Thermal Evolution Crystal Structure and Fe Crystallographic Sites in

Article pubs.acs.org/JPCC

Thermal Evolution Crystal Structure and Fe Crystallographic Sites in LaFexAl12−xO19 Hexaaluminates Yan Zhang,†,‡ Xiaodong Wang,*,† Yanyan Zhu,§ Xin Liu,† and Tao Zhang† †

State Key Laboratory of Catalysis, Dalian Institute of Chemical Physics, Chinese Academy of Sciences, Zhongshan Road 457, Dalian 116023, P. R. China ‡ University of Chinese Academy of Sciences, 19A Yuquanlu, Beijing 100049, P. R. China § College of Chemical Engineering, Northwest University, Xi’an 710069, P. R. China S Supporting Information *

ABSTRACT: Compared with BaFe12O19 hexaferrite, La-magnetoplumbite (MP) hexaaluminates with a high-substituted concentration have rarely been investigated. LaFexAl12−xO19 catalysts at various Fe loadings (x = 0−12) were synthesized by coprecipitation. Thermal evolution of the samples from 700 up to 1400 °C has been followed by X-ray diffractometry. Fe distribution in different crystallographic sites in the final MP structure was clarified by means of Rietveld refinement and roomtemperature and low-temperature 57Fe Mössbauer spectroscopy. The results showed that Fe doping (x = 1−9) lowered the threshold temperature of La-hexaaluminates. When x ≥ 6, the segregation of LaFeO3 intermediates essentially hindered the formation of monophasic MP phase and even led to the absence of the MP structure when x = 12. Compared with the unsubstituted sample, Fe-substituted La-hexaaluminates (x ≤ 11) were more stoichiometric. When x = 1−3, Fe preferentially entered the Al(2) sites within spinel blocks and the Al(3) and Al(5) sites in mirror planes. Increasing x to ≥4, Al(1) and Al(4) sites in the spinel block started to be systematically occupied up to x = 11. The reasonably constant turnover frequencies ((0.16 to 0.47) × 102 h−1 m−2) indicated a correlation between the N2O catalytic decomposition performance and Fe3+ ions in the mirror plane (Al(3), Al(5)).

1. INTRODUCTION Metal-substituted hexaaluminates were regarded as promising materials for high-temperature applications in view of their excellent thermal stability and high catalytic activity. The exceptional resistance to sintering has been related to their peculiar layered structure consisting of alternatively stacked spinel blocks and mirror planes.1−4 The general formula of metal-substituted hexaaluminates can be presented by AMxAl11−xO17‑α or AMxAl12−xO19‑α, corresponding to β-Al2O3 or magnetoplumbite (MP) structures (P63/mmc, No. 194), where A stands for the large cations (Ba, La, Sr, etc.) and M presents substituted transition-metal ions (Mn, Fe, Co, etc.) in Al crystallographic sites. The structure type of hexaaluminates is determined by the charge and radius of the large cations (A) in the mirror plane. For example, Ba-hexaaluminates usually correspond to the β-Al2O3, and La-hexaaluminates belong to the MP structure. © 2014 American Chemical Society

What was reported so far typically refers to the transitionmetal-substituted Ba-hexaaluminates, in which Mn and Fe are regarded as the most thermally stable and active promoters for methane catalytic combustion1,5 and high-temperature N2O decomposition.6,7 Artizzu-Duart et al.8 and Astier et al.9 reported that the most reactive Mn species in Al(5) interstitial sites near the mirror plane in Ba-hexaaluminate accounted for the catalytic activity for methane combustion. Naoufal et al.10 tentatively attributed the methane combustion activity to two slightly different octahedrally coordinated Fe3+ ions. Moreover, our previous work11 directly related the high activity of N2O decomposition to the Fe3+ ions in Al(5) sites in the mirror plane of β-Al2O3 structure and Al(3) sites in the mirror plane of Received: January 20, 2014 Revised: April 25, 2014 Published: April 29, 2014 10792

dx.doi.org/10.1021/jp500682d | J. Phys. Chem. C 2014, 118, 10792−10804

The Journal of Physical Chemistry C

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sites in the hexagonal structure of LaFe12O19 ferrite.23 However, to the best our knowledge, no systematic investigations have been performed either on how Fe content influence the thermal evolution of lanthanum hexaaluminate or on the gradual substitution of crystallographic Al sites by Fe ions in the final La-MP structure. In this respect, LaFexAl12−xO19 (x = 0−12, step =1) prepared via an aqueous coprecipitation route was employed in this paper. Phase compositions and morphology properties of the materials have been monitored as a function of Fe introduction. Fe crystallographic sites in the substituted La-hexaaluminates were also investigated by means of Rietveld refinement and 57Fe Mössbauer spectroscopy. To suppress the relaxation effects induced by Fe incorporation (x = 6−11), Mössbauer spectra were herein recorded at relatively low temperature (80 K). Clear separation of the subspectra and useful data on the cationic distribution among the various sublattices were thus obtained. This chemical state information on framework Fe was finally correlated with the activity of the catalysts in high-concentration N2O decomposition, with the aim to rationalize the catalytic properties of different Fe crystallographic sites in the mirror plane.

MP. These studies strongly indicate that the substituted metal ions (M) in the hexaaluminate structure are generally regarded as active centers in the reactions, and those inlaid in the mirror plane should be more reactive. Therefore, a high-substituted concentration of transition metals in the hexaaluminate structure, especially inlaid in the crystallographic site in the mirror plane, is desired. Machida et al.1 and Groppi et al.12 investigated the BaMnxAl12−xO19 combustion catalysts and revealed that manganese substitution for aluminum was possible up to x ≈ 3. Further increasing the metal concentration resulted in extra phases outside of barium hexaaluminate, such as metal oxide, perovskite, spinel, and so on, which easily aggregated and led to the degradation in catalytic activity. Partial substitution of Al ions was also observed in La-hexaaluminates with MP structure. Xu et al.13 reported that the Ni-modified LaNiyAl12−yO19 sample consisted of single hexaaluminate only at y ≤ 1 and a mixture of hexaaluminate and Ni-containing extraphases (NiO, NiAl2O4) at y > 1. Wang et al.14 and Tian et al.15 studied Mnsubstituted materials and reported that the monophasic LaMnxAl12−xO19 samples were obtained up to x = 1 and that the formation of LaMnO3 occurred upon further Mn addition. Such behavior may be associated with the limited capability of hexaaluminate to incorporate foreign metal ions in the structure.16 In contrast, no limits on the Fe-content were found in the substituted Ba-hexaaluminates, and a completely substituted BaFe12O19 with MP structure was obtained.17,18 Groppi et al.17 concluded that the formation of this monophasic MP material occurred via an easier diffusion of Ba ions inside γ-Fe2O3. Zhu et al.18 attributed the stabilization of high Fe concentration in Ba-hexaaluminate structure to the phase transformation from β-Al2O3 to MP. It seems that the MP phase could accommodate more Fe ions than β-Al2O3. 57 Fe Mö ssbauer spectroscopy is a sensitive and wellestablished technique to observe the local environment of Fe ions in the iron-based compounds. Lee et al.19 prepared SrFe12O19 using a sol−gel method, and the Mössbauer analysis showed that the γ-Fe2O3 intermediate crystalline phase was the only feature in the precursors, which transformed to the final hexaferrite at and above 600 °C, in line with the observation for BaFe12O19 sample prepared by coprecipitation.17 Zhu et al.18 identified the Fe crystallographic sites in BaFexAl12−xO19 by the room-temperature 57Fe Mössbauer spectroscopy, whereas the features of relaxation effects occurred when x ≥ 4 and attempts to fit the spectra failed. For Fe-substituted La-hexaaluminates, the Mössbauer spectroscopic measurements of LaFeAl11O1920 confirmed that Fe2+ ions were localized in the tetrahedral sites, while Fe3+ occupied both tetrahedral and bipyramidal sites. Zheng et al.21 prepared a series of Fe-substituted LaMnFexAl11−xO19 samples by carbonate precipitation up to x = 8, whereas no further investigations of the chemical state of Fe were performed. This may be due to the superparamagnetic relaxation in the samples derived from the Fe incorporation, which make the characterizations difficult. Moreover, few studies of La-hexaferrite have been carried out because of the difficulties in preparing. Küpferling et al.22 attempted to synthesis the LaFe12O19 hexaferrite using three different methods (mechanical alloying, coprecipitation, and sol−gel) with a La excess (2−100%). The resulted mixtures consisted of LaFe12O19 with a large amount of LaFeO3, Fe2O3, and Fe3O4 extraphases, whereas the La-hexaferrite phase could not be preserved without quenching. The Mössbauer measurements by the same authors showed that Fe ions occupied five different

2. EXPERIMENTAL SECTION 2.1. Catalyst Preparation. Samples with nominal composition LaFexAl12−xO19 (x = 0−12, step = 1) have been prepared using the coprecipitation method described elsewhere.12,24 Typically, to prepare LaFeAl11O19 (x = 1), we dissolved La(NO3)3, Fe(NO3)3·9H2O, and Al(NO3)3·9H2O with the molar ratio 1/1/11 in 100 mL of deionized water to form a 1 mol L−1 solution and then precipitated them in a saturated aqueous solution of (NH4)2CO3 (225 mL). The catalyst precursor was formed after vigorous stirring in an open flask at 60 °C for 6 h and aging for 3 h. Upon filtering, washing, and drying the precipitate overnight at 120 °C, the obtained precursor materials were calcined at 700, 900, 1000, 1100, 1200, 1300, and 1400 °C for 4 h each. The inductively coupled plasma (ICP) analysis for mother liquors and calcined samples confirmed the quantitative Fe/Al ratios and a slight loss in La during calcination. 2.2. Catalyst Characterization. X-ray powder diffraction (XRD) patterns were measured using Ni-filtered Cu Kα radiation (Kα1,λ = 1.54056 Å; Kα2,λ = 1.54439 Å; Kα2/Kα1 = 0.5) and a PANalytical X’Pert-Pro powder X-ray diffractometer in an angular range 2θ = 10−80°. Crystallite sizes were calculated from the Scherrer equation. FULLPROF and RIETICA programs were employed to identify the phase composition and atom crystallographic sites, with references (ICSD entries 38371, 203003, and 35174) as the baseline patterns. Further details were reported in the previous work.24 N2 adsorption−desorption isotherms have been measured on a Micromeritics ASAP 2010 apparatus, and the total surface area of the samples were calculated with the Brunauer− Emmett−Teller (BET) method. Scanning electron microscopy (SEM) was carried out using a JSM 6360-LV electron microscope operated at 20−25 kV. The Mössbauer effect spectra of naturally abundant 57Fe in LaFexAl12−xO19 samples have been measured at room temperature and at 80 K with a spectrometer working at constant acceleration using a Rh57Co source. The lower temperature was obtained by thermal contact of absorber with the liquid nitrogen reservoir. Absorbers were powdered samples with 10 mg cm−2 of 57Fe pressed between two beryllium windows. The spectrometer was calibrated with a standard α-Fe foil, and the 10793



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sample (x = 0), while amounts of well-crystallized LaAlO3 were still evident (1200−1300 °C). This easier formation of hexaaluminate phase in x = 1 and 3 could be related to the activation of La3+ mobility in γ-Al2O3 matrix, in line with Mnsubstituted La-MP hexaaluminate.25 The enlargement of a (a = b) lattice parameter of MP phase upon thermal treatment in x = 3 (5.6063 Å at 1000 °C versus 5.6510 Å at 1100 °C) further demonstrated that Fe ions incorporated into the hexaaluminate structure. For La-hexaaluminate, Groppi et al.25 and Xu et al.26 reported that the final MP formed via the reaction between γAl2O3 and dispersed lanthanum. An analogous mechanism could be reasonably hypothesized for Fe-substituted Lahexaaluminate herein investigated at low Fe content (x = 1, 3), that is, Fe ions in oxidic entities incorporated into the hexaaluminate structure through La3+ diffusion in the γ-Al2O3 matrix. When x was increased to 6, the formation of MP phase proceeded to a large extent at 1100 °C, which was accompanied by the decrease in α-Fe2O3 reflections and the more evident LaFeO3 segregations. Further calcination at 1200 °C brought about the formation of monophasic MP hexaaluminate, while the diffraction peaks associated with α-Fe2O3 and LaFeO3 completely disappeared. From the above results, we deduced that Fe species in α-Fe2O3 and LaFeO3 transferred into the MP lattice, which was further evidenced by the increased a value of MP phase from 5.6520 (x = 6 at 1100 °C) to 5.7275 Å (1200 °C). Accordingly, the formation of Fe-substituted Lahexaaluminate at high Fe concentration (x = 6) can be concluded in two routes, namely, solid-state reactions between γ-Al2O3 and dispersed La species, or LaFeO3. These results are consistent with the conclusions in our previous contributions.27 However, higher threshold temperature (1200 °C) was required to obtain the final monophasic MP phase in x = 6 than that in x = 1 and 3 samples (1100 °C), which could be attributed to the formation of LaFeO3 intermediates that essentially suppressed the mobility of La species. Similar thermal evolutions were observed in x = 9 and 11 samples, whereas modulations associated with residual α-Fe2O3 in x = 9 and α-Fe2O3 and LaFeO3 in x = 11 were still verified, even under calcination at 1300 °C. In particular, no evidence of the MP phase was observed when x = 12, and calcination at 1200− 1300 °C only resulted in a higher crystallization of the α-Fe2O3 and LaFeO3 phases. This is different from the case in barium hexaaluminate systems with no Ba-containing perovskite/ spinel-type intermediates during thermal evolution, and the completely substituted BaFe12O19 was obtained starting from 700 °C calcination.17 The XRD spectra of LaFexAl12−xO19 (x = 0−12) samples calcined at 1400 °C are reported in Figure 1. At this temperature, segregations of LaAlO3 were still observed when x = 0. The poorly evident MP reflections could be attributed to the presence of the defective cells that showed a high disorder of the crystal lattice in unsubstituted La-hexaaluminates.28 For x = 1−7 samples, diffraction peaks corresponding to a wellcrystallized MP phase were the only features in the XRD patterns. However, small reflections associated with the residual amount of α-Fe2O3 were also detected when x = 8−11. Further increasing Fe loading to x = 12, the MP phase was not obtained even upon calcination at 1400 °C. Figure 2 displays the XRD patterns of LaFexAl12−xO19 (x = 0−11) samples calcined at 1400 °C in the 30−40° (2θ) range. It clearly showed that the X-ray lines were continuously shifted to lower diffraction angles with the increase in x, corresponding to the enlargement of the

spectra were presented relative to the center of the spectrum of α-Fe reference. All spectra were computer-fitted using the MossWinn fitting package. The Fe K-edge extended X-ray absorption fine structure (EXAFS) was measured on beamline BL14W of the Shanghai Synchrotron Radiation Facility (SSRF, China), with the storage ring operating at 3.5 GeV beam energy and a stored beam current of 140−210 mA. A Si (111) double-crystal monochromator was used, and the energy was scanned from 200 eV below to 800 eV above the Fe K-edge (7112 eV). Fe foil was used as reference sample, and all spectra were measured in the transmission mode at room temperature. 2.3. Catalytic Activity Test. Catalytic decomposition of N2O was performed in a conventional fixed-bed flow system under atmospheric pressure. Catalyst (100 mg, 20−40 mesh) diluted with 400 mg quartz beads of the same particle size was loaded in a quartz microreactor (i.d. = 7 mm), with quartz fiber packed upstream and downstream of the catalyst bed. A mixture of 30 vol % N2O in Ar balance was fed into the catalyst bed at a flow rate of 50 mL min−1 (GHSV = 30 000 mL h−1 g−1 cat , STP). The reaction effluent was analyzed online using an Agilent 6890N gas chromatograph equipped with a TCD, a Porapak Q column for N2O monitoring and a Molecular Sieve 13X for O2 and N2 separation.

3. RESULTS AND DISCUSSION 3.1. X-ray Diffraction Analysis and Morphology. The phase composition of LaFexAl12−xO19 (x = 0, 1, 3, 6, 9, 11, 12) samples calcined at different temperatures was characterized by XRD, and the results are shown in Figure S1 (Supporting Information). When x = 0−3, calcination at 700 °C resulted in amorphous structure. However, for x ≥ 6, a set of reflections corresponding to α-Fe2O3 [JCPDS 1-84-0310] was observed due to the increase in Fe content. No evidence of lanthanum compound was identified, indicating the high dispersion of La species in x = 0−11 samples. In particular, when x = 12, reflections of LaFeO3 [JCPDS 1-75-439] originating from the thermal reaction between La species and α-Fe2O3 were also detected. Obviously, Fe content influenced the intermediate phase composition in the precursors. After calcination at 900 °C, modulations of the XRD baseline ascribable to γ-Al2O3 [JCPDS 10-425] were observed in x = 0 and 1. No iron oxide was detected in x = 1 sample, indicating the highly dispersed Fe species on La-modified γ-Al2O3. Meanwhile, reflections of α-Fe2O3 were evident in x = 3 due to the increased particle size of Fe2O3 (from 75%). It clearly indicated the preferential Fe occupation in the Al(2) tetrahedral sites of the spinel block and the Al(3) octahedral and Al(5) bipyramidal sites of the mirror plane. Further increasing Fe content (x = 9 and 11), besides the gradual increase in Fe occupancies in Al(2), Al(3), and Al(5) crystallographic sites of the MP structure, Al(1) and Al(4) sites in the spinel block were also largely occupied. 3.4. 57Fe Mö ssbauer Spectroscopy. Mössbauer hyperfine parameters can provide extremely useful microscopic information on various oxygen polyhedra in the structure. Figure 7 shows the room-temperature 57Fe Mössbauer spectra of LaFexAl12−xO19 (x = 1−12) calcined at 1400 °C. The fitted hyperfine parameters such as isomer shift (IS), quadrupole splitting (QS), and magnetic hyperfine field (H) are shown in Table 4. When x = 1−5, the IS values of 0.18 to 0.27 mm s−1 typical of Fe3+ in tetrahedral coordination20,32 were related to the occupancy of Fe3+ ions in Al(2) and Al(5) sites in the La-MP structure. Among them, the highly distorted trigonal bipyramidal (denoted as Tr, i.e. pseudotetrahedral) Al(5) sites in the mirror plane were expected to show larger QS value (2.29 to 2.48 mm s−1) than that observed for the symmetric tetrahedral (Th) Al(2) sites (QS = 0.56 to 0.70 mm s−1) in the spinel block. In addition, the subspectra (6−7%) with parameters typical of Fe2+ in the tetrahedral sites33 were observed in x = 1 and 2 samples, which was associated with the thermal reduction of Fe3+ at high temperature, in line with the XANES results (Figure 4). In particular, the isomer shift values (0.32 to 0.38 mm s−1) higher than that of Fe3+(Th) fell in the range of octahedral coordinations (IS = 0.30 to 0.50 mm s−1),34 indicating the distribution of Fe3+ in octahedral (Oh) sites of

Figure 6. X-ray Rietveld refined patterns of LaFexAl12−xO19 samples calcined at 1400 °C for (a) x = 1, (b) x = 6, and (c) x = 11.

the MP structure when x = 2−5. The relative area (A) of these Fe3+(Oh) ions increased progressively with Fe content (from 18% at x = 2 to 54% at x = 5), demonstrating that more Fe ions incorporated into the hexaaluminate lattice by occupying the octahedral sites of MP phase, which was in accordance with the Rietvled investigations (Table 3). Drastic modifications in the Mössbauer spectra can be noted when 6 ≤ x ≤ 11. The broad absorptions attributed to superparamagnetic relaxations indicated the enhanced longrange magnetic interactions, which were related to the increasing substitution of Fe ions in the MP structure. The individual subspectra due to different sites therefore could not be fitted. When x = 12, the spectrum consisted of two sextets with narrow lines. Considering the XRD results (Figure 1), the 10797



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Table 2. Results of Rietveld Refinement over LaFexAl12−xO19 (x = 1-11) Samples Calcined at 1400 °C cell parameters (Å) of MP Rp

phase proportion (w/w)

composition of MP phasea

a=b

c

1 2 3 4 5 6 7 8

7.669 10.933 10.609 8.876 9.210 9.454 9.355 9.396

La0.88Fe1.00Al11.00O19 La0.88Fe1.92Al10.08O19 La0.88Fe2.84Al9.16O19 La0.88Fe3.98Al8.02O19 La0.88Fe4.96Al7.04O19 La0.89Fe6.09Al5.91O19 La0.89Fe7.03Al4.97O19 La0.88Fe7.89Al4.11O19

5.5926(6) 5.6231(4) 5.6510(4) 5.6770(6) 5.7021(5) 5.7275(01) 5.7518(5) 5.7745(6)

22.081(1) 22.172(5) 22.269(8) 22.350(5) 22.440(9) 22.523(97) 22.596(4) 22.673(7)

x=9

8.826

La0.88Fe9.02Al2.98O19

5.7971(2)

22.763(8)

x = 10

9.189

La0.87Fe9.97Al2.03O19

5.8198(6)

22.846(9)

x = 11

8.718

100% MP 100% MP 100% MP 100% MP 100% MP 100% MP 100% MP 98.96 ± 1.13% MP 1.04 ± 0.08% Fe2O3 96.39 ± 1.04% MP 3.61 ± 0.08% Fe2O3 93.12 ± 1.27% MP 6.88 ± 0.12% Fe2O3 90.69 ± 1.35% MP 9.31 ± 0.17% Fe2O3

La0.86Fe10.87Al1.13O19

5.8436(2)

22.943(6)

samples x x x x x x x x

a

= = = = = = = =

Space group: P63/mmc.

Table 3. Refined Sites Occupancya of La, Fe, and Al in the MP Structureb over LaFexAl12−xO19 (x = 1, 3, 6, 9, and 11) Samples Calcined at 1400 °C atom site

mult.c

site

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(2) O(3) O(4) O(5)

2 6 12 4 4 2 4 12 4 4 2 4 6 12 12 4 4

2d 6h 12k 4f 4f 2a 4e 12k 4f 4f 2a 4e 6h 12k 12k 4e 4f

coor.d

block

x=1

x=3

x=6

x=9

x = 11

Ohf Thh Oh Oh Tri Oh Th Oh Oh Tr

Me M Sg S M S M S S M S M

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

0.496(8) 0.128(1) 0.984(3) 0.333(3) 0.609(2) 1.00 0.187(1) 0.016(05) 0.667(6) 0.391(1) 0.00 0.313(2) 1.00 1.00 1.00 1.00 1.00

0.394(7) 0.166(5) 0.698(2) 0.173(06) 0.127(03) 0.893(8) 0.112(2) 0.302(1) 0.827(3) 0.873(2) 0.107(1) 0.388(6) 1.00 1.00 1.00 1.00 1.00

0.253(01) 0.209(1) 0.413(1) 0.025(1) 0.013(01) 0.307(4) 0.059(07) 0.587(2) 0.975(39) 0.987(8) 0.693(9) 0.441(5) 1.00 1.00 1.00 1.00 1.00

0.201(7) 0.219(4) 0.169(1) 0.004(01) 0.008(02) 0.006(02) 0.043(2) 0.831(4) 0.996(33) 0.992(27) 0.994(32) 0.457(19) 1.00 1.00 1.00 1.00 1.00

a

Numbers in parentheses refer to the standard deviations. bSpace group: P63/mmc (No. 194). cMultiplicity. dCoordination. eMirror plane. Octahedral. gSpinel block. hTetrahedral. iTrigonal bipyramidal (distorted tetrahedral, one of the two pseudotetrahedral sites of the bipyramid). Constrains on occupancies of the MP phase, Al(1) + Fe(1) = 1; Al(2) + Fe(2) = 1; Al(3) + Fe(3) = 1; Al(4) + Fe(4) = 1; and Al(5) + Fe(5) = 0.5. f

sublattices associated with Fe3+ ions in octahedral sites (Al(1), Al(3), and Al(4)) could also be clearly separated. Among them, the hyperfine fields corresponding to sextets with the largest values (487−518 kOe, Table 4) were comparable to that of the octahedral Al(3) sites in MP-hexaaluminate, which were located in the mirror plane of the unit cell. The sextets with the lowest hyperfine fields (366−410 kOe, Table 4) were attributed to the octahedral Al(4) sites. When x = 9−11, the enhanced hyperfine fields corresponded to an increase in the exchange coupling of Fe3+ ions with their magnetic neighborhood, indicating a systematic increase replacement of nonmagnetic Al3+ by magnetic Fe3+ ions in the MP structure. Moreover, a new sextet with the highest hyperfine field values (533−534 kOe) was also observed. Taking into account the appearance of αFe2O3 in XRD patterns (Figure 1), the new doublet was attributed to Fe3+ in α-Fe2O3 structure. In addition, the QS values (0.37 to 0.40 mm s−1) corresponding to Fe2O3 in x = 9−

discerned sextets were attributed to the resonance absorption of Fe3+ ions in hematite (α-Fe2O3) and orthoferrite (LaFeO3) structures, respectively (Table 4), which was well consistent with the parameter values reported in literatures.35,36 To get more information about Fe3+ distributions among various sublattices experimentally, low-temperature Mössbauer spectroscopy was performed for LaFexAl12−xO19 (x = 6−11) samples at 80 K. The spectra were magnetically split, and the relaxation effects observed at room temperature disappeared, as shown in Figure 8. Table 4 presents the hyperfine parameters, which were fitted according to the model described in refs 37−39. When x = 6−8, all spectra were represented by five discrete sextets, corresponding to five different crystallographic sites (Al(1), Al(2), Al(3), Al(4), and Al(5)) in the MP structure (Figure 5). Similar parameters of Al(2) and Al(5) sublattices to those in x = 1−5 at room-temperature made the corresponding sextets clearly distinguishable. The remaining 10798



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Figure 7. Room-temperature 57Fe Mössbauer spectra of LaFexAl12−xO19 (x = 1−12) samples calcined at 1400 °C.

11 were much higher than that of the bulk Fe2O3 (∼ −0.20 mm s−1),35 which can be attributed to the perturbed local environment due to the strong interaction between Fe3+ and La−Fe−Al−O matrix. Meanwhile, the relative areas for αFe2O3 systematically increased from 5% in x = 9 to 11% in x = 11, which was in good agreement with the XRD results.

Figure 9 illustrates the site occupancy of Fe and La in MP structure, as obtained from the Rietveld analysis and Mössbauer characterization, as a function of the substituted Fe-content (x) in LaFexAl12−xO19 (x = 1−11) samples. At low concentration (x ≤ 3, Figure 9a), Fe preferentially substituted the Al(2) sites within the spinel block and the Al(3) and (5) sites in the mirror 10799



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Table 4. Fit Results of the 57Fe Mössbauer Effect Spectra of LaFexAl12−xO19 (x = 1-5 and 12) Samples at Room Temperature and LaFexAl12−xO19 (x = 6-11) Samples at T = 80 K room temperature

samples

IS (mm s−1)a

QS (mm s−1)b

x=1

0.26(1) 0.24(1) 0.99(2) 0.27(1) 0.18(1) 0.32(2) 0.95(1) 0.25(4) 0.23(03) 0.34(3) 0.18(2) 0.24(04) 0.36(1) 0.21(1) 0.26(03) 0.38(1) 0.33(1) 0.42(1) 0.36(1) 0.28(1) 0.32(1) 0.44(2) 0.21(2) 0.31(04) 0.28(2) 0.34(1) 0.45(1) 0.18(2) 0.33(4) 0.29(3) 0.35(04) 0.34(3) 0.24(4) 0.33(4) 0.27(4) 0.30(1) 0.33(1) 0.23(1) 0.34(1) 0.31(1) 0.27(1) 0.37(2) 0.34(3) 0.29(5) 0.33(1) 0.40(1) 0.19(1) 0.22(1) 0.26(2) 0.26(2) 0.36(1)

0.60(1) 2.48(2) 0.52(3) 0.70(5) 2.38(2) 0.44(4) 1.21(2) 0.63(1) 2.43(1) 0.58(2) 0.63(1) 2.41(1) 0.64(03) 0.56(03) 2.29(1) 0.62(02) −0.24(1) −0.15(1) 0.46(1) 0.36(4) 0.41(1) 0.57(6) 2.49(5) 0.40(1) 0.42(1) 0.38(1) 0.55(7) 1.59(3) 0.38(2) 0.38(1) 0.31(6) 0.32(6) 1.46(13) 0.39(1) 0.30(1) 0.34(1) 0.04(2) 1.52(5) 0.37(2) 0.29(1) 0.20(2) 0.28(4) 0.27(8) 1.98(3) 0.40(7) 0.29(1) 0.15(1) 0.20(1) 0.18(3) 1.91(2) 0.40(03)

x=2

x=3

x=4

x=5

x = 12 T = 80 K

x=6

x=7

x=8

x=9

x = 10

x = 11

H (KOe)c

A (%)d

site assignment

520(2) 524(2) 456(1) 417(2) 487(1) 366(3) 391(1) 482(1) 457(1) 506(1) 382(2) 413(1) 499(9) 480(8) 518(5) 410(3) 458(3) 479(1) 512(1) 533(1) 497(1) 412(2) 534(1) 505(4) 482(2) 531(1) 514(6) 437(3) 533(1) 503(4) 495(1) 530(1) 506(2) 423(3) 534(3)

65(3) 29(3) 6(1) 50(2) 25(1) 18(1) 7(1) 47(3) 22(2) 31(4) 37(4) 17(1) 46(6) 31(2) 15(1) 54(3) 49(4) 51(5) 31(4) 27(2) 28(3) 3(04) 11(1) 32(2) 26(2) 27(2) 5(02) 10(2) 35(6) 24(4) 25(1) 5(1) 11(3) 36(2) 23(1) 19(1) 8(1) 9(1) 5(04) 38(5) 19(3) 18(2) 9(2) 7(1) 9(2) 41(3) 17(2) 16(3) 8(1) 7(04) 11(1)

Fe3+(The) in Al(2) of MP Fe3+(Trf) 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 Fe3+(Ohg) in MP Fe2+(Th) in Al(2) 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 Fe3+(Th) in Al(2) of MP Fe3+(Tr) in Al(5) of MP Fe3+(Oh) in MP Fe3+ in α-Fe2O3 Fe3+ in LaFeO3 Fe3+(Oh) in Al(1) of MP Fe3+(Th) in Al(2) of MP Fe3+(Oh) in Al(3) of MP Fe3+(Oh) in Al(4) of MP Fe3+(Tr) in Al(5) of MP Fe3+(Oh) in Al(1) of MP Fe3+(Th) in Al(2) of MP Fe3+(Oh) in Al(3) of MP Fe3+(Oh) in Al(4) of MP Fe3+(Tr) in Al(5) of MP Fe3+(Oh) in Al(1) of MP Fe3+(Th) in Al(2) of MP Fe3+(Oh) in Al(3) of MP Fe3+(Oh) in Al(4) of MP Fe3+(Tr) in Al(5) of MP Fe3+(Oh) in Al(1) of MP Fe3+(Th) in Al(2) of MP Fe3+(Oh) in Al(3) of MP Fe3+(Oh) in Al(4) of MP Fe3+(Tr) in Al(5) of MP Fe3+ in α-Fe2O3 Fe3+(Oh) in Al(1) of MP Fe3+(Th) in Al(2) of MP Fe3+(Oh) in Al(3) of MP Fe3+(Oh) in Al(4) of MP Fe3+(Tr) in Al(5) of MP Fe3+ in α-Fe2O3 Fe3+(Oh) in Al(1) of MP Fe3+(Th) in Al(2) of MP Fe3+(Oh) in Al(3) of MP Fe3+(Oh) in Al(4) of MP Fe3+(Tr) in Al(5) of MP Fe3+ in α-Fe2O3

a Isomer shift relative to metallic iron. bQuadrupole splitting. cHyperfine field. dRelative area. eTetrahedral. fTrigonal bipyramidal (distorted tetrahedral, one of the two pseudotetrahedral sites of the bipyramid). gOctahedral.

Meanwhile, a marked rise of Fe distribution in Al(1) and Al(4) sites in the spinel block paralleled the increase in Fe content for x ≥ 4, in line with the progressive replacement of Al with Fe ions up to x = 11. For La3+ ions in MP-type hexaaluminates, the partial off-center shift from the normal position (La(1), 2d) in

plane, with an occupancy that increased almost linearly with Fe loading. When x ≥ 4, Fe occupation in Al(2), Al(3), and Al(5) sites gradually increased in the investigated range, while the rise was less pronounced and tended to an asymptotic value above 93, 97, and 85% for higher Fe loadings (x ≥ 8), respectively. 10800



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Figure 8. Low-temperature (80 K) 57Fe Mössbauer spectra of LaFexAl12−xO19 (x = 6−11) samples calcined at 1400 °C.

the Fe content and the surface morphology value for Fesubstituted La-hexaaluminates in catalyzing the N2O decomposition. In particular, increasing x up to 8−11, LaFexAl12−xO19 samples exhibited poor activity for N2O decomposition, even below that of LaAl12O19. Analysis of XRD showed that too great a number of Fe cations resulted in the formation of impurity phases (such as α-Fe2O3) outside the hexaaluminate structure, which partially covered the active sites and led to the loss of activity. According to the literature,11,12,24,27,40 the substituted metal ions incorporated into the sites in the mirror plane of the hexaaluminate structure should be highly active for N2O decomposition. The turnover frequencies (TOFs) were estimated by dividing the rate of N2O conversion (mol N2O h−1) by the number of Fe moles in Al(3) and Al(5) sites derived from the refinements and Mössbauer results and by the surface area of the samples. Notice that the TOF referred to Fe ions in the mirror plane at the surface under the assumption that the surface concentration of Fe is proportional to the overall Fe content in the bulk. The calculated values of TOF for LaFexAl12−xO19 (x = 1−7) catalysts at 550 °C are reported in Table 5. The analysis of the data indicated that there existed a correlation between the activity and Fe ions in the mirror plane,

the mirror plane to another crystallographic sites (La(2), 6h) occurred, quite different from the case in Ba-hexaaluminates with only 2d sites occupied by the large cations. As shown in Figure 9b, the site occupancy of La(1) gradually reduced on increasing Fe content, which was accompanied by an increase in La(2) sites. This displacement of La3+ ions (2d → 6h) in MP phase was consistent with the observation of Tronc et al.,20 which was attributed to the substitution of Al(5) by the more polarizable Fe3+ ions in the mirror plane. 3.5. Catalytic Activity. Activity tests have been performed on the LaFexAl12−xO19 (x = 0−11) samples calcined at 1400 °C. Test results are reported in Figure 10 in terms of N2O conversion as a function of reaction temperature. A different catalytic behavior was observed depending on the Fe contents (x values). When x = 0, the LaAl12O19 hexaaluminate was only slightly active for N2O decomposition. Upon Fe incorporation, despite the lower surface areas for x = 1−7 (2−10 m2 g−1) than that of LaAl12O19 (12 m2 g−1), the catalytic activity had a large increase, indicating the highly active framework Fe species for N2O decomposition. As shown in Figure 10, the catalytic activity progressively grew in line with the Fe incorporation up to x = 6 and decreased with further increasing the Fe loading. Accordingly, there appeared an optimum compromise between 10801



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Table 5. Summary of the Turnover Frequencies (TOFs) of LaFexAl12−xO19 (x = 1-7) at 550 °C sample x x x x x x x

= = = = = = =

1 2 3 4 5 6 7

TOF (h−1 m−2)a 0.47 0.22 0.16 0.21 0.30 0.32 0.31

× × × × × × ×

102 102 102 102 102 102 102

a

Turnover frequency (TOF) referred to Fe ions in the mirror plane (Al(3) and Al(5) sites) at the surface.

conversion per hour and per square meter) of LaFexAl12−xO19 (x = 0−7) are given in Figure 11. When x = 1, the observed

Figure 11. Normalized rate at 550 °C and the number of Fe ions in Al(3) and Al(5) sites of MP-type hexaaluminates as a function of x values in LaFexAl12−xO19 (x = 0−7) catalysts calcined at 1400 °C.

Figure 9. Site occupancy of Fe (a) and La (b) in the MP structure as a function of x value in LaFexAl12−xO19 (x = 1−11) samples calcined at 1400 °C.

increased rate compared with that of the Fe-free sample (x = 0) could be attributed to Fe ions in Al(5) sites in the MP structure. Increasing x to 2−4 resulted in a further increase in Fe3+ ions in Al(5) sites. Meanwhile, amounts of Fe3+ ions started to enter the Al(3) sites, which increased with the enhanced normalized rates. It indicated that Fe3+ ions in both Al(3) and Al(5) sites promoted the N2O catalytic decomposition for x = 2−4 catalysts. When x = 5−7, a linear increase in Fe3+ ions in Al(3) sites accompanied by the highly promoted normalized rates was observed. However, the rise of framework Fe3+ ions in Al(5) sites was less pronounced, which implied that Fe3+ ions in Al(3) sites were mainly responsible for the continuous enhancement of the normalized rates.

4. CONCLUSIONS The MP-type La−Fe hexaaluminate catalysts at various Fe loadings were prepared using a coprecipitation method in the present study. It has been demonstrated that the introduction of Fe ions influenced the thermal evolution of the substituted La-hexaaluminates. At low Fe concentration (x = 1−3), the final Fe-substituted La-MP phase formed via the solid-state reaction between γ-Al2O3 (with Fe oxidic entities dispersed on) and amorphous La species. In this case, Fe promoted the formation of MP phase by enhancing La3+ diffusion in γ-Al2O3 structure. Increasing Fe content (x ≥ 6) resulted in the segregation of LaFeO3, which also reacted with γ-Al2O3 to form

Figure 10. Catalytic activity in N2O conversion of LaFexAl12−xO19 (x = 0−11) catalysts calcined at 1400 °C.

as suggested by the TOF values ((0.16 to 0.47) × 102 h−1 m−2) that, despite some scattering, were reasonably constant on varying the Fe content. To investigate the effect of Fe ions in different crystallographic sites in the mirror plane (Al(3) and Al(5)) on the catalytic performance of Fe-substituted Lahexaaluminate catalysts, the normalized rates (moles of N2O 10802



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the final MP structure. However, this LaFeO3 intermediate essentially suppressed the mobility of La species and played a negative role for the formation of monophasic La-hexaaluminates, which even led to the absence of the MP structure in x = 12 samples. Rietveld refinement and 57Fe Mössbauer spectroscopic analysis indicated that at low content (x = 1−3) Fe preferentially occupied the Al(2) sites in the spinel block and the Al(3) and (5) sites in the mirror plane. When 4 ≤ x ≤ 8, Fe started to enter the Al(1) and Al(4) sites in the spinel block with an occupancy that increased almost linearly with Fe loading. Increasing x to ≥8, the Fe occupancy in the Al(2), Al(3), and Al(5) sites tended to asymptotic values, while those in Al(1) and Al(4) sites still systematically increased to x = 11. The Fe substitution in the MP structure promoted the offcenter shift of La from 2d to 6h sites, essentially distinct from that of the Fe-substituted Ba-hexaaluminates. The turnover frequencies ((0.16 to 0.47) × 102 h−1 m−2) for N2O decomposition referred to Fe ions in the mirror plane at the surface were reasonably constant. The catalytic properties of the reactive Fe3+ ions in Al(3) and Al(5) sites in the loosely packed mirror plane of MP structure were associated with the Fe-content. When x = 1, the enhanced rate could be mainly attributed to Fe3+ ions in the Al(5) sites. Further increasing x to 2−4, both of the Fe3+ ions in Al(3) and Al(5) sites were responsible for the higher rates. At high Fe loading (x = 5−7), Fe3+ ions in the Al(3) sites should be highly active for the improved normalized rates.

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ASSOCIATED CONTENT

S Supporting Information *

X-ray diffraction patterns of LaFexAl12−xO19 samples calcined at different temperatures. This material is available free of charge via the Internet at http://pubs.acs.org.

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AUTHOR INFORMATION

Corresponding Author

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

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS Financial support from the National Science Foundation of China (NSFC) grants (20773122, 21076211, 11205160, and 21303137) is greatly acknowledged.

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REFERENCES

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Thermal Evolution Crystal Structure and Fe Crystallographic Sites in

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