Identification of the Crystallographic Sites of Ir in BaIr0. 2FeAl10. 8O19

Article pubs.acs.org/JPCC

Identification of the Crystallographic Sites of Ir in BaIr0.2FeAl10.8O19 Hexaaluminate Yanyan Zhu,† Xiaodong Wang,*,† Yanqiang Huang,† Yan Zhang,†,‡ Guotao Wu,† Junhu Wang,† and Tao Zhang*,† †

State Key Laboratory of Catalysis, Dalian Institute of Chemical Physics, Chinese Academy of Sciences, 457 Zhongshan Road, Dalian 116023, P. R. China ‡ Graduate University of Chinese Academy of Sciences, Beijing 100049, P. R. China ABSTRACT: Noble metal ions in hexaaluminate structure are generally regarded as active centers for a variety of reactions, but their chemical states are still not clear for lack of effective characterization techniques. In this paper, in comparison with BaFeAl11O19, the crystallographic site of Ir in BaIr0.2FeAl10.8O19 hexaaluminate with Ba-βI-Al2O3 structure was identified by Rietveld refinement and 57Fe Mö ssbauer spectroscopy. In the BaIr0.2FeAl10.8O19 hexaaluminate structure, Fe occupied both the symmetric tetrahedral Al(2) sites in the spinel block and the distorted tetrahedral interstitial Al(5) sites in the mirror plane. In contrast to Fe, the framework Ir ions only occupied the distorted tetrahedral interstitial Al(5) sites in the loosely packed mirror plane, which originated from Ir ions in oxidic entities dispersed on the Ba-modified γ-Al2O3 in the precursor. Ir ions in the Al(5) sites were highly active for N2O decomposition.

1. INTRODUCTION N2O is a major greenhouse gas and its abatement has been a topic of environmental relevance due to its adverse contribution to global warming and ozone depletion.1 The largest source of N2O in the chemical industry (400 kt of N2O/ year) is the nitric acid plants, where N2O in low concentration (ppm level) is formed as a byproduct in the catalytic oxidation of ammonia over Pt−Rh alloy gauzes.2 Direct decomposition of N2O below the noble metal gauzes in the ammonia burner is regarded as a cost-effective abatement measure for existing plants. For such a process, extremely good thermal stability is required for the catalysts to withstand high temperatures more than 800 °C.2,3 On the other hand, N2O is also a promising green propellant used for small satellite propulsion systems, due to its low toxicity compared with traditional hydrazine propellant, as well as its capability for self-pressurizing and compatibility with the common construction materials.4−6 The chemistry of N2O as a propellant lies in the decomposition of N2O into N2 and O2 accompanied with a large amount of heat release and volume expansion, which can be used as propulsion power. Thus, the N2O used as the propellant must have a high concentration, even being a pure chemical, so as to generate a propulsion power as high as possible. However, the decomposition of pure N2O is a highly exothermic reaction (the enthalpy is −82 kJ/mol) and leads to a temperature rise over 1000 °C.4,7 Therefore, for the N2O decomposition to be applicable in both nitric acid plants and propulsion systems, besides activity, the high-temperature stability of catalytic materials is the common challenge. Metal-substituted hexaaluminates, due to remarkable resistance to sintering and thermal shock, have attracted much © 2012 American Chemical Society

attention in high-temperature catalytic processing, e.g., catalytic combustion of methane8−12 and partial oxidation of meth́ and Zhang’s groups found ane.13−15 Recently, Pérez-Ramirez’s that metal-substituted hexaaluminates also possessed good catalytic activity and thermal stability both in low concentration (1500 ppm) N2O abatement2,3,16 and in high concentration (30% v/v) N2O decomposition.17−23 These unique excellent properties are associated with their peculiar layered structure, consisting of alternate stacking along the c-axis of closely packed spinel block and the loosely packed mirror plane in which large cations are located. There are two typical hexaaluminate structures depending on the composition and the defectivity of the mirror plane, namely, β-Al2O3 and magnetoplumbite (MP), corresponding to the same space group P63/mmc (No. 194).24,25 The chemical formula can be represented by AMxAl12‑xO19, where A stands for a large alkaline, alkaline-earth, or rare-earth cation and M represents a transition or noble metal ion (e.g., Mn, Fe, Ni, Co, Cu, Ir, Ru) in the Al crystallographic site, which acts as an active center for a variety of catalytic reactions. So far, most of the reports have been concentrated on transition-metal substituted hexaaluminates, in which the oxide state and crystallographic site of substituted metal in structure were found to be closely related with the catalytic activity. Groppi et al.12 and Artizzu-Duart et al.26 observed the enhanced CH4 combustion activity with the increase of Mn3+/Mn2+ ratio. Our previous work19 also revealed that the Received: January 21, 2012 Revised: August 21, 2012 Published: October 2, 2012 24487

dx.doi.org/10.1021/jp3007238 | J. Phys. Chem. C 2012, 116, 24487−24495

The Journal of Physical Chemistry C

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octahedral Mn3+ in Al(1) site was much more active than tetrahedral Mn2+ in Al(2) site for N2O decomposition. Naoufal et al.27 tentatively attributed the methane combustion activity to Fe3+ species in S1 and S2 sites instead of those in S3 and S4 sites. Our work22 also indicated that Fe ions in the Al(5) sites of β-Al2O3 and the Al(3) sites of MP phase were more active for N2O decomposition than Fe ions in other crystallographic sites. Recently, it is found that the incorporation of active noble metal into the sintering-resistant hexaaluminate lattice can stabilize the noble metal against volatilization and sintering and then improve the catalytic activity. This endows the noble metal substituted hexaaluminate a combined advantage: high stability and high activity. Kikuchi et al.14 revealed that the framework Ru improved the partial oxidation of CH4, while Pd species was not incorporated in the hexaaluminate structure and the sintering of Pd resulted in the deterioration of the catalytic activity. Our previous work17,18 also suggested that the activity of N2O decomposition was greatly increased by incorporating Ir into BaFeAl11O19 compared with Ir outside the framework. Evidently, these experimental findings elucidated that the noble metal ions in the hexaaluminate structure possess peculiar excellent properties. Nevertheless, there is still not a clear picture on which Al crystallographic sites are preferentially occupied by noble metal ions and thus their effect on the catalytic activity due to the difficulty in characterization. Rietveld refinement is a sensitive and widely used technique for doped crystal compounds.28,29 In our previously developed BaIrxFe1‑xAl11O19 hexaaluminates,17,18 Ir species inside/outside the hexaaluminate structure may influence the chemical state of framework Fe to different degrees. In particular, such a difference may be magnified under a reducing atmosphere and then was identified by sensitive 57Fe Mössbauer spectroscopy,30,31 since the Mössbauer parameters of Fe2+ and Fe3+ were very different.32−34 Therefore, the 57Fe Mössbauer spectroscopy under reduced conditions may be employed to investigate the crystallographic site of Ir through the analysis of the chemical state of Fe in the hexaaluminate structure, in combination with Rietveld refinement. In this paper, BaIr0.2FeAl10.8O19 was employed as a model catalyst, in comparison with our previously reported BaFeAl11O19,22,23 to explore the crystallographic sites of Ir in barium hexaaluminate by means of Rietveld refinement and 57Fe Mö ssbauer spectroscopy, which were then correlated to the catalytic performance for high-concentration N2O decomposition.

a mixture of concentrated acids (HF + HCl + HNO3). ICP analysis for mother liquors and washing waters indicated that the precipitation of Ba, Fe, and Al contents occurred in a quantitative way, but a part of Ir component was lost due to insufficient solubilization during coprecipitation. During calcination, the actual Fe/Ba ratios kept constant and corresponded to the nominal ones (1/1), but Ir loading continuously decreased with increasing calcination temperature due to the evaporation loss. After 1200 °C calcination, the residual Ir loading was only 0.37 wt %, significantly lower than the initial value of 1.32 wt % (the value for the sample calcined at 500 °C). For comparison, the Ir-free BaFeAl11O19-t (denoted as BF1At) samples were prepared by the above-described procedure but without H2IrCl6 in the starting materials. In addition, Ir0.2/ BF1A sample was prepared by impregnating a desired amount of H2IrCl6 onto BF1A-1200 to make the theoretical molar ratio of Ir/Ba also equal to 0.2, like BI0.2F1A-t, which was then dried and calcined in air at 500 °C for 4 h. The calcination at lower temperature (500 °C) only made the Ir component outside the hexaaluminate structure, instead of being incorporated into the framework of hexaaluminate. ICP results indicated that the Ir loading in obtained Ir0.2/BF1A was 1.22 wt %, which was similar to that in BI0.2F1A-500 (1.32 wt %) but higher than that in BI0.2F1A-1200 (0.37 wt %). It is known that both Ir loading and Ir location influence the reduction of Fe3+ ions through H2 spillover effect. In order to avoid the disturbance of different Ir loading on the identification of Ir location in the 57 Fe Mössbauer spectroscopy section, we also prepared 0.37 wt % Ir/BF1A sample using the above-described impregnation method, which possessed the same Ir loading of 0.37 wt % as that in BI0.2F1A-1200. The important difference between 0.37 wt % Ir/BF1A and BI0.2F1A-1200 lies in the location of the Ir. In the former, Ir only existed outside the BF1A-1200 hexaaluminate, whereas in the latter, besides outside Ir, part of Ir species entered into the BF1A-1200 hexaaluminate structure after high-temperature treatment (1200 °C). 2.2. Catalyst Characterization. The X-ray diffraction (XRD) patterns were recorded with a PANalytical X’Pert-Pro powder X-ray diffractormeter, 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 = 10° and 80°. Before XRD test, the sample was ground roundly to a fine powder and then pressed lightly using a glass slide, in order to alleviate the preferential orientation as far as possible arising from the anisotropic planar crystallites of hexaaluminates. The intensity distribution of obtained XRD pattern is close to that of standard Ba0.75Al11O17.25 hexaaluminate (ICSD No. 29441, JCPDS No. 1-75-707). The PDF database was used for phase analysis. Phase composition and the crystallographic sites of Fe and Ir ions were adjusted by Rietveld full-profile analysis using FULLPROF and RIETICA programs. In the Rietveld refinement process, the contributions of both the Kα1 (λ = 1.540 56 Å) and Kα2 (λ = 1.544 39 Å) radiation were considered, and the intensity ratio of Kα2/Kα1 was fixed to 0.5. Diffraction line profiles were approximated by the 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 on the basis of

2. EXPERIMENTAL SECTION 2.1. Catalyst Preparation. BaIr0.2FeAl10.8O19-t sample (denoted as BI0.2F1A-t, where t indicates the calcination temperature) was prepared using the carbonate route as reported.19,22,35 Ba(NO3)2, H2IrCl6, Fe(NO3)3·9H2O, and Al(NO3)3·9H2O with the molar ratio 1:0.2:1:10.8 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 dried at 120 °C overnight. The sample was then calcined in air at 500, 700, 900, 1000, 1100, and 1200 °C for 4 h, to obtain BI0.2F1A-500, BI0.2F1A-700, BI0.2F1A-900, BI0.2F1A-1000, BI0.2F1A-1100, and BI0.2F1A-1200, respectively. The actual metal loadings of the catalysts were determined by Thermo IRIS Intrepid II inductively coupled plasma (ICP) after having dissolved solid in 24488



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further calcination at 900 and 1000 °C, intense XRD peaks corresponding to BaAl2O4 [JCPDS 1-72-387] were clearly observed, indicating that parts of Ba compounds and γ-Al2O3 were transformed into BaAl2O4. The cell parameters a (a = b) of BaAl2O4 in BI0.2F1A-900 and BI0.2F1A-1000 were 10.441 and 10.452 Å, respectively, which was very close to that in Irfree BF1A-t (t = 900 and 1000 °C, a = 10.441−10.453 Å).22 This indicated that the larger Ir ions [r(Ir4+) = 0.68 Å] were not incorporated into the rigid BaAl2O4 structure, probably due to the larger difference of radius and charge with Al ions [r(Al3+) = 0.51 Å]. Upon raising the calcination temperature to t ≥ 1100 °C, it is noteworthy that the formation of a β-Al 2 O 3 hexaaluminate phase took place together with the consumption of IrO2, γ-Al2O3, and BaAl2O4. It was reported36 that the formation of Ba-hexaaluminate prepared by the coprecipitation method herein adopted occurred via solid-state reactions between γ-Al2O3 and dispersed barium compounds or BaAl2O4. Therefore, Ir species in oxidic entities dispersed on the Ba-modified γ-Al2O3 in the precursor may enter into the hexaaluminate structure via the diffusion of barium in γ-Al2O3 matrix during calcination. For clarity, Figure 2 displays the XRD patterns of BF1A1200, BI0.2F1A-1200, and Ir0.2/BF1A samples in the 26°−41°

the references (ICSD No. 29441 for β-Al2O3 phase, ICSD No. 75426 for BaAl2O4 phase and ICSD No. 84577 for IrO2 phase). Crystalline dimensions were determined from the Scherrer equation. 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 Mössbauer spectra were recorded at room temperature with the spectrometer working in constant acceleration mode with the use of 57Co γ-quantum source in a Rh matrix. The absorbers were obtained by pressing the powdered samples (about 10 mg/cm2 of natural iron). Samples after H2 reduction were protected under Ar during Mössbauer measurements. 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. 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.17−19 Prior to the reaction, the catalyst sample was prereduced with pure H2 at 400 °C for 2 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.

3. RESULTS AND DISCUSSION 3.1. Phase Composition and Morphology. Figure 1 shows the XRD patterns of BI0.2F1A-t samples obtained by

Figure 2. X-ray diffraction patterns of BF1A-1200, BI0.2F1A-1200, and Ir0.2/BF1A samples in the 26°−41° (2θ) range.

(2θ) range. Compared with BF1A-1200 [a = b = 5.6141(2) Å, c = 22.775(1) Å],23 the pattern of BI0.2F1A-1200 slightly shifted to lower 2θ values, corresponding to the enlargement of the cell parameters [a = 5.6173(2) Å, c = 22.781(1) Å] of β-Al2O3 hexaaluminate, confirming that iridium cations have been incorporated into the hexaaluminate lattice by replacing smaller Al3+ ions. Since the oxidation state of iridium in the H2IrCl6 precursor is Ir4+, which is the most stable oxidation state of iridium in an oxide lattice,37 the introduced iridium in hexaaluminate structure probably existed as Ir4+. In contrast to BI0.2F1A-1200, the broad remarkable IrO2 peaks at around 28° and 35° were detected in Ir0.2/BF1A sample without the enlargement of the cell parameter of the β-Al2O3 phase, implying that 1.22 wt % Ir loading determined by ICP mainly existed as IrO2 with a crystalline size of about 34 nm outside the framework of BF1A-1200 hexaaluminate. Table 1 lists the BET surface area and Ir content of BI0.2F1A-t and Ir0.2/BF1A samples. As expected, the BET surface areas of BI0.2F1A-t decreased continuously with the

Figure 1. X-ray diffraction patterns of BI0.2F1A-t samples.

calcination of precursors at different temperatures. It can be seen that 500 and 700 °C calcination resulted in IrO2 [JCPDS 1-86-330] and γ-Al2O3 [JCPDS 1-1308] crystalline phases, and no Fe oxides were detected by XRD. Taking into account the molar ratio of Ba:Ir:Fe:Al = 1:0.2:1:10.8 and the high BET surface areas (193−288 m2/g) of both samples, Ir and Fe species should be dispersed on the Ba-modified γ-Al2O3. After 24489



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Table 1. BET Surface Area and Ir Content of BI0.2F1A-t and Ir0.2/BF1A Samples catalysts

surface area (m2/g)

Ir content (wt %)

BI0.2F1A-500 BI0.2F1A-700 BI0.2F1A-900 BI0.2F1A-1000 BI0.2F1A-1100 BI0.2F1A-1200 Ir0.2/BF1A

288 193 86 71 21 15 16

1.32 1.01 0.84 0.63 0.45 0.37 1.22

elevation of calcination temperature. In particular, significant loss of the surface areas took place when the samples were calcined at 900 and 1100 °C. As described above, phase transformation started at these temperatures, i.e., γ-Al2O3 and dispersed Ba compounds were transformed into BaAl2O4 at 900 °C, while γ-Al2O3, IrO2, and BaAl2O4 were transformed into Irsubstituted β-Al2O3 hexaaluminate at 1100 °C. After 1200 °C calcination, the BI0.2F1A sample can still preserve large surface area (15 m2/g), demonstrating the outstanding sinteringresistant property of the hexaaluminates. This could be further visualized from the SEM images in Figure 3. Both BI0.2F1A1200 and Ir0.2/BF1A (16 m2/g) samples showed characteristic anisotropic planar crystallites of hexaaluminate with large aspect ratio. The observed planar facets should have orientations parallel to the Ba-containing mirror plane resulting from the anisotropic crystal growth due to the peculiar layered structure of hexaaluminate.38−40 In addition, there was a decrease of Ir content with increasing calcination temperature, as shown in Table 1, implying the evaporation of partial Ir component during calcination. However, after 1100 °C calcination with the formation of hexaaluminate phase, the evaporation of Ir component was suppressed to some extent, probably due to the strong interaction between framework Ir and the hexaaluminate lattice. 3.2. Rietveld Refinement. To identify the crystallographic sites of Ir in β-Al2O3 hexaaluminate structure, Rietveld analysis of the XRD powder data was performed for BI0.2F1A-1200 (Figure 4), and the corresponding results are summarized in Tables 2 and 3. In comparison, Rietveld refinement results of previously reported BF1A-120022,23 were also shown. From

Figure 4. X-ray diffraction pattern fitted using Rietveld refinement method for BI0.2F1A-1200 sample; the upper tick marks correspond to β-Al2O3, the middle tick marks correspond to BaAl2O4, and the lower tick marks correspond to IrO2.

Table 2. Rietveld Refinement Results of BF1A-1200 and BI0.2F1A-1200 Samples results

BF1A-1200

BI0.2F1A-1200

Rp Rwp phase composition (molar percentage) composition of βI-Al2O3 phase a0 = b0 (Å) c0 (Å)

12.115 15.619 98.41 ± 0.92% βIAl2O3a, 1.59 ± 0.43% BaFe0.6Al1.4O4

11.381 15.145 98.17 ± 0.88% βI-Al2O3, 1.47 ± 0.39% BaFe0.6Al1.4O4, 0.36 ± 0.11% IrO2

Ba0.79Fe0.79Al10.21O17.21

Ba0.76Ir0.01Fe0.76Al10.22O17.24

5.6141(2) 22.775(1)

5.6173(2) 22.781(1)

a

One type of β-Al2O3 hexaaluminate.

Table 2, one can see that the BI0.2F1A-1200 sample consisted of 98.17 ± 0.88 mol % β-Al2O3 hexaaluminate, 1.47 ± 0.39 mol % BaFe0.6Al1.4O4, and 0.36 ± 0.11 mol % IrO2 phase. It is reported39 that unsubstituted Ba-β-Al2O3 hexaaluminate with Al/Ba ratio in the range of 9−14 is constituted by

Figure 3. Scanning electron micrographs of (a) BI0.2F1A-1200 and (b) Ir0.2/BF1A samples. 24490



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Table 3. Rietveld Refined Occupancy of Ba, Al, Fe, and Ir in βI-Al2O3 Structure of BF1A-1200 and BI0.2F1A-1200 Samples atomic coordinatesc atom site

mult

Ba(1) Al(1) Al(2) Al(3) Al(4) Al(5) Fe in Al(2) Fe in Al(5) Ir in Al(5) O(1) O(2) O(3) O(4) O(5) O(6)

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

a

b

site

coor

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

− Ohd The Th Oh Th Th Th Th − − − − − −

occupancy

x

y

z

BF1A-1200

BI0.2F1A-1200

0.3333 0.1676 0.3333 0.3333 0.0 0.8453 0.3333 0.8453 0.8453 0.00 0.3333 0.3333 0.1582 0.5036 0.8855

0.6667 0.3351 0.6667 0.6667 0.0 0.6906 0.6667 0.6906 0.6906 0.00 0.6667 0.6667 0.3164 0.0073 0.7710

0.75 −0.1046 0.0235 0.1756 0.0 0.1780 0.0235 0.1780 0.1780 0.1413 −0.0554 0.2500 0.0495 0.1474 0.2500

0.787(4) 0.929(15) 0.799(8) 1.00 1.00 0.006(1) 0.201(2) 0.065(1) 0.00 1.00 1.00 1.00 1.00 1.00 0.071(1)

0.757(3) 0.919(13) 0.798(8) 1.00 1.00 0.019(1) 0.202(2) 0.060(1) 0.002 1.00 1.00 1.00 1.00 1.00 0.081(1)

51:49

53:7

ratiof a

b

c

d

e

f

Multiplicity. Coordination. Atomic coordinates taken from reference (ICSD No. 29441). Octahedral. Tetrahedral. Ratio of Fe ions in the Al(2) sites to those in the Al(5) sites. Constrains on occupancies of βI-Al2O3 phase relevant to the Reidinger defect mechanism [Ba(1) + 3Al(5) = 1, Al(5) = O(6), Al(1) + Al(5) = 1, where the Fe and Ir occupancy in Al(5) must be included] in ref 11.

defective Ba-poor βI phase and Ba-rich βII phase depending on Ba content, with composition Ba 0. 72 Al 1 1 O 1 7.2 8 and Ba1.16Al10.68O17.16, respectively. On the basis of the XRD data, the Ba-poor Ba-βI-Al2O3 structure39,41 was chosen as starting model for the Rietveld refinement of BI0.2F1A-1200. The analysis results confirmed that a Ba-poor βI-Al2O3 phase with Ba0.76Ir0.01Fe0.76Al10.22O17.24 (Table 2) indeed formed despite the large amount of Ba, in line with previous literature.11,22 In Ba-poor βI-Al2O3 structure (Figure 5), 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.41 Refinements of the Al occupancies (Table 3) indicated that Fe ions occupied the symmetric tetrahedral Al(2) sites in the spinel block and the distorted tetrahedral interstitial Al(5) sites in the mirror plane with occupancy of 20.2% and 6.0%, respectively, similar to the distribution of Fe in BF1A-1200 (20.1% and 6.5%).22,23 In contrast to Fe, Ir ions only occupied the tetrahedral interstitial Al(5) sites in the mirror plane, while Al(1), Al(2), Al(3), and Al(4) sites did not show any significant evidence of Ir substitution. Such a difference should originate from different chemical states of Ir and Fe in the precursors. On the basis of the formation mechanism of barium hexaaluminate,22,36 both Ir and Fe in the distorted tetrahedral interstitial Al(5) sites in the mirror plane should originate from Ir and Fe species in oxidic entities dispersed on the Ba-modified γ-Al2O3 in the precursor via the diffusion of Ba2+ ions in γ-Al2O3, since the mirror plane is loosely packed in a way favorable to ion diffusion.22,42,43 Meanwhile, Fe ions in the tetrahedral Al(2) sites in the spinel block originated from Fe ions located in the intermediate BaAl2O4,22 due to the strong structural similarity between the spinel-type BaAl2O4 and the spinel block of hexaaluminate. Larger Ir ions were not incorporated into the intermediate rigid BaAl2O4 structure due to the large difference of radius and charge with Al3+ ions, and thus the occupation of Ir ions in the tetrahedral Al(2) sites in the spinel block was not observed.

Figure 5. The structure of βI-Al2O3 hexaaluminate. Numbers in parentheses refer to the different Al sites. Al(1) and Al(4), octahedral sites; Al(2), Al(3), and Al(5), tetrahedral sites.

3.3. 57Fe Mö ssbauer Spectroscopy. Since both Ir and Fe ions were coordinated with the lattice oxygen in the hexaaluminate structure, the framework Ir ions may influence the reduction of framework Fe ions in the Al(2) and Al(5) sites to different degrees, which could be identified by sensitive 57Fe Mössbauer spectroscopy, inversely facilitating the investigation of the chemical state of Ir in the hexaaluminate structure. Figure 6 displays the 57Fe Mössbauer spectra of BI0.2F1A-1200 after 24491



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Figure 6. Room temperature 57Fe Mössbauer spectra of (a) BF1A-1200, (b) BI0.2F1A-1200, and (c) 0.37 wt % Ir/BF1A after H2 reduction at different temperatures for 2 h.

(1.00−1.01 mm/s) of the other doublet was relatively high, which was assigned to Fe3+ ions in the distorted tetrahedral interstitial Al(5) sites resulting from a Reidinger defect, since the higher QS value means the greater extent of electric field distortion around Fe3+ ions.27,44 After reduction with H2 at 350 °C for 2 h, one new iron component with IS = 0.91 mm/s and QS = 1.11 mm/s appeared, which was assigned to Fe2+ species.32−34 In comparison with the relative area (A) of Fe3+ ions in the Al(2) and Al(5) sites of βI-Al2O3 over the calcined sample (51% and 49%), the corresponding values decreased to 41% and 28% after H2 reduction at 350 °C. Namely, 21% Fe3+ ions in the Al(5) sites in the mirror plane were reduced to Fe2+, much larger than Fe2+ ions (10%) in the Al(2) sites in the rigid spinel block (Table 5). Obviously, Fe3+ ions in the mirror plane were preferentially reduced. Two important factors should be considered, that is, the loosely packed mirror plane of hexaaluminate is a preferentially exposed surface38,39 accessible to gas-phase H2, and the mirror plane is also a preferential diffusion route of oxygen42,43 favorable for the removability of active oxygen.

H2 reduction at different temperatures for 2 h. For comparison, the reduced Ir-free BF1A-1200 and 0.37 wt % Ir/BF1A samples were also characterized by 57Fe Mössbauer spectroscopy. In the latter, the Ir loading was comparable to that of BI0.2F1A-1200, and the difference lain in that the Ir species in 0.37 wt % Ir/ BF1A did not enter into the hexaaluminate structure. The computer-fitted Mössbauer parameters are summarized in Table 4. For the calcined BF1A-1200 and reduced samples under H2 atmosphere at 250−300 °C for 2 h (Figure 6a), the observed two quadrupole doublets were assigned to Fe3+ species in βIAl2O3 structure based on the IS values.30,31 The IS values (0.21−0.23 mm/s) were small and much more related to that (IS = 0.18−0.19 mm/s) corresponding to Fe3+ ions in tetrahedral sites [denoted as Fe3+(Th)] reported in CaFe1.2Al10.8O1944 and LaFeAl11O19.45 This indicated that Fe3+ ions occupied Th sites in βI-Al2O3 structure, in line with our previous Rietveld refinement results.22 One doublet with small QS value (0.60−0.61 mm/s) was assigned to Fe3+(Th) in symmetric Al(2) sites of the βI-Al2O3 structure. The QS value 24492



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Table 4. Room Temperature 57Fe Mössbauer Parameters of BF1A-1200, BI0.2F1A-1200, and 0.37 wt % Ir/BF1A Samples after H2 Reduction at Different Temperatures for 2 h a

samples BF1A-1200

BF1A-1200-red 250 °Cd BF1A-1200-red 300 °C

BF1A-1200-red 350 °C

BI0.2F1A-1200

BI0.2F1A-1200-red 250 °C

BI0.2F1A-1200-red 300 °C

BI0.2F1A-1200-red 350 °C

0.37 wt % Ir/BF1A

0.37 wt % Ir/BF1A-red 250 °C

0.37 wt % Ir/BF1A-red 300 °C

0.37 wt % Ir/BF1A-red 350 °C

b

IS (mm/s)

QS (mm/s)

c

A (%)

0.23

1.01

49

0.21

0.60

51

0.23

1.00

49

0.22

0.61

51

0.23

1.00

49

0.21

0.60

51

0.25

0.98

28

0.23

0.62

41

0.91 0.23

1.11 1.02

31 47

0.22

0.62

53

0.21

1.04

36

0.21

0.63

51

0.80 0.21

1.10 1.06

13 28

0.22

0.69

47

0.84 0.23

1.10 1.07

25 18

0.26

0.73

43

0.90 0.23

1.11 1.01

39 49

0.21

0.60

51

0.23

0.98

43

0.22

0.64

49

0.85 0.23

1.14 1.00

8 39

0.22

0.64

47

0.86 0.22

1.15 1.07

14 25

0.23

0.69

39

1.09

36

0.87

Table 5. Reducibility of Fe3+ Ions in Different Crystallographic Sites in Reduced BF1A-1200, BI0.2F1A1200, and 0.37 wt % Ir/BF1A Samples

assignment 3+

Fe (Th) in βI-Al2O3 Fe3+(Th) in βI-Al2O3 Fe3+(Th) in βI-Al2O3 Fe3+(Th) in βI-Al2O3 Fe3+(Th) in βI-Al2O3 Fe3+(Th) in βI-Al2O3 Fe3+(Th) in βI-Al2O3 Fe3+(Th) in βI-Al2O3 Fe2+ Fe3+(Th) in βI-Al2O3 Fe3+(Th) in βI-Al2O3 Fe3+(Th) in βI-Al2O3 Fe3+(Th) in βI-Al2O3 Fe2+ Fe3+(Th) in βI-Al2O3 Fe3+(Th) in βI-Al2O3 Fe2+ Fe3+(Th) in βI-Al2O3 Fe3+(Th) in βI-Al2O3 Fe2+ Fe3+(Th) in βI-Al2O3 Fe3+(Th) in βI-Al2O3 Fe3+(Th) in βI-Al2O3 Fe3+(Th) in βI-Al2O3 Fe2+ Fe3+(Th) in βI-Al2O3 Fe3+(Th) in βI-Al2O3 Fe2+ Fe3+(Th) in βI-Al2O3 Fe3+(Th) in βI-Al2O3 Fe2+

Al(5) of Al(2) of Al(5) of Al(2) of Al(5) of

Fe2+ in Al(5)

BF1A-1200-red 250 °Cb BF1A-1200-red 300 °C BF1A-1200-red 350 °C BI0.2F1A-1200-red 250 °C BI0.2F1A-1200-red 300 °C BI0.2F1A-1200-red 350 °C 0.37 wt % Ir/BF1A-red 250 °C 0.37 wt % Ir/BF1A-red 300 °C 0.37 wt % Ir/BF1A-red 350 °C

0 0 21 11 19 29 6 10 24

Fe2+ in Al(2) differencea 0 0 10 2 6 10 2 4 12

− − 11 9 13 19 4 6 12

a

Calculated through the amount of Fe2+ species in the Al(5) sites minus those in the Al(2) sites. bThe BF1A-1200 sample after H2 reduction at 250 °C.

Al(2) of Al(5) of

framework Fe3+ species. Compared with 0.37 wt % Ir/BF1A (8−36%), there were more Fe2+ ions in BI0.2F1A-1200 (13− 39%) at the same reduction temperature, implying that the promotion effect of framework Ir on the reduction of framework Fe3+ was larger than outside Ir. Note that although the preferential reduction of Fe3+ ions in Al(5) sites in the mirror plane also occurred in both BI0.2F1A-1200 and 0.37 wt % Ir/BF1A, the difference of reducible Fe3+ amount in the Al(5) and Al(2) sites in BI0.2F1A-1200 was as high as 9−19 (Table 5), much higher than those in 0.37 wt % Ir/BF1A (4− 12). This indicated that the framework Ir significantly promoted the selective reduction of Fe3+ ions in the Al(5) sites. It was reasonable considering that only Fe3+ ions occupied the Al(2) sites in the spinel block, and in the mirror plane both Ir and Fe ions were located in the Al(5) sites and coordinated with lattice oxygen. Under a reducing atmosphere, the adsorptive activation of H2 onto the framework Ir ions in the Al(5) sites in the mirror plane probably promoted the removability of nearby lattice oxygen in the loosely packed mirror plane (the preferential diffusion route of oxygen). This resulted in the preferential reduction of adjacent Fe3+ ions in Al(5) sites in the same mirror plane, instead of the remote Fe3+ ions in Al(2) sites in the rigid spinel block. The results once again confirmed that Ir ions in BI0.2F1A-1200 were located in the Al(5) sites in the mirror plane, in line with the Rietveld refinement (Table 3). Unlike BI0.2F1A-1200 with Ir in the Al(5) sites of hexaaluminate after high-temperature treatment (1200 °C), 0.37 wt % Ir/BF1A showed relative lower selective reduction of Fe3+ ions in the Al(5) sites (Table 5), probably because supported Ir was randomly dispersed outside the hexaaluminate structure after 500 °C calcination. 3.4. Catalytic Activity. Figure 7 depicts the profiles of 30 vol % N2O conversion versus the reaction temperature over BF1A-1200, BI0.2F1A-1200, Ir0.2/BF1A, and 0.37 wt % Ir/ BF1A samples. In the absence of Ir (x = 0), the BF1A-1200 did not exhibit activity toward N2O decomposition until 550 °C. When Ir species was supported on the BF1A-1200 (0.37 wt % Ir/BF1A and Ir0.2/BF1A), the catalytic activity was only slightly enhanced, although the Ir loading in the latter reached as high as 1.22 wt %. When Ir was incorporated into BI0.2F1A1200 hexaaluminate structure with a low Ir loading of 0.37 wt %, the activity significantly increased. N2O decomposition started even at 350 °C and almost full N2O conversion (94%) was obtained at 400 °C, suggesting that framework Ir greatly promoted the N2O decomposition activity. This was further

Al(2) of

Al(5) of Al(2) of Al(5) of Al(2) of

Al(5) of Al(2) of

Al(5) of Al(2) of

Al(5) of Al(2) of Al(5) of Al(2) of

Al(5) of Al(2) of

Al(5) of Al(2) of

Isomer shift relative to α-Fe. Quadrupole splitting. cRelative area. The BF1A-1200 sample after H2 reduction at 250 °C. Uncertainty is ±3% of the reported value.

a

samples

b

d

Different from BF1A-1200, Fe2+ species were detected in both BI0.2F1A-1200 and 0.37 wt % Ir/BF1A (Figure 6b,c, Table 4) even when the reduction temperature was only 250 °C, indicating that the presence of Ir facilitated the reduction of 24493



Article

adsorption of N2O. Moreover, our H2-reduced 57Fe Mössbauer results showed that the lattice oxygen close to Ir ions in the Al(5) sites in the mirror plane was much easier to move and diffuse, thus greatly facilitating the desorption of surface oxygen generated by the dissociative adsorption of N2O onto Ir ions in Al(5) sites, since the oxygen desorption is generally regarded as the rate-determining step for the N2O decomposition process.7,49−51 Therefore, Ir ions in Al(5) sites in both the preferentially exposed mirror plane and the preferential diffusion route of oxygen should be highly active for N2O decomposition.

4. CONCLUSIONS In BaIr0.2FeAl10.8O19 hexaaluminate with Ba-poor βI-Al2O3 structure, Fe occupied both the Al(2) sites in the spinel block and the Al(5) sites in the mirror plane. In contrast to Fe, Ir ions only occupied the distorted tetrahedral interstitial Al(5) sites in the loosely packed mirror plane, originating from Ir ions in oxidic entities dispersed on the Ba-modified γ-Al2O3 in the precursor. The framework Ir ions in the Al(5) sites significantly promoted the selective reduction of adjacent Fe3+ ions in the Al(5) sites in the mirror plane, instead of the remote Fe3+ ions in the Al(2) sites in the rigid spinel block. Ir ions in the Al(5) sites of βI-Al2O3 hexaaluminates were highly active for N2O decomposition.

Figure 7. N2O conversion as a function of reaction temperature over BF1A-1200, BI0.2F1A-1200, Ir0.2/BF1A, and 0.37 wt % Ir/BF1A catalysts.

confirmed by the calculated reaction rate at 350 °C on the basis of the amount of Ir in different locations (Table 6). When Ir species supported on hexaaluminate (Ir0.2/BF1A) exist as iridium oxide crystallites outside the hexaaluminate structure, the reaction rate was only 0.29 mol h−1 gIr−1. This was reasonable considering that Ir species outside the hexaaluminate easily agglomerated together during calcination, which was supported by the CO-chemisorption experiment, where outside Ir, either on the Ir0.2/BF1A or outside the framework of BI0.2F1A-1200, was too large to be able to adsorb CO molecule. Therefore, the low activity of outside Ir species can be attributed to its large particle size (34 nm for Ir0.2/BF1A, 48 nm for BI0.2F1A-1200) determined by the Scherrer formula. However, when Ir was incorporated into the hexaauminate lattice, the reaction rate reached as high as 3.11 mol h−1 gIr−1, suggesting that framework Ir ions in the Al(5) sites in the mirror plane of hexaaluminate were highly active for N2O decomposition. According to the literature,7,46−48 the decomposition of N2O can be described as the adsorption of N2O at the active site with the formation of N2 and a surface O. Subsequently, this surface oxygen desorbs either by combination with another surface oxygen or by direct reaction with another N2O. On the basis of the above mechanism, there are two main factors affecting the N2O decomposition activity of Ir-substituted barium hexaaluminate: the adsorption of N2O and the desorption of oxygen. Considering that the loosely packed mirror plane is a preferentially exposed surface,11,39 Ir ions incorporated in the Al(5) sites in the mirror plane were easily accessible to the gas-phase N2O molecules and favorable for the

■

AUTHOR INFORMATION

Corresponding Author

*X.W.: phone, +86 411 84379680; e-mail, [email protected]. T.Z.: phone, +86 411 84379015; fax, +86 411 84691570; email, [email protected]. Notes

The authors declare no competing financial interest.

■

ACKNOWLEDGMENTS Financial support from National Science Foundation of China (NSFC) grants (21076211 and 21103173), External Cooperation Program of Chinese Academy of Sciences (GJHZ200827), and Chinese Academy of Sciences for “100 Talents” project is greatly acknowledged.

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Table 6. Relationship between the Location of Ir and Catalytic Performances of Ir0.2/BF1A and BI0.2F1A-1200 Catalysts catalysts Ir0.2/BF1A BI0.2F1A-1200

total Ira mol/gCat −5

6.35 × 10 1.92 × 10−5

outside Irb mol/gCat

framework Irc mol/gCat

R1d mol h−1 gIr−1

R2e mol h−1 gIr−1

− 1.41 × 10−5

0.29 −

− 3.11

−5

6.35 × 10 0.51 × 10−5

a The value determined by ICP. bThe amount of Ir outside the hexaaluminate structure. The value in BI0.2F1A-1200 corresponds to 0.36% IrO2 in Table 2 determined by Rietveld refinement. cThe amount of Ir in the hexaaluminate structure, which was calculated through the total Ir amount minus the outside Ir amount. dReaction rate, which was calculated on the basis of the amount of Ir outside the hexaaluminate structure; the reaction temperature was 350 °C. eReaction rate, which was calculated on the basis of the amount of framework Ir in the hexaaluminate structure; the reaction temperature was 350 °C.

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Identification of the Crystallographic Sites of Ir in BaIr0. 2FeAl10. 8O19

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