Solid active surface area

As discussed below, the porosity and surface area of the catalyst film is controllable to a large extent by the sintering temperature during catalyst preparation. This, however, affects not only the catalytically active surface area AG but also the length, t, of the three-phase-boundaries between the solid electrolyte, the catalyst film and the gas phase (Fig. 4.7). 

Selection of columns and mobile phases is determined after consideration of the chemistry of the analytes. In HPLC, the mobile phase is a liquid, while the stationary phase can be a solid or a liquid immobilised on a solid. A stationary phase may have chemical functional groups or compounds physically or chemically bonded to its surface. Resolution and efficiency of HPLC are closely associated with the active surface area of the materials used as stationary phase. Generally, the efficiency of a column increases with decreasing particle size, but back-pressure and mobile phase viscosity increase simultaneously. Selection of the stationary phase material is generally not difficult when the retention mechanism of the intended separation is understood. The fundamental behaviour of stationary phase materials is related to their solubility-interaction... 

Similarly, in the development of solid oxide fuel cells (SOFCs), it is well recognized that the microstructures of the component layers of the fuel cells have a tremendous influence on the properties of the components and on the performance of the fuel cells, beyond the influence of the component material compositions alone. For example, large electrochemically active surface areas are required to obtain a high performance from fuel cell electrodes, while a dense, defect-free electrolyte layer is needed to achieve high efficiency of fuel utilization and to prevent crossover and combustion of fuel. 

By active surface area we meant the kinetically active part of the total surface area. According to Helgeson et al. (1984), this area is restricted to etch pits. Alternative estimates of surface areas may be obtained from measurements of specific surface area s whenever solid particles have a narrow size range. The specific surface area for spherical particles is given by... 

Since the solid surface is responsible for catalytic activity, a large readily accessible surface in easily handled materials is desirable. By a variety of methods, active surface areas the size of football fields can be obtained per cubic centimeter of catalyst. 

A gas-solid reaction usually involves heat and mass transfer processes and chemical kinetics. One important factor which complicates the analysis of these processes is the variations in the pore structure of the solid during the reaction. Increase or decrease of porosity during the reaction and variations in pore sizes would effect the diffusion resistance and also change the active surface area. These facts indicate that the real mechanism of gas-solid noncatalytic reactions can be understood better by following the variations in pore structure during the reaction. 

According to the hypothesis, chemical conversions in a solid occur on the surface of a new crack (or in the layer adjacent to it), that is, the reaction rate in such a reacting system is a certain function of the specific surface area S (active surface area per unit volume). As noted above, the positive feedback in this model manifests itself in the fact that the rate of formation of cracks (i.e., the active surface growth rate in the sample volume) is proportional to the reaction velocity. Therefore, the equation describing the formation of a new surface can be written in a form analogous to that of a branched-chain process ... 

The activity of solid catalysts is usually proportional to the active surface area per unit volume of catalyst A high activity per unit volume consequently calls for (extremely) small particles Since most active species... 

POMs/HPA themselves are usually non-porous solids, with surface area less than 10 m2/g and low decomposition temperatures. Therefore, they have limited surface sites for surface-catalysed reactions. A number of attempts have been made to disperse POMs on inert supports, with the intention of effectively increasing the number of accessible active catalytic sites. A number of materials have been used as solid supports for the dispersion of HPA, for example silica, carbon, zirconia, alumina, and porous silica. 

In carbon materials, active surface sites only represent a fraction of the total surface area, called active surface area (ASA). Knowledge of the nature and concentration of the active sites is of paramount importance for a better understanding of the kinetics involved in heterogeneous gas-solid reactions. However, although ASA reveals interesting information about the sample, there is a need for a reliable and standardised method for its estimation. The aim of this work is to compare ASA determination by the usual methods (i.e., gravimetry, TPD) with another method, which is based on pressure measurements in order to perform an oxygen chemisorption isotherm (OCI). The results showed that the OCI method seems to be a valuable and alternative method for ASA determination, as it avoids the main potential source of errors Inherent in the usual methods. 

Therefore the electrochemical response with porous electrodes prepared from powdered active carbons is much increased over that obtained when solid electrodes are used. Cyclic voltammetry used with PACE is a sensitive tool for investigating surface chemistry and solid-electrolyte solution interface phenomena. The large electrochemically active surface area enhances double layer charging currents, which tend to obscure faradic current features. For small sweep rates the CV results confirmed the presence of electroactive oxygen functional groups on the active carbon surface. With peak potentials linearly dependent on the pH of aqueous electrolyte solutions and the Nernst slope close to the theoretical value, it seems that equal numbers of electrons and protons are transferred. 

A common feature of these catalysts is their acidic nature (i.e., they all act as solid phase acids in the hot gas oil vapor stream). Synthetic silica/alumina catalyst composites, for example, have an acidity of 0.25 mEq/g distributed over an active surface area of some 500m /g). This acidity is the key feature that distinguishes catalytic cracking from straight thermal cracking. 

The presence of physiological surface-active agents in the stomach and small intestine will influence the solubility and the dissolution of sparingly soluble drugs by improved wetting of solid particle surface areas and by micellar solubilisation. This has been reviewed in more detail by Gibaldi and Feldman (1970) and Charman et al. (1997). 

In the previous section we dealt with the surface area per unit weight or unit volume required to achieve technically acceptable conversions. The conclusion was that porous catalyst particles must usually meet the demands both of pressure drop (fixed bed catalysts) or of viability of separation (suspended catalyst particles) and of the extent of the catalytically active surface area. When the catalyst must be thermally pretreated, the active surface area should not severely drop as a result of sintering of the active particles. Although solid catalysts in fine-chemistry operations are usually employed in liquid phases, i. e. not at highly elevated temperatures, sintering of the active component(s) should not occur during the reaction. 

The active surface area of a distillation tray is an area of great turbulence and accumulations of solids or tars are unlikely to settle there and block liquid or vapour flow. The downcomers are much more liable... 

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