Physisorption and the BET Procedure

For practical purposes all adsorptions can be classified as one of two types. It can involve merely the van der Waals interaction between the substrate and the catalyst, a process that is termed physical adsorption or physisorption. Alternately, it can involve the formation of catalyst-substrate bonds as discussed above. This is termed chemical adsorption or chemisorption. While the latter is the basis for the chemistry of catalysts, physisorption is the basis for the BET procedure which is commonly used to measure the surface area of solids.  

The actual adsorption curves obtained are not as simple as that depicted in Fig. 2.5 but, rather, fall into one of several general pattems, 0.ll one of which is depicted in Fig. 2.6. With these curves extrapolation to zero pressure is not unambiguous so monolayer coverage is frequently determined by applying the BET equation in the form shown in Eqn. 2.6,where Vgds is the volume of gas adsorbed at pressure P, is the saturation pressure, commonly 200-400 Torr, is the volume of gas adsorbed at monolayer coverage and C is the BET constant that is related to the enthalpy of adsorption. 

By plotting the left side of Eqn. 2.6 against P/P a straight line results with the relationships given in Eqns. 2.7-9. The BET surface area of the sample. 

By applying a vacuum to the system after adsorption is complete and measuring the rate of nitrogen desorption it is possible to obtain the pore volume of the material as well as the size distribution of the pores (under 50 nm) of the material.  

While these values are frequently reported with significant accuracy, what is usually needed is merely an evaluation of whether the material under investigation has a low surface area (less than 10 m /g), a moderate surface area (between 50 and 100 m /g), a high surface area (200-500 m /g) or a very high surface area (greater than 800 m /g). The pore distribution can be classed as predominantly microporous (small pore, less than 0.5 nm), mesoporous (moderate pore, 1.0 to 3.0 nm) or macroporous (large pore, greater than 5.0 nm). 2,13 

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