One-pot hydrothermal preparation and catalytic performance of porous strontium ferrite hollow spheres for the combustion of toluene

Journal of Molecular Catalysis A: Chemical 370 (2013) 189–196

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One-pot hydrothermal preparation and catalytic performance of porous strontium ferrite hollow spheres for the combustion of toluene Kemeng Ji, Hongxing Dai ∗ , Jiguang Deng, Lei Zhang, Haiyan Jiang, Shaohua Xie, Wen Han Laboratory of Catalysis Chemistry and Nanoscience, Department of Chemistry and Chemical Engineering, College of Environmental and Energy Engineering, Beijing University of Technology, Beijing 100124, PR China

a r t i c l e

i n f o

Article history: Received 13 October 2012 Received in revised form 18 January 2013 Accepted 18 January 2013 Available online 26 January 2013 Keywords: Hydrothermal fabrication Hollow spheres Perovskite-type oxide Strontium ferrite Toluene combustion

a b s t r a c t Strontium ferrite (SFO) hollow spheres with or without porous shells were fabricated via a novel one-pot hydrothermal route with the assistance of glucose and/or ethylenediamine. Physicochemical properties of the materials were characterized by means of a number of analytical techniques, and their catalytic performance was evaluated for the combustion of toluene. It is shown that the SFO samples possessed an orthorhombic structure and displayed a porous hollow spherical morphology with a surface area of 18–27 m2 /g. The SFO sample (SFO-3) derived hydrothermally at 170 ◦ C with a glucose/ethylenediamine volumetric ratio of 0.3/1.0 exhibited the highest surface area and oxygen adspecies concentration and the best low-temperature reducibility. Among the SFO samples, the SFO-3 sample showed the best catalytic activity for toluene combustion, and the T10% , T50% , and T90% were ca. 145, 255, and 298 ◦ C at a space velocity of 20,000 mL/(g h), respectively. It is concluded that the good catalytic performance of SFO-3 was associated with its surface oxygen species concentration and better low-temperature reducibility. © 2013 Elsevier B.V. All rights reserved.

1. Introduction Volatile organic compounds (VOCs) emitted from industrial activities are major pollutants to the environment. Among various technologies (e.g., adsorption, absorption, biofiltration, pyrolysis, and catalytic combustion) for the control of VOC emissions, catalytic combustion is generally believed to be the most effective and promising [1]. In the past years, a large number of catalysts, such as supported noble metals, transition metal oxides, and perovskitetype oxides (ABO3 ), have been developed for the removal of VOCs [2]. Due to the high oxygen mobility, abundant oxygen vacancy, and long-term stability [3], ABO3 has been widely used as an effective solid catalyst for VOC removal [4]. Recently, SrFeO3−ı (SFO) have gained much attention in the photodegradation of phenol [5] and methyl orange [6], oxidative dehydrogenation of ethane [7], methane combustion [8], and electrochemical conversion [9] of methane because of its rich oxygen deficiency, high oxygen permeability, and high electronic conductivity [4]. It has been well known that the morphology of inorganic nanoparticles is a predominant factor influencing the physicochemical property of a material [3]. Thanks for the great synthetic flexibility and controllability of crystalline growth, the solvothermal process is regarded to be a powerful strategy to fabricate hollow

inorganic materials with enhanced catalytic activity, such as CaTiO3 [10], ZnSnO3 [11], and MFe2 O4 (M = Zn, Co, Ni, Cd) [12]. Xu and coworkers generated LnFeO3 (Ln = La, Pr–Tb) hollow spheres with porous shell via a citric acid-assisted hydrothermal route [13]. By using the templating approaches, one could obtain various hollow metal oxides [14]. For instance, adopting the hydrothermal method with carbonaceous saccharide microspheres as the template, some researchers prepared hollow Fe3 O4 @SiO2 [15], SnO2 [16], and MFe2 O4 (M = Zn, Co, Ni, Cd) [12]. Except for our studies on the fabrication and catalytic properties of three-dimensionally ordered macroporous (3DOM) [17] and 3D macroporous [18] SFO for toluene combustion, however, there are few reports on the catalytic applications in the removal of VOCs. Li and Sun [19] reported that carbon spheres could be prepared from glucose at a hydrothermal temperature of 160–180 ◦ C. According to this idea, we have recently developed a novel and facile one-pot hydrothermal strategy that exploits an in situ sacrificial template to fabricate SFO hollow spheres with or without porous shell. Herein, we report for the first time the fabrication, characterization, and catalytic performance of SFO hollow spheres for the combustion of toluene. 2. Experimental 2.1. Catalyst preparation

∗ Corresponding author. Tel.: +86 10 6739 6118; fax: +86 10 6739 1983. E-mail address: [email protected] (H. Dai). 1381-1169/$ – see front matter © 2013 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.molcata.2013.01.013

The SFO samples were prepared using the glucose-assisted hydrothermal strategy with metal nitrates as the precursor. In

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Table 1 Preparation conditions, crystallite sizes (D), BET surface areas, pore volumes, and average pore sizes of the SFO samples. Catalyst

SFO-0 SFO-1 SFO-2 SFO-3 SFO-4 a

Solution

0.5 mL EDA 0.5 mL EDA + 0.1 mol/L glucose 0.5 mL EDA + 0.3 mol/L glucose 1.0 mL EDA + 0.3 mol/L glucose 3.0 mL EDA + 0.3 mol/L glucose

Da (nm)

123.3 65.4 50.5 42.5 41.3

BET surface area (m2 /g)

Pore volume (cm3 /g)

Average pore size (nm)

Macropore (≥50 nm)

Mesopore (<50 nm)

Total

Macropore (≥50 nm)

Mesopore (<50 nm)

Total

0.9 1.6 2.2 1.6 1.7

10.5 16.6 17.0 24.9 8.2

11.4 18.2 19.2 26.5 9.9

0.012 0.021 0.030 0.031 0.044

0.009 0.010 0.006 0.006 0.006

0.021 0.031 0.036 0.037 0.050

9.8 11.6 15.1 36.7 26.4

The data were calculated according to the Scherrer’s equation using the FWHM of the (4 0 0) line for the SFO samples.

a typical preparation process, 0.01 mol of nitrates of strontium and iron were first dissolved in 25.0 mL of deionized water under magnetic stirring. After being well mixed, certain amounts of citric acid (total metal/citric acid molar ratio = 1/1), glucose (GLU), and ethylenediamine (EDA) were sequentially added to the above mixed solution, as shown in Table 1. Then, an ammonia (28 wt.%) solution was added dropwise to adjust the pH value of the mixed solution to be ca. 4.2. After being diluted to a total volume of 40.0 mL, the mixture was transferred into a 50-mL Teflon-lined stainless steel autoclave and placed in an oven for hydrothermal treatment at 170 ◦ C for 20 h. When the autoclave was naturally cooled to room temperature (RT), the obtained precursor was first dried at 120 ◦ C overnight and then well ground. Finally, the obtained black powders were calcined in a muffle furnace at a rate of 1 ◦ C/min from RT to 300 ◦ C and maintained at this temperature for 2 h, and further to 750 ◦ C and kept at this temperature for 4 h. All of the chemicals (A.R. in purity) were purchased from Beijing Chemical Reagent Company and used without further purification.

2.2. Catalyst characterization X-ray diffraction (XRD) patterns of the samples were recorded on a Bruker/AXS D8 Advance diffractometer operated at 40 kV and 40 mA using Cu K␣ radiation and a Ni filter ( = 0.15406 nm). Crystal phases were identified by referring the diffraction lines to those of the powder diffraction files – JCPDS-ICDD 2004 PDF Database. Thermogravimetric analysis (TGA) and differential scanning calorimetric (DSC) analysis were conducted in an air flow of 100 mL/min at a ramp of 15 ◦ C/min on a Seiko 6300 TG-DTA instrument. The temperature range was RT to 900 ◦ C. Fourier transform infrared (FT-IR) spectra of the samples in the range of 400–4000 cm−1 at a resolution of 0.1 cm−1 were measured on a Bruker Vertex 70 spectrometer. The scanning electron microscopic (SEM) images of the samples were recorded by a field emission-scanning electron microscopy (Gemini Zeiss Supra 55) operated at 10 kV, and the high-resolution transmission electron microscopic (HRTEM) images as well as the selected-area electron diffraction (SAED) patterns of the samples were collected on a JEOL-2010 instrument (operated at 200 kV). The nitrogen adsorption–desorption isotherms were measured under vacuum on a Micromeritics ASAP 2020 adsorption analyzer via N2 adsorption at −196 ◦ C, with the samples being outgassed at 250 ◦ C for 2 h. The surface areas and pore size distributions were calculated according to the Brunauer–Emmett–Teller (BET) and Barrett–Joyner–Halenda (BJH) methods, respectively. X-ray photoelectron spectroscopy (XPS, VG CLAM 4 MCD analyzer) was employed to determine the Sr 3d, Fe 2p, O 1s, and C 1s binding energies (BEs) of surface species of the samples, with Mg K␣ (h = 1253.6 eV) as the excitation source. Before XPS analysis, the samples were pretreated in an O2 flow of 20 mL/min at 500 ◦ C for 1 h to remove the surface carbonate and adsorbed water. The C 1s peak at 284.6 eV was taken as a reference for the BE calibration.

Hydrogen temperature-programmed reduction (H2 -TPR) was conducted on a Micromeritics AutoChem II 2920 apparatus in the RT to 900 ◦ C range with a sample (100 mg, 40–60 mesh) being placed in a U-shaped quartz microreactor (i.d. = 4 mm). Before each run, the sample was treated in O2 flow (30 mL/min) at 500 ◦ C for 1 h followed by cooling to RT under the same atmosphere. Then, the sample was reduced in a flow of 5% H2 /Ar (50 mL/min) at a heating rate of 10 ◦ C/min, with the effluent being monitored by a thermal conductivity detector (TCD). The TCD response was calibrated against the reduction of a standard CuO sample (Alfa Aesar, 99.9995%). 2.3. Catalytic evaluation Catalytic activities were measured in a continuously flow quartz fixed-bed microreactor (i.d. = 4 mm) at atmospheric pressure, with ca. 50 mg of the catalyst (40–60 mesh) and 0.3 g of quartz sands (40–60 mesh) being packed in the middle of the tubular microreactor. Before each measurement, the catalyst was pretreated at 80 ◦ C for 12 h. Unless specified otherwise, the total flow rate of the reactant feed (1000 ppm toluene + O2 + N2 (balance)), the toluene/O2 molar ratio, and the space velocity (SV) were 16.7 mL/min, 1/400, and ca. 20,000 mL/(g h), respectively. The flow rate was regulated by means of electronic mass flow controllers, and the 1000-ppm toluene fed to the microreactor was generated by a N2 flow passing through a toluene-filling container placed in an ice-water

Fig. 1. XRD patterns of (a) SFO-0, (b) SFO-1, (c) SFO-2, (d) SFO-3, and (e) SFO-4.

K. Ji et al. / Journal of Molecular Catalysis A: Chemical 370 (2013) 189–196

Fig. 2. SEM images of (a) SFO-0, (b and c) SFO-1, (d–f) SFO-2, (g–i) SFO-3, and (j–l) SFO-4.

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isothermal bath. The outlet gases were analyzed on-line by a gas chromatograph (GC-2010, Shimadzu) equipped with a hydrogen flame ionization detector (FID) and a TCD. A stabilwax@-DA capillary column (30 m in length) and a Carboxen 1000 packing column (3 m in length) were used for the separation of organics and permanent gases, respectively. For the variation in SV, the flow rate of N2 (balance) was adjusted at a fixed catalyst mass. The balance of carbon throughout the investigations was reckoned to be ca. 99.5%. 3. Results and discussion 3.1. Crystal phase composition Fig. 1 shows the XRD patterns of the as-prepared SFO samples under different conditions. By referring to the standard XRD pattern of orthorhombic SrFeO2.73 (JCPDS PDF# 40-0906), one can realize that (i) the diffraction lines of the SFO-0 sample could be well indexed as single-phase orthorhombic perovskite structure; and (ii) the other four SFO samples contained trace amounts of impurity phases SrFe12 O19 (JCPDS PDF# 80-1198) and SrCO3 (JCPDS PDF# 84-1778) in addition to the main orthorhombic perovskite phase. Moreover, when the concentration of glucose in the precursor solution increased, the diffraction peaks due to these impurities in the final products became stronger in intensity. The average crystallite sizes of the samples (Table 1) were dependent upon the concentration of glucose in the precursor solution, and decreased in the order of SFO-0 (123.3 nm) > SFO-1 (65.4 nm) > SFO-2 (50.5 nm) > SFO-3 (42.5 nm) > SFO-4 (41.3 nm). Obviously, the composition of the precursor solution had a great effect on the crystal size of the final product. The results (Figs. S1 and S2 of the Supplementary material) of TGA/DSC and FT-IR investigations reveal that the calcination temperature (750 ◦ C) was appropriate in the generation of a perovskite structure in the SFO samples. 3.2. Morphology, pore structure, and surface area Fig. 2 shows the typical SEM images of the SFO samples. It is observed that the SFO-0 sample was composed of a number of

interlaced nanoparticles with a size of 50–200 nm (Fig. 2a). When a certain amount of glucose was added to the precursor solution, however, the as-obtained SFO samples (except for the SFO-4 sample (Fig. 2j–l)) were porous hollow spheres with a size range from hundreds of nanometers to several micrometers (Fig. 2b–i), and the shells consisted of particles with different shapes (block-like in SFO-1 (Fig. 2c), rod-like in SFO-2 (Fig. 2f), and worm-like in SFO-3 (Fig. 2i)) and sizes (a few hundred nanometers). The thickness of the shells was 100–400 nm (insets in Fig. 2f and l). The contents of glucose and EDA exerted a great influence on the size of the sphere and the shape of the individual particle. To further observe the microstructures of the SFO hollow sphere samples, TEM analyses were also conducted, and the typical images of the SFO-0, SFO-1, SFO-2, and SFO-3 samples are shown in Fig. 3. The SFO-0 sample contained a number of regular cube-like nanoparticles with a size of 30–50 nm. The TEM images (Fig. 3c, e, and g) of the SFO-1, SFO-2, and SFO-3 samples clearly confirmed their hollow porous structures. Moreover, the well-resolved lattice spacings (Fig. 3b, d, f, and h) of these samples were measured to be ca. 0.19, 0.22, and 0.27 nm, corresponding to the (4 0 2), (4 2 0), and (4 0 0) planes of orthorhombic SrFeO3−ı , respectively. The recording of multiple bright electron diffraction rings in the SAED patterns (insets of Fig. 3b, d, f, and h) indicates the formation of polycrystalline SrFeO3−ı . Fig. 4 shows the nitrogen adsorption-desorption isotherms and pore-size distributions of the samples. It can be observed from Fig. 4A that all of the samples displayed a type II isotherm with a H3 hysteresis loop in the relative pressure (p/p0 ) range of 0.8–1.0, suggesting the presence of macroporous structure in these samples [20,21]. The appearance of a weak H2 hysteresis loop as well as a type I hysteresis loop in the p/p0 range of 0.2–0.8 was an indication of cylindrical-shaped mesopore generation of the nanoparticleaggregated samples [21,22]. Such a deduction was substantiated by the corresponding pore-size distributions (Fig. 4B). Each of the samples displayed one broad peak centered at ca. 40 nm and one small peak centered at ca. 16 nm for the SFO-2, SFO-3 and SFO-4 samples or at ca. 10 nm for the SFO-0 and SFO-1 samples. Furthermore, the dV/d(log D) variation tendency in the pore-size range of 1.7–4.0 nm indicates the possible presence of some micropores in

Fig. 3. TEM images and SAED patterns (insets) of (a and b) SFO-0, (c and d) SFO-1, (e and f) SFO-2, and (g and h) SFO-3.

K. Ji et al. / Journal of Molecular Catalysis A: Chemical 370 (2013) 189–196

80

Volume adsorbed (cm3/g, STP)

(A) (e)

60

(d) 40 (c)

(b)

20

(a) 0 0.2

0.0

0.4

0.6

Relative pressure p/p 0

0.8

1.0

0.12

(B)

dV /d(LogD )

0.09 (e)

(d)

0.06

193

Fe 2p3/2 and O 1s XPS spectra of the as-prepared SFO samples. The asymmetrical Fe 2p3/2 XPS spectrum of each SFO sample was decomposed into two components at binding energy (BE) = ca. 710.1 and 712.0 eV (Fig. 5A), respectively. The former was ascribable to the surface Fe3+ species, whereas the latter was assignable to the surface Fe4+ species [24]. That is to say, Fe4+ and Fe3+ species coexisted on the surfaces of the SFO samples. Similar results have been reported by other researchers [24,25]. The asymmetrical O 1s spectrum of each SFO sample (Fig. 5B) could be decomposed into three components at BE = ca. 529.1, 531.2, and 533.4 eV, attributable to the surface lattice oxygen (Olatt ), adsorbed oxygen (Oads , e.g., O2 − , O2 2− or O− ) [26–28], and carbonate species [24,29], respectively. From the data (Table 2) of quantitative analysis on the XPS spectra, one can see that: (i) there was a strontium enrichment on the surface of each SFO sample with the surface Sr/Fe molar ratio of at 2.65–4.82, and similar results were reported by Falcón and co-workers [24]; (ii) the surface Fe3+ /Fe4+ molar ratios of the SFO samples followed a sequence of SFO-3 (0.97) > SFO-2 (0.94) > SFO-1 (0.85) > SFO-0 (0.80) > SFO-4 (0.51); and (iii) the surface Oads /Olatt molar ratios (1.02–1.94) of these samples followed the order same as the Fe3+ /Fe4+ molar ratio. According to the electroneutrality principle, a higher surface Fe3+ /Fe4+ molar ratio would generate a higher surface oxygen vacancy density, thus leading to a higher Oads concentration [30,31]. The Oads species located at the surface oxygen vacancy sites of a perovskite played an important role in the catalytic combustion of VOCs [32,33,34]. It should be noted that due to the presence of a trace amount of SrFe12 O19 , there was a small contribution of Fe3+ in SrFe12 O19 to the rise in Fe3+ /Fe4+ molar ratio of the SFO samples. 3.4. Reducibility

(c) (b) 0.03

(a) 0 0

20

40

60

80

100

Pore diameter (nm) Fig. 4. (A) Nitrogen adsorption–desorption isotherms and (B) pore-size distributions of (a) SFO-0, (b) SFO-1, (c) SFO-2, (d) SFO-3, and (e) SFO-4.

these samples. The surface areas of the samples were in the range of 9.9–26.5 m2 /g (Table 1), with the SFO-3 sample possessing the highest surface area (26.5 m2 /g). The mesopores made a significant contribution to the surface area of each sample. For the five SFO samples, it seems to be that glucose concentration in the precursor solution was directly proportional to the surface area of the final product, but there might be an appropriate EDA concentration or an optimal EDA/glucose volumetric ratio for obtaining the highest surface area. The average pore sizes and pore volumes of the SFO sample were 9.8–36.7 nm and 0.021–0.050 cm3 /g, respectively. It is worth mentioning that the BET surface areas of our SFO samples were higher than that (<10 m2 /g) of nonporous SrFeO3−ı nanoparticles obtained via the traditional preparation routes [17]. The larger BET surface area and hierarchical porous structures would facilitate more efficient contact of the samples with organic contaminants, and hence enhancing their catalytic activities [23]. 3.3. Surface composition, iron oxidation state, and oxygen species XPS is an efficient technique to probe the surface element compositions and surface species of a solid catalyst. Fig. 5 shows the

Fig. 6A illustrates the H2 -TPR profiles of the SFO samples. There were multiple reduction steps below 900 ◦ C. Similar reduction processes were observed in previous studies [17,24]. The first two reduction steps below 620 ◦ C were ascribed to the removal of oxygen adspecies and the reduction of Fe4+ → Fe3+ (corresponding to the reactions of SrFeO3−ı + 1/2 H2 → SrFeO3−ı−x and SrFeO3−ı−x + 1/2 H2 → SrFeO2.5 [24]). The overlapping double peaks centered at ca. 750 and 820 ◦ C were due to the reduction of the brownmillerite SrFeO2.5 (i.e., Sr2 Fe2 O5 ) phase into FeO and metallic Fe0 , respectively, corresponding to the reactions of SrFeO2.5 + 1/2 H2 → FeO + SrO and FeO + 1/2 H2 → Fe0 [24]. Furthermore, one can see from Fig. 6A that the maximal temperature of the first reduction peak increased in the sequence of SFO-3 (300 ◦ C) < SFO-2 (309 ◦ C) < SFO-1 (331 ◦ C) < SFO-4 (344 ◦ C) < SFO-0 (369 ◦ C), indicating that the SFO-3 sample possessed the best low-temperature reducibility. In addition, in the range of 400–600 ◦ C, the weak reduction peaks of the SFO-0 sample were quite different from those of the other hollow spherical samples, indicative of an easier reduction of Fe4+ species in the hollow spherical samples. Meanwhile, the Fe4+ species in the porous hollow spherical samples were more readily to be reduced than those in the solid spherical SFO-4 sample. Therefore, the morphology of the sample exerted an effect on its reducibility [35,36]. By quantifying the reduction peaks, we can estimate the H2 consumptions (Table 2) of the SFO samples. Apparently, the H2 consumptions (3.7–4.9 mmol/g below 620 ◦ C) of the hollow spherical SFO samples were much higher than that (2.2 mmol/g) of the bulk SFO sample, the latter was rather close to that (2.17 mmol/g) of the SrFeO3 samples below 600 ◦ C [17,18]. The SFO-4 sample showed a higher H2 consumption (4.6 mmol/g) below 620 ◦ C than the SFO-0 (2.2 mmol/g) and SFO-2 (3.9 mmol/g) samples. To make a better comparison, we use the initial (where lattice oxygen consumption is less than 25% and no crystal phase transformation takes place [37,38]) H2 consumption rates in the

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710.1 eV

(A) Fe 2p3/2

(B) O 1s

531.2 eV

529.1 eV

712.0 eV

533.4 eV

(e)

(d)

Intensity (a.u.)

Intensity (a.u.)

(e)

(d)

(c)

(c)

(b) (b)

(a)

(a)

707

710

713

526

716

528

Binding energy (eV)

530

532

534

536

Binding energy (eV)

Fig. 5. (A) Fe 2p3/2 and (B) O 1s XPS spectra of (a) SFO-0, (b) SFO-1, (c) SFO-2, (d) SFO-3, and (e) SFO-4.

Table 2 Surface element compositions, H2 consumptions, and catalytic activities of the SFO samples. Catalyst

SFO-0 SFO-1 SFO-2 SFO-3 SFO-4 a b

Surface element composition

H2 consumptionb (mmol/g)

Toluene oxidation activity (◦ C)

Sr/Fea

Fe3+ /Fe4+

Oads /Olatt

50−620 ◦ C

620−880 ◦ C

T10%

T50%

T90%

2.65 (1.0) 4.82 (1.0) 2.46 (1.0) 4.27 (1.0) 3.67 (1.0)

0.80 0.85 0.94 0.97 0.51

1.23 1.31 1.35 1.94 1.02

2.2 3.7 3.9 4.9 4.6

2.5 2.3 2.4 1.7 2.3

230 194 174 145 202

306 287 269 255 313

345 352 333 298 347

The data in parenthesis are the nominal Sr/Fe atomic ratios. The data were obtained by quantitatively analyzing the H2 -TPR profiles.

range of 182–282 ◦ C to evaluate the low-temperature reducibility of the SFO samples, as shown in Fig. 6B. Obviously, the initial H2 consumption rate (i.e. the low-temperature reducibility) decreased in the sequence of SFO-3 > SFO-2 > SFO-1 > SFO-1 > SFO4, in rough agreement with the orders in surface area, Oads concentration, and catalytic performance (shown below) of these samples.

3.5. Catalytic performance The five SFO samples were tested in the catalytic combustion of toluene (Fig. 7A) under the conditions of toluene concentration = 1000 ppm, toluene/O2 molar ratio = 1/400, and SV = 20,000 mL/(g h). As confirmed by the good carbon balance (ca. 99.5%), in addition to CO2 and H2 O, no products of incomplete oxi-

1.6

(B) Initial H2 consumption rate (μmol/(gs))

(A) 563

(e)

344

460

756 822

H2 consumption (a.u.)

505 430

300

(d)

(c)

760

437

309

331

(b)

520

746 810

519

746 822

430

369 741

(a)

822

(b)

1.2

(a) (d) (e)

0.8

(c)

0.4

558

0.0 0

150

300

450

600 o

Temperature ( C)

750

900

1.8

1.9

2.0 -1

1000/T (K )

Fig. 6. (A) H2 -TPR profiles and (B) initial H2 consumption rate as a function of inverse temperature of (a) SFO-0, (b) SFO-1, (c) SFO-2, (d) SFO-3, and (e) SFO-4.

K. Ji et al. / Journal of Molecular Catalysis A: Chemical 370 (2013) 189–196

100

100

(A) 80

(B) Toluene conversion (%)

Toluene conversion (%)

195

SFO-0 SFO-1 SFO-2 SFO-3 SFO-4

60

40

20

0

80

SV (mL/(g h)) 5,000 20,000 30,000 40,000

60

40

20

0 100

170

240

310

380

100

o

200

300

400

o

Temperature ( C)

Temperature ( C)

Fig. 7. (A) Toluene conversion as a function of reaction temperature over the as-prepared SFO catalysts under the conditions of toluene concentration = 1000 ppm, toluene/oxygen molar ratio = 1/400, and SV = 20,000 mL/(g h), and (B) effect of SV on toluene conversion over the SFO-3 catalyst at toluene concentration = 1000 ppm and toluene/oxygen molar ratio = 1/400.

dation were detected over our SFO catalysts in each run. In the blank experiment (only quartz sands were loaded in the microreactor), no significant conversion of toluene was observed below 450 ◦ C, indicating that there was no occurrence of homogeneous reactions under the adopted reaction conditions. From Fig. 7A, one can see that the SFO-3 sample exhibited the best catalytic activity. Usually, the T10% , T50% , and T90% (defined as the temperatures for toluene conversion = 10, 50, and 90%, respectively) are used to compare the catalytic performance of the samples, as summarized in Table 2. The T10% , T50% , and T90% over the best-performing SFO-3 sample were ca. 145, 255, and 298 ◦ C, respectively. It is worth mentioning that the presence of trace amounts of SrFe12 O19 and SrCO3 phases in the porous SFO hollow sphere samples had no significant impact on their catalytic activities for the oxidation of toluene, because SrCO3 is inactive for toluene oxidation and Fe in SrFe12 O19 is present mainly in less-reactive Fe3+ . The catalytic activity of these samples followed a sequence of SFO-3 > SFO-2 > SFO-1 > SFO-0 > SFO-4, in good agreement with the orders in surface area, oxygen adspecies concentration, and low-temperature reducibility. Fig. 7B shows the effect of SV on the catalytic activity of the SFO-3 sample. As expected, the catalytic activity decreased with the rise in SV. The T10% , T50% , and T90% were 129, 251, and 283 ◦ C at SV = 5000 mL/(g h), 181, 311, and 367 ◦ C at SV = 20,000 mL/(g h), and 245, 351, and > 400 ◦ C at SV = 40,000 mL/(g h), respectively. The SFO-3 sample was superior in catalytic performance to the SrFeO3−ı sample (T50% = ca. 400 ◦ C and T90% > 480 ◦ C at SV = 3000 mL/(g h)) [17], the 3D macroporous SFO samples (T50% = 314–344 ◦ C and T90% = 347–394 ◦ C at SV = 20,000 mL/(g h)) [18], similar to the 3DOM SFO samples (T50% = 292–308 ◦ C and T90% = 340–355 ◦ C at SV = 20,000 mL/(g h)) [17]. To examine the catalytic stability, we carried out the on-stream reaction experiment over the SFO-3 sample at 298 ◦ C and 20,000 mL/(g h), and the result is shown in Fig. S3 of the Supplementary material. It is found that there was no significant decline in catalytic activity, indicating that the SFO-3 sample was catalytically durable within 100 h of on-stream reaction. Catalytic performance of ABO3 is usually related to the surface area, pore structure, defect nature and density, adsorbed oxygen species, and reducibility. It was reported that a higher surface area was favorable for the enhancement in catalytic performance [39,40]. The presence of porous structure can facilitate the adsorption and diffusion of reactant molecules, and is beneficial

for the mass transfer of reactants. The generation of oxygen vacancies promotes the activation of gas-phase oxygen molecules. The higher the oxygen vacancy density, the better is the catalytic activity of ABO3 [41,42]. Toluene combustion over the SFO samples can undertake via the following mechanism: toluene first reacts with the oxygen adspecies to give CO2 and water; in the meanwhile, the gas-phase oxygen molecules are activated to become the active oxygen adspecies, during which the redox process of Fe4+ ⇔ Fe3+ occurs. The strong redox ability of SFO would favor the recycle of Fe ions with two oxidation states, thus promoting the oxidation of organic compounds [43]. Compared to the nonporous SFO-0 and SFO-4 samples, the porous SFO-1, SFO-2, and SFO-3 samples possessed much higher surface areas (Table 1) and surface oxygen adspecies concentrations (Table 2 and Fig. 5) and much better in low-temperature reducibility (Table 2 and Fig. 6). These unique characters of the porous hollow spherical SFO samples rendered them better catalytic activity in the combustion of toluene (Fig. 7). Therefore, it is concluded that the good catalytic performance of the porous hollow spherical SFO samples was associated with their higher surface areas and oxygen adspecies concentrations and better low-temperature reducibility. 4. Conclusions Strontium ferrite hollow spheres with or without porous shells were prepared via a novel and facile one-pot hydrothermal route in the presence of glucose and/or ethylenediamine. The addition of glucose and ethylenediamine in the precursor solution exerted a great influence on the morphology and pore structure of the final product. The porous hollow spherical SFO samples possessed larger BET surface areas (18–27 m2 /g), higher oxygen adspecies concentrations, and better low-temperature reducibility. Under the conditions of toluene concentration = 1000 ppm, toluene/O2 molar ratio = 1/400, and SV = 20,000 mL/(g h), the SFO-3 sample showed the best catalytic activity for toluene combustion, giving the T10% , T50% , and T90% of ca. 145, 255, and 298 ◦ C, respectively. It is concluded that high oxygen adspecies concentration and good low-temperature reducibility as well as unique porous structure were responsible for the good catalytic performance of the SFO-3 sample. We believe that such porous SFO materials are promising in the oxidative removal of volatile organic compounds.

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Acknowledgements The work was supported by the NSF of China (20973017 and 21077007), the NSF of Beijing Municipality (2102008), the Discipline and Postgraduate Education (005000541212014), the Creative Research Foundation of Beijing University of Technology (00500054R4003 and 005000543111501), the Funding Project for Academic Human Resources Development in Institutions of Higher Learning under the Jurisdiction of Beijing Municipality (PHR201007105 and PHR201107104), and the Doctoral Innovation Fund of Beijing University of Technology. We also thank Prof. Chak Tong (Department of Chemistry, Hong Kong Baptist University) and Mrs. Jianping He (State Key Laboratory of Advanced Metals & Materials, University of Science and Technology Beijing) for doing the XPS and SEM analyses, respectively. Appendix A. Supplementary data Supplementary data associated with this article can be found, in the online version, at http://dx.doi.org/10.1016/ j.molcata.2013.01.013. References [1] J.C. Kim, Y.S. Son, K.J. Kim, Y.J. Lim, S.G. Chung, S. Young, Radiat. Phys. Chem. 79 (2010) 797. [2] Y.S. Xia, H.X. Dai, H.Y. Jiang, J.G. Deng, H. He, C.T. Au, Environ. Sci. Technol. 43 (2009) 8355. [3] J.G. Yu, X.X. Yu, B.B. Huang, X.Y. Zhang, Y. Dai, Cryst. Growth Des. 9 (2009) 1474. [4] W.L. Huang, Q.S. Zhu, J. Chem. Theory Comput. 5 (2009) 2787. [5] L.S. Jia, T. Ding, Q.B. Li, Y. Tang, Catal. Commun. 8 (2007) 963. [6] Y. Yang, Z.Q. Cao, Y.S. Jiang, L.H. Liu, Y.B. Sun, Mater. Sci. Eng. B 132 (2006) 311. [7] H.X. Dai, C.F. Ng, C.T. Au, Catal. Lett. 57 (1999) 115. [8] V.C. Belessi, A.K. Ladavos, P.J. Pomonis, Appl. Catal. B 31 (2001) 183. [9] A.G. Andersen, Catal. Lett. 27 (1994) 221. [10] X.F. Yang, J.X. Fu, C.J. Jin, J. Chen, C.L. Liang, M.M. Wu, W.Z. Zhou, J. Am. Chem. Soc. 132 (2010) 14279. [11] Y. Zeng, T. Zhang, H.T. Fan, W.Y. Fu, G.Y. Lu, Y.M. Sui, H.B. Yang, J. Phys. Chem. C 113 (2009) 19000. [12] Z.M. Li, X.Y. Lai, H. Wang, D. Mao, C.J. Xing, D. Wang, J. Phys. Chem. C 113 (2009) 2792.

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