Microporous Structure Of Carbonaceous Adsorbents Research Articles

An analysis of the porous structure of activated carbons obtained from hazelnut shells by various physical and chemical methods of activation

This paper presents the results of the research analysing the microporous structure of carbonaceous adsorbents obtained from hazelnut shells by various physical and chemical methods of activation. The calculations were carried out with the use of the procedure of fast multivariant identification of adsorption systems with implemented LBET class adsorption models, which is generally referred to as the LBET method, as well as by DR and BET methods on the basis of adsorption isotherms of benzene and nitrogen. Porosimetry measurements, as well as imaging with the use of scanning electron microscopy and an EDX elemental analysis were also conducted.

Use of fast multivariant identification of the parameters of adsorption systems to study the impact of activating agent on microporous structure formation during activation

The possibility of using fast multivariant identification of adsorption system parameters to evaluate the influence of the activation conditions on the microporous structure of carbonaceous adsorbents was examined. The fitted models used in the procedure are based on a unique approach to the modeling of adsorption processes, which is a significant development of the conventional BET approach. This takes into account the occurrence of branched clusters and surface heterogeneity. The approach considers heterogeneity from the perspectives of both geometry and energy, and discusses geometrical and energy limitations of creating clusters and the adhesive forces in layers above the first one.

Application of fast multivariant identification technique of adsorption systems to analyze influence of production process conditions on obtained microporous structure parameters of carbonaceous adsorbents

The paper presents the results of the research on the possible employment of the fast multivariant fitting technique as a tool for analysing the microporous structure of carbonaceous adsorbents. The proposed fast multivariant fitting of the LBET computer models to the empirical adsorption data technique was employed particularly to evaluate the impact of the production process conditions on the obtained microporous structure parameters of carbonaceous adsorbents. The purpose of the computer analysis was to obtain information as to what extent the proposed method of the microporous structure description can be useful for technologists in the development of new microporous materials with the optimum adsorption properties, as well as during the selection of optimum conditions of adsorption process as a reliable and quick analytic tool.

Parameters of microporous structure of carbonaceous adsorbents gasified with air or carbon dioxide

The microporous structure of two series of carbonaceous adorbents gasified with air or carbon dioxide has been studied in terms of the modified theory of volume filling of the micropores. According to this theory the amount adsorbed in the micropores has been deduced from the total adsorbed amount by comparing it with the adsorption isotherm on a reference non-porous carbon. Adsorption in the micropores is described by an isotherm equation associated with the γ-type micropore size distribution in order to evaluate parameters of the microporous structure. These parameters are calculated from the benzene adsorption isotherms.

Adsorption properties and microporous structures of carbonaceous adsorbents

The theory underlying the absorption behavior of carbons is discussed as a basis for quantitative analysis of their adsorption properties and microporous structure. The equations of the theory of volume filling of micropores for homogeneous and inhomogeneous microporous structures and a rational method for determining the specific surface area of micropores are considered. Four principal parameters are proposed which describe the physical vapor adsorption with good accuracy and quantitatively characterize the microporous structure of carbonaceous adsorbents.

Generalization of the theory of volume filling of micropores to nonhomogeneous microporous structures

It is suggested that the nonhomogenity of the microporous structures of carbonaceous adsorbents should be expressed through the normal distribution of the micropore volume by sizes (halfwidth for the slitlike model). On the basis of the theory of the volume filling of micropores in vapor adsorption, a new Dubinin-Stoeckli adsorption equation is obtained. Its three parameters—the total volume of the micropores, their halfwidth for the maximum of the distribution curve, and dispersion—are identical to the parameters of normal distribution. This is true only when the initial vapor adsorption isotherm has been corrected for adsorption in mesopores. Then the parameters of the adsorption and distribution equations, which are determined from the experimental vapor adsorption isotherm, are real quantities. It is shown that the most rational method for determining the mesopore specific surface area necessary for correcting the isotherm is the Dubinin-Kadlec γ F method identical to the t F method. The concepts developed are experimentally substantiated.

Adsorption of water vapor and the microporous structures of carbonaceous adsorbents. Communication 4. Adsorption isotherms of water vapor in the region of low relative pressures

1. The initial sections of the adsorption isotherms of water vapor on active carbons with different microporous structures have been studied in detail, and it has been shown that the isotherms have a convex section in the initial range of relative pressures up to p/ps=0.05. 2. The number of primary adsorption centers in these carbons has been estimated. A considerable increase in their number on the oxidization of the active carbon has been observed.

Microporous structures of carbonaceous adsorbents

The simplest model of the porous structure of carbonaceous adsorbents is proposed, in which the microporous zones are formed by the totality of contacting and merged carbon crystallites. The micropores limited in extent resulted from the burnout on activation of several atomic layers of carbon in the crystallites by gaseous oxidizers (intercrystallite micropores) and from some development of the spaces between contacting crystallites (intercrystallite micropores). The micropore zones are shaped into regular cubes. The slit-like spaces between part of the micropore zones represent the micropore volume, while the total surface area of the walls of such pores is the mesopore surface. The parameters of the model for typical samples of microporous carbonaceous adsorbents are calculated and discussed. The formation of intercrystallite slit-like micropores as a result of the burnout of carbon atomic layers in crystallites on avtivation is considered. It is shown that only a wide micropore distribution leading to the formation of the supermicropores practically results in deviation from the applicability of the one-term adsorption equation of the theory of volume filling of micropores.

Inhomogeneous microporous structures of carbonaceous adsorbents

Ways for theoretical calcultion of the geometric surface area of the micropore walls of carbonaceous adsorbents with inhomogeneous microporous structures are considered for the slitlike micropore model. Two calculations methods have been worked out based on the theory of volume filling of micropores (TVFM) from the parameters of the adsorption equation proposed by Stoeckli and from the parameters of the binomial TVFM equation. An independent method for determining the geometric surface area of the micropore walls from water vapor adsorption isotherms served as a reference. When we used an active carbon with a wide micropore distribution all the three methods led to practically identical results, which indicated that the concept of the geometric surface area of carbonaceous adsorbents is physically feasible. The physical unfeasibility of a specific surface determined by the BET method for a microporous adsorbent is shown.

Water vapor adsorption and the microporous structures of carbonaceous adsorbents

The principal role in adsorption of almost all vapors organic and inorganic substances on nonporous and microporous carbonaceous adsorbents is played by dispersion interactions. They are characterized by a considerable increase in adsorption potentials as a result of superposition of the fields of the opposite pore walls. This effect determines the entire specifics of adsorption in micropores and, in particular, the substantial increase in adsorbability. A theoretical estimate of the adsorption potentials of benzene and water in adsorption on graphite, and a comparison of the differential heats of adsorption of water vapors on a non-porous carbon black previously heated in a vacuum at 950°C and on an active carbon show that water adsorption is due to the formation of hydrogen bonds both between the oxygen complexes on the surface of carbonaceous adsorbents and between the adsorbed molecules themselves. Dispersion interactions are weak and can be neglected to a first approximation. It has been shown for microporous structures and the slitlike model that one can calculate, from the parameters W 0 and E 0 of the adsorption equation of the theory of volume filling of micropores (determined from the adsorption isotherm of a standard vapor, benzene) the volume of the micropores, their halfwidth, and the specific surface area of the micropore walls. The latter are in good agreement with the specific surface areas of the micropores, as estimated by the independent method of similarity of the adsorption isotherms of water in micropores and on the surface of a nonporous carbonaceous adsorbent. The application of the BET and t-methods to microporous carbonaceous adsorbents is physically unsubstantiated.

103. Microporous structures of carbonaceous adsorbents

103. Microporous structures of carbonaceous adsorbents

Microporous structures of carbonaceous adsorbents

The microporous structure of coals (anthracite and lean coal) carbonized and activated with water vapour at 900°C was investigated by the small-angle X-ray scattering and adsorption methods. As distinct from the previously studied active carbons from sucrose possessing uniform microporosity (inertia radii R = 6–7 A ̊ ), coals are characterized by a considerably greater polydispersity of micropores. Small-angle X-ray scattering data, which were in agreement with the results of the theoretical analysis of benzene adsorption isotherms, revealed coarser micropores of R = 6–7 A ̊ along with fine micropores of R = 6–7 A ̊ , in active coals. At low degrees of activation, fine micropores are predominant in the active carbons obtained. As burn-out increases the fraction of coarser micropores grows. It is believed that the size polydispersity of active coal micropores, which roughly corresponds to the radius range R = 4–15 A ̊ , stems from the non-homogeneity of the structure of the initial products used for activation, i.e. anthracite and lean coal.