Control of mesoporous structure of carbons synthesised using a mesostructured silica as template

doi:10.1016/S1387-1811(03)00403-7

Microporous and Mesoporous Materials

Volume 62, Issue 3, 28 August 2003, Pages 177-190

https://doi.org/10.1016/S1387-1811(03)00403-7 Get rights and content

Abstract

Mesoporous skeleton carbons with controllable pore size distributions (PSDs) were synthesised by modifying the extent of which the carbon precursor infiltrated the pores of the mesostructured silica, which was used as a template. When the silica porosity was fully infiltrated, carbons with a unimodal narrow PSD (∼3 nm) were obtained. This porosity was an inverse replica of the silica framework. If the silica pores were not completely filled, carbons with bimodal PSDs were synthesised. The carbons exhibited two kinds of mesopores: pores derived from the silica walls (∼3 nm) and larger pores, with sizes of up to 18 nm, formed from the coalescence of unfilled silica pores when the silica walls were dissolved. These results show that the synthesis of mesoporous carbons can be accurately controlled to obtain carbon materials with different PSDs using only one type of silica template.

Introduction

Porous carbons with controlled porosity in the mesopore range of 2–50 nm have recently received great attention because of their potential use as advanced adsorbents in adsorption and separation systems, as catalytic supports in fuel cell electrodes and as double-layer supercapacitors for energy storage. Numerous techniques for preparing mesoporous carbons can be found in the literature [1]. However only a few of these permit for an accurate control of mesoporosity. A few years ago, Ryoo et al. [2], [3] reported on the preparation of mesoporous carbons using mesostructured silica materials as templates. By means of this technique it is possible to obtain porous carbons, with a large surface area, a high porosity and controlled narrow pore size distributions (PSDs) in the mesopore range.

The preparation of these materials consists of (a) the infiltration of the porous structure of an inorganic material (template) by the carbon precursor (generally a polymer or prepolymer), (b) the polymerisation of infiltrated substance, (c) the carbonisation of the nanocomposites formed and (d) the elimination of the template. This procedure is very useful for synthesising mesoporous carbons with controlled porous characteristics. Thus, depending on the type of mesostructured silica that is used as template, carbons with different structures and pore sizes are prepared. First, Ryoo et al. [2] used a MCM-48 silica as template to synthesise an ordered mesoporous carbon (CMK-1) with a cubic structure (Pore size: 3 nm, S_BET: 1380 m² g⁻¹). Later, the same authors [4] used a SBA-15 mesostructured silica to prepare a carbon (CMK-3) with a 2-D hexagonal structure (Pore size: ∼4.5 nm, S_BET: 1520 m² g⁻¹), which was a faithful inverse replica of the silica framework. Kim and Pinnavaia [5] described the preparation of a mesoporous carbon with a hexagonal structure (pore size: 3.9 nm; S_BET: 1230 m² g⁻¹) from a MSU-H silica template. Recently, Lee et al. [6] demonstrated that the pore size of an ordered mesoporous carbon can be accurately modified to between 2.2 and 3.3 nm by changing the composition of the surfactant used in the synthesis of the mesostructured silica. All of these carbons are inverse replicas of the mesostructured silica frameworks, their porosity being a replica of the silica skeleton. Thus, hexagonally ordered carbons such as CMK-3 can be visualised as a network of hexagonally ordered carbon nanorods which are interconnected by means of carbon spacers. Given that the pores in carbons obtained from mesostructured silica templates are derived from the silica framework, they are limited by the thickness of the silica walls. For this reason, most of the mesoporous carbons prepared in this way have mesopores with sizes limited to the narrow 2–4 nm range [3], [4], [5], [6].

For some of the potential uses of mesoporous carbons, such as those related to catalysis, adsorption or double-layer electrical capacitors, the diffusion rate needs to be enhanced. In this case materials with a 3-D pore arrangement are more appropriate than materials containing well-ordered 2-D pore structures [7]. Apart from the pores derived from the silica framework, the presence of larger pores (complementary mesoporosity), would contribute to enhancing diffusivity in the pore network for the applications mentioned above. Recently, some authors reported on the preparation of mesoporous carbons containing two pore networks: (i) pores related with the silica framework and (ii) pores derived from the incomplete filling of silica porosity. Thus, Lee et al. [8] used a mesocellular silica foam to prepare a porous carbon, which exhibits a bimodal PSD with spherical cells of ∼27 nm interconnected by small mesopores of around 3.6 nm. Joo et al. [9] recently used an SBA-15 template to prepare a porous carbon whose structure was formed by interconnected carbon nanopipes. This porous carbon shows two pore sizes, 5.9 nm (internal diameter of carbon cylinders) and 4.2 nm (pores between adjacent cylinders).

The main purpose of this work is to investigate the preparation of mesoporous carbons with a 3-D pore structure, which combine two pore systems in the mesopore range, pores derived from silica skeleton and larger pores obtained from the partial filling of silica porosity. In addition, new methods to enable the carbon precursor to infiltrate into silica pores are being investigated. A mesoporous silica with a disordered mesostructure was used as template. Depending on the method used to introduce the carbon precursor into the silica nanopores, carbons with different mesostructures can be obtained.

Section snippets

Preparation of mesostructured silica

The synthesis of mesoporous silica was carried out with the aid of non-ionic surfactants following the procedure previously reported by Zhao et al. [10]. In the synthesis an oligomeric alkyl-ethylene oxide surfactant C₁₆EO₁₀ (Brij 56, Aldrich) was used as the structure directing agent. Briefly, a silica source (TEOS, Aldrich) was added to an aqueous solution containing HCl and surfactant (Starting mole ratio, TEOS:Brij 56:HCl:H₂O=1:0.14:2:85). The mixture was stirred by means of magnetic

Silica template

Fig. 1 shows a small angle XRD pattern of the mesostructured silica used as template, silica–carbon and carbon samples. The low-angle XRD reflections show the periodic nature of the porosity in both, silica and carbons. The silica sample shows only one XRD peak at 2θ=1.8° (d spacing=4.9 nm), which is characteristic of disordered mesostructured silica [15]. TEM images of the silica show that this material exhibits a low degree of structural order (Fig. 2a). However, in some small regions an

Discussion

In principle, the pore volume of the silica template used here, which is available for infiltration by the carbon precursor, is 1.1 cm³ g⁻¹. However, as indicated by various authors [17], [21], the silica framework of infiltrated samples shrinks during carbonisation. Consequently, the available silica pore volume proves to be lower than what might be expected from the original silica. The reasons for the shrinkage are not clear. In order to obtain a better understanding of this phenomenon, two

Conclusions

Mesoporous carbons with different PSDs can be synthesised from a single type of mesostructured silica that is used as template. This is achieved by controlling the degree of carbon infiltration into the silica porosity. Thus, carbons with a narrow PSD (around 3 nm), derived from the silica framework are obtained when the silica porosity is fully infiltrated. For moderate degrees of infiltration (φ∼0.5–0.6 gC g⁻¹ silica), the carbons show a porosity formed by pores around 3 nm (derived from the

Acknowledgements

A.B. Fuertes would like to gratefully acknowledge the financial support by the Spanish MCyT (MAT2002-00059). We thank C. Alvarez (Servicio Cientı́fico-Técnicos, Universidad de Oviedo) for the use of the transmission electron microscope.

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Control of mesoporous structure of carbons synthesised using a mesostructured silica as template

Abstract

Introduction

Section snippets

Preparation of mesostructured silica

Silica template

Discussion

Conclusions

Acknowledgements

Carbon

J. Colloid Interface Sci.

Carbon

J. Phys. Chem. B

Adv. Mater.

J. Am. Chem. Soc.

Chem. Commun.

J. Am. Chem. Soc.

Chem. Commun.

J. Am. Chem. Soc.

Nature