Synthesis of highly ordered Fe-containing mesoporous carbon materials using soft templating routes

Synthesis of highly ordered Fe-containing mesoporous carbon materials using soft templating routes

Microporous and Mesoporous Materials 128 (2010) 144–149 Contents lists available at ScienceDirect Microporous and Mesoporous Materials journal homep...

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Microporous and Mesoporous Materials 128 (2010) 144–149

Contents lists available at ScienceDirect

Microporous and Mesoporous Materials journal homepage: www.elsevier.com/locate/micromeso

Synthesis of highly ordered Fe-containing mesoporous carbon materials using soft templating routes Jiansheng Li, Juan Gu, Huijun Li, Yi Liang, Yanxia Hao, Xiuyun Sun, Lianjun Wang * Department of Environmental Science and Engineering, Nanjing University of Science and Technology, Nanjing 210094, PR China

a r t i c l e

i n f o

Article history: Received 30 April 2009 Received in revised form 18 July 2009 Accepted 7 August 2009 Available online 14 August 2009 Keywords: Iron Mesoporous carbon Synthesis Soft templating

a b s t r a c t Highly ordered Fe-containing mesoporous carbon materials were directly synthesized through simple soft templating routes by using resorcinol–formaldehyde (RF) as a carbon precursor, triblock copolymer Pluronic F127 as a template agent and hydrated iron nitrite as an iron source. The main strategy of this approach is to use the acidity self-generated in the aqueous solutions of the iron precursors as the catalyst for RF polymerization and no any additional mineral acid was necessary. The resultant materials were characterized using nitrogen sorption, X-ray diffraction, and transmission electron microscopy. It was found that the final products with a highly ordered mesostructure were obtained when the Fe/R ratio was around 0.1. For those with Fe/R ratios smaller or greater than this value, the ordering decreased. Iron species in the mesoporous carbon matrix existed in two states, metallic Fe and c-Fe2O3. Metallic Fe nanoparticles are dominantly buried in the walls of the mesoporous carbon while c-Fe2O3 nanoparticles are mostly located on the surface. The resulting materials with Fe/R ratio of 0.1 (Fe/OMC-0.10) exhibit specific activity for the catalytic wet peroxide oxidation (CWPO) of phenol solution with hydrogen peroxide. The phenol and TOC conversion can reach to 93.0% and 61.7% within 180 min at an initial of phenol 247 ppm under very mild conditions (P = 1 atm and T = 80 °C). Ó 2009 Elsevier Inc. All rights reserved.

1. Introduction Ordered mesoporous carbon (OMC) materials possess extensive potential for many high-tech applications such as electrode materials for batteries [1], sensors [2], membranes [3], separation [4], and catalysts [5]. Since the first report on OMC films in 2004 [6], much attention has been focused on the synthesis of OMC materials using a soft templating route [7–10], which is more easy, lowcost and thus industrially feasible compared to the hard templating method. So far, two main strategies have been proposed for direct synthesis of OMC materials via soft templating process. One strategy was put forward by Zhao and coworkers [11–14], which involves the self-assembly of a F127/phenol–formaldehyde mixture under basic conditions. Independently, another strategy was proposed by Liang and Dai [15,16], which is based on using phloroglucinol–formaldehyde polymer as a carbon-yielding component and commercially available triblock copolymers as structure-directing agents under acidic conditions. Apparently, the latter synthesis route exhibits comparative superiority in the case of incorporating active metal species into OMC matrix during the synthesis of OMC, which mainly due to the fact that in acidic environment the metal

* Corresponding author. Tel.: +86 25 84303216; fax: +86 25 84315941. E-mail address: [email protected] (L. Wang). 1387-1811/$ - see front matter Ó 2009 Elsevier Inc. All rights reserved. doi:10.1016/j.micromeso.2009.08.015

precursor can be compatible with the carbon precursors instead of precipitating as metal hydroxide under basic conditions [17]. For further applications of OMC, it would be interesting to employ metal nanoparticles into mesoporous carbon matrix to modify its properties. Conventional incorporation techniques can be summarized in two routes. One route is incorporating metal nanoparticles into the pre-synthesized OMC materials using impregnation, adsorption or ion exchange methods. Another route is called cocasting, which involves two main steps: (1) preparation of a mesostructured silica template and (2) filling the silica mesopore with an appropriate carbon precursor and metal source, followed by carbonization and removal of silica framework with NaOH or HF. Nevertheless, these two routes seem to be fussy, time-consuming and low effective to disperse metals throughout the carbon matrix. It is therefore desirable to incorporate active metal species into mesoporous carbon during synthesis of OMC through soft templating routes, while retaining the ordered mesostructure. Recently, incorporation of TiO2 [18], Ir [19], and Ru [20] nanoparticles into mesoporous carbons, has been accomplished by soft templating routes. Owning to excellent magnetic property and extreme reactivity, iron nanoparticles are more preferred candidates for many advanced nanotechnological applications, such as magnetic storage media [21], directed drug delivery [22], Fischer–Tropsch synthesis [23], and groundwater remediation or other environmental applications [24]. There has been some reports on the Fe/OMC

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prepared by hard templating method [25,26]. However, to our best knowledge, there is no report on the synthesis of Fe/OMC via soft templating routes can be found in current literatures. Herein, we have developed a simple method for facile synthesis of Fe/OMC composites under a moderate acidic condition. In this method, resorcinol–formaldehyde (RF) was used as a carbon precursor, triblock copolymer Pluronic F127 was used as a template agent and hydrated iron nitrite as an iron source. Our strategy is that the acid self-generated from the hydrolysis of precursory salt was just utilized as the catalyst for RF polymerization and no any additional mineral acid was necessary. Simultaneously, Fe species are introduced to the ordered mesostructure and subsequently reduced to Fe0 during the carbonization process at high temperature under inert atmosphere. To obtain a well-ordered structure, an important factor: the effects of iron loading contents was considered. The Fe/OMC materials from this novel approach exhibit good performance in the catalytic wet peroxide oxidation(CWPO) of phenol solution with hydrogen peroxide. 2. Experimental 2.1. Chemicals Triblock poly(ethylene oxide)-b-poly(propylene oxide)-bpoly(ethylene oxide) copolymer Pluronic F127 (MW = 12600, PEO106PPO70PEO106) was purchased from Aldrich Corp., and resorcinol, formaldehyde and hydrated iron nitrite (Fe(NO3)39H2O) were purchased from Shanghai Chemical Corp. All chemicals were analytical pure grade and were used as received without any further purification. 2.2. Synthesis The synthesis compositions were in the range of resorcinol(R)/ formaldehyde(F)/F127/hydrated iron nitrate/ethanol/water (molar ratio) = 1:2: 0.013:0.05–0.2:17.25:41.48. The resulting composite is designated as Fe/OMC–M, M denotes the molar ratio of iron to R at the beginning of polymerization. In a typical synthesis, 2.5 g F127 and 1.65 g R were dissolved absolutely in 14 g of ethanol/ water (1/1 vol%) solution under stirring at room temperature(signed A). An appropriate amount of hydrated iron nitrate corresponding to Fe/R molar ratio equal to 0.05, 0.10, 0.15, 0.20 was dissolved in 7 g of ethanol/water (1/1 vol%) solution (signed B). When solution A turned to be a light brown solution, solution B was dropped into it. After stirring for 2 h, 2.3 mL of F (37%) was subsequently dropped into the above solution. Followed by an additional 2 h stirring, the mixture was kept standing until it turned cloudy and began to separate into two layers. This two phase mixture was further kept aging for 60 h (shown in Fig. 1).

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Subsequently, the upper layer was discarded while the lower polymer-rich phase was stirred overnight until a sticky monolith was formed. Finally, the monolith was dried at 85 °C for 24 h and carbonized under a N2 atmosphere via heating ramps of 1 °C/min from 25 to 400 °C and 5 °C/min from 400 to 800 °C and kept at 800 °C for 2 h. 2.3. Characterization Powder X-ray diffraction (XRD) patterns were recorded on a Rigaku D/max-r system operating with Cu Ka (k = 1.5406Å) radiation. High-resolution transmission electron microscopy (HRTEM) images were obtained using a JEOL JEM-2100 microscope, operating at 200 kV. For TEM measurements, the samples were prepared by dispersing the powdered products as slurry in ethanol with a ultrasonic bath for about 10 min, and then one drop of the resulting suspension was dispersed and dried on a holey carbon film on a copper grid. N2 adsorption–desorption isotherms were measured using Micromeritics ASAP-2010 at liquid nitrogen temperature (77 K). Prior to measurements, all samples were degassed at 473 K for 12 h. The specific surface areas were evaluated using the Brunauer–Emmett–Teller (BET) method. The pore size distribution was derived from the desorption branches of the isotherms using the Barrett–Joyner–Halenda (BJH) method. 2.4. Catalytic wet peroxide oxidation (CWPO) of phenol Catalytic performance of the Fe/OMC materials was evaluated in the CWPO of phenol solution with hydrogen peroxide using a 250 mL glass flask reactor in a thermostatic water bath with magnetic stirring. A volume of 100 mL of an aqueous solution containing 247 mg/L phenol and 1813 mg/L H2O2 was used in the experiments. A dosage of 600 mg/L Fe/OMC-0.10 materials was added to the reactor. The experiments were carried out at 80 °C, P = 1 atm and the initial pH was adjusted to 3.0, by using 0.05 M H2SO4 solution. For comparison, a control experiment without adding catalysts was also performed at the same conditions. Periodically, a specified volume of the reaction samples was withdrawn by a 10 mL gas-tight syringe at selected reaction times. The samples were filtered by 0.22 lm nylon membranes, and immediately analyzed. Phenol was identified and quantified by means of HPLC (Waters 1525) consisted of a RP18 column (5 lm 3.9  150 mm), and equipped with a diode array ultraviolet detector selected at k = 271 nm. The system was operated in isocratic mode (methanol/water, 70/30, v/v) at a flow rate of 1 mL/min. The retention time of the phenol under these conditions was 1.8 min (±0.2 min). To assess the selectivity of the catalysts towards CO2 and H2O, the residual total organic carbon (TOC) content of the reaction solution was quantified using a TOC analyzer with a combustion/non-dispersive infrared detector ((Shimadzu TOC-VCPH, Japan). 3. Results and discussion

Fig. 1. Photo of two phase mixtures after aging for 60 h.

The low-angle XRD patterns of Fe/OMC composites with Fe/R molar ratios are presented in Fig. 2. The Fe/OMC-0.05 (Fig. 2a) and Fe/OMC-0.10 (Fig. 2b) samples show an intense diffraction peak at 2h range of 0.5–1°, which can be indexed to [1 0 0] reflection of a hexagonal mesostructure [19]. In particular, a sharp and narrow diffraction peak at 2h = 0.96° can be observed for Fe/ OMC-0.10, which indicates that this sample possesses well-ordered mesostructure. However, just a discernible weak peak at 2h = 0.94° was observed in the Fe/OMC-0.15 samples (Fig. 2c), and no obvious diffraction peak was detected in the Fe/OMC-0.20 (Fig. 2d) samples compared to Fe/OMC-0.05 and Fe/OMC-0.10,

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Graphite Fe -Fe2O3

(d)

(d)

Intensity (a.u.)

Intensity (a.u.)

(c)

(c)

(b)

(b)

(a)

(a) 1

2

3

4

10

5

20

30

40

50

60

70

80

2 theta (degree)

2 theta (degree) Fig. 2. Low-angle XRD patterns of Fe/OMC composites with different iron loading after carbonization (a) Fe/OMC-0.05, (b) Fe/OMC-0.10, (c) Fe/OMC-0.15, and (d) Fe/ OMC-0.20.

Fig. 3. Wide-angle XRD patterns of Fe/OMC composites with different iron loading after carbonization (a) Fe/OMC-0.05, (b) Fe/OMC-0.10, (c) Fe/OMC-0.15, and (d) Fe/ OMC-0.20.

implying that the optimal ordering of the hexagonal mesostructure can be obtained when Fe contents around Fe/R = 0.1 and the ordering decreased as the Fe content was either greater or smaller than this ratio. Clearly, a suitable amount of Fe present in the synthesis system would lead to a highly ordered mesostructure. This phenomenon can be explained from the preparing process of the Fe/OMC samples. In this synthesis, when iron precursor was added into the reaction mixture containing RF and F127, the acid will be generated due to the hydrolysis of precursory salts. And it can be used as the catalyst for cross-linking between R and F. The pH values of the synthesis solutions are listed in Table 1. As the Fe content increased from Fe/R = 0.05 to Fe/R = 0.2, the pH values of the solution was accordingly lowered from 1.96 to 1.47. However, the presence of excess iron species might disturb the self-assembly process and thus influence the final ordering of the mesostructure. The wide-angle XRD patterns (Fig. 3) exhibit a resolved diffraction peaks at 2h = 44.6° and a weak diffraction peak at 35.8° for all samples, which are in accordance with the (1 1 0) diffraction of body-centered cubic (bcc) a-Fe (JCPDS card No. 06-0696) and the (3 1 1) diffraction of c-Fe2O3 (JCPDS card No. 00-039-1346), respectively. It reveals that the iron species in the mesoporous carbon matrix exists in two states: metallic Fe and c-Fe2O3. According to Hoch et al. [27], iron nanoparticles (20–100 nm diameter) supported on carbon (C–Fe0) can be synthesized by carbothermal reduction of iron salts and carbon black under inert atmosphere, and the threshold temperature is 600 °C. Here, Fe/OMC was syn-

thesized through carbonization process at the temperature as high as 800 °C. Therefore, it can be believed that all Fe species can be reduced to Fe nanoparticles. In this study, the existence of c-Fe2O3 may be due to the fact that metallic Fe possesses extremely high reactivity and thus can be easily oxidized to iron oxides when exposed in the air [21]. Furthermore, when the ratio of Fe/R is beyond 0.10, an intensive diffraction peak at 2h = 26°, along with 3 resolved diffraction peaks at 2h = 43°, 54°, and 78° can be observed in the resulting Fe/OMC composites, which can be indexed to the (0 0 2), (1 0 1), (0 0 4), and (1 1 0) diffraction peak for typical graphite-like carbon, respectively [28]. The results indicate that the prepared materials are graphitized during carbonization at 800 °C which is much lower than graphitized temperature of pure OMC [29]. The lower graphitized temperature of Fe/OMC is due to the existence of the iron nanoparticles. It is well known that the nanosized metals such as Fe, Co, Ni, Mo can accelerate the development of graphitic structure of carbon when they are heat-treated together in inert gas atmosphere [30]. The mesostructures of the Fe/OMC samples and the distribution of iron species in the final materials are investigated with TEM. As shown in Fig. 4, parallel channels with a d spacing of about 9.2 nm are clearly observed on Fe/OMC-0.05(Fig. 4a) and Fe/OMC0.10(Fig. 4b), which is consistent with the XRD results. For Fe/ OMC-0.15, an orderly arranged strip-like channels structure can also be observed (Fig. 4c), although the regularity is not as good as that of the Fe/OMC-0.05 and Fe/OMC-0.10. In contrast, only wormhole-like disordered structure can be visualized on Fe/

Table 1 Textural properties of the Fe/OMC composites.

a b c

Sample name

pH

Unit cell parametera (nm)

Pore size diametersb (nm)

BET surface area (m2 g1)

Pore volume (cm3 g1)

Pore wall thicknessc (nm)

Fe/OMC-0.05 Fe/OMC-0.10 Fe/OMC-0.15 Fe/OMC-0.20

1.96 1.69 1.55 1.47

10.2 10.6 10.8 –

3.9 3.9 3.9 3.8

452 586 412 370

0.32 0.41 0.31 0.24

6.3 6.7 6.9 –

pffiffiffi Calculated from XRD results using the formula a = 2d100/ 3. Calculated from the N2 desorption branch of the isotherms by the BJH method with Halsey equation. Calculated using the formula c = a  b.

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Fig. 4. TEM images of Fe/OMC composites with different iron loading after carbonization (a) Fe/OMC-0.05, (b) Fe/OMC-0.10, (c) Fe/OMC-0.15, and (d) Fe/OMC-0.20.

OMC-0.20 (Fig. 4d). It can also be found that the nanoparticles, the dark spots, were dominantly dispersed on the walls of the OMC. For the samples with lower Fe contents, Fe/OMC-0.05(Fig. 4a) and Fe/OMC-0.10 (Fig. 4b), the particle size is about 9–12 nm. Moreover, there are more visible nanoparticles on the Fe/OMC0.10 can be seen than those of Fe/OMC-0.05, indicating that more iron species were introduced in RF polymerization. When the Fe/ R molar ratio increased to 0.15 or more, bigger particles with the diameter of 15–20 nm were obtained owning to aggregation of small nanoparticles. Nevertheless, the size of these incorporated nanoparticles is much smaller than that of support-free nanoparticles [31]. Therefore, we conclude that OMC materials not only stabilize the Fe nanoparticles by avoiding their aggregation to some extent, but also confine the growth of nanoparticles. Furthermore, as shown in these images, the ordered structures of Fe/OMC composites with highly dispersed nanoparticles can be maintained even Fe/R molar ratio up to 0.15. In order to further understand the existing state of iron species in OMC, detailed HRTEM analysis was performed. We have randomly observed many particles which were embedded in OMC or exposed on its surface. Fig. 5b and Fig. 5d are magnification images of the particle marked by the circle in Fig. 5a and c, respectively. These HRTEM images illustrate the perfect arrangements of the atomic layers and lack of defects. The lattice plane distance in Fig. 5b is 2.03 Å, which is consistent with the [1 1 0] plane of the a-Fe (JCPDS card No. 06-0696). However, Fig. 5d reveals a typical cubic c-Fe2O3 with a d spacing of 2.52 Å which is corresponding to the [3 1 1] plane [32]. From the above analysis, it reveals that iron species in the mesoporous carbon matrix existed in two states: metallic Fe and

c-Fe2O3. In principle, when carbonaceous materials containing metallic salts were heat-treated under inert atmosphere, RF polymers were carbonized and meanwhile the metallic salts first decomposed to metallic oxides. With the increasing temperature, metallic oxides were gradually reduced to metallic elements by carbonaceous materials. As reported by Chen et al., iron oxides existed in the bores of carbon nanotubes or on its outer surface can be reduced to metallic Fe when heat-treated temperature reached 800 °C [33]. Owing to the fact that metallic Fe particles are extremely active and prone to be oxidized by air at room temperature, Fe species located on the surface of OMC mostly exist as c-Fe2O3 particles while Fe species buried in the walls of OMC dominantly present as metallic Fe particles. c-Fe2O3 particles supported on the surface of OMC together with the carbon matrix, probably can protect the bcc-Fe particles embedded in the walls of OMC from oxidation. Fig. 6 shows N2 sorption isotherms and pore size distribution of the four Fe/OMC composites, and Table 1 lists the corresponding textural parameters. All of the composites exhibit typical type IV isotherms with an obvious H1-type hysteresis loop, indicating the mesoporous structures of the materials. The sharp inflections between the relative pressures p/p0 = 0.4–0.8 in these isotherms correspond to capillary condensation within uniform mesopores. The pore size distributions are very narrow, centering at around 3.9 nm for all samples (Fig. 6B). As shown in Table 1, Fe/OMC-0.10 has a surface area of 586 m2/g and pore volume of 0.41cm3/g. Differently, the samples with higher Fe-loading (i.e. Fe/OMC-0.15 and Fe/OMC-0.20) or lower Fe-loading (i.e. Fe/OMC-0.015) have a decreased surface area and pore volume. These N2 adsorption– desorption data are in well agreement with the Low-angle XRD

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Fig. 5. Typical TEM images and corresponding HRTEM image of particles embedded in OMC (Fe/OMC-0.10 (a and b)) and exposed on its surface (Fe/OMC-0.05 (c and d)).

250

25

(a)

(a) (b) (c) (d)

dV/d log(D) (cm 3/g/nm)

Volume adsorbed(cm3 /g, STP)

300

200

150

100

(b)

(a) (b) (c) (d)

20

15

10

5

0 0.0

0.2

0.4

0.6

0.8

1.0

Relative Pressure(P/P0 )

1

2

3

4

5

6

7

8

Pore size(nm)

Fig. 6. (A) N2 adsorption–desorption isotherms and (B) pore size distributions of Fe/OMC composites with different iron loading (a) Fe/OMC-0.05, (b) Fe/OMC-0.10, (c) Fe/ OMC-0.15, and (d) Fe/OMC-0.20.

(Fig. 2) and TEM (Fig. 4) results. Obviously, the moderate amount iron precursor is favorable to the formation of a well-ordered mesoporous structure. In order to examine the catalytic properties of Fe/OMC materials, we chose the CWPO of phenol solution with hydrogen peroxide as a probe reaction. All the experiments were carried out at very mild conditions (P = 1 atm and T = 80 °C). Fig. 7(A) shows the phenol conversion over Fe/OMC-0.10 catalyst and control experiments. As expected, Fe/OMC-0.10 materials exhibit distinct catalytic abilities and the eliminate rate of phenol can reach ca. 60.9% mass after 30 min. After 180 min, the conversion of phenol can reach 93.0%. While as shown in Fig. 7A, no significant consumption of phenol

can be seen during the whole reaction over the control experiment except for the initial 2 min. To have a better knowledge on the action of the catalyst, Fig. 7B shows the results obtained in terms of TOC conversion over the Fe/ OMC-0.10 catalyst and control experiments. During the oxidation process, phenol is initially degraded to form intermediate products and further degraded to the low-molecular-weight organic acids with the short-chain, CO2 and H2O [34]. Some of the short-chain organic acids are stable and refractory to mineralize into CO2 and H2O [35]. Thus, TOC conversion is lower than phenol conversion at the given reaction time. It is noted that the TOC conversion reach 61.7% after 180 min of reaction. From the above analysis, our

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80 Fe/OMC0.10 Control experiment

(A)

80 60 40 20

Fe/OMC0.10 Control experiment

70

TOC conversion(%)

Phenol Conversion(%)

100

(B)

60 50 40 30 20 10

0

0 0

20

40

60

80 100 120 140 160 180 200

Time(min)

0

20

40

60

80 100 120 140 160 180 200

Time(min)

Fig. 7. (A) Phenol conversion and (B) TOC conversion over Fe/OMC-0.10 catalyst and control experiments.

present Fe-OMC catalysts, possessing highly dispersed iron particles, exhibit promising performance for the oxidative degradation of phenol. 4. Conclusions Highly ordered Fe-containing mesostructure carbon materials can be directly prepared using soft templating routes without the addition of mineral acids. The acidity self-generated by the iron precursor should be enough to catalyze the polymerization of RF. The optimal ordering mesostructure can be obtained when the Fe/R molar ration is around 0.1. The iron species are spontaneously reduced to metallic iron by carbonaceous materials during carbonization. Fe species located on the surface of OMC mostly present as c-Fe2O3 particles while Fe species buried in the walls of OMC dominantly as metallic Fe particles due to extreme reactivity of metallic iron. The resulting materials possess specific activity for catalytic wet peroxide oxidation of phenol in solution. It is believed that the easy soft templating synthesis approach reported here can be readily expanded its scope to synthesize other metals containing OMC materials. Moreover, such highly ordered mesoporous Fe/ OMC composites may be applied in the fields of separation, catalysis, and environment remediation. Acknowledgments This work was financially supported by the Cultivation Fund of the Key Scientific and Technical Innovation Project, Ministry of Education of China (Grant No. 708049, Grant No. 20060288008), Scientific Research Foundation for the Returned Overseas Chinese Scholar, State Education Ministry and Natural Science Foundation of Jiangsu Province (Grant No. BK2009392). We acknowledge gratefully Prof. Jinqiang Liu for the helpful discussion. References [1] Z.J. Li, G.D. Del Cul, W.F. Yan, C.D. Liang, S. Dai, J. Am. Chem. Soc. 126 (2004) 12782. [2] K. Ariga, A. Vinu, Q.M. Ji, O. Ohmori, J.P. Hill, S. Acharya, J. Koike, S. Shiratori, Angew. Chem., Int. Ed. 47 (2008) 7254.

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