancy of Fe3+ in the tetrahedral Al(2) and Al(5) sites in the MP phase, which was in good agreement with the Mössbauer results (Table 2). For LF5A-1400, amounts of Fe ions in Al(2), Al(3), and Al(5) sites (67.7−77.3%) along with traces in Al(1) and Al(4) sites (5.3−20.8%) were detected. It thus clearly indicated the preferential localization of Fe species in the tetrahedral Al(2) and Al(5) sites as that in LF1A, and also the Al(3) octahedral sites of the mirror plane (Figure 1). This difference in Fe occupation from LF1A should originate from the newly formed Fe species in LF5A, that is, Fe3+(Oh) in perovskite-type LaFeO3 as an intermediate. As discussed by Iyi et al. for

by the XANES result in Figure S1 of the Supporting Information. 3.2. Structural Analysis and Fe Allocation in the Final MP Structure. Rietveld Refinement. The detailed Fe location and further effect on the crystal structure of La-hexaaluminate in LF1A and LF5A samples calcined at 1400 °C was studied by Rietveld structural refinement. The fitted XRD patterns are plotted in Figure 5, and the obtained crystal data are presented in Tables 3 and 4.

Figure 5. Observed (points) and calculated (continuous line) X-ray Rietveld refined patterns of (a) LF1A-1400 and (b) LF5A-1400 samples.

As indicated in Table 3, for LA-1400 without Fe doping, a MP phase with defective La0.85Al11.87O19 was formed. It should

Table 3. Results of Structural Refinement on LA-1400, LF1A-1400, and LF5A-1400 Samples results

LA-1400

Rp Rwp phase composition (mass ratio)

10.177 12.856 87% MP 13% LaAlO3 La0.85Al11.87O19 5.5681(3) 21.895(9)

composition of MP phase a = b (Å) c (Å)

LF1A-1400

2005

LF5A-1400

7.669 9.474 100% MP

9.210 11.463 100% MP

La0.88Fe1.00Al11.00O19 5.5926(6) 22.081(1)

La0.88Fe4.96Al7.04O19 5.7021(5) 22.440(9)



Article

Table 4. Rietveld Refined Occupancy of La, Al, Fe, and O in MP Structure of LA-1400, LF1A-1400, and LF5A-1400 Samples occupancyg

atomic coordinates

a

atom

mult.

La(1) La(2) Al(1) Al(2) Al(3) Al(4) Al(5) Fe in Al(1) Fe in Al(2) Fe in Al(3) Fe in Al(4) Fe in Al(5) O(1) O(1) O(2) O(2) O(3) O(4) O(5)

2 6 12 4 4 2 4 12 4 4 2 4 12 6 24 12 12 4 4 b

a

site

coor.

b

block

2d − Mc 6h − M 12k Oh Sd 4f Th S 4f Oh M 2a Oh S 4e Tr M 12k Oh S 4f Th S 4f Oh M 2a Oh S 4e Tr M 12j − − 6h − − 24l − − 12k − − 12k − − 4e − − 4f − − ratio of Fe in different sites c

d

X

Y

Z

LA -1400

LF1A -1400

0.6667 0.7241 0.8310 0.3333 0.3333 0.00 0.00 0.8310 0.3333 0.3333 0.00 0.00 0.2013 0.1781 0.1619 0.1561 0.5023 0.00 0.6667

0.3333 0.4482 0.6621 0.6667 0.6667 0.00 0.00 0.6621 0.6667 0.6667 0.00 0.00 0.3224 0.3560 0.3115 0.3122 0.0046 0.00 0.3333

0.2500 0.2500 0.1077 0.0271 0.1899 0.00 0.2389 0.1077 0.0271 0.1899 0.00 0.2389 0.2500 0.2500 0.0518 0.0518 0.1501 0.1481 0.0553

0.682(6) 0.057(4) 1.00 1.00 1.00 1.00 0.437(5) 0.00 0.00 0.00 0.00 0.00 0.50 − 0.50 − 1.00 1.00 1.00

0.600(1) 0.092(8) 1.00 0.640(5) 1.00 1.00 0.360(3) 0.00 0.360(3) 0.00 0.00 0.140(1) − 1.00 − 1.00 1.00 1.00 1.00

−

72:28e

LF5A -1400 0.441(1) 0.146(3) 0.792(15) 0.227(1) 0.323(4) 0.947(18) 0.123(1) 0.208(4) 0.773(5) 0.677(9) 0.053(1) 0.377(2) − 1.00 − 1.00 1.00 1.00 1.00 54:31:15f

e

Multiplicity. Coordination. Mirror plane. Spinel block. Ratio of Fe ions in Al(2) sites to those in Al(5) sites in the MP phase. fRatio of Fe ions in the octahedral sites (Al(1), Al(3), Al(4)), Al(2) sites, and Al(5) sites in the MP phase. gNumbers in parentheses refer to the standard deviations.

nonsubstituted La-hexaaluminates,60 the distorted magnetoplumbite structure consists of spinel blocks [Al 11 O 16 ] + intercalated by mirror planes with perovskite composition [LaAlO3]0. In this regard, the octahedral Fe3+ ions in the LaFeO3 phase showed a marked preference for the octahedral Al(3) sites in the mirror plane because of the structural similarity of LaFeO3 with the perovskite mirror plane in the LaMP phase. On the basis of the Rietveld results in Table 4, the percentage content of Fe3+ in Al(3) sites of all the Fe3+ in Al(1)−Al(5) sites of LF5A-1400 was calculated to be 27%. This value was consistent with the proportion of Fe3+(Oh) in the intermediate LaFeO3 phase of LF5A-1100 by characterization of Mössbauer spectroscopy (Table 2). This close Fe ratio further confirmed that Fe3+(Oh) in Al(3) sites in the mirror plane of MP phase originated from the perovskite-type LaFeO3 intermediates in LF5A precursors. For La-hexaaluminates prepared by the coprecipitation method, Groppi et al.12 and Xu et al.38 proposed that the final La-MP forms via the reaction between γ-Al2O3 and La uncrystal species. Our Mössbauer and Rietveld refinement results further determined that in the Fe-substituted Lahexaaluminate system herein investigated, γ-Al2O3 reacted with not only the amorphous lanthanum but also the Fecontaining perovskite-type LaFeO3 intermediates. The detailed thermal evolution of Fe from precursors to the final La-MP structure is summarized below. Initially, Fe3+ ions existed as αFe2O3 and were dispersed on La-modified γ-Al2O3. Via the diffusion of La ions in the γ-Al2O3 matrix, these Fe ions mainly entered the MP structure in tetrahedral Al(2) and Al(5) sites. In particular, at high Fe loading (x = 5), part of α-Fe2O3 reacted with La compounds to give an intermediate LaFeO3 phase. Because of the strong structural analogies, the octahedral Fe3+ in LaFeO3 intermediates were further accommodated in octahedral Al(3) sites in the mirror plane of La-MP hexaaluminates.

From what has been discussed thus far, a stabilizing effect of framework Fe ions in MP structure was observed, which could be related to the following considerations: (i) by occupying the vacant sites left by bipyramidal Al3+ ions, Fe3+ ions in the lattice filled in all Al vacancies in the mirror plane; (ii) with the presence of Fe2+, even in a small amount, the La vacancies in the mirror plane were thus less numerous because of a charge compensation mechanism. Therefore, the structural stoichiometry was improved and lattice distortions were reduced. DFT Calculations. To further understand the evolution of Fe species in MP-type La-hexaaluminate, DFT calculations have been performed to gain an insight into the thermodynamically favored site for the substitution of Fe ions. Table 5 summarizes Table 5. Effect of Fe Locations on the Lattice Properties per Unit Cell of La-Hexaaluminatea crystal structure

substituted sites

lattice energy (eV)

Al(1) −480.56 Al(2) −481.49 Al(4) −480.97 mirror plane Al(3) −480.51 Al(5) −480.72 initial value set for unsubstituted La-MP spinel block

a

a (Å)

b (Å)

c (Å)

5.60 5.60 5.60 5.61 5.62 5.59

5.60 5.60 5.60 5.61 5.62 5.59

22.04 22.05 21.98 22.23 22.06 21.96

Data were obtained based on the DFT/PBE level of theory.

the lattice energies as well as the lattice parameters for different substituted Al sites for Fe ions from a thermodynamic standpoint. It was found that the favored sites for Fesubstitution were generally in an order of Al(2) > Al(4) > Al(5) > Al(1) ≈ Al(3). Our DFT calculation results were consistent with the previous semiempirically potential model results about Mg-substituted hexaaluminate.40 Considering the significant structural difference between the mirror plane (loosely packed) and the spinel block (closely packed), 2006



Article

For LF1A-t catalysts, the optimum was found for LF1A-1200, and the catalytic activity progressively decreased with further calcination in the range of 1200−1400 °C. Similar modifications were found for LF5A-t catalysts. Figure 7 shows the

different Fe-substituted sites were involved in the formation of such Fe-modified La-hexaaluminates. In the spinel block, Fe ions preferred to locate in the tetrahedral Al(2) sites rather than in the Al(4) or Al(1) sites. While in the mirror plane, Fe ions tended to occupy the Al(5) sites compared to the Al(3) sites. This result was in good agreement with the experimental findings of Mössbauer and Rietveld refinement. In addition, when Fe content increased (x = 5), the substitution on Al(3) sites could be greatly promoted because of the loose pack property of the mirror plane which favored ion diffusion. As for the lattice parameters, no significant fluctuation of a and b lattice vectors with single Fe-substitution was detected. In contrast, the c lattice vectors had an enlargement compared to the unsubstituted pure La-hexaaluminate initially given in the calculation process, especially for the substitution site of Al(3). This could further explain the large enhancement of c value in LF5A (22.440 Å), in which some Fe3+ ions located in Al(3) sites, compared to that of LF1A (22.081 Å) with no Fe ions in Al(3) sites. 3.3. Catalytic Activity. The catalytic results for 30 vol % N2O decomposition performed over the investigated LF1A-t and LF5A-t with La-MP as the major component and over unsubstituted LA-1400 reference sample are presented in Figure 6. Compared to the poor activity of the LA-1400 sample, the conversion markedly grew after the introduction of Fe ions.

Figure 7. Arrhenius plot of N2O decomposition on LF1A-t and LF5At catalysts.

Arrhenius plots drawn for the Fe-containing hexaaluminate catalysts. The apparent activation energy (Ea) values were calculated to be 81−121 kJ mol−1 (shown in the inset of Figure 7) and compared well with those previously measured for transition-metal-modified zeolitic catalysts (Cu, Fe, Co-MFI, BEA, mordenite, ferrierite) in the N2O decomposition system.70−72 Correlation between the observed different Fe species and catalytic properties for N2O decomposition was attempted. For this purpose, the catalytic activity at 500 °C of LF1A-t1 (t1 = 1100−1400 °C) and LF5A-t2 (t2 = 1200−1400 °C) catalysts with monophasic MP phase was normalized by the surface area (moles of N2O per hour per square meter, Table 6). It is worth Table 6. Normalized Rates (r) at 500 °C of LF1A-t1 (t1 = 1100−1400 °C) and LF5A-t2 (t2 = 1200−1400 °C) Catalysts r (× 10−3 mol N2O h−1 m−2) t (°C) 1100 1200 1300 1400

LF1A

LF5A

R′(LF5A)a ( × 10−3 mol N2O h−1 m−2)

± ± ± ±

− 5.9 ± 0.6 5.9 ± 0.4 5.7 ± 0.8

− 1.2 ± 0.1 1.2 ± 0.1 1.2 ± 0.2

0.8 1.0 0.7 0.5

0.1 0.03 0.01 0.1

a

R′(LF5A) was calculated on the basis of the normalized rates of LF5A catalysts (r(LF5A)) divided by 5.

mentioning that under the same calcination and reaction conditions, the normalized rates of LF1A-1200 and LF5A-1200 catalysts (1.0−5.9 × 10−3 mol N2O h−1 m−2) were much higher than those already reported in our previous work for Fesubstituted Ba-hexaaluminates with similar compositions (BaFeAl11O19 and BaFe5Al7O19, 0.3−0.9 × 10−3 mol N2O h−1 m−2).18,73 This confirmed the catalytic advantage of Lahexaaluminate compared to Ba-hexaaluminate because of the structural difference as described in the Introduction. Furthermore, enhancement of the normalized rate was obtained on increasing the Fe content for LF1A to LF5A, indicating the

Figure 6. N2O conversion over (a) LF1A-t and (b) LF5A-t catalysts (for operating conditions, see Experimental Section). 2007



Article

4. CONCLUSIONS In this study Fe-based La-MP hexaaluminates were prepared and the following main conclusions have been drawn concerning structural and catalytic properties of LFxA (x = 1, 5) systems: (i) Fe concentration influenced the intermediate phase in the precursors of the MP phase, and increase of Fe content (x = 5) resulted in the formation of the LaFeO3 intermediates. The deliberate insertion of Fe ions during the crystal evolution lowered the forming temperature of MP phases and helped reach the stoichiometry compared to the unsubstituted La-hexaaluminates. (ii) The substituting mechanism of Fe ions in Lahexaaluminates with MP structure was monitored. Fe3+ ions originating in the initial α-Fe2O3 mainly incorporated into the tetrahedral Al(2) and Al(5) sites via the diffusion of La ions in the γ-Al2O3 matrix; meanwhile, Fe3+ ions in the octahedral sites of perovskite-type LaFeO3 intermediates preferentially accommodated in the octahedral Al(3) sites in the mirror plane of Lahexaaluminates. (iii) Compared to Ba-hexaaluminates with equal Fe content prepared in the same way, Fe-substituted La-hexaaluminate catalysts exhibited higher rate values (1.0−5.9 × 10−3 mol N2O h−1 m−2 versus 0.3−0.9 × 10−3 mol N2O h−1 m−2) in N2O decomposition at 500 °C. Correlation of normalized rates of La-hexaaluminate catalysts at 500 °C with Fe occupation in different Al crystallographic sites of the MP phase indicated that Fe3+ ions in the Al(3) and Al(5) sites in the mirror plane accounted for the high activity of N2O decomposition. The observed deactivation of catalytic properties could be associated with sintering or the presence of Fe2+.

beneficial effect of framework Fe for N2O decomposition. To exclude the influence of Fe concentration, normalized rate of LF5A was divided by 5 (the total Fe content in LF5A) and defined as R′(LF5A) (Table 6). Figure 8 shows the variation of normalized rate (r(LF1A) and R′(LF5A)) and calculated Fe occupancy in different Al

Figure 8. The normalized rate at 500 °C and the calculated Fe occupancy in different Al sites of La-MP hexaaluminate as a function of calcination temperature for LF1A and LF5A catalysts. r(LF1A) is the normalized rate of LF1A; R′(LF5A) was calculated on the basis of r(LF5A) and divided by 5. The number of Fe ions in different Al sites of MP phase in LF5A was modified by dividing by 5 in order to exclude the influence of Fe concentration.

sites of La-MP hexaaluminate as a function of calcination temperature. In LF1A samples, the enhancement of r(LF1A) from LF1A-1100 to LF1A-1200 was paralleled by the increase of Fe3+ ions in Al(5) sites and by the decrease of those in Al(2) sites. This indicated that the elevated rate in LF1A was mainly due to Fe3+ situated at Al(5) sites in the mirror plane, which were preferentially exposed at the surface and thus facilitated the interaction with N2O reagent and desorption of the products. In contrast, Fe3+ ions in Al(2) sites in the spinel block were less active for N2O catalytic decomposition. With further high-temperature calcination up to 1300−1400 °C, the normalized rate of LF1A decreased, which could be attributed to the thermal reduction of Fe3+ → Fe2+, implying the negative role of Fe2+ ions for N2O decomposition. These deactivation phenomena associated with the presence of Fe2+ agrees well with that reported in the literature for BaFe12O19 system in CH4 combustion.56 In contrast to the normalized rate in LF1A-1200 (1.0 × 10−3 mol N2O h−1 m−2), calcination at 1200−1400 °C resulted in a higher rate for LF5A catalysts (1.2 × 10−3 mol N2O h−1 m−2). It is noteworthy that the latter samples possessed active Fe species in Al(5) (15%) slightly less than that in the former one (20%). This suggested that there existed another factor in promoting the N2O catalytic decomposition activity. Considering the appearance of Fe3+ ions in the Al(3) sites (27%), this higher rate of LF5A catalysts should be accounted for by the introduction of Fe3+ ions in these octahedral sites in the mirror plane.

■

ASSOCIATED CONTENT

S Supporting Information *

Detailed calculations for mass and heat transfer limitations with the Weisz−Prater criterion, and the information for Fe K-edge XANES spectra of La-hexaaluminates. This material is available free of charge via the Internet at http://pubs.acs.org.

■

AUTHOR INFORMATION

Corresponding Author

*Tel: +86 411 84379680. Fax: +86 411 84691570. E-mail: [email protected]. Notes

The authors declare no competing financial interest.

■

ACKNOWLEDGMENTS Financial support from the National Science Foundation of China (NSFC) grants (21076211, 21206159, and 11205160) and Science Foundation of Northwest University (No12NW14) are greatly acknowledged.

■

REFERENCES

(1) Cai, G. B.; Sun, W.; Fang, J.; Li, M.; Cong, Y.; Yang, Z. Design and Performance Characterization of a Sub-Newton N2O Monopropellant Thruster. Aerosp. Sci. Technol. 2012, 23, 439−451. (2) Micci, M. M.; Ketsdever, A. D. Micropropulsion for Small Spacecraft; AIAA Press: Reston, VA, 2000. 2008



Article

(3) Reng, X. G.; Li, M. H.; Wang, A. Q.; Li, L.; Wang, X. D.; Zhang, T. Catalytic Decomposition of Hydroxyl Ammonium Nitrate at Room Temperature. Chin. J. Catal. 2007, 28, 1−2. (4) Rodhe, H. A Comparison of the Contribution of Various Gases to the Greenhouse Effect. Science 1990, 248, 1217−1219. (5) Wojtowicz, M. A.; Pels, J. R.; Moulijn, J. A. Combustion of Coal as a Source of N2O Emission. Fuel Process. Technol. 1993, 34, 1−71. (6) Zakirov, V.; Sweeting, M. N.; Lawrence, T. J.; Sellers, J. J. Nitrous Oxide as a Rocket Propellant. Acta Astronaut. 2001, 48, 353−362. (7) Zakirov, V.; Goeman, V.; Lawrence, T. J.; Sweeting, M. N. Surrey Research on Nitrous Oxide Catalytic Decomposition for Space Applications. Proceedings of the 14th Annual AIAA/USU Conference on Small Satellites, August 21−24, 2000. (8) Machida, M.; Eguchi, K.; Arai, H. Catalytic Properties of BaMAl11O19‑α (M = Cr, Mn, Fe, Co, and Ni) for High-Temperature Catalytic Combustion. J. Catal. 1989, 120, 377−386. (9) Arai, H.; Machida, M. Thermal Stabilization of Catalyst Supports and Their Application to High-Temperature Catalytic Combustion. Appl. Catal., A 1996, 138, 161−176. (10) Eguchi, K.; Arai, H. Recent Advances in High Temperature Catalytic Combustion. Catal. Today 1996, 29, 379−386. (11) Bellotto, M.; Artioli, G.; Cristiani, C.; Forzatti, P.; Groppi, G. On the Crystal Structure and Cation Valence of Mn in Mn-Substituted Ba-β-Al2O3. J. Catal. 1998, 179, 597−605. (12) Groppi, G.; Cristiani, C.; Forzatti, P. Preparation, Characterization and Catalytic Activity of Pure and Substituted LaHexaaluminate Systems for High Temperature Catalytic Combustion. Appl. Catal., B 2001, 35, 137−148. (13) Santiago, M.; Pérez-Ramírez, J. Decomposition of N2O over Hexaaluminate Catalysts. Environ. Sci. Technol. 2007, 41, 1704−1709. (14) Pérez-Ramírez, J.; Santiago, M. Metal-substituted Hexaaluminates for High-Temperature N2O Abatement. Chem. Commun. 2007, 619−621. (15) Santiago, M.; Hevia, M. A. G.; Pérez-Ramírez, J. Evaluation of Catalysts for N2O Abatement in Fluidized-Bed Combustion. Appl. Catal., B 2009, 90, 83−88. (16) Zhu, S. M.; Wang, X. D.; Wang, A. Q.; Cong, Y.; Zhang, T. A Novel Ir-Hexaaluminate Catalyst for N2O as a Propellant. Chem. Commun. 2007, 1695−1697. (17) Tian, M.; Wang, A. Q.; Wang, X. D.; Zhu, Y. Y.; Zhang, T. Effect of Large Cations (La3+ and Ba2+) on the Catalytic Performance of MnSubstituted Hexaaluminates for N2O Decomposition. Appl. Catal., B 2009, 92, 437−444. (18) Zhu, Y. Y.; Wang, X. D.; Wang, A. Q.; Wu, G. T.; Wang, J. H.; Zhang, T. Identification of the Chemical State of Fe in Barium Hexaaluminate Using Rietveld Refinement and 57Fe Mössbauer Spectroscopy. J. Catal. 2011, 283, 149−160. (19) Zhang, Y.; Wang, X. D.; Zhu, Y. Y.; Zhang, T. Stabilization Mechanism and Crystallographic Sites of Ru in Fe-Promoted Barium Hexaaluminate under High-Temperature Condition for N2O Decomposition. Appl. Catal., B 2013, 129, 382−393. (20) Arai, H.; Eguchi, K.; Machida, M. Cation Substituted Layered Hexaaluminates for a High-Temperature Combustion Catalyst. MRS International Meeting on Advanced Materials 1989, 2, 243−253. (21) Groppi, G.; Bellotto, M.; Cristiani, C.; Forzatti, P. Thermal Evolution Crystal Structure and Cation Valence of Mn in Substituted Ba-β-Al2O3 Prepared via Coprecipitation in Aqueous Medium. J. Mater. Sci. 1999, 34, 2609−2620. (22) Groppi, G.; Assandri, F.; Bellotto, M.; Cristiani, C.; Forzatti, P. The Crystal Structure of Ba-β-Alumina Materials for High-Temperature Catalytic Combustion. J. Solid State Chem. 1995, 114, 326−336. (23) Gardner, T. H.; Spivey, J. J.; Kugler, E. L.; Campos, A.; Hissam, J. C.; Roy, A. D. Structural Characterization of Ni-Substituted Hexaaluminate Catalysts Using EXAFS, XANES, XPS, XRD, and TPR. J.Phys. Chem. C 2010, 114, 7888−7894. (24) Machida, M.; Sato, A.; Kijima, T.; Inoue, H.; Eguchi, K.; Arai, H. Catalytic Properties and Surface Modification of Hexaaluminate Microcrystals for Combustion Catalyst. Catal. Today 1995, 26, 239− 245.

(25) Machida, M.; Shiomitsu, T.; Eguchi, K.; Arai, H.; Shimizu, Y. Observation of Anisotropic Oxygen Diffusion in Hexaaluminate. J. Solid State Chem. 1991, 95, 220−223. (26) Yin, F.; Ji, S.; Wu, P.; Zhao, F.; Li, C. Preparation, Characterization, and Methane Total Oxidation of AAl12O19 and AMAl11O19 Hexaaluminate Catalysts Prepared with Urea Combustion Method. J. Mol. Catal. A 2008, 294, 27−36. (27) Lietti, L.; Cristiani, C.; Groppi, G.; Forzatti, P. Catalytic Combustion of NH3-Containing Gasified Biomasses over MeHexaaluminates (Me = Mn, Co, Fe, Ni, Cr) Catalysts. Catal. Today 2000, 59, 191−204. (28) Xu, Z.; Zhen, M.; Bi, Y.; Zhen, K. Carbon Dioxide Reforming of Methane to Synthesis Gas over Hexaaluminate ANiAl11O19−δ (A = Ca, Sr, Ba and La) Catalysts. Catal. Lett. 2000, 64, 157−161. (29) Utaka, T.; Al-Drees, S. A.; Ueda, J.; Iwasa, Y.; Takeguchi, T.; Kikuchi, R.; Eguchi, K. Partial Oxidation of Methane over Ni Catalysts Based on Hexaaluminate- or Perovskite-Type Oxides. Appl. Catal., A 2003, 247, 125−131. (30) Kikuchi, R.; Iwasa, Y.; Takeguchi, T.; Eguchi, K. Partial Oxidation of CH4 and C3H8 over Hexaaluminate-Type Oxides. Appl. Catal., A 2005, 281, 61−67. (31) Artizzu-Duart, P.; Brullé, Y.; Gaillard, F.; Garbowski, E.; Guilhaume, N.; Primet, M. Catalytic Combustion of Methane over Copper- and Manganese-Substituted Barium Hexaaluminates. Catal. Today 1999, 54, 181−190. (32) Astier, M.; Garbowski, E.; Primet, M. BaMgAl10O17 as Host Matrix for Mn in the Catalytic Combustion of Methane. Catal. Lett. 2004, 95, 31−37. (33) Johansson, M. Catalytic Combustion of Gasified Biomass for Gas Turbine Applications; Ph.D. Thesis, Royal Institute of Technology, Department of Chemical Engineering and Technology, Chemical Technology, KTH, Stockholm, Sweden, 1998. (34) Jang, B. M. L.; Nelson, R. M.; Spivey, J. J.; Ocal, M.; Oukaci, R.; Marcelin, G. Catalytic Oxidation of Methane over Hexaaluminates and Hexaaluminate-Supported Pd Catalysts. Catal. Today 1999, 47, 103− 113. (35) Zhai, Y. Q.; Meng, M.; Chen, J. L.; Li, Y. D. Influences of La3+ and Ba2+ as Mirror Plane Cations on Structure, Thermal Stability of Hexaaluminates and Its Catalytic Combustion Activity for Methane. Chin. J. Appl. Chem. 2005, 22, 320−325. (36) Groppi, G.; Christiani, C.; Forzatti, P. Preparation and Characterization of Hexaaluminate Materials for High-Temperature Catalytic Combustion. Catalysis: A Specialist Periodical Report; Royal Sociaty of Chemistry: London, 1997, 13, 85−113. (37) Groppi, G.; Cristiani, C.; Forzatti, P.; Bellotto, M. Phase Composition and Mechanism of Formation of Ba-β-alumina-Type Systems for Catalytic Combustion Prepared by Precipitation. J. Mater. Sci. 1994, 29, 3441−3450. (38) Xu, J. G. Studies on the Preparation of High Temperature Resistant Catalytic Materials for Methane Combustion; Doctoral Dissertation, Dalian Institute of Chemical Physics, Chinese Academy of Sciences, Dalian, China, 2003. (39) Laville, L.; Gourier, D.; Lejus, A. M.; Vivien, D. Optical and ESR Investigations of Lanthanum Aluminates LaMg1−xMnxO19 Single Crystals with Magnetoplumbite-Like Structure. J. Solid State Chem. 1983, 49, 180−187. (40) Park, J.-G.; Cormack, A. N. Defect Energetics and Nonstoichiometry in Lanthanum Magnesium Hexaaluminate. J. Solid State Chem. 1997, 130, 199−212. (41) Tronc, E.; Laville, F.; Gaspetin, M.; Lejus, A. M.; Vivien, D. Structural and Spectroscopic Investigations of LaFeAl11O19 Compounds. J. Solid State Chem. 1989, 81, 192−202. (42) Groppi, G.; Bellotto, M.; Cristiani, C.; Forzatti, P.; Villa, P. L. Preparation and Characterization of Hexaaluminate-Based Materials for Catalytic Combustion. Appl. Catal., A 1993, 104, 101−108. (43) Kresse, G.; Hafner, J. Ab Initio Molecular Dynamics for Liquid Metals. Phys. Rev. B 1993, 47, 558−561. 2009



Article

(66) Cótica, L. F.; Zanatta, S. C.; Rocha, M. A.; Santos, I. A.; Paesano, A., Jr.; da Cunha, J. B. M.; Hallouche, B. Phase Evolution and Magnetic Properties of a High-Energy Ball-Milled Hematite-Alumina System. J. Appl. Phys. 2004, 95, 1307−1315. (67) Petrović, S.; Terlecki-Baričević, A.; Karanović, Lj.; KirilovStefanov, P.; Zdujić, M.; Dondur, V.; Paneva, D.; Mitov, I.; Rakić, V. LaMO3 (M = Mg, Ti, Fe) Perovskite Type Oxides: Preparation, Characterization and Catalytic Properties in Methane Deep Oxidation. Appl. Catal., B 2008, 79, 186−198. (68) Iyi, N.; Takekawa, S.; Kimura, S. Crystal Chemistry of Hexaaluminates: β-Alumina and Magnetoplumbite Structures. J. Solid State Chem. 1989, 83, 8−19. (69) Wells, A. F. Structural Inorganic Chemistry Clarendon; Clarendon Press: Oxford, U.K., 1975. (70) Kapteijn, F.; Rodriguez-Mirasol, J.; Moulijn, J. A. Heterogeneous Catalytic Decomposition of Nitrous Oxide. Appl. Catal., B 1996, 9, 25−64. (71) Smeets, P. J.; Sels, B. F.; Van Teeffelen, R. M.; Leeman, H.; Hensen, E. J. M.; Schoonheydt, R. A. The Catalytic Performance of Cu-Containing Zeolites in N2O Decomposition and the Influence of O2, NO and H2O on Recombination of Oxygen. J. Catal. 2008, 256, 183−191. (72) Pérez-Ramírez, J.; Kapteijn, F.; Mul, G.; Moulijn, J. A. NOAssisted N2O Decomposition over Fe-Based Catalysts: Effects of GasPhase Composition and Catalyst Constitution. J. Catal. 2002, 208, 211−223. (73) Zhu, Y. Y.; Wang, X. D.; Wu, G. T.; Huang, Y. Q.; Zhang, Y.; Wang, J. H.; Zhang, T. Evolution of Fe Crystallographic Sites from Barium Hexaaluminate to Hexaferrite. J. Phys. Chem. C 2012, 116, 671−680.

(44) Kresse, G.; Hafner, J. Ab Initio Molecular-Dynamics Simulation of the Liquid-Metal-Amorphous-Semiconductor Transition in Germanium. Phys. Rev. B 1994, 49, 14251−14269. (45) Kresse, G.; Furthmuller, J. Efficiency of Ab-Initio Total Energy Calculations for Metals and Semiconductors Using a Plane-Wave Basis Set. Comput. Mater. Sci. 1996, 6, 15−50. (46) Kresse, G.; Furthmuller, J. Efficient Iterative Schemes for Ab Initio Total-Energy Calculations Using a Plane-Wave Basis Set. Phys. Rev. B 1996, 54, 11169−11186. (47) Blöchl, P. E. Projector Augmented-Wave Method. Phys. Rev. B 1994, 50, 17953−17979. (48) Kresse, G.; Joubert, D. From Ultrasoft Pseudopotentials to the Projector Augmented-Wave Method. Phys. Rev. B 1999, 59, 1758− 1775. (49) Perdew, J. P.; Burke, K.; Ernzerhof, M. Generalized Gradient Approximation Made Simple. Phys. Rev. Lett. 1996, 77, 3865−3868. (50) Monkhorst, H. J.; Pack, J. D. Special Points for Brillouin-Zone Integrations. Phys. Rev. B 1976, 13, 5188−5192. (51) Dudarev, S. L.; Botton, G. A.; Savrasov, S. Y.; Szotek, Z.; Temmerman, W. M.; Sutton, A. P. Electronic Structure and Elastic Properties of Strongly Correlated Metal Oxides from First Principles: LSDA + U, SIC-LSDA and EELS Study of UO2 and NiO. Phys. Status Solidi A 1998, 166, 429−443. (52) Rollmann, G.; Rohrbach, A.; Entel, P.; Hafner, J. First-Principles Calculation of the Structure and Magnetic Phases of Hematite. Phys. Rev. B 2004, 69, 165107−165119. (53) Mears, D. E. Tests for Transport Limitations in Experimental Catalytic Reactors. Ind. Eng. Chem. Process Des. Dev. 1971, 10, 541− 547. (54) Vannice, M. A. Kinetics of Catalytic Reactions; Springer Press: New York, 2005. (55) Fogler, H. S. Elements of Chemical Reaction Engineering; Prentice Hall: New York, 2005. (56) Groppi, G.; Cristiani, C.; Forzatti, P. BaFexAl(12−x)O19 System for High-Temperature Catalytic Combustion: Physico-Chemical Characterization and Catalytic Activity. J. Catal. 1997, 168, 95−103. (57) Belous, A. G.; Pashkova, E. V.; Kravchyk, K. V.; Ivanitskii, V. P.; V’yunov, O. I. Mössbauer and X-ray Diffraction Studies of Cubic Solid Solutions of the ZrO2−Y2O3−Fe2O3 System. J. Phys. Chem. C 2008, 112, 3914−3919. (58) Glasser, F. P.; Woodhams, F. W. D.; Meads, R. E.; Parker, W. G. A; Mössbauer, E. S. R. Study of CaO·6Al2O3-CaO·6Fe2O3 Solid Solutions. J. Solid State Chem. 1972, 5, 255−261. (59) Iyi, N.; Inoue, Z.; Takekawa, S.; Kimura, S. The Crystal Structure of Barium Hexaaluminate Phase I (Barium β-Alumina). J. Solid State Chem. 1984, 52, 66−72. (60) Iyi, N.; Inoue, Z.; Takekawa, S.; Kimura, S. The Crystal Structure of Lanthanum Hexaaluminate. J. Solid State Chem. 1984, 54, 70−77. (61) Gasperin, M.; Saine, M. C.; Kahn, A.; Laville, F.; Lejus, A. M. Influence of M2+ Ions Substitution on the Structure of Lanthanum Hexaaluminates with Magnetoplumbite Structure. J. Solid State Chem. 1984, 54, 61−69. (62) Modaressi, A.; Courtois, A.; Gerardin, R.; Malaman, B.; Gleitzer, C. Fe3PO7, Un Cas De Coordinence 5 Du Fer Trivalent, Etude Structurale et Magnetique. J. Solid State Chem. 1983, 47, 245−255. (63) Wang, X. D.; Zhao, X. Q.; Shen, J. Y.; Sun, X. Y.; Zhang, T.; Lin, L. W. A Mössbauer Study of In−Fe2O3/HZSM-5 Catalysts for the Selective Catalytic Reduction of NO by Methane. Phys. Chem. Chem. Phys. 2002, 4, 2846−2851. (64) Cótica, L. F.; Zanatta, S. C.; de Medeiros, S. N.; Santos, I. A.; Paesano, A., Jr.; da Cunha, J. B. M. Mechanical Milling of the (αFe2O3)x(α-Al2O3)1−x System: An X-ray Diffraction and Mössbauer Spectral Study. Solid State Ionics 2004, 171, 283−288. (65) Vaishnava, P. P.; Ktorides, P. I.; Montano, P. A.; Mbadcam, K. J.; Melson, G. A. In Situ Mössbauer Spectroscopic Characterization of FeAl2O3 and FeThO2 Fischer−Tropsch Catalysts. J. Catal. 1985, 96, 301−313. 2010


Characterization of Fe Substitution into La-Hexaaluminate Systems

Recommend Documents