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On the specific double-layer capacitance of activated carbons, in relation to their structural and chemical properties
Twelve well-characterized activated carbons with average micropore widths between 0.7 and 2 nm, total surface areas of 378–1270 m2g−1 and specific capacitances C up to 320 F g−1 have been investigated, using H2SO4 2 M as electrolyte. Some of the carbons have also been oxidized with (NH4)2S2O8, which leads to specific oxygen contents between 0.4 and 7.1 μmol m−2 of carbon surface area. It appears that Co, the limiting capacitance at a current density of 1 mA cm−2 of electrode surface, does not depend significantly on the oxygen content. An empirical equation is proposed to describe the decrease of C with increasing current density d (1–70 mA cm−2 of electrode surface), as a function of the oxygen content.
As suggested by different authors, Co can be expressed as a sum of contributions from the external surface area Se and the surface of the micropores Smi. A closer investigation shows that Co/Smi increases with the pore size and reaches values as high as 0.250–0.270 F m−2 for supermicropores. It is suggested that the volume Wo* of the electrolyte found between the surface layers in pores wider than 0.7–0.8 nm contributes to Co. However, this property is limited to microporosity, like the enthalpy of immersion of the carbons into benzene. The latter is also correlated to Co, which provides a useful means to identify potential supercapacitors.
Dubinin's theory and its contribution to adsorption science
Dubinin's theory for the volume filling of micropores (TVFM), originally developed for the adsorption of single vapours by microporous solids such as activated carbons and Zeolites, has gradually been extended to other areas. They include immersion calorimetry, the adsorption of water vapour and of mixtures, as well as adsorption front aqueous solutions. Recent studies in the field of adsorption from aqueous solutions, by activated carbons, suggest that the principle of temperature invariance is fulfilled and in the case of phenolic compounds a modified DRK equation can be used to predict the adsorption equilibrium over a certain range of temperatures. Computer modelling of CO2 adsorption by carbons at 273 K leads to micropore distributions, which are in good agreement with those derived from other techniques. It also appears that the model isotherm, in single slit-shaped micropores can be fitted to the Hill-de Boer isotherm, in agreement with mathematical studies of the origin of the Dubinin-Astakhov equation.
The oxidation of an Asturian bituminous coal in air and its influence on subsequent activation by steam
It is shown that the oxidation of an Asturian bituminous coal by air, at 150 ° and 270 °C, has a considerable influence on the microporous structure of the carbon obtained by subsequent carbonization and steam activation to 50% burn-off. The direct activation of the coal leads to an open micropore system of 0.2 cm3/g, with an average pore size near 0.8 nm. On the other hand, much larger systems of micropores can be obtained in the final products, depending on the temperature of pretreatment in air. The oxidation at 270 °C eventually leads to supermicropores (W0 = 0.4 cm3/g) and to a small system of narrow micropores near 0.3–0.5 nm. Molecular sieving experiments indicate that these micropores are part of the constrictions leading into 50–60% of the larger pores.
On the characterization of carbon blacks by various techniques
It is shown how the use of various adsorption and calorimetric techniques can provide unambiguous information on the presence or the absence of porosity on the surface of carbon blacks. Dubinin's theory, its extension to immersion calorimetry, and gas-phase calorimetry can be applied successfully. As an example, the characterization of 6 porous and non-porous samples is described.
On the determination of surface areas in activated carbons
The paper examines the validity of two approaches frequently used to determine surface areas in activated carbons, namely the BET method and the use of immersion calorimetry. The study is based on 21 well characterized carbons, whose external and microporous surface areas, Se and Smi, have been determined by a variety of independent techniques. It appears clearly that SBET and the real surface area Smi + Se are in agreement only for carbons with average pore widths Lo around 0.8–1.1 nm. Beyond, SBET increases rapidly and SBET− Se is practically the monolayer equivalent of the micropore volume Wo. This confirms that a characterization of surface properties based on SBET is, a priori, not reliable. The study of the enthalpy of immersion of the carbons into benzene at 293 K, based on Dubinin’s theory, shows that ΔiH consists of three contributions, namely from the interactions with the micropore walls (−0.136 J m−2), the external surface (−0.114 J m−2), and from the volume W*o of liquid found between the surface layers in the micropores (−141 J cm−3). It appears that for carbons where Lo> 1 nm, the real surface area cannot be determined in a reliable way from the enthalpy of immersion and a specific heat of wetting alone.
On the mechanisms of phenol adsorption by carbons
The removal of phenol and related compounds from dilute aqueous solutions by activated carbons corresponds to the coating of the micropore walls and of the external surface by a monolayer. This process is described by an analog of the Dubinin—Radushkevich—Kaganer equation. On the other hand, as suggested by immersion calorimetry at 293 K, in the case of concentrated solutions, the mechanism corresponds to the volume filling of the micropores, as observed for the adsorption of phenol from the vapor phase. The equilibrium is described by the Dubinin—Astakhov equation. It follows that the removal of phenol from mixtures with water depends on the relative concentrations, and the limiting factor for adsorption is either the effective surface area of the carbon, or the micropore volume.
Activation chimique des charbons par les hydroxydes, carbonates et oxydes des métaux alcalins
Chemical activation of coals using aïkali metal hydroxides, oxides and carbonates. The preparation of active carbons is described, starting from two coals originating from Freyming and Oignies. Varions hydroxides, oxides and aïkali carbonates were used but the originality of this study is the use of cesium derivatives. The interpretation of the Coa adsorption isotherms shows clear differences among the various reagents. It is seen that CsOH is more efficient than KOH, and that Cs20 is even more active than CsOH. Furthermore, by mixing an inactive hydroxide (NaOH) with KOH, a new, more efficient, activating agent was obtained. Immersion calorimetry studies confirm the gas adsorption results. Finally, scanning electron microscopy and scanning tunnelling microscopy evidence the formation of porosity.
Modelling of water adsorption by activated carbons: effects of microporous structure and oxygen content
The present paper examines the adsorption of water by microporous carbons containing various amounts of surface oxygen and a smaller proportion of basic centres. The modelling of water adsorption for 293 and 310 K, using variable pore size distributions (PSD), confirms that the overall type IV isotherm is the sum of a type I isotherm associated with the specific interactions, and a type V isotherm reflecting the non-specific interactions. The principle of temperature invariance is followed by these isotherms, which indicates that modelling leads to the Dubinin-Astakhov equation. The present approach allows the prediction of water adsorption near room temperature, on the basis of the PSD and the density of oxygen present on the surface area of the micropores. It is assumed, to a first and good approximation, that the pores are slit-shaped and the oxygen distribution is random. (C) 2004 Elsevier Ltd. All rights reserved.
Micropore sizes in activated carbons determined from the Dubinin–Radushkevich equation
On the activation of Asturian anthracite following various pretreatments
An Asturian anthracite has been subjected to different pretreatments and subsequently activated by steam at 850°C to a total burn-off of 55 percent. The physical properties (micropore sizes and distributions, external surface areas, etc.) of the solids are compared with those of the carbon obtained by direct activation. Although the yield is generally low, it appears that better results are obtained by activation following preoxidation in air at 270°C for 3 days. In the case of pretreatments with a mixture of nitrogen, air, and water vapor at 450°C, the subsequent activation is less efficient. The micropore volumes and the pore size distributions are similar to those observed for soft precursors, but at a much lower degree of burn-off.