Adsorption at the Gas/Solid Interface

BY N. D. PARKYNS and K. S. W. SING

1 Introduction

The first survey of the literature on adsorption at the gas/solid interface, which appeared in Volume 1 of this series,1 was mainly concerned with the interpretation of the equilibrium data of physisorption (i.e. the adsorption energy and isotherm). In the present Report, which deals primarily with the literature of 1972 and the early part of 1973, emphasis is placed on the characterization of the gas/solid interface and the study of particular adsorption systems.

In recent years it has become increasingly clear that the distinctive shape of a physisorption isotherm is dependent not only on the texture of the adsorbent but also on the nature of the adsorbate-adsorbent and adsorbate-adsorbate interactions. In the region of low surface coverage, the isotherm character is directly related to the density and uniformity of the solid surface; if specific adsorbateadsorbent interactions are involved (e.g. hydrogen bonding between hydroxygroups), then the presence of certain functional groups on the surface becomes important. Considerable progress has been made in the characterization of oxide surfaces (particularly those of alumina, silica, and titania) by the use of i.r. spectroscopy. For example, the surface hydroxy-groups have been identified and the specific interactions with various polar molecules have been confirmed. The application of certain spectroscopic techniques (e.g. laser-Raman spectroscopy) is still at an exploratory stage, whereas n.m.r. spectrocopy is now firmly established for surface chemical studies.

Oxide adsorbents of high surface area are usually microporous or mesoporous (or both) and structurally ill-defined (amorphous or poorly crystalline). They readily undergo changes in texture and crystallinity as a result of sintering or lowtemperature ageing and therefore are not very suitable as adsorbents for fundamental physisorption studies. However, because of their industrial importance such materials have been studied in great detail. Various proposals have been made for the adoption of specially prepared oxides as nonorous reference adsorbents and tables of standard adsorption data (e.g. for argon and nitrogen adsorption) on these solids have been published.

The variation with coverage of the differential enthalpy of adsorption has revealed that the surface of a non-porous oxide is energetically heterogeneous. Of all the readily available high-area solids, graphitized carbon blacks probably provide the most uniform and stable type of surface. Moreover, the graphite basal plane, which forms each face of the polyhedron, interacts only non-specifically with a wide range of polar and non-polar adsorbate molecules. Furthermore, the overall adsorbate-adsorbent interaction energy may be reduced and the uniformity of the basal plane preserved if the graphitized carbon surface is coated by the preadsorption of non-volatile molecules. Such surface-modified graphitized carbon blacks, coated with polymers, have been used for gas chromatography, and those coated with pre-adsorbed xenon have been used for fundamental studies of the adsorption of nobleMgas atoms. These studies are extremely important in view of the difficulties encountered in attempts to improve the theoretical basis for the calculation of the intermolecular forces involved in physisorption.

The series of Specialist Periodical Reports3 'Surface and Defect Properties of Solids' deals inter alia with the spectroscopic properties of surfaces and adsorbed species and the identification of chemisorption sites. Some overlap with the subject matter of the present Report is therefore unavoidable, but the approach is quite different in the two cases. Here we are concerned with the relationship between the characteristic features of physisorption and the properties of the gas/solid interface, rather than with chernisorption and the defect properties of solids. The sorption properties of clays and zeolites, which have received a great deal of attention in the literature, will be reserved for a future Report. Diffusion and rate studies are also excluded from the present review.

2 Experimental Methods

Determination of Adsorption Isotherms. — Volumetric Methods. Recent developments in instrumentation have allowed several refinements to be introduced in the techniques used for the determination of adsorption isotherms. Pierotti and his co-workers,4 for example, have described in some detail a high-precision volumetric apparatus, which incorporates a null-capacitance manometer and a precision cryostat. This apparatus was designed for the investigation of the adsorption of Ne, Ar, Kr, and Xe at low coverage over the temperature range 65 — 300 K. Special precautions were taken in the calibration of the various parts of the dead-space volume, giving a precision in the adsorption measurement of 10 — 100 times greater than that usually obtained with a conventional volumetric apparatus.

The failure of the adsorbent to attain the same low temperature as the cryogenic bath has been investigated by Teichner and his co-workers. In the volumetric technique, good thermal contact exists between the adsorbent and the liquid nitrogen, but a slight difference in temperature (ca. 0.02°C) is found between the liquid nitrogen surface (the coldest region) and its bulk. This difference should be taken into account in accurate measurements.

In discussing the use of helium for dead-space determinations, Everett 6 has pointed out that the assumption usually made, that helium is neither adsorbed nor absorbed by the solid, is certainly not true for many microporous adsorbents, and that the only proof that the adsorption of helium is zero is that the apparent value of the adsorbent volume is independent of temperature (or if the data are sufficiently precise, varies with temperature in accordance with the coefficient of thermal expansion of the solid).

Krypton adsorption at 77 K is often used for the determination of low surface areas (e.g. of ceramics) and various modifications in the technique have been introduced to improve the accuracy and to assist in routine measurements. For this purpose, thermistor pressure gauges have been used, and a radiochemical method has been employed to measure the, β-activity of the adsorbate labelled with 85Kr.

Much attention has been given in recent years to the automation of the volumetric method for routine applications. Commercial equipment has been evaluated 11 and new designs have been introduced.12 A method of feeding results from a volumetric apparatus direct to a computer for determining BET...