Soft-templating synthesis of ordered mesoporous carbons in the presence of tetraethyl orthosilicate and silver salt

Microporous and Mesoporous Materials 156 (2012) 121–126

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Soft-templating synthesis of ordered mesoporous carbons in the presence of tetraethyl orthosilicate and silver salt Laura Sterk a, Joanna Górka a,1, Ajayan Vinu b,c, Mietek Jaroniec a,⇑ a

Department of Chemistry, Kent State University, Kent, OH 44240, USA International Center for Materials Nanoarchitectonics, WPI Research Center, National Institute for Materials Science, 1-1 Namiki, Tsukuba 305 0044, Japan c Australian Institute for Bioengineering and Nanotechnology, The University of Queensland, #75 Corner Cooper and College Road, Brisbane 4072, QLD, Australia b

a r t i c l e

i n f o

Article history: Received 21 December 2011 Received in revised form 14 February 2012 Accepted 15 February 2012 Available online 24 February 2012 Keywords: Mesoporous carbons Nitrogen adsorption Silver nanoparticles Soft-templating synthesis

a b s t r a c t Soft-templating synthesis of ordered mesoporous carbons (OMCs) in the presence of tetraethyl orthosilicate (TEOS) and silver nitrate was carried out in order to introduce silver nanoparticles and to create additional microporosity in these materials. This strategy was employed to obtain the phenolic resinbased OMCs with two different loadings of silver. Also, this approach was used to obtain silver-containing mesoporous carbon–silica hybrids, which after dissolving silica with NaOH solution gave microporous– mesoporous carbons with Ag particles. Nitrogen adsorption, small and wide angle X-ray diffraction, transmission electron microscopy and thermogravimetric analysis showed good adsorption and structural properties of the aforementioned OMC materials. Ó 2012 Elsevier Inc. All rights reserved.

1. Introduction Carbon-based materials are commonly used in industrial applications because of their low cost and commercial availability. For instance, activated carbons are produced at very low cost for various applications ranging from adsorption, catalysis, purification and separation processes to capacitors; they have high surface areas due to the presence of micropores (pore widths below 2 nm) but broad pore size distributions and disordered porosity. Thus, these materials have several shortcomings: slow mass transport due to complex microporosity, low conductivity due to surface groups and defects, and tendency for pore structure collapse upon high temperature heating under neutral atmosphere (graphitization) [1]. In the late 1990s, nanocasting strategy, often referred to as hard-templating named because of using sacrificial siliceous templates, yielded first carbons with ordered and tunable porous structures [2–4]. Even though the hard-templating became very popular way to produce mesoporous carbons, it is considered unfeasible because of high cost, laborious process and environmental risk associated with using HF or NaOH for the removal of siliceous templates. In 2006, the soft-templating strategy was developed on the basis of organic–organic self-assembly of block ⇑ Corresponding author. Tel.: +1 330 672 3790; fax: +1 330 672 3816. E-mail address: [email protected] (M. Jaroniec). Present address: Oak Ridge National Laboratory, Chemical Sciences Division, Nanomaterials Chemistry Group, 1 Bethel Valley Road, Bldg. 4100, Room A223, Oak Ridge, TN 37831-6201, USA. 1

1387-1811/$ - see front matter Ó 2012 Elsevier Inc. All rights reserved. http://dx.doi.org/10.1016/j.micromeso.2012.02.028

copolymers (soft templates) and polymeric-type carbon precursors [5–9]. This one-pot synthesis affords carbons possessing high surface areas and large mesopores with narrow pore size distributions. Nanoporous carbons because of their high stability in acidic and basic media are attractive supports for the development of various catalysts. Chen et al. [10] reported the preparation of silver–carbon mesoporous materials as catalysts for fuel cell applications. Preferential oxidation of CO to CO2 inhibits the poisoning effect of CO on the fuel cell catalyst. Using carbon as a support allows for a more efficient single stage cell configuration due to the conductive properties of carbon. Also, silver is an inexpensive alternative to its precious metal congeners, which is an important feature, related to the common effort to replace the use of Pt-based catalysts with those containing semi-precious metals. Other possible applications of silver-containing carbons are based on silver antibacterial properties and include materials used for water purification and treatment [11–14]. Although not fully understood, silver species inhibit the replication of the bacteria and yeast fungus such as Escherichia coli, Staphylococcus aureus and Candida albicans making silver-containing materials ideal for such as applications as a water treatment [13]. Interestingly, the strongest antibacterial activity was exhibited by carbons just doped with silver [14]. Another work reported that the silverloaded carbons exhibit much higher adsorption towards metalcyanide complexes from aqueous solutions than activated carbons alone [15]. The commonly used method for silver loading is a simple impregnation of amorphous carbon powders, graphitic fibers or

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monoliths with silver salts [12,16–19]. An interesting synthesis route towards the preparation of high surface area and high pore volume carbons with silver nanoparticles was reported by Jaroniec et al. [20]. In this case, silica colloids used as a hard template were pressed with silver nanoparticles together to form a monolith. The impregnation of the latter with a phenolic resin-type carbon precursor and subsequent carbonization and silica removal led to the final mesoporous carbons with silver nanoparticles. The advantage of this synthesis route is the possibility to precisely control the size of mesopores formed and the size of silver nanoparticles embedded in the carbon framework [21]. Here we report the synthesis of ordered mesoporous carbons (OMCs) with embedded silver nanoparticles by self-assembly of a triblock copolymer and phenolic resin precursors in the presence of AgNO3. Also, the reaction mixture was supplied with tetraethyl orthosilicate (TEOS) in order to improve microporosity and specific surface area of the final materials. The whole synthesis procedure was analogous to that demonstrated earlier for the preparation of nickel-containing carbons [22] in order to determine the effect of metal salt on the mesostructure formation.

2. Materials and methods 2.1. Chemicals Poly(ethylene oxide)-poly(propylene oxide)-poly(ethylene oxide) triblock copolymer (EO106PO70EO106; Pluronic F127) was provided by BASF Corp. Resorcinol, (C6O2H6; 98%), formaldehyde (HCHO; 37%), silver nitrate (AgNO3; 99%) and tetraethyl orthosilicate (TEOS, 98%) were purchased from Arcos Organics. HCl (35– 38%) was acquired from Fischer and ethanol from Pharmco.

2.2. Materials The carbon samples studied were synthesized by using a slightly modified recipe reported by Wang et al. [23]. Initially, 1.25 g of Pluronic F127 block copolymer, 1.25 g of resorcinol and the specified amount of AgNO3, (1 or 2 mmol) were dissolved in water/ethanol solution; the weight ratio of water to ethanol was 5.5:10. After a complete dissolution 1.1 ml of HCl was added under continuous stirring for another 30 min. Then 1.25 ml of formaldehyde was introduced to the reaction mixture and stirred until mixture turned milky. Additional 30 min of stirring was applied to ensure the phenolic resin formation. At this time, the mixing was stopped and the solutions remained undisturbed until a phase separation took place. The upper phase, which consisted mostly of ethanol and water, was removed and the lower phase was transferred onto a Pyrex dish to evaporate the solvent for 16 h under ambient conditions followed by aging at 100 °C for 24 h. In the case of SiO2-containing samples, TEOS in the amounts of 40 and 60 wt.% with respect to the carbon precursor were added about 30 min after formaldehyde addition. A tube furnace was used to carbonize the resulting samples under nitrogen atmosphere with the heating rate of 2 °C /min up to 180 °C and keeping at the target temperature for 5 h, resuming heating with the same rate up to 400 °C and with 5 °C/min up to 800 °C followed by keeping them at 800 °C for 2 h. In order to remove silica, samples were soaked with 3% sodium hydroxide (10 ml per gram of the sample) and kept at 70 °C for 16 h followed by washing with DI water. The final samples were denoted according to the formula: MxAgyT, where M stands for the type of material (CS = carbon–silica composite, C = carbon), x indicates the amount of silver salt added (1 or 2 mM) and y refers to wt.% of TEOS (T) in the sample.

2.3. Measurements Nitrogen adsorption isotherms were measured at 196 °C on ASAP 2020 volumetric analyzer (Micromeritics, Inc., GA). Samples were outgassed at 200 °C for 2 h prior to adsorption measurements. Wide angle X-ray diffraction analysis was performed on PANalytical X’Pert PRO MPD X-ray diffraction system using Cu Ka radiation (40 kV, 40 mA). All patterns were recorded using 0.02° step size and 4 s per step in the range of 15° 6 2h 6 80°. Small angle XRD data were measured in the range of 0.4° 6 2h 6 5°. Thermogravimetric analysis was performed on a TA Instrument Hi-Res TGA 2950 thermogravimetric analyzer from 30 to 800 °C under air flow with a heating rate of 10 °C/min. TEM images of the samples were taken on a Hitachi HD-2000 Scanning and Transmission Electron Microscope (STEM). The unit was operated at an accelerating voltage of 200 kV and an emission current of 30 mA. 2.4. Calculations The BET specific surface area [24] was calculated from nitrogen adsorption isotherms in the relative pressure range of 0.05–0.2. The total pore volume [25] was estimated from the amount adsorbed at a relative pressure of 0.99. The pore size distributions were calculated from nitrogen adsorption isotherms at 196 °C using the improved KJS method calibrated for cylindrical mesopores with diameters up to 10 nm [26]. 3. Results Nitrogen adsorption and pore size distributions were used to examine the changes in the physicochemical characteristics of the carbon–silica materials with incorporated silver nanoparticles. Fig. 1 shows adsorption isotherms and pore size distributions for the silver-containing carbon materials prepared with 1 mmol loading of silver that includes carbon (C–Ag), carbon–silica composites (CS–AgT) and final carbons obtained after dissolving the silica component from the parent carbon–silica composites (C–AgT). All samples exhibit isotherms of type IV with H1-type hysteresis loop indicating the presence of mesopores. Adsorption isotherms for the final carbon materials (C–AgT) show the highest nitrogen uptake due to increased microporosity created by silica removal. The carbon sample synthesized without TEOS (C–Ag) shows total adsorption lower than that obtained for the aforementioned samples but higher than that for the carbon–silica composites (CS– AgT). These data stay in a good agreement, taking into account that the silica portion in the silver-containing composites is non-porous unlike carbon which possesses some microporosity as seen in the C–Ag carbon. The aforementioned account of microporosity is evident in both final carbons containing silver, having the micropore volumes of 0.18 and 0.21 cm3/g for the C–1Ag40T and C–1Ag60T samples, respectively; these values are higher than those for the remaining samples in this series. This increase in microporosity is also visible on the PSD curves up to 4 nm. The specific surface area varies from 353 m2/g evaluated for the composite to 779 m2/g for the final Ag–carbon material obtained after dissolving silica (see Table 1). In the case of mesoporosity, no radical changes were observed including both the mesopore volumes and the pore diameters, the latter were found to be 1 nm larger for the samples synthesized with TEOS. However, the bimodal desorption branches suggest the existence of some pore constrictions. Nitrogen adsorption isotherms and the corresponding pore size distributions for the materials synthesized with doubled loading of silver (2 mmol) are presented in Fig. 2. In the case of this series of

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C-1Ag CS-1Ag40T C-1Ag40T CS-1Ag60T C-1Ag60T

0.14 0.12 0.10

3

300

C-1Ag CS-1Ag40T C-1Ag40T CS-1Ag60T C-1Ag60T

PSD (cm /g.nm)

Volume Adsorbed (cm3STP/g)

0.16 400

200

0.08 0.06 0.04

100

0.02 0 0.0

0.2

0.4

0.6

0.8

0.00

1.0

2.0

4.0

Relative Pressure (p/po)

6.0

8.0

10.0 12.0 14.0

Pore Width (nm)

Fig. 1. Nitrogen adsorption isotherms and the corresponding pore size distributions for the carbons and carbon composites prepared with 1 mmol loading of silver.

Table 1 Structural properties of the carbon samples studied.a Sample

SBET(m2/g)

Vt (cm3/g)

Vmi (cm3/g)

Vme (cm3/g)

wKJS (nm)

RTGA%

d (nm)

C–1Ag C–2Ag CS-1Ag40T CS-2Ag40T CS-1Ag60T CS-2Ag60T C–1Ag40T C–2Ag40T C–1Ag60T C–2Ag60T

571 561 500 364 353 212 779 495 712 578

0.50 0.50 0.47 0.32 0.32 0.19 0.65 0.42 0.57 0.42

0.15 0.15 0.13 0.05 0.01 0.06 0.18 0.16 0.21 0.19

0.35 0.35 0.34 0.27 0.31 0.13 0.47 0.26 0.36 0.23

7.1 7.0 7.8 8.0 8.2 8.0 7.6 8.2 8.2 8.0

23.3 25.4 38.1 47.8 54.6 70.8 23.9 47.1 35.5 52.3

10.1 10.0 10.1 10.0 10.4 11.0 10.4 10.5 10.7 10.8

Vt – single-point pore volume; Vmi – volume of fine pores defined as the difference Vt Vme; Vme – volume of mesopores obtained by integration of PSD in the range 3.5– 30 nm; wKJS – mesopore diameter at the maximum of the PSD curve; RTGA – residue obtained by the TG analysis in air. a SBET – BET surface area.

samples, the silver-containing carbon prepared without TEOS exhibits the highest nitrogen uptake. Surprisingly, the Ag-carbon materials with etched silica component are characterized by smaller total adsorption, which is a reverse trend to that observed for the series with 1 mmol loading of silver. Even though the capillary condensation steps almost overlap each other, there is an easily noticeable difference at low p/po values revealing different amounts of micropores created after dissolution of the TEOS-generated silica. Taking this into account, it is not surprising that the isotherms for composite materials also overlap and have the lowest adsorption values. Based on the entries in Table 1 and the PSD curves one can track all changes in micro- and mesoporosity.

Generally, the BET surface areas show lower values mostly due to bigger loadings of non-adsorbing silver in the samples. For this reason, the specific surface area even for the final carbons (C– AgT) stays close to the value obtained for the C–2Ag material, which is 560 m2/g. The same decreasing trend is also carried for micro- and mesopore volumes. TEM images shown in Fig. 3 clearly indicate the presence of uniform and hexagonally ordered mesopores. The carbon samples synthesized without TEOS, C–1Ag and C–2Ag, show a very good ordering of mesopores. The periodicity of these materials was maintained for the silver-containing carbon samples prepared with smaller loading of TEOS (40 wt.%). After increasing TEOS loading to

300

C-2Ag CS-2Ag40T C-2Ag40T CS-2Ag60T C-2Ag60T

0.14 0.12

PSD (cm3/g.nm)

Volume Adsorbed (cm3STP/g)

0.16 C-2Ag CS-2Ag40T C-2Ag40T CS-2Ag60T C-2Ag60T

200

100

0.10 0.08 0.06 0.04 0.02

0 0.0

0.00 0.2

0.4

0.6

0.8

Relative Pressure (p/po)

1.0

2.0

4.0

6.0

8.0

10.0 12.0 14.0

Pore Width (nm)

Fig. 2. Nitrogen adsorption isotherms and the corresponding pore size distributions for the carbons and carbon composites prepared with 2 mmol loading of silver.

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Intensity (a.u.)

CS-1Ag40T

C-1Ag

C-1Ag60T

C-1Ag C-1Ag40T

0.5

1.0

1.5

2.0

CS-1Ag60T

2.5

3.0

0.5

1.0

2θ(°)

Intensity (a.u.) 1.0

2.0

C-2Ag40T

CS-2Ag60T

CS-2Ag40T

C-2Ag60T

1.5

2.0

2θ(°)

2.5

3.0

2.5

3.0

C-2Ag

C-2Ag

0.5

1.5

2θ(°)

Intensity (a.u.)

60 wt.% the mesoporous structure of the carbon samples became more worm-like as shown in the bottom panels of Fig. 3, although ordered domains were also observed (not shown in this figure). Periodicity was also evaluated based on small angle powder Xray diffraction patterns (Fig. 4). The presence of an intense peak in the range between 0.5° and 1° suggests good ordering in all samples. Also, a slight shift of this peak towards smaller values of 2h denotes a small (0.2 nm) increase in the pore diameter visible on the PSD curves obtained from adsorption data. Based on the synthesis of similar materials [27,28] the expected ordering is 2D hexagonal (p6mm). Although the symmetry group cannot be determined from the XRD data, TEM images suggest a hexagonal arrangement of pores (Fig. 3). The presence of silver was confirmed by wide angle XRD measurements (Fig. 5). The four reflection peaks (1 1 1), (2 0 0), (2 2 0), (3 1 1) appeared for each material, which were identified as silver crystals possessing a cubic structure with Fm3m symmetry (based on JCPDS card number 087-0597). Crystallite diameters, calculated from the first (most intense) peak using the Scherrer equation, were determined to be in the range of 43–61 nm. These particles are surrounded by porous carbon. The size of formed silver nanoparticles explains the pore blockage suggested by the shape of hysteresis loops and also justifies a considerably lower adsorption obtained for the samples with higher silver loading (Fig. 2). Moreover, it is well known that at higher temperatures metallic species can migrate across a surface and agglomerate creating larger particles [28]. Taking this fact into account and the data obtained for the samples prepared without TEOS, which suggest the presence of smaller silver nanoparticles, one can speculate that the narrow and intense wide angle XRD peaks dominate and reflect the aforementioned larger silver nanoparticles formed during carbonization process by aggregation of a portion of smaller ones.

Intensity (a.u.)

Fig. 3. TEM images of the silver-containing carbon samples studied; all scale bars are 50 nm.

2.5

3.0

0.5

1.0

1.5

2.0

2θ(°)

Fig. 4. Small angle XRD patterns for the samples studied.

L. Sterk et al. / Microporous and Mesoporous Materials 156 (2012) 121–126

∗

* *

∗

*

*

CS-1Ag40T

*

*

C-1Ag40T *

*

Intensity (a.u.)

Intensity (a.u.)

*

∗ ∗

40

50

60

2θ(°)

∗

∗

∗

∗ ∗

*

*

∗

C-2Ag40T

C-1Ag

30

∗

∗

CS-2Ag40T

*

C-2Ag

70

80

30

40

50

60

70

80

2θ(°)

Fig. 5. Wide angle XRD patterns for the silver-containing carbon and silica–carbon samples studied.

Thermogravimetric analysis was used to determine silver and silica residue in the Ag-containing carbons and composite materials (Table 1). C–1Ag and C–2Ag show similar values of 23.3% and 25.4% respectively, indicating lower than expected value of silver residue found in the latter sample. The residue value for C–1Ag is supported by the value obtained for C–1Ag40T, 23.9%, because both should be equal upon complete removal of TEOS. The same relation does not appear at higher concentration of silver where C–2Ag40T has a residue of 47.1% indicating that there is some non-dissolved silica. The residue values for the composite materials fall in the range of 38–71% depending on the amounts of silica and silver in the system. The final carbons obtained after removal of silica from the samples prepared with 60 wt.% of TEOS showed higher values than their carbon counterparts (C–1Ag and C–2Ag) indicating the presence of residual silica, which can contribute to the pore blocking mentioned above. 4. Discussion 4.1. TEOS and silver effects It has been reported that TEOS is capable of co-assembly with phenolic-resin providing a facile method for the incorporation of silica species in the framework, which gives structural support and minimizes the sample shrinkage by as much as 20% occurring under heat treatment [27]. This is especially beneficial for the carbons obtained under alkaline conditions, which suffer extensive framework shrinkage during carbonization. Another benefit of using TEOS is the creation of an additional microporosity after dissolution of the TEOS-generated silica [29,30]. High microporosity and surface area are very desirable for number of catalytic applications. In our case, the first noticeable effect of TEOS addition is an increase in the diameter of mesopores compared to those of C–Ag materials, which can originate from preferential interaction of prehydrolyzed TEOS species with PEO blocks of the soft-template used or may be a result of smaller framework shrinkage during carbonization. Also, as reported elsewhere, there are at least two possible TEOS accumulation scenarios resulting in carbons with tuned both micro- and mesoporosity [31]. If TEOS is evenly distributed in the carbon matrix, an increase in microporosity is expected. This was observed for all final materials, however it is especially pronounced for C–1Ag-40T and both carbons prepared with 60% of TEOS. In the second case, already mentioned at the beginning, TEOS species exhibit favorable interactions with PEO blocks causing a pore enlargement, which also translates to higher mesopore volume. Interestingly, it is also possible that both phenomena occur simultaneously, as it is for C–2Ag60T material, where an increase

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in micro- and mesopore volume is observed. Based on that, it is relatively easy to track TEOS incorporation pattern. However, it is not so simple to predict how the latter looks like in the case of silver. Data obtained from thermogravimetric analysis for the samples prepared without TEOS show very similar residues for both carbons (C–1Ag and C–2Ag) indicating that the use of twice higher concentration of silver nitrate in the synthesis did not produce the sample with much higher silver loading. Note that both samples, C–1Ag and C–2Ag, were synthesized under the same acidic conditions by using HCl. This synthesis results in the phase separation and only the polymeric phase is subjected to carbonization. Note that the presence of Cl ions in the synthesis mixture facilitates the accumulation of silver in the polymeric phase. The excessive silver ions stay in aqueous phase; therefore, doubling the concentration of silver nitrate did not result in doubling the silver loading in C–2Ag. However, the residue percentage obtained for the composite materials stays pretty close to theoretical values suggesting that TEOS may help with silver incorporation. There are numerous reports showing strong interaction between silver ions and silanes [32–34], which can be adapted, for example, to obtain uniform layers of silver nanoparticles on the desired surfaces. Despite that, silver ions also exhibit strong interactions with arylrings [33,34], which additionally facilitate Ag incorporation. Also, it is noteworthy that due to high atomic weight of silver, its weight percentage in the C-AgT samples is high. This has a pronounced effect on the overall adsorption characteristics of the silver-containing carbons due to significantly smaller amount of highly adsorbing carbon per unit mass of the composite sample. This explains a noticeable drop in the BET surface area, the total pore volume and the volumes of mesopores and micropores for the silver-containing carbons with increasing silver loading. Note that the observed reduction in the specific surface area and pore volumes of the metal-containing samples is not solely caused by the decrease of the carbon mass in the composite sample. An additional reduction may origin from partial blockage of the pores in carbons by embedded metal nanoparticles. These effects have been observed for analogous systems containing nickel nanoparticles [35,36]. In our previous work [22] the generation of nickel nanoparticles in carbon matrix in the presence of TEOS was demonstrated. In both cases we kept the same synthesis conditions for TEOS and metal salt loadings to be able to determine and compare the carbon mesophase formation in relation to the amount of metal salt. The use of two different metal nitrates for the formation of nanoparticles in mesoporous carbons showed that the aforementioned effects are more pronounced for silver. This is evident by comparing the total pore volumes, which for nickel-containing carbons are in the range 0.53–1.38 cm3/g (see Ref. [22]) and for the silver-containing carbons this range consists of much smaller values, i.e., 0.19–0.65 cm3/g. The surface areas were also considerably higher for the nickel-containing carbons (491–1692 m2/g) than those for the silver-containing analogues (212–805 m2/g). Finally, the pore widths varied only slightly, where the average pore width for the silver-containing carbons was 8.0 nm compared to 7.5 nm for the nickel-containing analogues. It should be noted that in the case of high nickel loading a secondary mesoporous network was formed, which contributed to some extraordinary high values of the surface area (1692 and 1273 m2/g) and pore volumes (1.59 and 1.38 cm3/g) but the aforementioned trends are still observed for the remaining nickel-containing carbons. These differences are likely a result of greater blockage of pores by silver particles. Another mentionable topic pertains to the metals themselves (Ag and Ni) and how they interact with carbon. It is well known that nickel interaction with carbon at high temperatures often results in catalytic graphitization [37]. This is on account of nickel acting as a catalyst for the formation of graphite. As a prove for that, we

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were able to find a peak at 26° in wide angle XRD patterns indicating some graphitic domains in the Ni-containing materials, especially those made with high concentration of nickel. This is shown in the nickel-containing etched and composite materials. This peak is not present on the wide angle XRD profiles for the silver-containing samples. Silver is not a good catalyst for graphite formation. Another difference for consideration is the migratory nature of silver on the carbon surfaces [38], which may result in higher blockage of carbon pores. 5. Conclusions In conclusion, the soft-templating was successfully employed for the preparation of mesoporous carbon–silica composites containing Ag nanoparticles. The resulting silver-containing carbons possess relatively high surface area and pore volume, although higher values of these quantities were observed for the counterparts prepared in the presence of nickel salts [22]. The mesopore ordering of the resulting carbons was not significantly affected by silver loading, which is especially true at small silver loadings. Acknowledgments This material is based upon work supported by the National Science Foundation under CHE-0848352. The authors thank BASF for providing the triblock polymer. The TEM imaging was performed at the NIMS, Japan. References [1] K. Knioshita, Carbon Electrochemical and Physicochemical Properties, John Wiley & Sons, New York, 1987. [2] R. Ryoo, S.H. Joo, S. Jun, J. Phys. Chem. B 103 (1999) 7743. [3] J. Lee, S. Yoon, T. Hyeon, S.M. Oh, K.B. Kim, Chem. Commun. (1999) 2177.

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