Physical adsorption surface heterogeneity

M. Jaroniec, R. Madey. Physical Adsorption on Heterogeneous Surfaces. Amsterdam Elsevier, 1988. 

Additional difficulties in formulating an adsorption theory for the liquid - solid interface result from a variety of interactions between components of a liquid mixture and from a complex structure of the adsorbent, which may possess different types of pores and strong surface heterogeneity. Our considerations will be limited to physical adsorption on heterogeneous solid surfaces of components of comparable molecular sizes from non-electrolytic (non ideal or ideal) miscible binary liquid mixtures. 

Jaroniec, M and Madey, R. (1988). Physical Adsorption on Heterogeneous Solid Surfaces. Elsevier, Amsterdam. 

Jaroniec, M. Madey, R. Physical Adsorption on Heterogeneous Solids Elsevier Amsterdam, 1988. Katsanos, N.A. Gavril, D. Kapolos, J. Karaiskakis, G. Surface energy of solid catalysts measured by inverse gas chromatography. J. Colloid Interf. Sci. 2004,270,455-461. Topalova, I. Katsanos, N.A. Kapolos, J. Vasilakos, Ch. Simple measurement of deposition velocities and wall reaction probabihties in denuder tubes. Atm. Environ. 1994,28, 1791-1802. 

All gases below their critical temperature tend to adsorb as a result of general van der Waals interactions with the solid surface. In this case of physical adsorption, as it is called, interest centers on the size and nature of adsorbent-adsorbate interactions and on those between adsorbate molecules. There is concern about the degree of heterogeneity of the surface and with the extent to which adsorbed molecules possess translational and internal degrees of freedom. 

Brunauer (see Refs. 136-138) defended these defects as deliberate approximations needed to obtain a practical two-constant equation. The assumption of a constant heat of adsorption in the first layer represents a balance between the effects of surface heterogeneity and of lateral interaction, and the assumption of a constant instead of a decreasing heat of adsorption for the succeeding layers balances the overestimate of the entropy of adsorption. These comments do help to explain why the model works as well as it does. However, since these approximations are inherent in the treatment, one can see why the BET model does not lend itself readily to any detailed insight into the real physical nature of multilayers. In summary, the BET equation will undoubtedly maintain its usefulness in surface area determinations, and it does provide some physical information about the nature of the adsorbed film, but only at the level of approximation inherent in the model. Mainly, the c value provides an estimate of the first layer heat of adsorption, averaged over the region of fit. 

For an oxidized surface, the value of y is 10" - 1.7-10 and it decreases with increasing the experimental temperature. In this case the activation energy of a change in yis 2.1 kcal/mole. From these data it can be inferred that the heterogeneous de-excitation of singlet oxygen proceeds in terms of the physical adsorption mechanism similar to that described for glass. 

Adsorption is the preferential concentration of a species at the interface between two phases. Adsorption on solid surfaces is a very complex process and one that is not well understood. The surfaces of most heterogeneous catalysts are not uniform. Variations in energy, crystal structure, and chemical composition will occur as one moves about on the catalyst surface. In spite of this it is generally possible to divide all adsorption phenomena involving solid surfaces into two main classes physical adsorption and chemical adsorption (or chemisorption). Physical adsorption arises from intermolecular forces... 

A major portion of this chapter is concerned with physical adsorption, particularly from a global thermodynamic point of view. This is followed by a molecular-scale examination of crystalline surfaces and a brief discussion of chemisorption and its relevance to heterogeneous catalysis. 

Solid surfaces are usually heterogeneous therefore, since adsorption at the more active sites is favoured, heats of both monolayer physical adsorption and chemisorption might, in this respect, be expected to become significantly less exothermic as the surface coverage increases, as, for example, shown at low pressures in Figures 5.12a and 5.12b. This, in turn would cause the initial slope of an adsorption isotherm to be steeper than that predicted according to the Langmuir equation or the BET equation. 

As we discussed in Sec. VI,1 physical adsorption on charcoal and on metal surfaces is caused by the polarization of the adsorbed molecules in the electronic field over the surface of the conducting adsorbent (Sec. V,7), together with the nonpolar van der Waals forces between the adsorbent and the adsorbed molecules (Sec. V,2). As mentioned in Sec. V,12, the magnitude of the polarization of the adsorbed molecules by the electronic field is not seriously influenced by so-called active spots or by surface heterogeneity. The contribution by the nonpolar van der Waals forces, however, is more influenced by a heterogeneous character of the surface of the adsorbent. As those forces cooperate and as the surface of a metallic... 

In Secs. VIII,2 and 3 we saw that two phenomena may cause a decrease of the heat of physical adsorption of gases on metals or on ionic adsorbents first the mutual repulsion of parallel oriented dipoles and, second, a heterogeneous character of the surface. 

The undeniable fact that the surface may show a dominating heterogeneity for physical adsorption, hence for van der Waals attraction forces or for electrostatic polarization by local fields of the surface (Secs. VIII,2 and 3), does not mean that they should be heterogeneous for chemisorption as well. As was stated in Sec. V,12 the forces between ions and metal surfaces and the covalent forces between chemisorbed atoms or molecules and metal surfaces are far less influenced by active places of the surface than are some of the forces leading to physical adsorption. It is especially the cracks and fissures of the surface, which may give it a pronounced heterogeneous character for physical adsorption, that do not influence the chemisorption bonds very much (274). 

It is, therefore, quite possible that a surface, heterogeneous in character for physical adsorption shows a homogeneous nature for chemisorption. 

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