Surface Area and Porosity Determinations by Physisorption

Measurement, Classical Theories and Quantum Theory'

James B. Condon

• Editorial: Elsevier Science
• Año: 2006
• ISBN:
  9780080481265
• Categoría: Technology & Engineering
• Categoría: Chemical & Biochemical Engineering

Surface Area and Porosity Determinations by Physisorption is a practical guide for industry or academics to the measurement of surface area and pore size using the tool of physical adsorption. Starting with a brief description of what physical adsorption is and the raw data that is obtained. The instrumentation for measuring this isotherm is described in some details. Recommendations are presented as to what instrumentation would be most appropriate for a particular application. An appendix of current commercial instruments is included. The mathematics required for the simple analysis of the obtained isotherm is presented with step-wise instructions for the analysis of the more useful analysis methods. Subsequent chapters describe the analyses and the theories behind the analyses in more detail.* Includes over 150 figures and tables which illustrate the equipment and examples data acquired * Provides a practical guide for measuring and interpreting physical adsorption * Up-to-date aspects of the more subtle physical adsorption theories such as density functional theory and the quantum mechanical chi theory are presented

Chapter 1 An Overview of Physisorption

INTRODUCTION SCOPE AND TERMINOLOGY

The term “physical adsorption” or “physisorption” refers to the phenomenon of gas molecules adhering to a surface at a pressure less than the vapor pressure. The attractions between the molecules being adsorbed and the surface are relatively weak and definitely not covalent or ionic. In Table 1 definitions used in this book and in most of the literature on physisorption are given [1].

Table 1 Some definitions needed to comprehend the first part of this book

^aThe practical distinction between these depends upon conditions and especially the adsorbate. The above definition is based on nitrogen adsorbate and the IUPAC standard.

For most adsorption experiments the temperature at which the measurements are made is less than the triple point of the gas being used but above its freezing point. This being the case, one would normally expect that the adsorbate characteristics resemble the liquid phase rather than the solid phase of the adsorptive. This is the normal assumption used for most adsorption theories. The principle measurement performed as an adsorption experiment is the measurement of the adsorption isotherm. The adsorption isotherm is the measurement of amount adsorbed versus adsorptive pressure at constant temperature. This is the easiest measurement to make. Another type of measurement is calorimetry. One form of calorimetry measures the amount of heat evolved as the adsorptive is adsorbed. Another form measures the heat capacity of the adsorbate. There are various forms of calorimetry but the most accurate methods are very difficult to perform and only a few examples are available in the literature. Another form of calorimetry, which is easier to perform, is scanning calorimetry. This calorimetry form is a good tool to determine qualitative features of the adsorption and to yield a fair indication of the physical quantities.

GENERAL DESCRIPTION OF PHYSISORPTION

For purpose of this book distinctions will be made between physical adsorption for the liquid-like state and in the solid-like state. Figs. 1 and 2 illustrate the atomic scale difference between the two types of adsorption. For solid-like state of Fig. 1 the adsorbate molecules are located on definite sites in relation to the underlying atoms of the adsorbent. For example, they lie directly over one of the atoms or in between two or three atoms in a defined geometry. One could refer to this as an “in-register” adsorption or even “epitaxy”. Chemisorption, where the attraction between the adsorbate and adsorbent is a covalent or ionic bond, would be an example of such adsorption. Adsorption well below the triple point in temperature would also be expected to follow this pattern. Additional adsorption above the first layer, might also be “in register”.

Fig. 1 A model of adsorption of the “in-register” type, e.g. chemisorption, epitaxy.

Fig. 2 Models of two types of physisorption. A, gas-like; B, liquid-like;

, before encounter;

, after encounter; hatched is during encounter.

The other mode of adsorption, which is the subject of this book, is illustrated in Fig. 2, of which two theoretical sub-possibilities exists. For this adsorption, referred to as physisorption, in adsorbent provides an overall attraction for which no particular site has a strong enough attraction to localize the adsorbate. In other words, the adsorbate molecules are free to skate over the entire surface, at least for a fair distance, even though there might be bumpy spots. For this physisorption picture there can be further distinctions, one where the adsorbate is behaving as a gas and there is only adsorption on top of the adsorbate, or one where the adsorbate behaves like a liquid, where adsorbate molecules can roll over one another and an adsorptive molecule can adsorb upon an adsorbate molecule. Most adsorption isotherms are performed under conditions where the liquid-like condition is assumed to exist. Calculations of the gas-like state indicate that the amount that can be adsorbed in this fashion is very low for most practical experimental conditions. Nevertheless, one would expect some of this to exist even with the presence of the liquid-like adsorbate.

MEASURING THE SURFACE AREA BY PHYSISORPTION

There are two principal methods to measure the adsorption isotherm, volumetric and gravimetric. In both methods the adsorbent is held at a constant temperature, usually near or at the boil...