Improved Pore-Size Analysis of Carbonaceous Adsorbents

An improvement was proposed for the pore-size analysis of active carbons based on low-temperature (77 K) nitrogen adsorption isotherms measured over a wide range of relative pressures (5 × 10−7–0.995). It was shown that the applicability of the Barrett, Joyner and Halenda (BJH) computational method based on the Kelvin equation could be extended significantly towards small mesopores and large micropores when a proper t-curve was used to represent the film thickness of nitrogen adsorbed on the carbon surface. It was proposed that the aforementioned t-curve be obtained from the nitrogen adsorption isotherm at 77 K on a macroporous carbon black by fitting its multilayer part to the calibrated t-curve for nitrogen adsorbed at 77 K on a macroporous silica. To date, the Harkins–Jura or Halsey t-curves have been used to describe the pressure-dependence of the film thickness. This appears to be inaccurate, especially in the range of low relative pressures. It was shown that this inaccuracy makes the pore-size analysis questionable. However, the t-curve proposed in this work gave the pore-size distribution functions for the carbons studied thereby reproducing the total pore volume and showing realistic behaviour in the range at the borderline between micropores and mesopores.

Surface Activity Coefficients in Adsorption at Liquid/Solid Interface

The global surface activity coefficients characterizing nonideality of the surface solution generated by differences in molecular interactions and adsorbent heterogeneity are defined and studied. These activity coefficients may be evaluated from the excess adsorption data. It is shown that they are useful to formulate a simple unified description of liquid/solid adsorption involving effects of solution nonideality and adsorbent heterogeneity.

Estimation of the Surface Properties of Unmodifed and Strongly Oxidized Active Carbons on the Basis of Water Vapour Adsorption Isotherms

Water vapour adsorption isotherms were measured for a series of commercial active carbons and one carbon black oxidized with concentrated nitric acid at its boiling temperature. A comparative analysis of the surface properties of the unmodified and strongly oxidized active carbons was performed on the basis of the adsorption potential distributions (APDs) calculated from these adsorption isotherms. It was shown that the APDs for oxidized carbons exhibit some extra details (e.g., peaks) in the range of high adsorption potentials which can be assigned to interactions of water molecules with oxygen surface groups formed during the oxidation process. This is especially visible for oxidized carbon black. The observed sensitivity of the APDs obtained from water vapour adsorption isotherms make them attractive functions for monitoring the surface changes in active carbons due to their oxidation.

Liquid/Solid Interfaces: Studies of Kinetics of Isotope Exchange

Many radiotracer methods based on the phenomenon of isotope exchange are used in the studies of interfaces. These methods enable an investigation of heterogeneous systems at equilibrium. Studies of the kinetics of isotope exchange also give valuable information about systems in which ions appearing in the solution are at the equilibrium state with the same ions adsorbed on the solid particles or built into their crystalline lattice. Such systems, in which the solids are either ionic crystals or ion-exchanger beads, are taken into account in the present paper. In spite of wide applications, the phenomenon of heterogeneous isotope exchange is rather poorly understood and the fragmentary results reported in the literature do not lead to unambigous conclusions concerning the mechanism of this phenomenon. In the present review, the theoretical models of isotope exchange are discussed and compared with experimental results. The results are also discussed with respect to enlargement of applications of isotope exchange to investigation of heterogeneous systems, e.g. containing porous solid particles.

Influence of the Pore Geometry on the Micropore Size Distribution Function of Active Carbons

A simple procedure for the evaluation of the micropore size distribution function of active carbons from the experimental adsorption isotherm of a single gas is discussed. In this method, the adsorption potential distribution and the dependencies of this potential on the micropore size for three different models, i.e. slit-like, cylindrical and spherical, are utilized. It was shown that the geometrical shape of pores may have a substantial influence on the micropore size distribution. While the micropore size distributions calculated assuming slit-like and cylindrical pore geometries are similar, spherical pore geometry leads to a substantially different distribution. A qualitative agreement was found between the micropore size distribution obtained by the current procedure, which is equivalent to the Horvath–Kawazoe method, and that evaluated by the density functional theory (DFT) method.

Scaffold-assisted synthesis of crystalline mesoporous titania materials

Scaffold-assisted synthesis of crystalline mesoporous titania materials with controllable structural and crystalline properties.

Silica-metal core-shell nanostructures

Silica-metal nanostructures consisting of silica cores and metal nanoshells attract a lot of attention because of their unique properties and potential applications ranging from catalysis and biosensing to optical devices and medicine. The important feature of these nanostructures is the possibility of controlling their properties by the variation of their geometry, shell morphology and shell material. This review is devoted to silica-noble metal core-shell nanostructures; specifically, it outlines the main methods used for the preparation and surface modification of silica particles and presents the major strategies for the formation of metal nanoshells on the modified silica particles. A special emphasis is given to the Stöber method, which is relatively simple, effective and well verified for the synthesis of large and highly uniform silica particles (with diameters from 100 nm to a few microns). Next, the surface chemistry of these particles is discussed with a special focus on the attachment of specific organic groups such as aminopropyl or mercaptopropyl groups, which interact strongly with metal species. Finally, the synthesis, characterization and application of various silica-metal core-shell nanostructures are reviewed, especially in relation to the siliceous cores with gold or silver nanoshells. Nowadays, gold is most often used metal for the formation of nanoshells due to its beneficial properties for many applications. However, other metals such as silver, platinum, palladium, nickel and copper were also used for fabrication of core-shell nanostructures. Silica-metal nanostructures can be prepared using various methods, for instance, (i) growth of metal nanoshells on the siliceous cores with deposited metal nanoparticles, (ii) reduction of metal species accompanied by precipitation of metal nanoparticles on the modified silica cores, and (iii) formation of metal nanoshells under ultrasonic conditions. A special emphasis is given to the seed-mediated growth, where metal nanoshells are formed on the modified silica cores with deposited metal nanoparticles. This strategy assures a good control of the nanoshell thickness as well as its surface properties.

Further advancements in predicting adsorption equilibria using excess formalism: calculation of adsorption excesses at the liquid/solid interface

Prediction of adsorption equilibria for ternary liquid mixtures on solid surfaces by means of adsorption data for the corresponding three binary liquid mixtures can be improved by combining the thermodynamic excess formalism with geometrical models. This new strategy for the prediction of excess adsorption isotherms is examined for four ternary adsorption systems ranging from ideal to highly non-ideal ternary mixtures. The predicted isotherms are discussed and compared with experimental ones as well as with those obtained for a model based on the absolute quantities. The results confirm: (i) superiority of predicting adsorption in terms of excess quantities, and (ii) utility of geometrical models for constructing ternary molar compositions on the basis of binary ones to predict equilibria not only for liquid mixtures alone but also for adsorption of liquid mixtures on solid surfaces.

Features of nitrogen adsorption on nonporous carbon and silica surfaces in the framework of classical density functional theory

Equilibrium adsorption data of nitrogen on a series of nongraphitized carbon blacks and nonporous silica at 77 K were analyzed by means of classical density functional theory to determine the solid-fluid potential. The behavior of this potential profile at large distance is particularly considered. The analysis of nitrogen adsorption isotherms seems to indicate that the adsorption in the first molecular layer is localized and controlled mainly by short-range forces due to the surface roughness, crystalline defects, and functional groups. At distances larger than approximately 1.3-1.5 molecular diameters, the adsorption is nonlocalized and appears as a thickening of the adsorbed film with increasing bulk pressure in a relatively weak adsorption potential field. It has been found that the asymptotic decay of the potential obeys the power law with the exponent being -3 for carbon blacks and -4 for silica surface, which signifies that in the latter case the adsorption potential is mainly exerted by surface oxygen atoms. In all cases, the absolute value of the solid-fluid potential is much smaller than that predicted by the Lennard-Jones pair potential with commonly used solid-fluid molecular parameters. The effect of surface heterogeneity on the heat of adsorption is also discussed.

Equilibrium Adsorption in Cylindrical Mesopores: A Modified Broekhoff and de Boer Theory versus Density Functional Theory

This paper presents a thermodynamic analysis of capillary condensation phenomena in cylindrical pores. Here, we modified the Broekhoff and de Boer (BdB) model for cylindrical pores accounting for the effect of the pore radius on the potential exerted by the pore walls. The new approach incorporates the recently published standard nitrogen and argon adsorption isotherm on nonporous silica LiChrospher Si-1000. The developed model is tested against the nonlocal density functional theory (NLDFT), and the criterion for this comparison is the condensation/evaporation pressure versus the pore diameter. The quantitative agreement between the NLDFT and the refined version of the BdB theory is ascertained for pores larger than 2 nm. The modified BdB theory was applied to the experimental adsorption branch of adsorption isotherms of a number of MCM-41 samples to determine their pore size distributions (PSDs). It was found that the PSDs determined with the new BdB approach coincide with those determined with the NLDFT (also using the experimental adsorption branch). As opposed to the NLDFT, the modified BdB theory is very simple in its utilization and therefore can be used as a convenient tool to obtain PSDs of all mesoporous solids from the analysis of the adsorption branch of adsorption isotherms of any subcritical fluids.

An improved methodology for adsorption characterization of unmodified and modified silica gels

An improvement in the adsorption characterization of the surface and structural properties of unmodified and modified mesoporous silica gels is presented. This improvement was achieved by selection of proper macroporous silica as the reference solid for adsorption characterization of porous silica gels. Experimental illustration is provided for unmodified and n-octyl-modified silica gels of different bonding density. The surface and structural properties of these silica gels were characterized by utilizing the standard adsorption data for both unmodified and octyl-modified LiChrospher Si-1000 macroporous silica gels. It was shown that the standard nitrogen adsorption data have an appreciable influence on the analysis of the pore size and surface properties of silica gels. This analysis can be improved by selecting the reference solid of the surface properties close to those of the silica gel studied.

Novel bifunctional periodic mesoporous organosilicas, BPMOs: synthesis, characterization, properties and in-situ selective hydroboration-alcoholysis reactions of functional groups

A new class of bifunctional periodic mesoporous organosilicas (BPMOs) containing two differently bonded organic moieties in a mesoporous host has been synthesized and characterized. By incorporating bridge-bonded ethylene groups into the walls and terminally bonded vinyl groups protruding into the channel space, both the chemistry and physical properties of the resulting BPMO could be modified. The materials have periodic mesoporous structures in which the bridging ethylene plays a structural and mechanical role and the vinyl groups are readily accessible for chemical transformations. The vinyl groups in the material underwent hydroboration with BH(3).THF and the resulting organoborane in the BPMO was quantitatively transformed into an alcohol using either H(2)O(2)/NaOH or NaBO(3).4H(2)O. The materials retained ordered structures after subsequent in situ reactions with largely unchanged pore volumes, specific surface areas and pore size distributions. Other organic functionalized BPMO materials may be synthesized in a similar manner or by further functionalizing the resulting borylated or alcohol functionalized BPMO materials. The thermal properties of the BPMO materials have also been investigated and are compared to those of the periodic mesoporous organosilica (PMO) materials. Noteworthy thermal events concern intrachannel reactions between residual silanols or atmospheric oxygen and organics in BPMOs. They begin around 300 degrees C and smoothly interconvert bridging ethylene to terminal vinyl groups and terminal vinyl to gaseous ethene and ethane, ultimately producing periodic mesoporous silica at 900 degrees C that exhibits good structural order and a unit-cell size decreased relative to that of the parent BPMO.