Fritz Stoeckli

On the characterization of acidic and basic surface sites on carbons by various techniques

Abstract Active carbons of different origins have been oxidized with H2O2 and (NH4)2S2O8 and their oxygen surface complexes have been characterized by TPD, classical titration following Boehms method and by neutralization calorimetry. The net enthalpies of neutralization, determined by immersion calorimetry into NaOH and HCl 2 N lead to −41.1±1.8 and −52.3±2.0 kJ eq−1 for the acidic and basic sites on the surface. Experiments with NaHCO3 lead to −39.7±1.7 kJ eq−1 for the carboxylic groups alone. These results suggest that the surface groups of active carbons can also be characterized by immersion calorimetry. Results are also given for the variation of the pH of the point of zero charge with the total oxygen content of the surface.

Contributions of hemicellulose, cellulose and lignin to the mass and the porous properties of chars and steam activated carbons from various lignocellulosic precursors.

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.

EVOLUTION OF MICROPOROSITY DURING ACTIVATION OF CARBON

Abstract 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.

A generalization of the Dubinin-Radushkevich equation for the filling of heterogeneous micropore systems in strongly activated carbons

Abstract A generalization of the Dubinin-Radushkevich equation, recently proposed by Stoeckli as an alternative to the Dubinin-Astakhov equation, is discussed theoretically and is tested with new experimental data. The extended equation applies to the filling of heterogeneous micropore systems in strongly activated carbons. It is based on the assumption that the original DR equation only applies to relatively homogeneous systems of micropores, if adsorption is considered over a large range of temperature and pressure. This is suggested by adsorption experiments on carbons with marked molecular-sieve properties. Heterogeneity is dealt with by introducing weighted contributions from the various systems, all following the DR equation, but with different structural parameters. A Gaussian distribution of the micropore volumes with respect to constant B leads to a satisfactory general isotherm, with an extended range of applicability. This isotherm is also compared with the generalization of Dubinin and Astakhov.

The comparison of experimental and calculated pore size distributions of activated carbons

Abstract 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.

On the characterization of microporous carbons by immersion calorimetry alone

Immersion calorimetry is a useful tool for the characterization of solid surfaces in general, but in the case of microporous solids it usually requires complementary information, obtained from an adsorption isotherm. This article demonstrates the possibilities and the limitations of this technique when used alone. It appears that the use of a standard value for the enthalpy immersion does not always provide a reliable assessment of the total surface area.

Specific and non-specific interactions of water molecules with carbon surfaces from immersion calorimetry

Abstract The enthalpy of immersion of carbons into water reflects the specific and the non-specific interactions between the liquid and the solid. The study of a variety of active carbons oxidized to different degrees with (NH 4 ) 2 S 2 O 8 , HNO 3 and H 2 O 2 , reveals a simple correlation between the enthalpy of immersion, Δ i H (H 2 O), the oxygen content of the surface, the basic groups, the micropore filling and the wetting of the external (non-microporous) surface. The specific interactions between the surface oxygen and water is 12.1 kJ/mol of oxygen and it involves on average two molecules of water per oxygen. The basic groups on the surface, which contain no oxygen, have an interaction energy with water of 10.3 kJ/HCl eq of base. The non-specific filling of the micropores by water corresponds to 0.8 kJ/mol of water. Preliminary results show that the correlation also holds for oxidized carbon blacks with 10.7 kJ/mol for the oxygen–water interaction and 0.035 J/m 2 for the wetting of the pure carbon surface.

Water adsorption in active carbons described by the Dubinin–Astakhov equation

It is shown that the adsorption of water by a variety of active carbons can be described within the framework of Dubinins theory. Owing to the low values of the characteristic energy, E(0.8–2.5 kJ mol–1), the Dubinin–Astakhov equation becomes S-shaped in the range 0.3 < p/po <0.7 and provides a good basis for the fit of the adsorption branch of type V isotherms near room temperature. The parameters of the equation are almost temperature invariant and consequently a good agreement is also found in many cases for the enthalpies of immersion into water, as predicted by the extension of Dubinins theory.

Water adsorption on activated carbons with different degrees of oxidation

A typical activated carbon, derived from olive stones, has been oxidized to different degrees with (NH 4 ) 2 S 2 O 8 and analysed by water vapour adsorption, immersion calorimetry, acid–base titration and temperature-programmed desorption of CO 2 and CO monitored by mass spectrometry. These techniques led to a coherent description of the surfaces and of their chemistry. The water adsorption isotherms, of type IV, were decomposed into type I and V contributions and analysed in terms of the Dubinin–Astakhov equation. The corresponding calculated enthalpies of immersion into water are in agreement with the experimental values. The number of carboxyl, lactone, phenol and basic groups identified by titration, can also be related to the parameters of the Dubinin–Astakhov equation and to the enthalpy of immersion into water. Finally, a good linear correlation is found between the amounts of CO 2 and CO desorbed from the surface, the enthalpies of immersion into water and the total number of sites identified on the surface.

The adsorption of water by active carbons, in relation to their chemical and structural properties

An interesting letter has recently been published in this total micropore volume of the solid, the thermal expansion Journal by Lodewyckx and Vansant [1], introducing the coefficient of the adsorptive and its molar volume in the idea of an affinity coefficient for water with respect to liquid state. G(1 1 1/n) is the tabulated ‘Gamma’ function, benzene, the usual reference in Dubinin’s theory. This which takes values between 0.89 and 0.92 when n varies concept is valid for carbons with relatively low oxygen from 1.5 to 5. This means that the enthalpy of immersion contents, where the water adsorption isotherm is of type V depends essentially on the characteristic energy E 5 bE . o and corresponds to a Dubinin-Astakhov equation, as Since active carbons also possess an external (nonshown earlier [2,3]. microporous) surface S , the experimental enthalpy of e We wish to show that Lodewyckx’s idea can be included immersion, Dh (J /g) is i exp in a more general approach based essentially on the Dh (J /g) 5 Dh (J /g) 1 h S (3) i exp i mi i e comparison of the enthalpies of immersion of carbons into water and benzene and taking into account the chemistry where h , a negative quantity, represents the wetting of the i of the surface through an excess enthalpy of immersion. surface. For benzene and water at 293 K, 2 h corresponds i Our approach follows two recent studies dealing with the 2 to 0.114 and 0.030 J /m [3–5] and the last term of Eq. (3) interaction of water [4] and of methanol and ethanol [5] is only a fraction of the total enthalpy of immersion. with active carbons containing variable amounts of oxygen From Eqs. (2) and (3), it follows that the enthalpy of and basic groups. The starting point is the DA equation immersion of a given adsorptive can be calculated from the [3,6] parameters of the DA isotherm. A good agreement has