Stoeckli, Fritz

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.

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.

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.

2000, Stoeckli, Fritz, Guillot, André, Hugi-Cleary, Deirdre, Slasli, Abdou

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.