Characterization Of Microporous Carbons Research Articles

The impact of residual adsorbate on the characterization of microporous carbons with small angle scattering

Microporous carbon materials were investigated with small angle X-ray scattering to determine whether a pre-treatment with heat and vacuum of the samples is necessarily required to obtain accurate and reliable data. The findings of this study indicate strongly erroneous data in terms of micropore volume and apparent sample density, if the samples are not carefully degassed prior to the measurement, even when the measurement is performed in high vacuum. The observed scattering curves were compared to a systematically wetted sample. The results further show an ageing of the samples that causes predominant cluster adsorption most likely of water at oxygen surface groups. Some recommendations are given for an adequate sample preparation prior to characterizing (micro)porous materials with small angle scattering.

PET as precursor of microporous carbons: preparation and characterization

In this paper the preparation and characterization of microporous carbons from polyethyleneterephtalate (PET) is studied. The influence of the carbonization temperature and the activation time on the characteristics of the samples are considered. The textural characteristics are studied by adsorption of gases and vapors as well as by mercury porosimetry. These allow to establish the influence of the experimental treatment parameters in the porous network of the samples. Moreover, the analysis of the adsorption data has been carried out by BET and Dubinin–Radushkevich (DR) equations as well as by the α s-plot. The results of these analysis are very consistent and permit to determine the porous structure of the samples and also to identify the individual adsorption mechanism.

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.

Characterization of microporous carbons by using TGA curves measured under controlled conditions

Microporous active carbons of differing structural and surface properties were studied by thermogravimetry (TGA) under controlled conditions. Weight loss curves were measured for various carbon samples wetted and saturated with water, n-butanol, benzene and n-heptane using a high-resolution thermogravimetric analyzer. This analyzer is capable of maintaining controlled conditions. Analysis of the TGA curves showed that molecules desorb from the surface via a multi-step process. For the samples wetted with liquids the initial step reflects evaporation of the excess liquid. The subsequent steps, observed for both types of samples (wetted in liquids and saturated in their vapors), reflect evaporation of adsorbate from pores and its thermodesorption from fine pores and from the carbon surface. These latter features of the TGA curves were used to obtain information about porosity and surface heterogeneity of the active carbons studied.

Characterization of Microporous Carbons Using Adsorption at Near Ambient Temperatures

The adsorption isotherms of CF4 and SF6 measured at various temperatures near ambient are used to obtain information about micropore structure of two commercial carbons. This information is extracted from the comprehensive analysis of the data on three levels of assumptions. First from the temperature dependence of adsorption the isosteric heats of adsorption are calculated using nonparametric regression. Heats of adsorption allow for qualitative comparison of adsorption energetics between different carbons. The more detailed description of this energetics is given in terms of the adsorption energy distributions. Finally the micropore size distributions are calculated from the adsorption energy distributions assuming that the adsorption energy of molecules confined in micropores is determined by their sizes and geometry. No assumptions are made about the analytical form of the distribution. Effects of model assumptions, such as gas−solid interaction parameters or the pore wall thickness, on the calculated micropore size distributions are discussed. In order to obtain better insight on pore size distribution and geometry of actual carbons the results of this work are compared with the results of an independent chromatographic method based on the exclusion effect.

The combined use of different approaches in the characterization of microporous carbons

Adsorption of N 2 (77 K), CO 2 (273 K), and several hydrocarbons (273 or 298 K) and n-nonane preadsorption for two series of activated carbons with more-or-less distorted type I isotherms have been analyzed by the Dubinin-Radushkevich (D-R), Langmuir, and α s methods. When the microporosity is narrow and uniform, all theoretical and experimental approaches lead to values of micropore volume that are in good agreement. For carbons with wide micropore size distributions the Langmuir, α s and D-R methods applied to N 2 (77 K) (if for the latter a straight portion in the D-R plot can be drawn in the 0.05–0.3 range of relative pressure) yield the total micropore volume (as the conventional D-R plots for hydrocarbons) whereas n-nonane preadsorption and CO 2 give the volume corresponding to narrow micropores, in this way allowing a more complete characterization of the microporosity.

Characterization of microporous carbons by flow microcalorimetry

Heats of immersion in n-heptane of several microporous carbons and the heat of displacement of n-heptane by n-butanol and n-dotriacontane have been evaluated in a flow microcalorimeter. The heats of n-butanol adsorption were used to estimate polar sites on the surfaces of the carbons accessible to this alcohol. Comparison with the adsorption of tertiary butanol suggested that in some microporous PVDC carbons most of the polar sites are present in pores having diameters not exceeding 6.5 Å units. Other types of microporous carbons adsorbed very similar amounts of the branched and normal alcohols. The heats of adsorption of n-dotriacontane (a measure of the graphitic and hydrophobic surface sites) were relatively high for all the carbons investigated including the microporous carbons having very high surface areas as estimated from the BET (Brunauer-Emmett-Teller) N 2 adsorption method. The latter carbons also gave very high heats of immersion in n-heptane (ca. 200 J/g) and, in some cases, high heats of immersion in water that could be related to the heats of displacement of n-heptane by n-butanol (i.e., the polarity of the carbons). The adsorption of n-butanol and n-dotriacontane was carried out at different flow rates (i.e., at different rates of supply of the solutes to the adsorbents placed in the flow microcalorimeter) and the rates of heat evolution on adsorption and desorption were related to the nature of porosity in individual carbons.

The characterization of microporous carbons by means of the dubinin-radushkevich equation

Abstract The Dubinin-Radushkevich (D-R) equation of adsorption predicts a Rayleigh distribution of adsorption free energy with adsorption (micropore) volume. Only when this distribution is present in microporous solids will a completely linear D-R plot result. Adsorption data of carbon dioxide at 195°K and nitrogen and argon at 77°K on various microporous polymer carbons are interpreted in terms of the D-R equation and distributions of free energy with pore volume. In no case is the complete Rayleigh distribution found to apply and corresponding deviations from linear D-R plots occur. Highly activated carbons show a bimodal distribution of free-energy values to nitrogen and argon at 77°K, but not to CO 2 at 195°K. Tentative suggestions for this behavior are presented.