Joan E. Shields

Surface Area Analysis from the Langmuir and BET Theories

The Langmuir [1] equation is more applicable to chemisorption (see chapter 12), where a chemisorbed monolayer is formed, but is also often applied to physisorption isotherms of type I. Although this type of isotherm is usually observed with microporous adsorbents, due to the high adsorption potential, a separation between monolayer adsorption and pore filling is not possible for many such adsorbents.

Pore Size and Surface Characteristics of Porous Solids by Mercury Porosimetry

The experimental method employed in mercury porosimetry is presented in detail in chapter 18. It involves filling an evacuated sample holder with mercury and then applying pressure to force the mercury into interparticle voids and intraparticle pores. Both applied pressure and intruded volume are recorded.

Interpretation of mercury porosimetry data

The experimental method employed in mercury porosimetry, discussed more extensively in Chapter 20, involves the evacuation of all gas from the volume containing the sample. Mercury is then transferred into the sample container while under vacuum. Finally, pressure is applied to force mercury into the interparticle voids and intraparticle pores. A means of monitoring both the applied pressure and the intruded volume are integral parts of all mercury porosimeters.

Chemisorption: Site Specific Gas Adsorption

When the interaction between a surface and an adsorbate is relatively weak, only physisorption takes place via dispersion and coulombic forces (see Chapter 2). However, surface atoms often possess electrons or electron pairs that are available for chemical bond formation. Resulting chemical adsorption or chemisorption has been defined by IUPAC [1] as “adsorption in which the forces involved are valence forces of the same kind as those operating in the formation of chemical compounds” and as “adsorption which results from chemical bond formation (strong interaction) between the adsorbent and the adsorbate in a monolayer on the surface” [2].

Langmuir and BET theories

The success of kinetic theories directed toward the measurements of surface areas depends upon their ability to predict the number of adsorbate molecules required to exactly cover the solid with a single molecular layer. Equally important is the cross-sectional area of each molecule or the effective area covered by each adsorbed molecule on the surface. The surface area then, is the product of the number of molecules in a completed monolayer and the effective cross-sectional area of an adsorbate molecule. The number of molecules required for the completion of a monolayer will be considered in this chapter and the adsorbate cross-sectional area will be discussed in Chapter 6.

Mercury Porosimetry: Non-wetting Liquid Penetration

The method of mercury porosimetry for the determination of the porous properties of solids is dependent on several variables. One of these is the wetting or contact angle between mercury and the surface of the solid.

Langmuir and BET theories (kinetic isotherms)

The success of kinetic theories directed toward the measurements of surface areas depends upon their ability to predict the number of adsorbate molecules required exactly to cover the solid with a single molecular layer. Equally important is the cross-sectional area of each molecule or the effective area covered by each adsorbed molecule on the surface. The surface area, then, is the product of the number of molecules in a completed monolayer and the effective cross-sectional area of an adsorbate molecule. The number of molecules required for the completion of a monolayer will be considered in this chapter and the adsorbate cross-sectional area will be discussed in Chapter 6.

Volumetric Chemisorption: Catalyst Characterization by Static Methods

The vacuum volumetric, or static, method is used to determine the monolayer capacity of a catalyst sample from which certain important characteristics such as active metal area, dispersion, crystallite size, etc., may be derived by the acquisition of adsorption isotherms.

Mercury Porosimetry: Intra and Inter-Particle Characterization

The forced intrusion of liquid mercury between particles and into pores is routinely employed to characterize a wide range of particulate and solid materials. Most materials can be analyzed so long as the sample can be accommodated in the instrument, which typically restricts the sample dimensions to no more than 2.5cm. Those materials that amalgamate with mercury (zinc and gold for example) cannot be analyzed unless extreme steps are taken to passivate the surface. The exact pore size range that can be measured depends predominantly on the instrument pressure range but also on the contact angle employed in the Washburn equation. The largest pore size that can be determined is limited by the lowest filling pressure attainable and the smallest pore size by the highest pressure achievable.

Hysteresis, entrapment, and contact angle

Cumulative volume curves generated by intruding mercury into porous samples are not followed as the pressure is lowered and mercury extrudes out of the pores. In all cases, the depressurization curve lies above the pressurization curve and the hysteresis loop does not close even when the pressure is returned to zero, indicating that some mercury is entrapped in the pores. Usually after the sample has been subjected to a first pressurization— depressurization cycle, no additional entrapment occurs during subsequent cycles. In some cases, however, a third or even fourth cycle is required before entrapment ceases.