Stoeckli, Fritz

2005, Stoeckli, Fritz, Centeno, Teresa A.

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, S_e and S_mi, have been determined by a variety of independent techniques. It appears clearly that S_BET and the real surface area S_mi + S_e are in agreement only for carbons with average pore widths L_o around 0.8–1.1 nm. Beyond, S_BET increases rapidly and S_BET− Se is practically the monolayer equivalent of the micropore volume W_o. This confirms that a characterization of surface properties based on S_BET 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 Δ_iH 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 L_o> 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.

1991, Rebstein, Patrick, Stoeckli, Fritz

A commercially available active carbon, impregnated with sulphur, has been characterized and compared to the original carbon by using adsorption and immersion techniques. The specific reaction of liquid CS₂ with sulphur, following the preadsorption of various amounts of n-nonane, shows that the accessibility of sulphur decreases rapidly. It is also found that the impregnated carbon becomes hydrophilic but, in the case of SO₂ adsorption, no change in adsorption capacity is detected.

2001, Stoeckli, Fritz, Hugi-Cleary, D.

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.

1991, Stoeckli, Fritz, Ballerini, Luca

Various adsorption and immersion techniques and a recent model for micropore distributions have been used to assess quantitatively the evolution of the main properties of active carbon. The precursors used in this study were of vegetable and polymeric origin. One activation series based on natural coal was also included.