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Stoeckli, Fritz

Nom
Stoeckli, Fritz
Affiliation principale
Fonction
Professeur.e émérite
Email
fritz.stoeckli@unine.ch
Identifiants
https://libra.unine.ch/handle/123456789/471

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  • Publication
    Accès libre
    On the specific double-layer capacitance of activated carbons, in relation to their structural and chemical properties
    (2006)
    Centeno, Teresa A.
    ;
    Stoeckli, Fritz 
    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.
  • Publication
    Accès libre
    On the determination of surface areas in activated carbons
    (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, 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.
  • Publication
    Accès libre
    On the mechanisms of phenol adsorption by carbons
    (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.
  • Publication
    Accès libre
    Micropore sizes in activated carbons determined from the Dubinin–Radushkevich equation
    (2001)
    Stoeckli, Fritz 
    ;
    López-Ramón, M. V.
    ;
    Hugi-Cleary, Deirdre
    ;
    Guillot, André
  • Publication
    Accès libre
    The oxidation of an Asturian bituminous coal in air and its influence on subsequent activation by steam
    (1996)
    Centeno, Teresa A.
    ;
    Stoeckli, Fritz 
    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.
  • Publication
    Accès libre
    Activation chimique des charbons par les hydroxydes, carbonates et oxydes des métaux alcalins
    (1994)
    Lebgaa, Dénia
    ;
    Ehrburger, Pierre
    ;
    Papirer, Eugène
    ;
    Donnet, Jean-Baptiste
    ;
    Stoeckli, Fritz 
    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.
  • Publication
    Accès libre
    On the activation of Asturian anthracite following various pretreatments
    (1994)
    Centeno, Teresa A.
    ;
    Stoeckli, Fritz 
    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.
  • Publication
    Accès libre
    On the characterization of carbon blacks by various techniques
    (1994)
    Stoeckli, Fritz 
    ;
    Huguenin, Didier
    ;
    Laederach, A.
    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.
  • Publication
    Accès libre
    The physical and chemical characterization of a sulphur-impregnated active carbon, by combined adsorption and immersion techniques
    (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.
  • Publication
    Accès libre
    Evolution of microporosity during activation of carbon
    (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.