Matthias Thommes

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.

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].

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.

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.

Dynamic Chemisorption: Catalyst Characterization by Flow Techniques

Under conditions of dynamic flow, controlled heating rates can be used to acquire characteristic reaction rate curves that can be used to classify, or fingerprint, different catalysts.

Evaluation of Fractal Dimension by Gas Adsorption

The concepts of fractal geometry elaborated by Mandelbrot [1] can be applied successfully to the study of solid surfaces. Fractal objects are self-similar, i.e., they look similar at all levels of magnification. The geometric topography (roughness) of the surface structure of many solids can be characterized by the fractal dimension, D. In the case of a Euclidean surface D is 2, however for an irregular (real) surface D may vary between 2 and 3. The magnitude of D may depend on the degree of roughness of the surface and/or the porosity. There exist several experimental methods to determine the fractal dimension, e.g., small-angle X-ray (SAXS) and small-angle neutron scattering measurements (SANS), adsorption techniques and mercury porosimetry. All these techniques search for a simple scaling power law of the type: Amount of surface property ∝ resolution of analysis D [2], where D is the fractal dimension of the surface for which the property is relevant. Amount of surface property can for instance be related to the intensity of scattered radiation, pore volume or monolayer capacity. The change in resolution is here achieved by changing the scattering angle, pore radius or the size of the adsorbate.

Physical Adsorption Measurement: Preliminaries

The adsorbed amount as a function of pressure can be obtained by volumetric (manometric) and gravimetric methods, carrier gas and calorimetric techniques, nuclear resonance as well as by a combination of calorimetric and impedance spectroscopic measurements (for an overview see refs [1–3]). However, the most frequently used methods are the volumetric (manometric) and the gravimetric methods. The gravimetric method is based on a sensitive microbalance and a pressure gauge. The adsorbed amount can be measured directly, but a pressure dependent buoyancy correction is necessary. The gravimetric method is convenient to use for the study of adsorption not too far from room temperature. The adsorbent is not in direct contact with the thermostat and it is therefore more difficult to control and measure the exact temperature of the adsorbent at both high and cryogenic temperatures. Therefore, the volumetric method is recommended to measure the adsorption of nitrogen, argon and krypton at the temperatures of liquid nitrogen (77.35 K) and argon (87.27 K) [4].

Vacuum Volumetric Measurement (Manometry)

Many types of static volumetric vacuum adsorption apparatus have been developed [e.g., 1–7] and no doubt every laboratory where serious adsorption measurements are made has equipment with certain unique features. The number of variations is limited only by the need and ingenuity of the users. However, all volumetric adsorption systems have certain essential features, including a vacuum pump, one or more gas supplies, a sample container, a calibrated manometer, and a coolant. Fig.14.1a describes a historical set-up using an Hg-volume manometer instead of a pressure transducer; the volumes Va, Vb, and Vc correspond to the calibrated reference volume in Fig. 14.1b, which refers to a simplified, modern static volumetric sorption apparatus.