Surface area solids, adsorption

In writing the present book our aim has been to give a critical exposition of the use of adsorption data for the evaluation of the surface area and the pore size distribution of finely divided and porous solids. The major part of the book is devoted to the Brunauer-Emmett-Teller (BET) method for the determination of specific surface, and the use of the Kelvin equation for the calculation of pore size distribution but due attention has also been given to other well known methods for the estimation of surface area from adsorption measurements, viz. those based on adsorption from solution, on heat of immersion, on chemisorption, and on the application of the Gibbs adsorption equation to gaseous adsorption. 

Figure 1.6 Top Low-temperature nitrogen adsorption ( ) and desorption (x) isotherms measured on a calcined SBA-15 mesoporous silica solid prepared using an EO20PO70EO20 block copolymer [54]. Bottom Pore size distribution derived from the adsorption isotherm reported at the top [54]. A high surface area (850 m2/g), a uniform distribution of cylindrical nanopores (diameter —90 A), and a large pore volume (1.17 cm3/g) were all estimated from these data. These properties make this material suitable for use as support in the preparation of high-surface-area solid catalysts. (Reproduced with permission from The American Chemical Society.)...

A. Dabrowskl, M. Jaronlec, Excess Adsorption Isotherms for Solid-Liquid Systems and Their Analysis to Determine the Surface Phase Capacity, Adv. Colloid Interface Set, 31 (1990) 155. (Essential an attempt to obtain and define surface areas by adsorption from binary mixtures.)... 

Colloids can be organic or inorganic. Even if they are not separated from the dissolved load by classical filtration, colloids have the physicochemical properties of a solid. Colloids are finely divided amorphous substances or sohds with very high specific surface areas and strong adsorption capacities. It is shown by Ferret et al. (1994) for the Rhine River that the colloids contribute less than 2% of the total particle volume and mass, but represent a dominant proportion of the available surface area for adsorption of pollutants. The abundance of colloids, their fate, through coagulation and sedimentation processes in natural waters therefore control the abundance of a number of elements. 

Due to their large surface area for adsorption, porous materials are useful excipients for solid dispersions. For example, 2-naphthoic acid (2-NPA) solid dispersion with porous crystalline cellulose (PCC) has been successfully prepared by heat treatment of 2-NPA and PCC mixture. " PCC is derived from MCC, but with a larger surface area. Different from 2-NPA mixed with PCC, 2-NPA mixed with MCC still maintained a crystalline form under the same mixing and heating conditions. Various experimental data such as X-ray powder diffraction, Fourier transform infrared (FT-IR) spectroscopy, and solid-state fluorescence measurements suggest that 2-NPA is adsorbed onto the surface of PCC and becomes molecularly dispersed into the system. 

Minimizing Adsorbed Impurities on Colloids The purity of many coagulated colloids is improved by digestion. During this process, water is expelled from the solid to give a denser mass that has a smaller specific surface area for adsorption. 

Ritter and Drake give the true density of the solid material in an activated alumina particle as 3.675 g/cm. The density of the particle determined by mercury displacement is/l. 547. The surface area by adsorption measurement is 175 m"/g. From this information compute the pore volume per gram, the porosity of the particles, and the mean pore radius. The bulk density of a bed of the alumina particles in a 250-cm graduate is 0.81 g/cm. What fraction of the total volume of the bed is void space between the particles and what fraction is void space within the particles ... 

Weigh out in the weighing dish three samples of about 0.25 to 0.30 g each (vary the sample weights, but weigh each exactly), transfer to 250-mL Erlenmeyer flasks, and dissolve in about 50 mL distilled water. Add 10 mL 1% dextrin suspension (shake well before using) and 10 drops dichlorofluorescein indicator solution. The dextrin prevents excessive coagulation of the precipitate at the end point. This keeps a larger surface area for adsorption of the indicator, which enhances the sharpness of the end point. Instead of a suspension of dextrin, 0.1 g of the solid may be added. 

Clays are considered detrimental to EOR processes that are based on the injection of chemicals, such as foam-forming surfactants, because clays provide a large amount of surface area for adsorption. Table VII shows a comparison of specific surface areas of some clays (97, 117, 118) and of the solids used in the adsorption experiments of Figure 15 (12, 119, 120). Figure 15 allows comparison of adsorption levels in Berea sandstone, which consists mainly of quartz and 6-8% clays, with adsorption on clean quartz sand. 

Dividing this number by Avogadro s number L and the volume occupied by the surface, that is, by the volume of the high surface area solid H gives the concentration of adsorption sites ... 

Experimental Verification of Adsorption Isotherms and Linear Least-Squares Analysis. If gas A is exposed to a very high surface area solid catalyst (i.e., sslOO m /g) in a closed chamber, then a sensitive electronic balance should provide measurements of the increase in catalyst mass at a given gas pressure pa as active sites become occupied. A flow control valve is necessary to maintain constant pressure pa while measurements are made, because adsorption of gas molecules on the catalytic surface will cause a decrease in gas pressure if additional gas is not introduced into the system. Knowledge of the gas density at STP conditions and the additional mass of gas from the flow control valve required to maintain constant pressure pa allows one to calculate the volume of adsorbed gas per initial mass of catalyst, va- Experiments are repeated at different gas pressures. The raw data correspond to pa va pairs that can be modeled via the Langmuir isotherm to extract two important parameters of the adsorption process. 

Gas chromatography studies of surface modified silicas provided new insight concerning the adsorption properties of new stationary phases [129]. Inverse gas chromatography (IGC) can be a useful tool to determine the adsorption properties of low surface area solids. The work done by Bakaeva et al. [130] is a good example of IGC use to study low-surface-area-silica glasses. In the cited paper Bakaeva et al. employed IGC to analyze the adsorption of butanol and hexane on E-glass fiber. 

Adsorption chromatography, in which the stationary phase consists of a high-surface-area solid adsorbent the solutes physically adsorb on the stationary phase, while the liquid mobile phase tries to dislodge them. 

Very often that when dealing with adsorption of many gases and vapours in high surface area solids such as activated carbon and silica gel that surface diffusion can contribute significantly to the overall uptake. For this mass transfer mechanism, the mass balance equation describing the concentration distribution within the particle are ... 

Because sites that are active for adsorption also tend to be active catalytic sites, compounds in the streams that would not normally cause deactivation may react to form nondesorbables. Olefins, diolefins, and other unsaturated hydrocarbons are especially difficult since they ea.sily polymerize to long-chain species in the presence of high-surface-area solids. Hydrocarbons in the presence of oxygen can form oxygenated species such as aldehydes and ketones, which can further react by aldol condensation to form heavier components. The presence of oxygen with sulfur compounds can create elemental sulfur. 

The detailed examination of the MCM-41 porous structure was based on analysis of adsorption branch of isotherms using the method of comparison plots [11]. In this method the amount adsorbed on the solid under investigation (a) is plotted against that adsorbed on a reference adsorbent (a O at the same equilibrium pressure. In this study the adsorption in the pores of MCM-41 was compared with that on the open flat surface of the reference solid material. It was prepared by the thermal destruction of corresponding MCM-41 samples at 1000°C for 2 hours, / s these reference adsorbents had different surface area the adsorption on them was... 

The Harkins and Jura (36) absolute method of calculating specific surface area from adsorption data apparently gives more consistent results than the BET method when different adsorbates are used on dinerent kinds of solids. It is based on an empirical equation ... 

There is, of course, a large number of possible combinations of carrier fluids and adsorbable solutes that could be used for the determination of surface areas. A necessary condition for candidates suitable for this application is the conformity with Langmuir s isotherm, i.e. no change in the heat of adsorption of the adsorptive with surface coverage, and the ability to saturate the total fluid-solid interface at low solution or low gas mixture concentrations. Other important conditions are the completeness of the displacement of carrier fluid from the interface and the formation of a close packed monolayer by the adsorbate. The complete displacement of carrier fluid may not occur on microporous solids for which the determinations of total surface areas by adsorption are always difficult to interpret irrespective of the adsorptive used in such determinations. 

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