Estimation of the energetic nonhomogeneity of the surface of oxide and carbon materials

Abstract

The molecular statistical method for evaluating the distribution of active sites of various adsorbents relative to their energies has been improved. This method is used not only for the treatment of experimental data on the adsorption of hydrocarbons on various adsorbents, which is the usual procedure, but also data on the adsorption of polar water and methanol molecules on the active sites of adsorbent surfaces. Two types of active sites differing in energy have been shown to exist on the surface of graphitized carbon black, the complex shungite carbon/mineral adsorbent, and modified Silochrom. Chromatographic, calorimetric, and structural adsorption data were used to establish the relationship between the observed maxima of the energy distribution function of the adsorption sites with concrete adsorption sites or pores of the surface, on which the molecules are adsorbed.Adsorbents and catalysts, by their nature, are energetically nonhomogeneous. We distinguish between structural and chemical nonhomogeneity. Structural nonhomogeneity for monophase adsorbents is a consequence, for example, of the energy inequivalence of various crystal faces as, for example, in the case of talc [1], the existence of different types of dislocations on the surface of microcrystals, the existence of surface micropores, a breakdown of orderedness in the deposition of adjacent layers in the structure, and the domain distribution of isomorphic impurities.Complex adsorbents such as alumosilica gels, mountain leather-montmorillonite silicates, and carbon-mineral adsorbents are also commonly used in adsorption and catalytic processes. Each of the phases in these adsorbents has a characteristic set of structural-energetic sites of nonhomogeneity, while the boundary of the two adjacent disperse materials is a special type of structural heterogeneity, which plays a role in heterogeneous catalysis.Chemical heterogeneity is related both to the different chemical composition of the adsorbent, which predetermines the type of active adsorption sites, and to their coordination number. Such heterogeneity is characteristic for silica gels with isolated, vicinal, and silanediol groups on the surface [2], for alumina gels with characteristic tetrahedral and octahedral coordination of the surface aluminum atoms, and, correspondingly, different activity of the bound hydroxy groups [3], and for carbon adsorbents with characteristic carboxyl, phenol, and hydroxyl active sites [4].Various physical chemical methods are employed for studying both structural and chemical nonhomogeneity. However, theoretical thermodynamic analysis based on experimental adsorption and chromatographic data remains the most reliable method permitting determination of the quantitative characteristics of surface nonhomogeneity. Various workers [5][6][7] 0040-5760/08/4405-0325

Statistical-mechanical derivation of generalised langmuir equation and its application for the interpretation of chromatographic data for the adsorption of water and methanol on non-porous carbon materials

“…(8)-(10) become identical to Eqs. (5)-(7) in [12]. If, in addition, each centre can accommodate not more than one molecule, then (as shown above) all vanish, and Eq.…”

“…Phenolic hydroxyl groups are less acidic, and possess much lower sorption activity [4,10]; however the density of these groups on the surface of graphitised thermal black and thermally expanded graphite is higher than that of the carboxyl groups [11]. Ab initio calculations [12] have shown that the adsorption of one water or methanol molecule on the carboxyl centre results in the formation of two hydrogen bonds: between the oxygen atom of the hydroxyl group and the proton of adsorbed molecule, and between the proton of the hydroxyl group and the oxygen atom of adsorbed mol ecule. On the contrary, the adsorption on the weakly acidic phenolic hydroxyl group results in the forma tion of one hydrogen bond only.…”

Energetic heterogeneity of the surface of a micro/mesoporous zeolite adsorbent

“…Carbon adsorbents are characterized by carboxyl, phenolic, hydroxyl, and other active sites. The reasons for energetic heterogeneity have been analyzed in detail by various workers [1][2][3][4][5].Various physical methods are used to study the energetic heterogeneity. Atomic force microscopy [6] and high resolution electron microscopy [7] are among the most common methods for oxides and silicates.…”

“…Carbon adsorbents are characterized by carboxyl, phenolic, hydroxyl, and other active sites. The reasons for energetic heterogeneity have been analyzed in detail by various workers [1][2][3][4][5].…”