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Über dieses Buch

The growth of interest in newly developed porous materials has prompted the writing of this book for those who have the need to make meaningful measurements without the benefit of years of experience. One might consider this new book as the 4th edition of "Powder Surface Area and Porosity" (Lowell & Shields), but for this new edition we set out to incorporate recent developments in the understanding of fluids in many types of porous materials, not just powders. Based on this, we felt that it would be prudent to change the title to "Characterization of Porous Solids and Powders: Surface Area, Porosity and Density". This book gives a unique overview of principles associated with the characterization of solids with regard to their surface area, pore size, pore volume and density. It covers methods based on gas adsorption (both physi­ and chemisorption), mercury porosimetry and pycnometry. Not only are the theoretical and experimental basics of these techniques presented in detail but also, in light of the tremendous progress made in recent years in materials science and nanotechnology, the most recent developments are described. In particular, the application of classical theories and methods for pore size analysis are contrasted with the most advanced microscopic theories based on statistical mechanics (e.g. Density Functional Theory and Molecular Simulation). The characterization of heterogeneous catalysts is more prominent than in earlier editions; the sections on mercury porosimetry and particularly chemisorption have been updated and greatly expanded.

Inhaltsverzeichnis

Frontmatter

Theoretical

1. Introduction

Abstract
There is a convenient mathematical idealization which asserts that a cube of edge length, ℓ cm, possesses a surface area of 6ℓ2 cm2 and that a sphere of radius r cm exhibits 4πr 2 cm2 of surface. In reality, however, mathematical, perfect or ideal geometric forms are unattainable since under microscopic examinations all real surfaces exhibit flaws. For example, if a ‘super microscope’ were available one would observe surface roughness due not only to voids, pores, steps, and other surface imperfections but also due to the atomic or molecular orbitals at the surface. These surface irregularities will always create a real surface area greater than the corresponding theoretical area.
S. Lowell, Joan E. Shields, Martin A. Thomas, Matthias Thommes

2. Gas Adsorption

Abstract
Gas adsorption is one of many experimental methods available for the surface and pore size characterization of porous materials. These include small angle x-ray and neutron scattering (SAXS and SANS), mercury porosimetry, electron microscopy (scanning and transmission), thermoporometry, NMR-methods, and others. Each method has a limited length scale of applicability for pore size analysis. An overview of different methods for pore size characterization and their application range was recently given by IUPAC [1]. Among these methods gas adsorption is the most popular one because it allows assessment of a wide range of pore sizes (from 0.35 nm up to > 100 nm), including the complete range of micro- and mesopores and even macropores. In addition, gas adsorption techniques are convenient to use and are not that cost intensive as compared to some of the other methods. A combination of mercury porosimetry and gas adsorption techniques allows even performing a pore size analysis over a range from ca. 0.35 nm up to ca. 400 μm.
S. Lowell, Joan E. Shields, Martin A. Thomas, Matthias Thommes

3. Adsorption Isotherms

Abstract
The shape of sorption isotherms of pure fluids on planar surfaces and porous materials depends on the interplay between the strength of fluid-wall and fluid-fluid interactions as well as the effects of confined pore space on the state and thermodynamic stability of fluids confined to narrow pores. The International Union of Pure and Applied Chemistry [1] proposed to classify pores by their internal pore width (the pore width defined as the diameter in case of a cylindrical pore and as the distance between opposite walls in case of a slit pore), i.e., Micropore: pore of internal width less than 2 nm; Mesopore: pore of internal width between 2 and 50 nm; Macropore: pore of internal width greater than 50 nm.
S. Lowell, Joan E. Shields, Martin A. Thomas, Matthias Thommes

4. Adsorption Mechanism

Abstract
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 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 aspects of the adsorbate cross-sectional area will be discussed in chapter 5.
S. Lowell, Joan E. Shields, Martin A. Thomas, Matthias Thommes

5. Surface Area Analysis from the Langmuir and BET Theories

Abstract
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.
S. Lowell, Joan E. Shields, Martin A. Thomas, Matthias Thommes

6. Other Surface Area Methods

Abstract
Because of its simplicity and straightforward applicability, the BET theory is almost universally employed for surface area measurements. However, other methods [e.g., 1, 2] including methods based on small angle x-ray scattering and small neutron scattering [3] have been developed. Whereas the scattering methods cannot be used in routine operations (to date at least), immersion calorimetry and in particular permeability measurements are more frequently used in various applications. We will discuss below some aspects of the latter two methods together with the so-called Harkins Jura relative method, which is based on gas adsorption and is applied in a relative pressure range P/P 0, which is similar as in case of the BET theory (i.e., 0.05 – 0.3). However, no attempt is made to derive and discuss these alternate methods completely, but rather to present their essential features and to indicate how they may be used to calculate surface areas.
S. Lowell, Joan E. Shields, Martin A. Thomas, Matthias Thommes

7. Evaluation of Fractal Dimension by Gas Adsorption

Abstract
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 propertyresolution 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.
S. Lowell, Joan E. Shields, Martin A. Thomas, Matthias Thommes

8. Mesopore Analysis

Abstract
As discussed in chapter 4, the state and thermodynamic stability of pure fluids in mesopores depends on the interplay between the strength of fluid-wall and fluid-fluid interactions on the one hand, and the effects of confined pore space on the other hand. The most prominent phenomenon observed in mesopores is pore condensation, which represents a first-order phase transition from a gas-like state to a liquid-like state of the pore fluid occurring at a pressure P less than the corresponding saturation pressure P 0 of the bulk fluid, i.e., pore condensation occurs at a chemical potential μ less than the value μ0 at gas-liquid coexistence of the bulk fluid. The relative pressure where this condensation occurs depends on the pore diameter. The relationship between the pore size and the relative pressure where capillary condensation occurs can be described by the classical Kelvin equation. However, in the classical Kelvin equation the shift from bulk coexistence (μ0 — μ), is expressed in terms of macroscopic quantities, whereas a more comprehensive understanding of the underlying physics was achieved only recently by applying microscopic approaches based on the Density Functional Theory (DFT), and computer simulation studies (Monte Carlo and Molecular Dynamics). We have discussed these different approaches from a more theoretical point of view in chapter 4. Here, we will discuss their significance for the pore size analysis of mesoporous materials.
S. Lowell, Joan E. Shields, Martin A. Thomas, Matthias Thommes

9. Micropore Analysis

Abstract
As discussed in chapters 3 and 4, IUPAC [1] classifies pores as macropores for pore widths greater than 50 nm, mesopores for the pore range 2 to 50 nm and micropores for the pores in the range less than 2 nm.
S. Lowell, Joan E. Shields, Martin A. Thomas, Matthias Thommes

10. Mercury Porosimetry: Non-wetting Liquid Penetration

Abstract
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.
S. Lowell, Joan E. Shields, Martin A. Thomas, Matthias Thommes

11. Pore Size and Surface Characteristics of Porous Solids by Mercury Porosimetry

Abstract
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.
S. Lowell, Joan E. Shields, Martin A. Thomas, Matthias Thommes

12. Chemisorption: Site Specific Gas Adsorption

Abstract
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].
S. Lowell, Joan E. Shields, Martin A. Thomas, Matthias Thommes

Experimental

13. Physical Adsorption Measurement: Preliminaries

Abstract
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].
S. Lowell, Joan E. Shields, Martin A. Thomas, Matthias Thommes

14. Vacuum Volumetric Measurement (Manometry)

Abstract
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.
S. Lowell, Joan E. Shields, Martin A. Thomas, Matthias Thommes

15. Dynamic Flow Method

Abstract
In 1951, Loebenstein and Deitz [1] described an innovative gas adsorption technique that did not require the use of a vacuum. They adsorbed nitrogen out of a mixture of nitrogen and helium that was passed back and forth over the sample between two burettes by raising and lowering attached mercury columns. Equilibrium was established by noting no further change in pressure with additional cycles. The quantity adsorbed was determined by the pressure decrease at constant volume. Successive data points were acquired by adding more nitrogen at the system. The results obtained by Loebenstein and Deitz agreed with vacuum volumetric measurements on a large variety of samples with a wide range of surface areas. They were also able to establish that the quantities of nitrogen adsorbed were independent of the presence of helium.
S. Lowell, Joan E. Shields, Martin A. Thomas, Matthias Thommes

16. Volumetric Chemisorption: Catalyst Characterization by Static Methods

Abstract
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.
S. Lowell, Joan E. Shields, Martin A. Thomas, Matthias Thommes

17. Dynamic Chemisorption: Catalyst Characterization by Flow Techniques

Abstract
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.
S. Lowell, Joan E. Shields, Martin A. Thomas, Matthias Thommes

18. Mercury Porosimetry: Intra and Inter-Particle Characterization

Abstract
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.
S. Lowell, Joan E. Shields, Martin A. Thomas, Matthias Thommes

19. Density Measurement

Abstract
In many areas of powder technology, the need to measure the powder volume or density often arises. For example, powder bed porosities in permeametry, volume specific surface areas, sample cell void volume, and numerous other calculated volumes, all require accurately measured powder densities or specific volumes. It is appropriate, therefore, to introduce density measurements of solids.
S. Lowell, Joan E. Shields, Martin A. Thomas, Matthias Thommes

Backmatter

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