André Guillot

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.

Optimal design of an activated carbon for an adsorbed natural gas storage system

Abstract The Dubinin–Astakhov equation was used to determine the influence of the microporous characteristics of activated carbon on the performances of both charge and discharge of an ANG system. From the dynamic performance criterion as a function of total microporous volume (Wo), average micropore width (Lo), and micropore size dispersion (n), it is possible to identify the optimal activated carbon for methane storage under dynamic conditions by way of the heat and mass transfer limitations. This study shows that the activated carbon must be conductive (with an average micropore width of 1.5 nm) for the charge step only, permeable and sufficiently conductive for the discharge process (with an average micropore width of 2.5 nm). The well-known Maxsorb activated carbon shows the better performance. This theoretical investigation has been validated by experimental results.

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.

The characterization of microporosity in carbons with molecular sieve effects

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.

The effect of the carbonization/activation procedure on the microporous texture of the subsequent chars and active carbons

Abstract 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

Microporosity in carbon blacks

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.

Activated carbon porosity tailoring by cyclic sorption/decomposition of molecular oxygen

Activated-carbon pore size tailoring is usually achieved by pore size reduction, pore size widening or more rarely by direct activation. Successive widening cycles composed of an initial molecular oxygen sorption step, followed by a carbonization step under nitrogen, have been applied to three different materials in order to study their gradual pore size modifications. For years, the whole microporosity obtained after several cycles is known to present better pore size distribution than those inherited from conventional activations. Moreover, as shown in the present paper, the gradual obtained microporosities cover a wide and valuable range of porous textures highly dependent on the initial material origin and initial activation. Gradual mean pore size evolutions of 1 or 2 A per cycle were observed linked to a simultaneous increase in microporous specific volume in the case of pitch-based and coconut activated carbons, respectively. Comparatively, the microporous specific volume of a commercial coconut carbon molecular sieve was increased by 26% without modification of its mean pore size and therefore of its sieving effect. Those results have been used to test a simple textural model of activated carbon porosity proposed in the recent literature.

Pore size distributions of active carbons assessed by different techniques

(3n 21) n 3 CM (Fig. 1) the frequency of the micropore widths f(L) 5 3W L a exp[2aL ] /G(n) (4) 0 observed by STM on the surface of the solid, DN /DL, is This function, like the distribution based on the DS slightly different from the volumic distribution DW /DL, equation [1,7–9], has a single maximum and it is not since the STM analysis does not take into account the suited for the description of bimodal micropore distribuactual depth of the micropores. However, the two distions, provided that they exist. In the case of carbon CM tributions are related and STM, like TEM, confirms that the pore size distribution Eq. (4) shown in Fig. 1(b), has the micropores of active carbons are locally slit-shaped. been obtained from the adsorption isotherms of CH (253, 4 This, in turn, provides the basic model for computer 273, 308 K) and of CO (253, 273, 298 K) fitted to Eq. 2 simulations. (1). This distribution is in good agreement with the In the case of microporous carbons, it has also been histogram of Fig. 1(a), derived from liquids with molecular possible to derive PSD from adsorption data within the sizes between 0.4 and 1.5 nm. As confirmed independently framework of Dubinin’s theory for the volume filling of by STM, the experimental distribution has no secondary micropores [5,6]. A possible relation is the so-called maximum around 1.5 nm. Dubinin–Stoeckli (DS) equation [1,7,8], which applies to Computer modeling of adsorption and the determination strongly activated carbons, with relatively wide distribuof pore size distributions based on standard isotherms has tions. As reported by Daley at al. [9], good agreement is become increasingly popular and it has been discussed found between the PSD derived from the DS equation and recently [10]. At the present time, modeling is frequently the experimental distribution observed by STM on strongly based on the adsorption of N [11–13], CO [14,15] and 2 2 activated carbons. CH [16–19], but a systematic comparison with pore size 4 Recently [1], a modified Dubinin isotherm has also been distributions obtained by independent techniques is still suggested to obtain PSD in the micropore range, lacking. For example, to our knowledge, no direct com-

Reference isotherm for high pressure adsorption of CO2 by carbons at 273 K

Abstract 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.

Activated carbons prepared from thermally and chemically treated petroleum and coal tar pitches

Abstract 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.