Energetics of Adsorption: Single Crystal Calorimetry

Abstract

Adsorption energy is a fundamental thermodynamic quantity in the description of gas–surface interactions. In general, it depends not only on the ad-species and on the chemical nature of the substrate but also on the surface coverage and the density and nature of the surface defects. It can also be significantly affected by the presence of coadsorbates and by temperature when the latter determines a different arrangement of species on the substrate or even the formation of different moieties at the surface. On the one hand, the measure of the heat of adsorption by the isosteric method can be used only in selected cases, where adsorption occurs reversibly and a reliable control of the coverage over a large enough range of temperatures is experimentally accessible. On the other hand, temperature-programmed desorption, which until now has been the most widely used technique, is not applicable when adsorption occurs irreversibly, and care is needed to properly analyze the experimental data, especially when the heat of adsorption is strongly coverage dependent. Single-crystal calorimetry (SCAC) provides a means of overcoming most such limitations. Since it is impossible to cover all the results obtained by this technique in a single review, a rich bibliography is provided and only selected results (in the opinion of the authors) are summarized here. The first section addresses the results obtained by the infrared calorimeter of the Cambridge group, which mostly deals with the coverage dependence of the heat of adsorption of different gases on single-crystal (both low and high Miller index) surfaces. The second section deals with the results obtained by the pyroelectric calorimeter by the Washington group, which cover both the adsorption of large organic molecules and of nongaseous species. The impressive results obtained to date have drawn other research groups into the field, and new designs have appeared that enable, for example, the study of the adsorption of gas molecules at the surface of nanoparticles and at oxide surfaces. The last section of this chapter is devoted to such results.