Adsorption at the Gas/Solid Interface

BY K. S. W. SING

1 Introduction

Numerous studies of gas/solid interfacial systems were reported in the years 1970 and 1971, with much attention given to the physisorption of gases on carbons, oxides, clays, and zeolites and to chemisorption on metal and oxide catalysts. The main objective of much of this work has been to elucidate the surface structure and texture of porous and finely divided materials of industrial importance (e.g. catalysts, desiccants, and pigments) and various experimental techniques were developed for this purpose. Advances were also made in the application of the principles of statistical thermodynamics and quantum mechanics to both chemisorption and physisorption.

It would be impossible to deal adequately in this Report with all of these areas of research. By restricting the scope of this chapter to physisorption and its role in the characterization of solid surfaces, an introduction is provided to certain of the later chapters in this first volume and also to the more specialized aspects of the gas/solid interface to be surveyed in subsequent volumes. This approach should avoid any appreciable overlap of subject matter with the series of Specialist Periodical Reports on 'Surface and Defect Properties of Solids', which deals inter alia with various aspects of chemisorption and catalysis.

Physisorption occurs whenever a gas (the adsorptive) is brought into contact with an evacuated solid (the adsorbent). The phenomenon is thus a general one and is dependent on those intermolecular attractive and repulsive forces which are responsible for the condensation of vapours and the deviations from ideality of real gases. In physisorption (as distinct from chemisorption) there is no electron exchange between the adsorbed species (the adsorbate) and the adsorbent.

In the least complicated cases (the adsorption of a non-polar molecule on a homopolar surface), dispersion forces provide the main attraction between the adsorbate molecule and the assembly of force centres in the adsorbent. In other cases (the adsorption of a polar molecule on a heteropolar surface), various types of specific adsorbent-adsorbate interactions may contribute to the adsorption energy. The importance of specificity within the context of physisorption has only been fully appreciated in recent years and increasing attention is being given to its assessment. It seems appropriate therefore to discuss this aspect of physisorption in some detail.

Most adsorbents of high surface area are porous; to discuss the effect of porosity on physisorption it is helpful to classify pores into three groups on the basis of their effective width. The narrowest pores, of width not exceeding about 2.0 nm (20 Å) are called micropores; the widest pores, of width exceeding about 50 nm (0.05 µm or 500 Å) are called macropores. The pores of intermediate width, which were for a time termed intermediate or transitional pores, are now referred to as mesopores.

The whole of the accessible micropore volume may be pictured as adsorption space, since in pores of these dimensions the adsorption fields of opposite walls overlap. The micropore volume is thus filled by adsorbate molecules at fairly low relative pressure (i.e. within the region of the adsorption isotherm below the conventional 'monolayer capacity'). The filling of micropores may therefore be regarded as a primary physisorption process. On the other hand, capillary condensation in mesopores is always preceded by the formation of an adsorbed layer on the pore walls and is consequently a secondary process; this aspect of the problem is dealt with in Chapter 4.

Although a clear distinction may be drawn in principle between the processes occurring in micropores and mesopores, in practice it is difficult to specify this difference in terms of the characteristic features of a real system. There are two underlying problems: first, the adsorbent properties of a solid are determined both by its texture (area and porosity) and by the adsorbent-adsorbate and adsorbate-adsorbate interactions, which occur in a unique way in each adsorption system; secondly, the pores in a real solid are generally distributed over a wide range of both size and shape. In view of these difficulties, it is hardly surprising that the interpretation of isotherms, in terms of monolayer-multilayer adsorption and micropore filling, has been the subject of much debate over the past few years.

With the growing awareness of this complexity of physisorption has come the appreciation of the need for the determination of standard adsorption data on carefully prepared and well-characterized solids. Graphitized carbon blacks probably represent the best examples of adsorbents with uniform (homotattic) surfaces. Certain low-temperature isotherms (e.g. of Ar or Kr) on graphitized carbon blacks exhibit a stepwise character indicating well-defined layer-by-layer adsorption. Oxide surfaces are generally energetically heterogeneous; they are hydrated (hydroxylated) unless they have been heated to a high temperature. The effect of the surface dehydroxylation of silica on the physisorption of a number of vapours has been studied in great detail and the results have revealed the sensitivity of the adsorption heats and isotherms to the change in the character of the adsorbent-adsorbate interactions.

In this Report, current theories of physisorption are discussed in relation to the adsorption potential and to the adsorption isotherm, with a final section devoted to empirical methods of isotherm analysis. Emphasis is thus placed on the interpretation of adsorption data, rather than on any theoretical treatment per se. Reference is made to a wide range of gas-solid systems, but the surface properties of particular adsorbents are left for detailed consideration in a subsequent Report.

2 The Adsorption Potential

Adsorbate-Adsorbent Interactions on Non-porous Solids. — The importance of a priori calculations of the potential energy (φ)of an atom or molecule in the force field of a solid surface has long been recognized, and a considerable literature dealing with this subject has accumulated. The earlier work has been discussed in some detail by Young and Crowell, and more recent developments have been reviewed by Crowell, Steele, and Pierotti and Thomas.

The potential energy depends both on the distance, z, of the atom from the surface and on the location relative to the lattice of the solid. If the position of the foot of the normal from the atom to the surface is defined in terms of a vector τ in the plane of the surface, relative to some chosen point in the lattice,...