ARTICLE pubs.acs.org/JPCC
Evolution of Fe Crystallographic Sites from Barium Hexaaluminate to Hexaferrite Yanyan Zhu,†,‡ Xiaodong Wang,*,† Guotao Wu,† Yanqiang Huang,† Yan Zhang,†,‡ Junhu Wang,† and Tao Zhang*,† hysics, Chinese Academy of Sciences, 457 Zhongshan Road, State Key Laboratory of Catalysis, Dalian Institute of Chemical P Dalian 116023, P. R. China ‡ Graduate University of Chinese Academy of Sciences, Beijing 100049, P. R. China †
ABSTRACT: The substituted metal ions in the hexaaluminate structure are regarded as active centers, but their very limited substituted content prevents the obtainment of higher activity. Fe-substituted hexaaluminates exceptionally overcome such a limit. In this paper, we have studied the Fe crystallographic sites in BaFexAl12 xO19 (x = 1 12) and proposed the unique mechanism of stabilization of Fe ions in hexaaluminate structure at a high substitution level by employment of Rietveld refinement and 57Fe M€ossbauer spectroscopy. When x = 1 4, the site occupancy of Fe3+ in both βI-Al2O3 and newly formed magnetoplumbite phases was kept constant, and the increased Fe3+ ions were accommodated by the increased amount of magnetoplumbite phase. When x = 5 12, the site occupancy of Fe3+ in the magnetoplumbite phase continuously increased, and thus the increased Fe3+ ions were accommodated by the magnetoplumbite structure. BaFexAl12 xO19 catalysts exhibited different N2O decomposition properties under conventional furnace heating and microwave heating modes.
1. INTRODUCTION Hexaaluminates have attracted much attention in hightemperature catalytic processing, e.g., combustion of methane,1 5 CO2-reforming of methane to synthesis gas,6 partial oxidation of methane,7 10 process-gas N2O abatement,11 14 and the decomposition of N2O as a propellant,15 20 due to remarkable resistance to sintering and thermal shock. These materials have peculiar layered structures consisting of closely packed spinel blocks and loosely packed mirror planes in which the large cations (Ba, La, and Ca) are located.21 More importantly, introduction of transition metal or noble ions into the hexaaluminate structure by replacing Al3+ ions can enhance the catalytic activity without decrease of sintering resistance. For a variety of catalytic reactions, the framework-substituted metal ions are generally regarded as active centers,8,12,15,22 so that a highsubstituted concentration is desired. However, for most of the transition metals (like Mn, Cu, Ir, Ru, etc.), the substituted metal content is rather low, due to their large differences in radius and charge with Al3+ ions. For example, manganese substitution for aluminum was possible up to ≈3 Mn per unit cell,17,22,23 and copper substitution was limited to about 1.3 Cu per unit cell.24,25 The excess metal usually exists as metal oxides, spinels, or perovskites out of hexaaluminates, which covers partially the active sites and then leads to a loss of activity. Therefore, the limit of introduced metal content prevents the obtainment of higher activity to some extent. In barium hexaferrites (BaFe12O19), which are generally used in magnetic recording and microwave devices,26 28 Al3+ ions can be completely replaced by Fe ions.29 Obviously, Fe-substituted barium hexaaluminates overcame the limits on the soluble metal r 2011 American Chemical Society
content in the structure observed in Mn- and Cu-substituted barium hexaaluminates, suggesting a unique mechanism of stabilization of Fe ions in the hexaaluminate matrix. Naoufal et al.30 reported that Fe3+ ions occupied the octahedral S1 and S2 sites in BaFeAl11O19, and when Fe/Ba = 2 4 Fe3+ ions also entered into two new octahedral sites (S3 and S4). Townes et al.,31 Belous et al.,26 and Qiu et al.27 revealed that there were five nonequivalent crystallographic sites of Fe in BaFe12O19. Among them, three were octahedral, one was tetrahedral, and the last site was surrounded by five oxygen atoms forming a trigonal bipyramid. Evidently, the Fe loading affected significantly the Fe crystallographic sites in the hexaaluminate matrix. Groppi et al.29 found that the substituted Fe content also greatly influenced the structure type of formed hexaaluminate, namely, β-Al2O3 and magnetoplumbite (MP), with different Al crystallographic sites,21 which was independent of the substituted content of Mn and Cu.17,22 25 Up to now, there is still not a clear picture on how the Al3+ crystallographic sites in BaAl12O19 were gradually substituted by Fe ions to give BaFe12O19, which is very important for understanding the unique mechanism of stabilization of Fe ions in the hexaaluminate matrix at a high substitution level. In this work, BaFexAl12 xO19 (x = 0 12, step = 1) was employed to explore the evolution of Fe crystallographic sites from hexaaluminate to hexaferrite by means of Rietveld refinement and 57Fe M€ossbauer spectroscopy. The effect of Fe crystallographic sites on the catalytic activity of high concentration N2O Received: July 15, 2011 Revised: December 11, 2011 Published: December 12, 2011 671
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decomposition was investigated. Besides the conventional furnace heating, microwave heating was also attempted considering the excellent microwave-absorbing property of barium hexaferrite.
Prior to the reaction, the catalyst sample was pretreated in Ar at 300 °C for 0.5 h. After cooling to room temperature in Ar, the gas flow was switched to the reacting gas mixture containing 30 vol. % N2O in Ar at a flow rate of 50 mL/min, corresponding to a gas hour space velocity (GHSV) of 30 000 h 1. The outlet gas composition was analyzed online with an Agilent 6890N gas chromatograph equipped with Porapak Q and Molecular Sieve 13X columns and a thermal conductivity detector. N2O conversion was calculated from the difference between the inlet and outlet concentrations. Before analysis, the reaction proceeded at each temperature for 30 min to reach a steady state. The microwave reaction system consisted of a microwave generator, a rectangular waveguide, a circulator, a resonant cavity, and a plunger as described previously.33 36 The microwave energy was supplied by a 200 W, 2.45 GHz microwave generator, and the effective microwave power for this experiment ranged from 10 to 100 W. In microwave heating mode, the catalyst was loaded in a tubular quartz reactor which was aligned vertically at the center of the microwave cavity, so that the region was seated in the microwave field at maximum intensity.
2. EXPERIMENTAL SECTION 2.1. Catalyst Preparation. The BaFexAl12 xO19 samples (x = 0 12, step = 1) were prepared using the carbonate route as reported.17,32 For example, to prepare BaFe1Al11O19, Ba(NO3)2, Fe(NO3)3 3 9H2O, and Al(NO3)3 3 9H2O with the molar ratio 1/ 1/11 were dissolved individually in deionized water at 60 °C and then added into a saturated aqueous solution of (NH4)2CO3 under strong stirring to form the hexaaluminate precursor precipitate. After continuous stirring at 60 °C for 6 h, the precipitate was filtered, washed with deionized water, and then dried at 120 °C for 12 h and calcined at 500 °C in air for 4 h. Finally, the sample was calcined in air at 1200 °C for 4 h. Analysis by inductively coupled plasma (ICP) for Ba, Fe, and Al contents in mother liquors and washing waters indicated that the precipitation occurred in a quantitative way. Chemical analysis of samples after 1200 °C calcination confirmed that the actual Fe/Ba ratios corresponded to the nominal ones. To investigate the reproducibility of the above preparation method, three batches of BaFe3Al9O19 samples were prepared. Both the XRD characterizations and the activity tests for N2O decomposition showed the identical results in an accepted error range, demonstrating good reproducibility of this method. 2.2. Catalyst Characterization. The X-ray diffraction (XRD) patterns for phase identification were recorded with a PANalytical X’Pert-Pro powder X-ray diffractometer, using Cu Kα radiation (diffraction from a flat specimen, Bragg Brentano arrangement, U = 40 kV, I = 40 mA) with a Ni filter. The diffractograms were scanned between 2θ = 5 80°. Whole pattern least-squares Rietveld refinements were carried out by using the FULLPROF and RIETICA programs. In the Rietveld refinement process, the contributions of both the Kα1 (λ = 1.54056 Å) and Kα2 (λ = 1.54439 Å) radiations were considered, and the intensity ratio of Kα2/Kα1 was fixed to 0.5. Diffraction line profiles were approximated by Pseudo-Voigt function. The refined instrumental and structural parameters were: zero shift, scale factor, background polynomial parameters, unit cell parameter, FWHM and shape parameters, and site occupancies. Ionic scattering factors were used throughout the refinements. Site occupancy factors (SOF) were constrained to the chemical composition. The atomic coordinates were given based on the references (ICSD No. 29441, 202800-202803, and 157056). BET surface areas of the catalysts were measured by N2 adsorption at 196 °C using a Micromeritics ASAP 2010 apparatus. Scanning electron microscopy (SEM) experiments were performed with a JSM 6360-LV electron microscope operating at 20 25 kV. The samples were vapor deposited with gold before analysis. The M€ossbauer spectra were recorded at room temperature with a spectrometer working in the mode of constant accelerations with the use of 57Co γ-quantum source in the Rh matrix. All spectra were computer-fitted to a Lorentzian shape with a least-squares fitting procedure. The isomer shifts (IS) were given with respect to the centroid of α-Fe at room temperature. The absorbers were obtained by pressing the powdered samples (about 10 mg/cm2 of natural iron). 2.3. Activity Tests. Catalytic performance was evaluated in a quartz tube fixed-bed reactor using 100 mg (20 40 mesh) of a catalyst diluted with quartz sand as described previously.15 17
3. RESULTS AND DISCUSSION 3.1. Phase Composition and Morphology. Figure 1 shows the XRD patterns of BaFexAl12 xO19 (x = 0 12) samples. When x = 0 and 1, a well-crystallized β-Al2O3 hexaaluminate phase (JCPDS No. 1-75-707) was observed. Compared with x = 0, the diffractogram at x = 1 slightly shifted to lower 2θ values corresponding to the enlargement of cell parameters of the β-Al2O3 phase (a = b = 5.5892 Å and c = 22.744 Å at x = 0, a = b = 5.6141 Å and c = 22.775 Å at x = 1), implying that Fe cations (r(Fe3+) = 0.64 Å) have been incorporated into the β-Al2O3 lattice by replacing smaller Al ions (r(Al3+) = 0.51 Å). However, when x = 2, besides the diffraction peaks of the β-Al2O3 phase, peaks at 2θ = 31.3° and 35.1° were also clearly observed, which were associated with the MP-type hexaaluminate (JCPDS No. 1-84-1788),6,29,37 indicating that higher Fe content promoted the formation of the MP phase. The diffraction lines due to the MP phase became stronger with increasing x up to 3 and 4, while those corresponding to the β-Al2O3 phase became weaker. In particular, the β-Al2O3 peaks had disappeared completely at x = 5. This result strongly suggested that the increase of introduced Fe ions in Ba-hexaaluminate structure facilitated the phase transformation from β-Al2O3 into MP. This was consistent with the previous report that the β-Al2O3 phase formed in the region of Al-rich composition and the MP phase formed in the region of Fe-rich composition.29 It is worthwhile to note that no iron oxide was detected even in BaFe4Al8O19 and BaFe5Al7O19. This is different from what happened in other transition metal substituted Ba-hexaaluminates (BaMxAl12 xO19, M = Mn and Cu),17,22 25 in which excess metal ions (x g 3) usually lead to the appearance of metal oxides outside the framework of hexaaluminate instead of the transformation of the β-Al2O3 phase. Further increasing Fe loading to x > 5, there was still no appearance of iron oxide just with the sharper of MP peaks and the shift of diffractograms to lower 2θ values, indicating the increase of crystalline size and the enlargement of cell parameters of the MP phase. Table 1 displays the surface area of BaFexAl12 xO19 (x = 0 12) samples. One can see that the surface area decreased with increasing x value, indicating that more Fe ions incorporated into the hexaaluminate structure resulted in the sintering of materials, 672
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Table 1. Surface Areas of BaFexAl12 xO19 (x = 0 12) Samples samples
surface area (m2/g)
x=0
20
x=1 x=2
18 17
x=3
17
x=4
16
x=5
14
x=6
10
x=7
6
x=8
5
x=9 x = 10
4 -
x = 11
-
x = 12
-
Figure 1. X-ray diffraction patterns of BaFexAl12 xO19 (x = 0 12) samples. Figure 2. Scanning electron micrograph images of BaFexAl12 xO19: (a) x = 0, (b) x = 1, (c) x = 3, (d) x = 6, (e) x = 9, and (f) x = 12 samples.
which was consistent with the observation of Groppi et al.29 To be noted, the surface area (16 18 m2/g) almost kept constant in the process of phase transformation (x = 1 4), indicating that the phase transformation prevented the sintering of materials to some extent. In contrast, in the range of x = 5 12, the surface area decreased obviously even below the detection limit of the N2 adsorption technique with x g 10.
The SEM images of BaFexAl12 xO19 (x = 0, 1, 3, 6, 9, and 12) samples are shown in Figure 2. It can seen that all samples consisted of the plate crystallites of hexagonal shape (0.25 2 μm diameter, 0.005 0.5 μm thick), and the incorporation of Fe ions into hexaaluminate structure made the crystallites larger. The 673
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Table 2. Results of Rietveld Refinement on BaFexAl12 xO19 (x = 0 12) Samples phase composition
cell parameters (Å) βI-Al2O3
samples
a
Rp
βI-Al2O3
phase proportion (molar ratio) βI-Al2O3a,
b
MP
a
MP c
a
c
x=1
12.115
98%
Ba0.79Fe0.79Al10.21O17.21
-
5.6141(2)
22.775(1)
-
x=2
12.484
80% βI-Al2O3, 18% MP, 2% S
Ba0.78Fe0.79Al10.21O17.22
BaFe4.37Al7.63O19
5.6145(2)
22.776(1)
5.7103(4)
22.644(3)
x=3
10.274
46% βI-Al2O3, 53% MP, 1% S
Ba0.78Fe0.79Al10.21O17.22
BaFe4.39Al7.61O19
5.6153(3)
22.778(2)
5.7124(4)
22.647(3)
x=4
11.771
6% βI-Al2O3, 89% MP, 5% S
Ba0.78Fe0.79Al10.21O17.22
BaFe4.45Al7.55O19
5.6162(4)
22.781(3)
5.7153(4)
22.650(2)
x=5
11.243
96% MP, 4% S
-
BaFe5.15Al6.85O19
-
-
5.7372(4)
22.719(2)
x=6
10.861
97% MP, 3% S
-
BaFe6.06Al5.94O19
-
-
5.7538(5)
22.753(3)
x=7
10.538
97% MP, 3% S
-
BaFe7.04Al4.96O19
-
-
5.7725(4)
22.789(2)
x=8 x=9
11.165 10.032
97% MP, 3% S 99% MP, 1% S
-
BaFe8.02Al3.98O19 BaFe9.06Al2.94O19
-
-
5.7907(5) 5.8222(3)
22.880(3) 22.957(2)
x = 10
10.523
99% MP, 1% S
-
BaFe9.97Al2.03O19
-
-
5.8334(3)
22.998(1)
x = 11
10.116
99% MP, 1% S
-
BaFe11.01Al0.99O19
-
-
5.8654(3)
23.125(2)
x = 12
9.321
100% MP
-
BaFe12O19
-
-
5.8783(2)
23.188(1)
2% S
-
One type of β-Al2O3 hexaaluminate. b Spinel BaAlyFe2‑yO4 (0 e y < 2).
observed planar facets should have orientations parallel to the mirror plane resulting from the anisotropic crystal growth due to the peculiar layered structure of BaFexAl12 xO19 hexaaluminates since the crystal growth along the c axis by stacking of spinel blocks, which are separated by Ba-containing mirror plane layers, is very slow compared with that along the a axis in both β-Al2O3 and MP structures.38 40 3.2. Rietveld Refinement. To reveal the unique mechanism of stabilization of Fe ions in the hexaaluminate structure at high substitution level, Rietveld analysis of the XRD powder data was performed for BaFexAl12 xO19 (x = 1 12). Rietveld refinement is a powerful tool to identify the crystalline structure and crystalline phase abundance,41,42 and thus the phase proportion and Fe concentration in both β-Al2O3 and MP phases of Fesubstituted Ba-hexaaluminates can be quantitatively determined. As shown in Table 2, in the range of x = 1 4, the increase of Fe content (x value) only significantly influenced the proportion of different phases but not the compositions of both β-Al2O3 and MP phases as well as corresponding cell parameters a and c. The content of Fe ions incorporated into the β-Al2O3 structure was rather low (Ba0.78Fe0.79 Al10.21O17.28 at x = 2 4), indicating the limited capability of the β-Al2O3 phase to accommodate foreign ions, which may be the reason for the appearance of metal oxides (such as Mn and Cu) outside of barium hexaaluminate with the β-Al2O3 phase. In contrast, Fe concentration in the MP phase was much higher with the ratio of Fe/Ba = 4.37 4.45 at x = 2 4, which was supported by the larger cell parameter (a = b) of the MP phase (5.7103 5.7153 Å at x = 2 4) compared with that of β-Al2O3 (5.6141 5.6162 Å at x = 1 4). However, somewhat surprising is that the c value of the MP phase (22.644 22.650 Å) was smaller than that of β-Al2O3 (22.775 22.781 Å), although the former had a larger amount of Fe ions. This could be attributed to the increase of Ba concentration from 0.78 0.79 in β-Al2O3 to 1 in the MP phase, which enforced the bonds between the spinel block and the mirror plane and resulted in the contraction of the mirror plane. This contraction compensated the expansion along the c axis due to the incorporation of the larger Fe ions with higher concentration. Such a compensation effect was also observed in Mn-substituted Ba-hexaaluminate,22
Figure 3. Proportion of β-Al2O3 and MP phases and the Fe/Ba ratios in both phase compositions as a function of x value in BaFexAl12 xO19 (x = 1 12) samples.
where the larger size of Mn ions (r(Mn2+) = 0.80 Å, r(Mn3+) = 0.66 Å) was balanced by the contraction of the mirror plane. In the range of x = 5 12, the ratio of Fe/Ba in the MP phase significantly increased from 5.15 to 12.00, suggesting that more Fe ions entered into the Al crystallographic sites in the MP structure. Meanwhile, the cell parameters a and c of the MP phase continuously increased from 5.7372 and 22.719 Å to 5.8783 and 23.188 Å, respectively. Figure 3 displays the proportion of β-Al2O3 and MP phases and the Fe/Ba ratios in both phase compositions as a function of x value in BaFexAl12 xO19 (x = 1 12) samples. When x = 1, Fe ions only entered into the β-Al2O3 phase with an Fe/Ba ratio equal to 1. In the range of x = 2 4, the Fe/Ba ratios in both β-Al2O3 and newly formed MP phases almost kept constant, which were 1 (β-Al2O3) and 4.37 4.45 (MP), respectively. In this range, only the phase proportion changed, accompanied with the gradual phase transformation from β-Al2O3 to MP. The increased Fe3+ ions were accommodated by the increased amount 674
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β-Al2O3 phase.22 In the Mn-substituted Ba2+-β-Al2O3, the excess Ba2+ charge (with respect to +1.5, which was regarded as the ideal valence for the large cation in the mirror plane of stoichiometric β-Al2O3 phase according to the electrostatic valence rule21) was compensated by the substitution of lower charge of initially introduced Mn2+ ions (with respect to Al3+) at Mn/Ba < 1 accompanied with the decrease of the Reidinger defect. However, when Mn/Ba > 1 with the presence of Mn3+ ions, the excess positive charge could not be effectively balanced, which resulted in the appearance of Mn oxides outside of hexaaluminate at Mn/Ba = 3. In contrast, in the Fe-substituted Ba-hexaaluminate, the excess cation charge in Ba2+-β-Al2O3 was compensated by the formation of the charge-insufficient Ba2+-MP phase,19 where +2.4 was assumed as the ideal valence at the large cation site.20,21 The proportion of the charge-insufficient MP phase with higher accommodating capacity for Fe ions continuously increased with increasing Fe content, which resulted in the stabilization of more Fe ions in hexaaluminate structure. The difference of accommodating capability between β-Al2O3 and MP phases probably originated from the different crystallographic sites substituted by Fe in both phases. Subsequently, the Fe crystallographic sites in both phases were carefully analyzed. Figure 4 shows the typical fitted XRD patterns of BaFeAl11O19 with a β-Al2O3 structure, BaFe3Al9O19 with both β-Al2O3 and MP phases, and BaFe12O19 with a MP structure. The representative results are summarized in Table 3. It is reported that39 unsubstituted Ba-β-Al2O3 hexaaluminate is constituted by the defective Ba-poor βI phase and Ba-rich βII phase, with composition Ba0.72Al11O17.28 and Ba1.16Al10.68O17.16, respectively. Comparing our obtained XRD data of BaFeAl11O19 with the diffraction patterns of βI-Al2O3 and βII-Al2O3 phases, the Ba-βI-Al2O3 structure39,43 was chosen as a starting model for the Rietveld refinement. The analysis results confirmed that a Ba-poor βI-Al2O3 phase with Ba0.79Fe0.79Al10.21O17.29 indeed formed in BaFeAl11O19, in line with Mn-substituted Ba-βI-Al2O3 hexaaluminates.22 In the βI-Al2O3 structure (Figure 5a), small fractions of Al3+ ions shifted from their normal octahedral Al(1) sites in the spinel block to new tetrahedral interstitial Al(5) sites in the mirror plane bridged by interstitial oxygen due to the vacancy of Ba2+ through a Reidinger defect mechanism.43 Refinements of the Al occupancies for BaFeAl11O19 indicated the presence of Fe ions in the tetrahedral Al(2) sites in the spinel block and the tetrahedral interstitial Al(5) sites in the mirror plane of βI-Al2O3 structure with occupancy of 20.1% and 6.5%, respectively, while Al(1), Al(3), and Al(4) sites did not show any significant evidence of Fe substitution (Table 3). This was quite different from the case of Mn-substituted Ba-βI-Al2O3 where only these sites (Al(1) and Al(2)) in the spinel block were substituted.22 Such a difference in the mirror plane may be responsible for the phase transformation of Fe-substituted barium hexaaluminates from βI-Al2O3 into MP since the structure type of hexaaluminates was determined by the composition and the defectivity of the mirror plane.21,29 In 46% βI-Al2O3 phase of BaFe3Al9O19, Fe ions only occupied Al(2) and interstitial Al(5) sites as that in BaFeAl11O19 (Table 3). In contrast, in 53% MP phase the refinement results revealed that all of the Al crystallographic sites (Al(1), Al(2), Al(3), Al(4), and Al(5)) could be occupied by Fe ions, suggesting that there are more Al crystallographic sites in the MP structure favorable for the substitution of Fe ions. In particular, the occupancies of Fe in Al(2), Al(3), and Al(5) sites were above 60%, indicating that Fe ions preferentially occupied the tetrahedral
Figure 4. X-ray diffraction patterns of BaFexAl12 xO19 fitted using the Rietveld refinement method for (a) x = 1, (b) x = 3, and (c) x = 12 samples.
of MP phase. When x g 5 with the disappearance of the β-Al2O3 phase, the ratio of Fe/Ba in the remaining MP phase started to continually increase from 5.15 at x = 5 to 12.00 at x = 12, and the increased Fe3+ ions gradually entered into the MP structure. It is noteworthy that the phase transformation from β-Al2O3 to MP should be the key factor for the stabilization of Fe ions in Ba-hexaaluminate at high substitution level through a charge compensation mechanism, which was quite different from the stabilization of Mn ions in Ba-hexaaluminate with only the 675
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Table 3. Refined Sites Occupancy of Ba, Fe, Al, and O in BaFexAl12 xO19 (x = 1, 3, 6, 9, and 12) Samples x=1 atom site
mult.a
x=3
βI-Al2O3i
coor.b
βI-Al2O3
MPj
x=6
x=9
x = 12
MP
MP
MP
Ba(1)
2
-
0.787(4)
0.784(4)
1.00
1.00
1.00
1.00
Al(1)
12
Ohc
0.929(15)
0.928(15)
0.808(10)
0.602(6)
0.328(3)
0.00
Al(2)
4
Thd
0.799(8)
0.799(9)
0.386(5)
0.275(5)
0.025(1)
0.00
Al(3)
4
Th/Ohe
1.00
1.00
0.320(6)
0.282(7)
0.110(3)
0.00
Al(4)
2
Oh
1.00
1.00
0.960(24)
0.932(27)
0.630(17)
0.00
Al(5)f
12
Th
0.006(1)
0.007(1)
-
-
-
-
Al(5)g
2
Trh
-
-
0.391(8)
0.281(4)
0.074(2)
0.00
Fe in Al(1) Fe in Al(2)
12 4
Oh Th
0.201(2)
0.201(2)
0.192(2) 0.614(8)
0.398(4) 0.725(13)
0.672(7) 0.975(18)
1.00 1.00
Fe in Al(3)
4
Th/Oh
-
-
0.680(13)
0.718(18)
0.890(21)
1.00
Fe in Al(4)
2
Oh
-
-
0.040(1)
0.068(2)
0.370(10)
1.00
Fe in Al(5)f
12
Th
0.065(1)
0.065(1)
-
-
-
-
Fe in Al(5)g
Tr
-
-
0.609(12)
0.719(10)
0.926(25)
1.00
O(1)
12
2
-
1.00
1.00
1.00
1.00
1.00
1.00
O(2)
12
-
1.00
1.00
1.00
1.00
1.00
1.00
O(3) O(4)
4 4
-
1.00 1.00
1.00 1.00
1.00 1.00
1.00 1.00
1.00 1.00
1.00 1.00
O(5)
2
-
1.00
1.00
1.00
1.00
1.00
1.00
O(6)
6
-
0.071(1)
0.072(1)
0.00
0.00
0.00
0.00
Expected composition
BaFe1Al11O19
BaFe3Al9O19
BaFe6Al6O19
BaFe9Al3O19
BaFe12O19
Calculated composition
BaFe1.00Al12.73O21.60
BaFe2.99Al9.72O20.07
BaFe6.06Al5.94O19
BaFe9.06Al2.94O19
BaFe12O19
multiplicity. b coordination. c Octahedral. d Tetrahedral. e Tetrahedral for the βI-Al2O3 phase or octahedral for the MP phase. f Interstitial Al(5) in the βI-Al2O3 phase. g Al(5) in the MP phase. h Trigonal bipyramid. i Constraints on occupancies of the βI-Al2O3 phase relevant to the Reidinger defect mechanism (Ba(1) + 3(Al(5) + Fe(5)) = 1, Al(5) + Fe(5) = O(6), Al(1) + Al(5) + Fe(5) = 1, Al(2) + Fe(2) = 1) in ref 22. j Constraints (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) = 1) on occupancies of the MP phase. a
and BaFe4Al8O19 samples was close to that of BaFe3Al9O19; accordingly, no detailed data were shown. From Figure 5, one can see that the major difference between βI-Al2O3 and MP phases lies in the contents and arrangement of the ions in the mirror plane, which may account for their different accommodating capability for Fe ions. In the mirror plane of the βI-Al2O3 phase in BaFexAl12 xO19 (x = 1 4), only a small fraction of distorted tetrahedral Al(5) shifted from the octahedral Al(1) site could be substituted by Fe ions, while the symmetric tetrahedral Al(3) site was rarely occupied. In contrast, in the mirror plane of the MP phase (x = 2 4), besides the distorted trigonal bipyramid Al(5) site, the octahedral Al(3) site, which can offer larger space to accommodate larger Fe ions as compared with the smaller tetrahedral Al(3) site in βI-Al2O3, was also largely occupied with occupancy above 60%. Such a higher occupancy of Fe in both Al(3) and Al(5) sites in the mirror plane of MP structure should be responsible for the observed higher Fe concentration in the MP phase, compared with the very limited occupancy of Fe in the distorted tetrahedral Al(5) site in the mirror plane of the βI-Al2O3 phase with lower Fe concentration. Further increasing Fe content (x g 5), the occupancies of Fe in five Al crystallographic sites in the MP structure started to continuously increase (Table 3). For example, the Fe occupancy in the Al(5) site from ≈60% at x = 2 4 increased up to 71.9%, 92.6%, and 100%, respectively, corresponding to x = 6, 9, and 12. When x = 12 (Figure 4, Table 3), all the Al crystallographic sites in the MP structure were occupied by Fe ions with occupancy of 100%, even in the less occupied Al(1) and Al(4) sites at x = 2 4.
Figure 5. Structure of Ba-hexaaluminate (a) βI-Al2O3 and (b) MP. Numbers in parentheses refer to the different Al sites. Al(1), octahedral site; Al(2), tetrahedral site; Al(3) in βI-Al2O3, tetrahedral site; Al(3) in MP, octahedral site; Al(4), octahedral site; Al(5) in βI-Al2O3, tetrahedral site; Al(5) in MP, trigonal bipyramid site.
Al(2) site in the spinel block and octahedral Al(3) and trigonal bipyramid Al(5) sites in the mirror plane (Figure 5b). The site occupancy of Fe in both βI-Al2O3 and MP phases of BaFe2Al10O19 676
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Figure 6. Site occupancy of Fe in both (a) βI-Al2O3 and (b) MP phases as a function of x value in BaFexAl12 xO19 (x = 1 12) samples.
Figure 6 illustrates the site occupancy of Fe in both βI-Al2O3 and MP phases as a function of x value in BaFexAl12 xO19 (x = 1 12). When x = 1, Fe ions only occupied the tetrahedral Al(2) site in the spinel block and the tetrahedral interstitial Al(5) site in the mirror plane of the βI-Al2O3 phase with occupancy of 20.1% and 6.5%, respectively, corresponding to the chemical formula of Ba0.79Fe0.79Al10.21O17.21. Increasing x up to 2 4, besides βI-Al2O3, part of the Fe ions started to enter into the newly formed MP phase where there are more Al crystallographic sites favorable for the substitution of Fe ions. Among them, the tetrahedral Al(2) site in the spinel block and octahedral Al(3) and trigonal bipyramid Al(5) sites in the mirror plane were preferentially occupied with occupancy above 60%, whereas octahedral Al(1) and Al(4) sites in the mirror plane were less occupied with occupancy of 19.1 20.0% and 4.0%, respectively. To be noted, in this range (x = 2 4) the site occupancy of Fe in both βI-Al2O3 and MP phases had a plateau, suggesting that Fe content incorporated in both phases kept constant. Associating with the proportion of both βI-Al2O3 and MP phases (Table 2), the increased Fe loading in samples at x = 2 4 should be accommodated by the increased MP phase. When x g 5, the Fe occupancy in Al(2), Al(3), and Al(5) sites of the MP phase gradually increased and almost reached above 90% at x g 9. Meanwhile, a linear increase of Fe occupancy in the Al(1) site
Figure 7. Room-temperature 57Fe M€ossbauer spectra of BaFexAl12 xO19 (x = 1 12) samples.
and a significant rise of Fe occupancy in the Al(4) site at x g 7 were observed. Obviously, in the range of x = 5 12, the increased Fe ions in samples gradually entered into MP structure up to the formation of BaFe12O19 at x = 12. € ssbauer Spectroscopy. Figure 7 displays room3.3. 57Fe Mo temperature 57Fe M€ossbauer spectra of BaFexAl12 xO19 (x = 1 12). Table 4 presents the fitted M€ossbauer parameters and the number of Fe ions in different crystallographic sites (NFe), as calculated by multiplying the M€ossbauer relative area (A) and the total Fe content (x) in samples. It can be seen that the isomer shift (IS) values (0.19 0.41 mm/s) are much more related to Fe3+ species (IS = 0.17 0.42 mm/s)44 47 rather than Fe2+, for which usual values of IS are closer to 0.9 1.3 mm/s.47 49 This result indicates that Fe ions in our samples are exclusive in the +3 oxidation state. The spectrum of BaFeAl11O19 was fitted with two doublets, which were assigned to different Fe3+ species in the βI-Al2O3 677
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Table 4. Room-Temperature 57Fe M€ ossbauer Parameters and the Number of Fe Ions in Different Crystallographic Sites (NFe) in BaFexAl12 xO19 (x = 1, 2, 3, 11, and 12) Samples samples
IS,a mm/s
QS,b mm/s
Heff,c KOe
A,d %
NFe
assignment
x=1
0.23
1.01
-
49
0.49
Fe (Th ) in Al(5) of βI-Al2O3
x=2
0.21 0.23
0.60 0.98
-
51 41
0.51 0.82
Fe3+(Th) in Al(2) of βI-Al2O3 Fe3+(Th) in Al(5) of βI-Al2O3
0.19
0.55
-
46
0.92
Fe3+(Th) in Al(2) of both βI-Al2O3 and MP
0.41
0.60
-
10
0.20
Fe3+(Ohf) in MP
0.19
2.51
-
3
0.06
Fe3+(Trg) in MP
0.21
0.93
-
24
0.72
Fe3+(Th) in Al(5) of βI-Al2O3
0.20
0.56
-
38
1.14
Fe3+(Th) in Al(2) of both βI-Al2O3 and MP
0.41
0.60
-
31
0.93
Fe3+(Oh) in MP
0.22 0.35
2.47 0.42
412
7 51
0.21 5.61
Fe3+(Tr) in MP Fe3+(Oh) in Al(1) of MP
0.26
0.24
471
20
2.20
Fe3+(Th) in Al(2) of MP
0.37
0.15
511
17
1.87
Fe3+(Oh) in Al(3) of MP
0.35
0.09
497
8
0.88
Fe3+(Oh) in Al(4) of MP
0.24
2.01
389
4.0
0.44
Fe3+(Tr) in Al(5) of MP
0.36
0.41
418
48
5.76
Fe3+(Oh) in Al(1) of MP
0.28
0.21
494
20
2.40
Fe3+(Th) in Al(2) of MP
0.39 0.34
0.22 0.05
518 513
17 11
2.04 1.32
Fe3+(Oh) in Al(3) of MP Fe3+(Oh) in Al(4) of MP
2.22
403
4
x=3
x = 11
x = 12
0.28
3+
e
Fe3+(Tr) in Al(5) of MP
0.48
Isomer shift relative to α-Fe. Electric quadrupole splitting. Hyperfine field. Relative area. Tetrahedral. f Octahedral. g Trigonal bipyramid. Uncertainty is (3% of reported value. a
b
c
d
e
was higher than that of Fe3+(Th) due to the larger band separation of Fe3+ O2 for octahedral sites. According to the Rietveld refinement results (Table 3), the Fe3+(Oh) doublet was assigned to Fe3+ ions in the octahedral Al(1), Al(3), and Al(4) sites in MP structure (Figure 5b). The other doublet with a very high QS value (2.47 2.51 mm/s) was assigned to Fe3+ ions in the much distorted trigonal bipyramid Al(5) sites (denoted as Tr) in the mirror plane of MP structure (Figure 5b) since the large QS value (2.44 2.65 mm/s) in the Tr site was also detected in MP-type CaFe1.2Al10.8O1944 and LaFeAl11O19.50 It is worthwhile to note that the local chemical environment of Fe3+ ions in both βI-Al2O3 (x = 1 3) and MP phases (x = 2 3) almost kept constant, evidenced by the close M€ossbauer parameters (Table 4), which can be attributed to the similar site occupancy of Fe3+ in different Al crystallographic sites in both phases at x = 1 3 (Figure 6). When x g 4, an evolution of M€ossbauer spectra from doublets to sextets occurred, and the features of relaxation effects were observed. This might be because more Fe ions incorporated into the MP structure decreased the mean distance of neighboring magnetic Fe3+, which affected the magnetic hyperfine interaction and the spin spin relaxation time. When x = 11 and 12, the M€ossbauer spectra lines were fitted with five typical sextets by the model described in refs 26, 27, 51, and 52, corresponding to Fe3+ ions in Al(1), Al(2), Al(3), Al(4), and Al(5) sites in MP structure (Figure 5b). As shown in Table 4, for the BaFe11AlO19 sample (x = 11), the hyperfine fields (Heff) of 412 KOe and 511 497 KOe were attributed to Fe3+ ions in octahedral Al(1) and another two octahedral sites (Al(3) and Al(4)), respectively, whereas Heff = 471 KOe to Fe3+ ions in tetrahedral Al(2) sites and Heff = 389 KOe to Fe3+ ions in trigonal bipyramid Al(5) sites. When x = 12, a systematic increase of Heff was observed, indicating that more magnetic Fe3+ ions entered into the hexaaluminate
structure. The IS values (0.21 0.23 mm/s) were small and much more related to Fe3+ ions in tetrahedral (Th) sites (IS = 0.18 0.19 mm/s) reported in CaFe1.2Al10.8O1944 and LaFeAl11O19.50 This indicated that Fe3+ occupied tetrahedral sites in the βI-Al2O3 structure, which was well consistent with the Rietveld refinement result that only tetrahedral Al sites (Al(2) and interstitial Al(5)) were occupied by Fe ions at x = 1 (Table 3). One doublet with a small QS value (0.60 mm/s) was assigned to Fe3+(Th) in the symmetric Al(2) site in the spinel block. The QS value (1.01 mm/s) of the other doublet was relatively high, which was assigned to Fe3+ ions in the distorted tetrahedral interstitial Al(5) sites in the mirror plane resulting from Reidinger defect since the larger QS value means the larger extent of electric field distortion around Fe3+ ions.30,44 The relative areas (A) of Fe3+ ions in tetrahedral Al(2) and interstitial Al(5) sites measured by M€ossbauer spectroscopy were 51% and 49% (Table 4), respectively, which was well consistent with the Rietveld quantitative analysis (51% and 49%). Similarly, when x = 2, the doublet (41%) corresponding to Fe3+(Th) in the distorted Al(5) site of the βI-Al2O3 phase was also observed. In addition, one doublet (46%) with IS = 0.19 mm/s and QS = 0.55 mm/s was assigned to Fe3+(Th) in the symmetric Al(2) site of both βI-Al2O3 and MP phases, considering the identical occupation of tetrahedral Al(2) sites by Fe3+ ions in both phases as revealed by Rietveld analysis. To be noted, two new doublets with entirely different M€ossbauer parameters (Table 4) were also observed, and the relative area (A) increased from 10% and 3% at x = 2 to 31% and 7% at x = 3, respectively. Apparently, the appearance of the two new doublets should be associated with the formation of the MP phase considering the XRD results. The IS value (0.41 mm/s) of one new doublet was close to the reported value (0.42 mm/s) corresponding to Fe3+ ions in octahedral (Oh) sites of MP-type hexaaluminate,44 which 678
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Figure 8. N2O conversion as a function of reaction temperature over BaFexAl12 ‑xO19 (x = 0 12) catalysts.
Figure 9. N2O conversion as a function of microwave power over BaFexAl12 xO19 (x = 1, 7, 9, and 12) catalysts.
structure by replacing nonmagnetic Al3+ ions. Meanwhile, NFe in different crystallographic sites also systematically increased. To be noted, NFe in the Al(4) site increased 50% from 0.88 to 1.32, suggesting that Fe3+ ions preferentially occupied the Al(4) site in the range of x = 11 12, which was well consistent with the Rietveld refinement results (Figure 6). In other words, it was very difficult for Fe3+ ions to occupy the octahedral Al(4) site in the spinel block, which can be attributed to the good symmetry of Al(4) site, evidenced by the rather small QS value ( 0.05 to 0.09 mm/s, Table 4). 3.4. Catalytic Activity. Figure 8 depicts the profiles of 30 vol. % N2O conversion versus the reaction temperature over BaFexAl12 xO19 (x = 0 12) samples. In the absence of Fe (x = 0), the BaAl12O19 did not exhibit activity toward N2O decomposition until at 700 °C. When Fe3+ ions were incorporated into the βI-Al2O3 structure (x = 1), the activity was enhanced significantly with N2O conversion of 73% at 700 °C, implying that Fe3+ ions in the βI-Al2O3 structure were highly active for N2O decomposition. When x = 2, about 1.60 Fe3+ ions among 2 Fe3+ ions were still located in the βI-Al2O3 phase, and a small fraction of Fe3+ ions (0.36) started to enter into the newly formed MP phase. Meanwhile, the activity further increased significantly. Increasing x up to 3 and 4 resulted in a decrease of Fe3+ amount in βI-Al2O3, while Fe3+ amount in the MP phase continued to increase accompanied with the highest activity. These results above suggested that in the process of phase transformation (x = 2 4) Fe3+ ions in the MP phase were also responsible for N2O decomposition. However, a further increase of Fe3+ ions in the MP phase (x g 5) induced a decrease in the N2O conversion. In particular, when x g 10 the catalytic activity was even lower than that of BaAl12O19, which was attributed to the rather low surface area below the instrumental detection limit. Considering that BaFe12O19 hexaferrite is an excellent microwave absorber,26,27 microwave heating was attempted. Figure 9 illustrates the N2O conversion as a function of the microwave power over BaFexAl12 xO19 (x = 1, 7, 9, and 12) catalysts. Although BaFeAl11O19 with the βI-Al2O3 phase exhibited high activity for N2O conversion under conventional furnace heating, it was inactive when operated in the microwave mode even at the maximal microwave power of 100 W in our device, probably due to its poor microwave absorption ability. When x = 7 with
comparable activity of BaFeAl11O19 under conventional heating mode, the microwave discharge was already ignited at 50 W, and the ignited microwave power decreased to 20 and 10 W with increasing x value up to 9 and 12. Obviously, in the microwave heating mode, the N2O decomposition was greatly promoted by the Fe-rich MP phase. In contrast to the βI-Al2O3 phase which only accommodated very limited Fe3+ ions, the MP phase possessed a higher accommodating Fe3+ ion capacity, which made it a good microwave medium, so that the microwave energy was effectively converted into the activation energy of the polar N2O molecules absorbed on BaFexAl12 xO19 catalysts and thus a high N2O conversion was obtained.
4. CONCLUSIONS In summary, we have given a clear picture of the evolution of Fe crystallographic sites in BaFexAl12 xO19 with increasing x value and proposed that the phase transformation from βI-Al2O3 to MP was the key factor for the unique stabilization of Fe ions in the barium hexaaluminate structure at high substitution level. When x = 1, the sample only crystallized in the βI-Al2O3 phase, where Fe3+ ions just occupied the tetrahedral Al(2) site in the spinel block and the tetrahedral interstitial Al(5) site in the mirror plane with low occupancy (e20%). Increasing x up to 2 4, besides the βI-Al2O3 phase, part of the Fe3+ ions started to enter into the newly formed MP phase, in which the tetrahedral Al(2) site in the spinel block and octahedral Al(3) and trigonal bipyramid Al(5) sites in the mirror plane were preferentially occupied with occupancy above 60%. In this range, the site occupancy of Fe in both βI-Al2O3 and MP phases kept constant, and the increased Fe3+ ions in samples were accommodated by the increased amount of the MP phase. When x g 5, the site occupancy of Fe in the MP phase started to continuously increase, and the increased Fe species gradually entered into the MP structure up to the formation of BaFe12O19 at x = 12. Under conventional heating, Fe3+ ions in both βI-Al2O3 and MP phases of BaFexAl12 xO19 (1 e x e 7) samples were responsible for N2O decomposition, whereas in the microwave heating mode Fe3+ ions in the MP phase of BaFexAl12 xO19 (x g 7) samples exhibited higher activity. 679
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’ AUTHOR INFORMATION
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Corresponding Author
*Tel.: +86 411 84379015. Fax: +86 411 84691570. E-mail:
[email protected] (T.Z.);
[email protected] (X.W.).
’ ACKNOWLEDGMENT Financial support from the National Science Foundation of China (No. 20773122 and No. 21076211), External Cooperation Program of Chinese Academy of Sciences (No. GJHZ200827), and Chinese Academy of Sciences for “100 Talents” project is greatly acknowledged. ’ REFERENCES (1) Arai, H.; Machida, M. Appl. Catal., A 1996, 138, 161–176. (2) Eguchi, K.; Arai, H. Catal. Today 1996, 29, 379–386. (3) Ren, X. G.; Zheng, J. D.; Song, Y. J.; Liu, P. Catal. Commun. 2008, 9, 807–810. (4) Zarur, A. J.; Hwu, H. H.; Ying, J. Y. Langmuir 2000, 16, 3042–3049. (5) Zarur, A. J.; Ying, J. Y. Nature 2000, 403, 65–67. (6) Xu, Z. L.; Zhen, M.; Bi, Y. L.; Zhen, K. J. Catal. Lett. 2000, 64, 157–161. (7) Chu, W. L.; Yang, W. S.; Lin, L. W. Appl. Catal., A 2002, 235, 39–45. (8) Kikuchi, R.; Iwasa, Y.; Takeguchi, T.; Eguchi, K. Appl. Catal., A 2005, 281, 61–67. (9) Gardner, T. H.; Spivey, J. J.; Campos, A.; Hissam, J. C.; Kugler, E. L.; Roy, A. D. Catal. Today 2010, 157, 166–169. (10) Gardner, T. H.; Spivey, J. J.; Kugler, E. L.; Campos, A.; Hissam, J. C.; Roy, A. D. J. Phys. Chem. C 2010, 114, 7888–7894. (11) Santiago, M.; Hevia, M. A. G.; Perez-Ramírez, J. Appl. Catal., B 2009, 90, 83–88. (12) Perez-Ramírez, J.; Santiago, M. Chem. Commun. 2007, 619–621. (13) Santiago, M.; Perez-Ramírez, J. Environ. Sci. Technol. 2007, 41, 1704–1709. (14) Santiago, M.; Groen, J. C.; Perez-Ramírez, J. J. Catal. 2008, 257, 152–162. (15) Zhu, S. M.; Wang, X. D.; Wang, A. Q.; Cong, Y.; Zhang, T. Chem. Commun. 2007, 1695–1697. (16) Zhu, S. M.; Wang, X. D.; Wang, A. Q.; Zhang, T. Catal. Today 2008, 131, 339–346. (17) Tian, M.; Wang, A. Q.; Wang, X. D.; Zhu, Y. Y.; Zhang, T. Appl. Catal., B 2009, 92, 437–444. (18) Tian, M.; Wang, X. D.; Zhu, Y. Y.; Wang, J. H.; Zhang, T. Chin. J. Catal. 2010, 31, 100–105. (19) Zhu, Y. Y.; Wang, X. D.; Wang, A. Q.; Wu, G. T.; Wang, J. H.; Zhang, T. J. Catal. 2011, 283, 149–160. (20) Zhu, Y. Y.; Wang, X. D.; Zhang, Y.; Wang, J. H.; Huang, Y. Q.; Kappenstein, C.; Zhang, T. Appl. Catal., A 2011, 409-410, 194–201. (21) Iyi, N.; Takekawa, S.; Kimura, S. J. Solid State Chem. 1989, 83, 8–19. (22) Bellotto, M.; Artioli, G.; Cristiani, C.; Forzatti, P.; Groppi, G. J. Catal. 1998, 179, 597–605. (23) Artizzu-Duart, P.; Millet, J. M.; Guilhaume, N.; Garbowski, E.; Primet, M. Catal. Today 2000, 59, 163–177. (24) Artizzu-Duart, P.; Brulle, Y.; Gaillard, F.; Garbowski, E.; Guilhaume, N.; Primet, M. Catal. Today 1999, 54, 181–190. (25) Artizzu, P.; Guilhaume, N.; Garbowski, E.; Brulle, Y.; Primet, M. Catal. Lett. 1998, 51, 69–75. (26) Belous, A. G.; V’yunov, O. I.; Pashkova, E. V.; Ivanitskii, V. P.; Gavrilenko, O. N. J. Phys. Chem. B 2006, 110, 26477–26481. (27) Qiu, J. X.; Wang, Y.; Gu, M. Y. Mater. Lett. 2006, 60, 2728–2732. (28) Gonzalez-Angeles, A.; Mendoza-Suarez, G.; Gruskova, A.; Toth, I.; Jancarik, V.; Papanova, M.; Escalante-García, J. I. J. Magn. Magn. Mater. 2004, 270, 77–83. 680
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