Brunauer—Emmett—Teller isotherms

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 ... 

In the analysis of isotherm data in the literature the so-called BET (Brunauer. Emmett. Teller) isotherm equation is frequently used ... 

We now cite the types of experimental data in the literature, by which an analysis of surface adsorption effects is carried out. One common experiment involves measuring adsorption isotherms. By weighing or by volumetric techniques one determines as a function of equilibrium gas pressure the amount of gas held on a given surface at a specified temperature. Usually this quantity varies sigmoidally with rising pressure P, as sketched in Fig. 5.2.1 for a variety of temperatures 7). By standard methods that rely on the Brunauer, Emmett, Teller isotherm equa-tion one can determine the point on the isotherms at which monolayer coverage of the surface is complete it is usually is located fairly close to the knee of the isotherm. From the cross sectional area of the adsorbate molecules and from the amount needed for monolayer coverage one may then ascertain more or less quantitatively the surface area of the adsorbent. As-... 

Multilayer adsorption and other complications are accounted for in the more elaborate Brunauer-Emmett-Teller isotherm (BET isotherm) [28], which involves the heats of adsorption of the first and subsequent layers and with which not only Type I behavior can be approximated. In its simplest form, for an unlimited number of adsorption layers and partial pressures well below saturation, the BET equation can be written... 

Brunauer-Emmett-Teller Isotherm (BET Isotherm) An adsorption isotherm equation that accounts for the possibility of multilayer adsorption and different enthalpy of adsorption between the first and subsequent layers. Five types of adsorption isotherm are usually distinguished. These are denoted by roman numerals and refer to different characteristic shapes. See Adsorption Isotherm. 

Multilayer Adsorption Adsorption in which the adsorption space contains more than a single layer of molecules therefore, not all adsorbed molecules will be in contact with the surface of the adsorbent. See also Brunauer-Emmett-Teller Isotherm, Monolayer Adsorption. 

---