Synthesis of Monolithic Hierarchically Porous Iron-Based Xerogels

May 8, 2012 - On the other hand, poly(acrylamide) works as a phase separation inducer as well as a precipitation inhibitor. Appropriate choice of iron...
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Synthesis of Monolithic Hierarchically Porous Iron-Based Xerogels from Iron(III) Salts via an Epoxide-Mediated Sol−Gel Process Yasuki Kido,† Kazuki Nakanishi,*,† Akira Miyasaka,† and Kazuyoshi Kanamori† †

Department of Chemistry, Graduate School of Science, Kyoto University, Kitashirakawa, Sakyo-ku, Kyoto 606-8502, Japan ABSTRACT: Various iron-based polycrystalline monoliths, Fe3O4, iron, and Fe3C, with hierarchically distributed pores have been synthesized from ionic precursors using a sol− gel process accompanied by phase separation. Propylene oxide acts as a proton scavenger to increase pH moderately and uniformly in a reaction solution, which leads to homogeneous gelation. On the other hand, poly(acrylamide) works as a phase separation inducer as well as a precipitation inhibitor. Appropriate choice of iron precursor, solvent, polymer, and epoxide allowed the formation of iron(III)-based xerogels with cocontinuous macroporous structures. The dried gels were amorphous, whereas heating in air above 300 °C led to the formation of α-Fe2O3. Calcination under an inert condition above 400 °C formed Fe3O4, iron, and Fe3C without collapse of macrostructures. Examination has been carried out using SEM, TG-DTA, FT-IR, Hg intrusion, pH measurement, X-ray diffraction, and N2 adsorption−desorption. KEYWORDS: iron hydroxide, iron oxide, iron carbide, xerogel, monoliths, hierarchically porous, sol−gel, propylene oxide, phase separation wide variety of metals.16,17 The synthesis process has several advantages compared to the traditional alkoxide-derived sol− gel route; the sources of metal are inexpensive and soluble in water, which allows the technique to involve various transition, main group, and rare earth metals.18−27 According to Scheme 1,

1. INTRODUCTION Iron(III) oxides are stable and ubiquitous compounds that have been utilized in various fields. Application includes catalysts, electrochemical sensors, and data storage media.1−5 Hematite (α-Fe2O3) is the most stable iron(III) oxide phase under ambient conditions, being utilized in a variety of applications.4,6−8 Maghemite (γ-Fe2O3) is useful in recording and data storage applications, and appropriate additives improve coercivity and storage capacity through the controls on the shape and size of the particles.9,10 Magnetite (Fe3O4) is employed for various catalysts and has an important magnetic property.11,12 In order to apply these oxides practically, one has to accurately control the particle size, morphology, and surface areas.1,5 In the case of applications such as catalyst, welldesigned pore structures are desired that enhance efficient contacts of the materials’ inner surfaces with external molecules. A monolithic hierarchically porous material with both macropores and meso-micropores has great advantages in industrial applications: macropores offer pathways for molecular transport and meso-micropores provide a large area of active surface. Actually, hierarchically porous monoliths have been utilized, for example, as a column for HPLC13 and catalyst.14 As the first demonstration of synthesis of hierarchically porous iron oxide monoliths, here we report monolithic macroporous iron(III)based xerogels using iron(III) salts as precursors. Epoxide-mediated sol−gel reaction using aqueous solution of metal salts was first reported by Gash et al.15 In a partially hydrolyzed solution of metal salts, epoxide works as a proton consumer to raise the solution pH moderately and uniformly, which allows the hydrolysis and condensation of aquo-cations. The uniform polycondensation enables one to prepare monolithic gels of metal hydroxides or oxyhydroxides in a © 2012 American Chemical Society

Scheme 1. Epoxide-Mediated Sol-Gel Reaction

(a) epoxide such as propylene oxide is protonated by an acid (metal hydrate species [M(H2O)x]m+), and (b) a subsequent irreversible ring-opening reaction is brought about by a nucleophilic anionic base such as chloride anion. The method has proven to be useful for preparing various metal oxides in different morphologies, including monoliths, powders, and thin films.28,29 Recently, combining the polymerization-induced phase separation with the above sol−gel technique, pure macroporous alumina monoliths with mesopores have been prepared by Tokudome et al.30 A sol−gel reaction progresses parallel to the phase separation, which freezes the phase-separating transient structures and results in the “cocontinuous” structure in the Received: February 13, 2012 Revised: May 7, 2012 Published: May 8, 2012 2071

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micrometer range.31,32 Extending the above synthetic strategy, they prepared macroporous monoliths of mixed metal oxide, containing tri/divalent cations.33,34 Compared to the previous works, however, it is not easy to obtain iron(III) oxide monolithic gels since they tend to form dispersed precipitates of iron(III) hydroxide due to the poor solubility at low pH.35−37 Similarly to the case of preparing monolithic macroporous calcium phosphate,34 the effective inhibition of the precipitation should play a key role in preparing monolithic iron(III) oxide gels. For the effective inhibition of crystal growth, we focused on poly(acrylamide) and glycerol. In this study, poly(acrylamide) serves as a phase separation inducer,32 and glycerol serves as a solvent. In addition, both the solvent viscosity and the coordination effect derived from them play important roles for obtaining monolithic gels. Except the case of aerogels, there have been few reports of preparation of iron oxide monoliths with well-defined pores. Here we report a straightforward synthetic route for the formation of hierarchically porous iron-based monolith from iron(III) chloride hexahydrate. The effect of solvent compositions, polymer, and epoxides on the morphology and gel formation is examined. In addition, the influence of heat treatment condition on the crystalline phase and structure is discussed.

and stirred for 1 min. The homogeneous solution thus prepared was sealed and kept at 60 °C for gelation. The wet gels were aged at 60 °C for 24 h and immersed in IPA at 60 °C for 24 h three times and then evaporation-dried at 40 °C. Heat treatment in air was carried out at temperatures up to 300 °C for 4 h with a heating rate of 5 °C min−1. Some samples were calcined at various temperatures for 4 h with a heating rate of 5 °C min−1 in the Ar stream at a rate of 1 L min−1 in an electric furnace. Gel samples were purged with Ar prior to the heattreatment. 2.2. Characterization. Morphology of dried gels was observed by a scanning electron microscope (SEM: JSM-6060, JEOL, Ltd., Japan, with Pt coating). Thermogravimetric-differential thermal analysis (TGDTA: Thermo plus TG 8120, Rigaku Corp., Japan) up to 1000 °C was performed on the sample at a heating rate of 5 °C min−1 while continuously supplying air or Ar at a rate of 100 mL min−1. Chemical bonding information on the sample was investigated with Fourier transform infrared spectroscopy (FT-IR: IR Affinity-1, Shimadzu Corp., Japan) using the potassium bromide (KBr) pellet technique; each spectrum was collected after 100 scans for the wavenumber range 400−4000 cm−1 at a resolution of 2 cm−1. Macropore size distributions over the diameter from 10 nm to 300 μm were evaluated by mercury porosimetry (Poremaster 60-GT, Quantachrome Instruments, USA). Time evolution of solution pH was monitored at 60 °C (pH meter F-21, Horiba, Ltd., Japan). The X-ray diffraction (XRD) analysis was carried out with the RINT system (patterns from 10° to 90°, 2 kW, CuKα: λ = 0.154 nm, RINT-Ultima III, Rigaku Corp., Japan). Micromesopores were characterized by nitrogen adsorption− desorption isotherms (Belsorp mini II, Bel Japan Inc., Japan). The pore size distribution was calculated from them by the Barrett− Joyner−Halenda (BJH) method, and surface area was obtained by the Brunauer−Emmett−Teller (BET) method.

2. EXPERIMENTAL SECTION 2.1. Synthesis. Iron(III) chloride hexahydrate (FeCl3·6H2O: Sigma-Aldrich Co., USA, 99+%) was used as an iron source. Mixture of distilled water and glycerol (C3H8O3: Kishida Chemical, Japan, 99.8%) was used as a solvent. Poly(acrylamide) (PAAm: SigmaAldrich Co., USA, 50 wt % in water) with average molecular weight, MW, of 10,000 was used as a phase separation inducer. Propylene oxide (PO: Sigma-Aldrich Co., USA, 99>%) was added as a gelation agent. Trimethylene oxide (TMO: Acros Organics, Belgium, >97%) was also used as a gelation agent for the purpose of modifying the rate of pH increase. 2-Propanol (IPA: Kishida Chemical, Japan, 99>%) was used for washing. Sample gels were prepared with the starting compositions listed in Table 1. First, 2.42 g of FeCl3·6H2O and WPAAm g of PAAm were dissolved in a mixture of VH2O mL of distilled water and VGLY mL of glycerol. Then, 1.88 mL of PO (PO/Fe = 3 in molar ratio) was added to the transparent orange solution under ambient condition (25 °C)

3. RESULTS AND DISCUSSION 3.1. Effect of PAAm. The starting solutions with the compositions listed in Table 1 were homogeneous and transparent orange in color. The addition of PO quickly formed iron(III) hydroxide gels (∼11 min in P3), while gelation was not observed without PO. It is probable that dried gels contain both water molecules and hydroxyl ions. Highly probable crystalline phases are iron(III) oxide hydrate (Fe2O3·nH2O) or iron(III) oxyhydroxide (FeOOH) (discussed later).38,39 For simplicity, we refer to the dried gel phase as “iron(III) hydroxide”. Figure 1a shows appearance of a dried gel, and Figure 1b-f shows SEM images of dried gels prepared with varied WPAAm.

Table 1. Starting Compositions of Samples FeCl3·6H2O (g)

H2O (mL)

glycerol (mL)

PAAm (g)

PO (mL)

P1 P2 P3 P4 P5 P6 P7 P8 P9 P10 P11 P12 P13 P14 P15

2.42 2.42 2.42 2.42 2.42 2.42 2.42 2.42 2.42 2.42 2.42 2.42 2.42 2.42 2.42 FeCl3·6H2O (g)

2.75 2.25 2.00 1.75 1.50 1.50 2.50 2.00 2.00 2.50 3.00 3.50 1.50 1.00 0.50 H2O (mL)

2.00 2.00 2.00 2.00 2.00 2.00 2.00 1.50 2.50 1.50 1.00 0.50 2.50 3.00 3.50 glycerol (mL)

0.00 1.00 1.50 2.00 2.50 1.50 1.50 1.50 1.50 1.50 1.50 1.50 1.50 1.50 1.50 PAAm (g)

1.88 1.88 1.88 1.88 1.88 1.88 1.88 1.88 1.88 1.88 1.88 1.88 1.88 1.88 1.88 TMO (mL)

T1

2.42

2.00

2.00

1.50

1.80

Figure 1. (a) Appearance of the resultant dried gels macrostructures with varied WPAAm; (b) P1, (c) P2, (d) P3, (e) P4, and (f) P5.

The total volume of water, (VH2O + WPAAm × 0.5), was set at 2.75 mL for P1 to P5. In the absence of PAAm, gelation did not occur in 24 h. After 480 h, gel formation was observed;, however, obtained wet gel was too soft to keep its monolithic form, “soft deformable solid”. As the SEM picture of the dried 2072

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gel represents, no macropore is found, and fine particles compose the gel phase (Figure 1b). On the other hand, with increased WPAAm, the monolithic stiff gels were formed, and morphologies varied remarkably. In P2 the gel skeleton is fragmented cocontinuous structures (Figure 1c), whereas the addition of an appropriate amount of WPAAm produced a homogeneous cocontinuous gel skeleton with smoother skeleton surfaces (P3, Figure 1d). Further increase in WPAAm formed a coarsened cocontinuous structure (P4, Figure 1e) and then a continuous gel matrix with isolated macropores (P5, Figure 1f). To clarify the role of PAAm, we carried out thermal analysis and chemical bonding analysis to determine the distribution of PAAm between gel phase and liquid phase. TG-DTA curves for gels prepared without PAAm (P1) and with PAAm (P3) are depicted in Figure 2a. The evaporation of volatile components

did not appear in the sample prepared without PAAm.40 On the other hand, TG-DTA analysis under an Ar atmosphere indicates no exothermic peak around 200 °C and 350−500 °C. Gradual weight loss from 200 to 800 °C is ascribed to the decomposition of carbon derived from organic species such as PAAm and glycerol and also to the crystallization into reduced iron species (discussed later). Fourier transform infrared spectra of (i) P1 and (ii) P3 are represented in Figure 2b. The presence of PAAm in gels is confirmed by the appearance of new bands: deformation vibration of -NH groups (1,200 cm−1), stretching vibration of C−N bonds (1,450 cm−1), bending vibration of N−H bonds (1,604 cm−1), stretching vibration of CO groups (1,658 cm−1), and stretching vibration of N−H bonds (3,186 cm−1).41−43 The results of these data reveal that PAAm is distributed to the gel phase preferentially. The homogeneous solution phase-separates into PAAm-iron(III) hydroxide composite phase and solvent phase, and the sol−gel transition freezes the phase-separated structures. Both PAAm and iron(III) hydroxide form gel skeleton due to the strong interaction between amide groups and hydroxyl groups, whereas the spaces occupied by the solvent phase become macropores after drying. The interaction between polymer having hydrogen-bonding ligands and metal is reported and widely used to obtain porous materials with metal−organic skeletons.44−46 PAAm acts as not only a phase separation inducer but also a network former in this study. The pore size distribution of dried gels prepared with varied WPAAm is shown in Figure 3. Mercury porosimetry data indicate

Figure 3. Macropore size distributions of dried samples with varied WPAAm = 1.0 g (open square: P2) and 1.5 g (open circle: P3).

iron(III) hydroxide gels possess a narrow pore size distribution, and median macropore sizes of P2 and P3 are 1.9 and 1.3 μm, respectively. The porosities calculated from the bulk density and pore volume are 74% and 63% for P2 and P3, respectively. As PAAm is a network former as discussed above, with an increase in WPAAm, the volume ratio of skeleton to macropores increases, resulting in the decrease of total pore volume. 3.2. Effect of Solvents. Figure 4 shows SEM photographs of dried gels prepared with varied VH2O. As the amount of VH2O increases, the morphologies changed from isolated pores (P6, Figure 4a), through a cocontinuous structure (P3, Figure 4b), to a thinner cocontinuous skeleton (P7, Figure 4c). The

Figure 2. (a) TG-DTA curves for gel phase of dried samples prepared without PAAm (P1, dash lines) and with PAAm (P3, solid lines) under an air atmosphere and analysis on sample P3 under an Ar atmosphere (gray lines). (b) FT-IR spectra of dried samples prepared without PAAm (P1, i) and with PAAm (P3, ii).

takes place below 100 °C. The weight loss and exothermic peak between 200 and 300 °C are ascribed to the crystallization of iron(III) hydroxide into iron oxide (and decomposition of glycerol, discussed later). The weight loss and exothermic peak between 350 and 500 °C as observed for the gel prepared with PAAm is attributed to the combustion of PAAm since the peak 2073

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inhibits the growth of nanocrystal and skeletons in our case. Given these effects derived from glycerol, the thickening tendency of skeletons observed in the GP system can be explained by the partial distribution of glycerol to the gel phase. As shown in TG-DTA curves (Figure 2), the exothermic peak and weight loss around 250 °C include the decomposition of glycerol in skeleton, also illustrated by FT-IR spectra in P13− P15 (not shown). Obtained dried gels thus mainly consist of iron(III) hydroxide, PAAm, and glycerol. In a water rich sample (denoted as the “HP” system), shorter gelation (from the scale of minutes to seconds) was observed, and homogeneous cocontinuous structures did not form. Morphologies varied from a cocontinuous structure (P3 in Figure 6a), through an isolated pore (P10 in Figure 6b), and an

Figure 4. SEM images of dried samples prepared with varied VH2O: (a) P6, (b) P3, and (c) P7.

transition was similarly observed in samples with varied VGLY (P8 and P9, not shown here). As the volume fraction of the skeleton becomes larger relative to the solvent, a discontinuous liquid phase forms droplets and results in the formation of isolated pores after drying (Figure 4a); on the other hand, with further increase in the amount of solvents the gel fraction becomes smaller and a fragmented skeleton is formed (Figure 4c). For the purpose of further exploring the effect of water and glycerol on the morphology some samples were prepared, setting the total volume of solvents, (VH2O + VGLY + WPAAm × 0.5), at 4.75 mL, and vary VH2O and VGLY (P10−P15). The gel morphologies in glycerol rich samples (denoted as the “GP” system) are shown in Figure 5. With increased VGLY (that is,

Figure 6. SEM images of dried samples prepared with varied VH2O or VGLY: (a) P3, (b) P10, (c) P11, and (d) P12. Times indicate gelation time respectively.

isolated pore with a fine cocontinuous structure (P11 in Figure 6c), to aggregation of fine particles in submicrometer range (P12 in Figure 6d). A less viscous solvent in the HP system suppresses the crystal growth inefficiently and accelerates the gelation. The morphological dependence on VH2O can also be accounted for by the ratio of distribution of water to the “gel phase”. The increase in VH2O reduces the strength of the gel skeleton supported by glycerol, and homogeneously dispersed water forms droplets in sols; the homogeneous solution phaseseparates into large droplets and skeletons (Figure 6b). When an increased distribution of water to the gel phase occurs, subsequent phase separation is induced in the submicrometer range; as a result, secondary phase separation takes place (Figure 6c). Further increase in VH2O brings about the breakup of connection, resulting in the formation of particle aggregation outside of droplets (Figure 6d). As water preferentially works as a solvent to compose pores compared to glycerol, increased distribution of water to the “skeleton phase” caused a significant change in skeleton morphologies. 3.3. pH Evolution. In the present synthesis, the rate of pH increase is considered to be one of the important factors. To further understand the process in the formation of iron(III) hydroxide gels, we monitored the change of pH in solutions at 10 s intervals. The results of the sample prepared in P3 are shown in Figure 7. Aqueous solutions of iron(III) chloride hexahydrate exhibit considerably strong acidity due to the

Figure 5. SEM images of dried samples prepared with varied VH2O or VGLY: (a) P3, (b) P13, (c) P14, and (d) P15. Times indicate gelation time respectively.

relatively decreased VH2O), macrostructures changed from cocontinuous structures (P3, P13 in Figure 5a, b, respectively) to fragmented cocontinuous structures and the skeleton becomes thicker (P14, P15 in Figure 5c, d, respectively). In addition, the gelation time is also varied from the scale of minutes to hours. The viscosity in solutions is one key parameter in this study; the viscosity in solvents influences remarkably on the rate of the growth of iron(III) hydroxide gels, i.e. precipitation of iron(III) hydroxide. Viscous solvents in GP system derived from both polymer and glycerol suppresses the growth and the precipitation of nanocrystal, which causes the slower gelation and the formation of fragmented structures. Alternatively, the coordination effect also affects them; glycerol forms a complex with iron(III) hydroxide. It is well-known that organic molecules, polyol such as glycerol or ethylene glycol, can interact with hydroxyl groups,47−51 thereby the interaction 2074

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Figure 7. Time evolutions of pH in the reaction solutions; open circle and open triangle indicate P3 and T1, respectively.

partial hydrolysis to liberate chloride ions. After the addition of PO, the pH value increases dramatically from 0.06 to 1.6 in the initial stage until about 2 min and then decreases slightly. The gelation occurred in 11 min at pH ∼ 1.4. Initial rapid increase of pH derives from the scavenging protons by an epoxide ringopening reaction, whereas a moderate decrease is explained by the precipitation of iron(III) hydroxide consuming hydroxyl groups. In previous sol−gel studies using epoxide, several kinds of proton scavengers were used as gelation agents.34,52,53 They report the rate of pH change dominates morphology, pore volume, and crystalline phase.36,37,53 We investigated the effect of a proton scavenger on gel morphologies. Instead of PO, we synthesized iron(III) hydroxide gels with trimethylene oxide (TMO), using an equivalent mole of TMO (T1) to that of PO. Time evolution of pH prepared with TMO is also illustrated in Figure 7. A slower increase of pH is observed until about 7 min, and then pH decreased gradually. The pH value of ∼1.3 at gelation point, observed 16 min after the addition of PO, is similar to the sample prepared with PO. The macrostructure, however, was not a cocontinuous structure but fragmented skeletons (not shown). The difference in gelation time and macrostructures derives from the reactivity of the cyclic ethers used.34,53,54 Sufficient supersaturation due to the high reactivity in PO accelerates the gelation, and the phase-separating structure in the early stage is frozen. On the other hand, the deficient reactivity in TMO causes the low degree of supersaturation which leads to slower gelation, resulting in the formation of the fragmented skeleton. Considering the precipitation of iron(III) hydroxide at low pH, the choice of epoxide having sufficient reactivity allows the formation of gels with controlled smooth macrostructures. 3.4. Heat Evolution and Crystallization. Variation of XRD patterns with heat-treatment temperature in air is depicted in Figure 8a. The measurement was performed on P3. No specific peaks are found for as-dried gel, indicating the amorphous phase. Crystalline phases such as akaganeite (βFeOOH) or ferrihydrite were reported in previous studies prepared by an epoxide-mediated sol−gel process;38,53 however, obtained xerogel were amorphous under the present XRD conditions possibly due to the low crystallinity derived from our synthetic condition. The gel remains amorphous after the heat-treatment at 200 °C, while at 300 °C the diffraction patterns ascribed to hematite (α-Fe2O3).50 After the heat-

Figure 8. X-ray diffraction patterns of samples: (a) as-dried and heattreated in air at different temperature and (b) as-dried and heat-treated in Ar at different temperature. Inset of (a) shows SEM picture of sample heat-treated in air at 300 °C. Symbols indicate open circle: αFe2O3, closed circle: Fe3O4, open triangle: iron, and closed triangle: Fe3C, respectively. Asterisk represents unidentified phase.

treatment, the monolithic form was not preserved: monoliths became fragmented into powders. The Figure 8a inset shows the SEM picture, which indicates macrostructure with micrometer-ranged pores after the heat-treatment at 300 °C; however, detailed structural information was not examined due to the fragmented powdery form. We assume that the decomposition of glycerol and water in the gel skeleton triggers the crystallization into oxides, resulting in the collapse of monolithic form. Figure 8b shows XRD patterns of samples heat-treated in Ar atmosphere. The calcination was performed on P3. The gel remains amorphous up to 300 °C under XRD, while crystallization occurred over 400 °C; Fe3O4, iron, and Fe3C phases appeared. Represented in TG-DTA data in Figure 2a, organic species in the gel phase become carbon in an inert condition, resulting in the gradual weight loss from 200 to 800 °C derived from the combustion of carbon. As is well-known, organic species serve not only as a carbon precursor but also as a reducing agent of iron(III) species in an inert gas flow.55 Hence, with varied temperature, development of carbothermal reduction allowed the formation of partly reduced crystalline phases, Fe3O4, iron, and Fe3C. Since these organic species are homogeneously distributed in the network, finely dispersed carbon remains in the sample after the heat-treatment. Thus, obtained heat-treated samples are iron species/carbon composites. 2075

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Revealed from BJH data, about 5−6 nm mesopores are formed in the as-dried gel, whereas heat-treated ones possess 3−4 nm mesopores. Small mesopores in heat-treated samples originate from the interstices among primary particles. Considering the calcined sample consists of iron species/carbon composites, micropores appeared above 400 °C derive from the combustion of carbon in the skeleton. BET analysis indicated that an asdried sample has 115 m2 g−1 BET surface area owing to a large amount of mesopore; on the other hand, BET surface area of heat-treated samples dramatically decreased compared to an asdried sample and then increases with varied temperature: 5 m2 g−1 at 300 °C, 22 m2 g−1 at 400 °C, 224 m2 g−1 at 700 °C, and 262 m2 g−1 at 1000 °C, respectively. Since the increase in temperature accelerates the decomposition of carbon in the network, samples heat-treated at much higher temperature possess a larger amount of micropore: the BET surface area of heat-treated samples mainly depends on the amount of micropores.

Figure 9a shows macrostructures of samples heat-treated on P3 in Ar stream. Samples showed shrinkage; however, the

4. CONCLUSIONS Various polycrystalline iron-based monoliths, iron oxide, metallic iron, and iron carbide, with both macroporoes and meso-micropores have been synthesized by the epoxidemediated sol−gel route accompanied by phase separation. The appropriate choice of the concentration of precursors, solvents, polymers, and epoxides allowed the formation of the cocontinuous macroporous gel. Using poly(acrylamide) as a phase separation inducer as well as precipitation inhibitor, the interaction between PAAm (and glycerol) and hydroxyl groups in gels suppresses the precipitation of iron(III) hydroxide, resulting in the formation of PAAm-glycerol-iron(III) hydroxide composite gels with controlled morphology. Both micrometer-sized and nanometer-sized pores are formed for as-dried and heat-treated samples. Calcination in air resulted in the crystallization into α-Fe2O3 with collapse of the monolithic form; on the other hand, heat-treatment under Ar atmosphere led to the formation of partly or fully reduced iron species such as Fe3O4, iron and Fe3C without spoiling macrostructure.

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AUTHOR INFORMATION

Corresponding Author

*Phone/Fax: +81 757 532 925. E-mail: [email protected]. Figure 9. (a) SEM images of samples heat-treated in Ar with varied temperatures: (i) at 300 °C, (ii) at 400 °C, (iii) at 700 °C, and (iv) at 1000 °C. Inset of (iv) is appearance of the calcined sample. (b) Nitrogen adsorption−desorption isotherms of samples as-dried and heat-treated in Ar with varied temperature. Inset shows the corresponding pore size distribution curve calculated by the BJH method. Symbols represent open triangle: as-dried, open circle: at 300 °C, closed circle: at 400 °C, open square: at 700 °C, and closed square: at 1000 °C, respectively.

Notes

The authors declare no competing financial interest.

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REFERENCES

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macrostructure as well as monolithic samples shape were perfectly preserved through the heat-treatment process. Similarly to the as-dried gel, all samples heat-treated in Ar exhibited narrow macropore distribution, confirmed by mercury intrusion (not shown). Both pore size and pore volume decreased with an increased temperature because of the shrinkage of network. Figure 9b indicates nitrogen adsorption− desorption isotherm of as-dried and heat-treated samples, and BJH pore size distributions are depicted in the inset. They show both as-dried and heat-treated samples have meso-micropores. 2076

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dx.doi.org/10.1021/cm300495j | Chem. Mater. 2012, 24, 2071−2077