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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
    Contributions of hemicellulose, cellulose and lignin to the mass and the porous properties of chars and steam activated carbons from various lignocellulosic precursors
    (2009)
    Cagnon, Benoît
    ;
    Py, Xavier
    ;
    Guillot, André
    ;
    Stoeckli, Fritz 
    ;
    Chambat, Gérard
    In this study, contributions of hemicellulose, cellulose and lignin to the mass and the porous properties of chars and activated carbons from various lignocellulosic materials were studied. A predictive calculation was established using the experimental results obtained for the three components separately to evaluate the carbonization and activation yields and their respective contributions to the chars and to the subsequent activated carbons of various precursors in term of weight fraction. These equations were validated. The results showed that lignin can be considering as being the major contributor of all chars and activated carbons. Besides, the evolution of the mean pore size versus the specific porous volume showed that each component contributes to the porosity of chars and activated carbons whatever is its weight contribution.
  • Publication
    Accès libre
    Specific and non-specific interactions between ammonia and activated carbons
    (2004)
    Stoeckli, Fritz 
    ;
    Guillot, André
    ;
    Slasli, Abdou
    It is shown that the adsorption of NH3 by activated carbons at different temperatures follows Dubinin's theory and, as in the case of water, for carbons with high oxygen contents one observes an upward deviation in the initial section of the DR plot. This indicates a strong primary adsorption, due to specific interactions. At higher relative pressures, this adsorption is followed by a classical micropore filling mechanism, where NH3 conforms to the pattern of non-specific adsorption. The affinity coefficient β1 (NH3) associated with primary adsorption varies between 0.4 and 1.2. It depends on the characteristic energy E0 and on the ratio between the amount of oxygen present on the surface and the limiting amount of ammonia adsorbed in the micropores. This behaviour had been reported earlier for the adsorption of short alcohols, but with a single DR plot, due to the fact that specific and non-specific interactions were similar. For carbons with little oxygen on the surface, NH3 adsorption leads to a single DR plot, irrespective of the average pore-size.
  • Publication
    Accès libre
    The effect of the carbonization/activation procedure on the microporous texture of the subsequent chars and active carbons
    (2003)
    Cagnon, Benoît
    ;
    Py, Xavier
    ;
    Guillot, André
    ;
    Stoeckli, Fritz 
    Chars obtained by carbonizing coconut shells at different intermediate heat treatment temperatures (IHTT) between 400 and 800 °C were activated at 800 °C in a stream of N2+H2O, following two distinct procedures. In the first procedure, activation follows directly the carbonization, whereas in the second procedure, the sample was first brought back to 25 °C and subsequently heated again to the activation temperature of 800 °C. The data for CO2 adsorption at 25 °C and N2 at −196 °C with immersion calorimetry confirms that the activated carbons derived from chars obtained at low IHTT and in two steps, present a “gate effect” for burn-offs <20% or 25%, otherwise, the final carbons present similar structural characteristics for higher burn-offs. It also appears that the evolution of the average pore width L0 with the micropore volume W0 follows a general pattern outlined early.
  • Publication
    Accès libre
    The comparison of experimental and calculated pore size distributions of activated carbons
    (2002)
    Stoeckli, Fritz 
    ;
    Guillot, André
    ;
    Slasli, Abdou
    ;
    Hugi-Cleary, Deirdre
    Activated carbons are disorganized materials with variable pore size distributions (PSD). If one assumes that the porosity consists mainly of locally slit-shaped micropores, model isotherms can be obtained by computer simulations and used to assess the PSD on the basis of experimental isotherms. In the present study, CO2 isotherms have been measured at 273 K on seven well-characterized microporous carbons with average micropore widths between 0.65 and 1.5 nm and analysed with model isotherms obtained with standard Monte Carlo simulations. The resulting PSD are in good agreement with those obtained from a modified Dubinin equation, from liquid probes of molecular dimensions between 0.4 and 1.5 nm, from STM and from modelling based on CH4 adsorption at 308 K. The present study validates the determination of micropore distributions in active carbons based on CO2 isotherms, provided that no gate effects are present.
  • Publication
    Accès libre
    Microporosity in carbon blacks
    (2002)
    Stoeckli, Fritz 
    ;
    Guillot, André
    ;
    Slasli, Abdou
    ;
    Hugi-Cleary, Deirdre
    Microporous carbon blacks can be characterized by the same techniques as activated carbons, using the classical DR equation and comparison plots based on non-porous materials. The CO2 adsorption isotherm at 273 K, combined with computer modelling, also leads to an assessment of microporosity. The results agree with independent techniques such as immersion calorimetry into liquids of variable molecular dimensions and a modified Dubinin equation. The study also confirms that the comparison plots based on N2 (77 K), CO2 (273 K) and C6H6 (293 K) do not necessarily lead to overlapping results for the total micropore volume and the external surface area of the carbons.
  • Publication
    Accès libre
    The characterization of microporosity in carbons with molecular sieve effects
    (2002)
    Stoeckli, Fritz 
    ;
    Slasli, Abdou
    ;
    Hugi-Cleary, Deirdre
    ;
    Guillot, André
    The apparent and the real micropore size distributions (PSDs) of molecular sieve carbons can be assessed by combining the adsorption of CO2 at 273 K with immersion calorimetry into liquids of increasing molecular dimensions. On the basis of model isotherms resulting from computer simulations, the adsorption of carbon dioxide, a relatively small probe, leads to the overall PSD of the carbon (essentially the internal micropore system). Immersion calorimetry, on the other hand, reveals the distribution of the pores accessible directly from the liquid phase, that is without constrictions. Liquid CS2 probes the same volume as CO2 and can be used as a reference. The paper describes the case of an industrial molecular sieve carbon obtained by blocking partly the entrance to a relatively broad micropore system, thus limiting its accessibility to molecules with diameters below 0.5–0.6 nm. It is shown how activation by steam at 900 °C removes the constrictions and leads to a gradual overlap of the two PSDs. The distribution of the pore widths on the surface, observed directly by scanning tunnelling microscopy, is also given.
  • Publication
    Accès libre
    Activated carbons prepared from thermally and chemically treated petroleum and coal tar pitches
    (2001)
    Daguerre, E.
    ;
    Guillot, André
    ;
    Stoeckli, Fritz 
    Petroleum and coal tar pitches have been treated thermally at 400°C and 470°C, extracted with toluene and subsequently activated with CO2 at 900°C. With one exception, the development of the micropore widths and volumes in the resulting carbons follows the trend observed for carbons based essentially on precursors of vegetable origin. On the other hand, petroleum pitch treated at 400°C leads to an active carbon which follows the pattern observed for cokes, semi-cokes and pyrolyzed tyres. It is suggested that the difference between the two types of active carbons is due to the large amount of compounds with low molecular weight left in the petroleum pitch after thermal soaking.
  • Publication
    Accès libre
    Reference isotherm for high pressure adsorption of CO2 by carbons at 273 K
    (2001)
    Guillot, André
    ;
    Stoeckli, Fritz 
    It is suggested that the adsorption of CO2 on Vulcan 3G at 273 K and up to 3.2 MPa can be used as a reference isotherm for the characterization of porous and non-porous carbons. The results obtained with the proposed reference are in good agreement with those of the nitrogen (77 K) and benzene (293 K) comparison plots. The external surface areas have also been compared with those obtained from immersion calorimetry into C6H6 at 293 K. It also appears that at low relative pressures CO2 adsorbed on Vulcan 3G follows the Dubinin–Radushkevich–Kaganer equation between 253 and 273 K.
  • 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 Microporosity of Activated Carbon Fibre KF1500 Assessed by Combined CO2 Adsorption and Calorimetry Techniques and by Immersion Calorimetry
    (2000)
    Guillot, André
    ;
    Stoeckli, Fritz 
    ;
    Bauguil Y.
    The microporous structure of the activated carbon fibre KF1500 was investigated by applying a new technique as presented here. This was based on a combination of high-pressure CO2 adsorption measurements between 253 K and 353 K with direct measurement of the differential heats of adsorption at 298 K at low and medium coverages. The validity of this approach was tested by using two activated carbons which has been characterized previously by a variety of techniques, including immersion calorimetry. It appears that a combination of high-pressure adsorption and calorimetry with CO2 alone is a useful tool for the characterization of the whole range of microporosity. This approach confirms earlier observations, according to which the initial stage of adsorption by strongly activated carbons corresponds to adsorption on the walls of large micropores rather than to the primary filling of small micropores.