Elsevier

Carbon

Volume 37, Issue 8, 1 January 1999, Pages 1257-1264
Carbon

Characterization of pore size distributions on carbonaceous adsorbents by DFT

https://doi.org/10.1016/S0008-6223(98)00322-4Get rights and content

Abstract

Carbonaceous adsorbents are amorphous graphitic carbon, consisting of SP2 hexagonal carbon layers with different sizes of pores ranging from micropores to macropores. Nitrogen adsorption is a standard tool for determination of pore size distribution (PSD). In the present work, we carried out extensive adsorption studies of a series of carbonaceous adsorbents including PAN-based activated carbon fibers, coal activated carbon and pitch carbon bead. The PSDs of samples studied were calculated by employing the regularization method according to Density Functional Theory (DFT). It was shown that adsorption measurement provided profound insight into the structural heterogeneity of carbonaceous adsorbents.

Introduction

Porous solids play an important role in the fields of separation and purification. Carbonaceous adsorbents are amorphous graphitic carbon, consisting of SP2 hexagonal carbon layers with different sizes of pores ranging from micropores to macropores. The adsorption properties result from their remarkable surface and structural properties, which manifest themselves in high specific surface area and adsorption capacity. These in turn arise from the presence of a reasonable amount of micropores [1]. Carbonaceous adsorbents are porous materials exhibiting slit-like micropores; this feature is a source of their high adsorption capacities, which makes them suitable as adsorbents, catalysts and catalyst supports.

Adsorbents of high adsorption capacity are generally porous. There are many different kinds of rigid and non-rigid pore structures and an adsorbent is likely to contain a range of pores of different sizes and shapes [2]. Definition of pore size follows recommendations of IUPAC: Macropore – width greater than 50 nm; mesopore – width from 2 to 50 nm; micropore – width less than 2 nm. Micropores can be divided into ultramicropores (width less than 0.7 nm) and supermicropores (width from 0.7 to 2 nm) [3]. Macropores, and in many cases mesopores, play the role of transport in adsorption. The volume of a macropore may be up to 0.8 cm3/g, whereas the specific surface area of macropores is so small that it can be negligible, so macropores do not have any appreciable effect on adsorption value. They are, however, transport arteries, making the internal parts of carbon grains readily accessible to the molecules adsorbed. The volume filling of a macropore takes place only at a relative pressure close to unity. The sizes of mesopores are generally much larger than those of the molecules adsorbed. On the surface of this variety of pores there occurs monomolecular and polymolecular adsorption of vapors, that is, the formation of successive adsorption layers. Besides, volume filling of mesopores takes place by the capillary-condensation mechanism in an experimental easily realized interval of relative pressures. At the same time mesopores may be the adsorption sites of larger molecules (such as biology molecules). Due to the overlaps of opposite wall adsorption force, adsorption potentials in micropores are high. Since there is an adsorption–force field in the entire volume of micropores, adsorption of vapor in micropores leads to their volume filling 4, 5, 6. However, these limits are to some extent arbitrary since the pore filling mechanisms are dependent on the pore shape and are influenced by the properties of adsorptive and by the adsorbent–adsorbate interactions. It is micropores that play the most important role in adsorption, sometimes, micropores can be considered as the voids that accommodate one, two or three admolecules [7]. In the view of these points of the different features of pores as well as in order to successfully synthesize and apply novel carbonaceous adsorbents, their surface and structural properties need to be thoroughly characterized. Pores of adsorbents are generally formed by selective gasification in steam and/or CO2 or by chemical activation with KOH etc. at 500°C∼1000°C. Physisorption measurements are widely used for the investigation of the texture of porous carbons. The phenomenon of physisorption is a general one – unlike chemisorption – and occurs whenever a gas is brought into contract with an outgassed solid. Adsorption measurements provide a useful “fingerprint” of the microstructure and are essential if the carbon is used as an adsorbent or catalyst support [2]. The pore size distribution (PSD) appears to be especially useful information about porous solids 1, 8. The PSD is usually obtained from a gas adsorption measurement.

In the present work, we carried out extensive adsorption studies of a series of carbonaceous adsorbents including PAN-based activated carbon fibers, coal activated carbon and pitch carbon bead. Nitrogen adsorption measurements were used to evaluate the specific surface area, micropore surface area, micropore volume and PSD. The specific surface area of samples studied was calculated from the standard BET method [9]. The micropore volume and external surface area were obtained from the t-plot method 3, 4. The PSDs of the samples were calculated by employing the regularization method according to Density Functional Theory (DFT) that is based on a molecular model for adsorption of nitrogen in porous solids 10, 11, 12, 13.

Section snippets

Sample preparation

Polyacrylonitrile (PAN)-based activated carbon fibers (ACFs) of different degrees of activation were prepared in our laboratory by continuous synchronism carbonization and activation apparatus in steam, carbon dioxide and nitrogen gas atmospheres. Coal activated carbon (AC) was supplied by Xinhua Industry Plant (China) and pitch carbon bead (PIT-bead) was from Japan.

Adsorption measurement

A Micromeritics ASAP 2000 accelerated surface area and porosimetery apparatus (Micrometics Ins. Corp.) was used to measure

Adsorption isotherms

Nitrogen adsorption, because the relatively small molecule diameter of nitrogen is frequently used at 77 K to probe porosity and surface area and to be a standard procedure for the characterization of porosity texture of carbonaceous adsorbents also. The adsorption isotherm is the information source about the porous structure of the adsorbent, heat of adsorption, characteristics of physics and chemistry and so on. In order for us to utilize the information within the adsorption isotherms, it is

Conclusions

Carbonaceous adsorbents exhibit bimodel or trimodel micropore distributions. The DFT method allows PSDs to be determined over a wide range from micropore to macropore using a single analysis method. The PSDs of carbonaceous adsorbents can be changed in a wide range with the difference of the conditions of preparation. Multi-stages of adsorption and pore filling procedure can be observed by Dubinin-Radushkevich Plots. The information of porosity is interpreted by adsorption isotherms of samples

Acknowledgements

The authors acknowledge Ms. Y.Z. Fan (State Key Laboratory of Coal Conversion) for providing the nitrogen adsorption measurement and DFT analysis data.

References (23)

  • N.A. Seaton et al.

    Carbon

    (1989)
  • M. Kruk et al.

    J. Colloid Interface Sci.

    (1996)
  • Z. Ryu et al.

    Carbon

    (1998)
  • R.W. Cranston et al.

    Adv. Catal.

    (1957)
  • S. Brunauer et al.

    J. Colloid Interf. Sci.

    (1967)
  • H. Marsh

    Carbon

    (1987)
  • K.S.W. Sing

    Carbon

    (1989)
  • Kruk M, Jaroniec M, Gadkaree KP. Carbon'97, Ext. Abstr. 23rd Biennial Conf. Carbon, Penn. State, USA....
  • Sing KSW, Adsorption: science and technology, Dordrecht: Kluwer Academic Publishers,...
  • Gregg SJ, Sing KSW. Adsorption, Surface area and porosity, 2nd ed. London: Academic Press,...
  • K.S.W. Sing et al.

    Pure Appl. Phys. Chem.

    (1985)
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