Brunauer- Emmett-Teller adsorption isotherms

BET Brunauer-Emmett-Teller (adsorption isotherm) DPP differential pulse polarography... 

H. Brunauer-Emmett-Teller Adsorption isotherm Studies... 

The area is an important surface parameter for catalytic studies. It is needed to evaluate the rate constant of the surface reaction from the kinetics as well as to allow a fair comparison to be made of the effectiveness of different catalysts. Areas are commonly determined by nitrogen or krypton gas adsorption interpreted by the Brunauer-Emmett Teller (BET) isotherm [30, 32], A number of other methods has been proposed and utilised including microscopy, isotopic exchange, chromatography, gas permeability, adsorption from solution, and negative adsorption (desorption) of co-ions [30, 33]. 

Evidence for a specific inclusion process promoted by molecular imprinting is directly given through sensor effects in comparison to non-imprinted layers of equivalent height. Detailed interpretation of specific inclusion or non-specific adsorption phenomena is accessible with the Brunauer-Emmett-Teller (BET) isotherm adsorption analysis [18] a typical BET-isotherm is shown in Fig. 21.2. 

Gas adsorption is a suitable method for a fractal analysis because it is sensitive to the fine structure of the pores and has negligible adverse affects on the pore system. The results are usually analyzed by using fractal generalizations of the Brunauer-Emmett-Teller (BET) isotherm (30) or of the Frenkel-nalsey-TfiU (FHH) isotherm (31). The latter may also be seen as a fractal generalization of the Kelvin equation and is therefore also applicable in the capillary condensation regime (32). It has been claimed that the fractal BET theory is more appropriate for mass fractals (see sect. Fractals ), whereas surface fractals are to be analyzed using the fractal FHH theory (33). These methods have been applied to cellulose powders (34) and tablets (35). 

The possibility of multilayer adsorption is envisaged in the Brunauer-Emmett-Teller (BET) isotherm. The assumption is made that the first layer is adsorbed with a heat of adsorption H and the second and subsequent layers are all characterized by heats of adsorption equal to the latent heat of evaporation, //l- By considering the dynamic equilibrium between each layer and the gas phase the BET isotherm is obtained,... 

The BET (Brunauer-Emmett-Teller)-type isotherm in Figure 3.3.8B reflects multilayer adsorption of the adsorbate. After a monolayer adsorbate coverage is achieved in the adsorbent pores, additional molecular layers are formed on top of the adsorbed monolayer by condensation of vapors. In adsorbents having small-diameter pores, multilayer condensation of the adsorbate vapor can fill the pore completely with the liquid adsorbate. This phenomenon is called capillary condensation (Figure 3.3.8C). Consider the curved interface between the vapor phase and the condensed phase of species i in the micropore in this figure. The vapor pressure of the condensed liquid above the concave curved liquid surface in the capillary F utved less than that over a plane condensed liquid surface F pi ... 

Specific surface area (SSA), total pore volume and average pore diameter were measured by N2 adsorption-desorption isotherms at 77K using Micromeritics ASAP 2020. The pore size was calculated on the adsorption branch of the isotherms using Barrett-Joyner-Helenda (BJH) method and the SSA was calculated using the Brunauer-Emmett-Teller (BET) method. 

Nitrogen adsorption was performed at -196 °C in a Micromeritics ASAP 2010 volumetric instrument. The samples were outgassed at 80 °C prior to the adsorption measurement until a 3.10 3 Torr static vacuum was reached. The surface area was calculated by the Brunauer-Emmett-Teller (BET) method. Micropore volume and external surface area were evaluated by the alpha-S method using a standard isotherm measured on Aerosil 200 fumed silica [8]. Powder X-ray diffraction (XRD) patterns of samples dried at 80 °C were collected at room temperature on a Broker AXS D-8 diffractometer with Cu Ka radiation. Thermogravimetric analysis was carried out in air flow with heating rate 10 °C min"1 up to 900 °C in a Netzsch TG 209 C thermal balance. SEM micrographs were recorded on a Hitachi S4500 microscope. 

The N2 adsorption-desorption isotherms for MSU-Ge-2 show a type-lV adsorption branch associated with a well-defined capillary condensation step at P/Po 0.13, characteristic of uniform mesopores (Fig. 4). The adsorption data indicate a very high Brunauer-Emmett-Teller (BET) surface area of 363 m /g and a pore volume of 0.23 cm /g. Given that the Ge mesostructure is much heavier than the corresponding silica, this surface area is actually equivalent to silica with a surface area of 1,316 m /g. 

EXAMPLE 9.5 Calculating the Adsorption Energy from the Brunauer-Emmett-Teller Isotherm. The BET analysis uses p/p0 rather than p as a variable just as we used this pressure ratio to compare Langmuir adsorption at different temperatures in Example 9.3. What corrections, if any, are needed in the apparent adsorption energy of about 14 kJ mole-1 as calculated in Example 9.3 ... 

The Brunauer-Emmett-Teller (or BET) adsorption isotherm applies only to the physisorption of vapours but it is important to heterogeneous catalysis because of its use for the determination of the surface areas of solids. The isotherm is given by the following equation,... 

The most common method used for the determination of surface area and pore size distribution is physical gas adsorption (also see 1.4.1). Nitrogen, krypton, and argon are some of the typically used adsorptives. The amount of gas adsorbed is generally determined by a volumetric technique. A gravimetric technique may be used if changes in the mass of the adsorbent itself need to be measured at the same time. The nature of the adsorption process and the shape of the equilibrium adsorption isotherm depend on the nature of the solid and its internal structure. The Brunauer-Emmett-Teller (BET) method is generally used for the analysis of the surface area based on monolayer coverage, and the Kelvin equation is used for calculation of pore size distribution. 

Because the Langmuir isotherm is not an adequate description of most systems, Equation (2.9) is not used much for area measurement. A number of other isotherm formulations utilize adsorption in surface area measurements, however (cf. Young Crowell, 1962, for example). The best known and most widely used is the BET (Brunauer, Emmett Teller, 1938) theory, a generalization of the Langmuir model to multilayer absorption. Assuming that for the second and succeeding molecular... 

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