Adsorption sites adsorbate-solid complexes

Adsorption Sites and Adsorbate-solid Complexes Vibrational Spectroscopy, NMR and Dilfraction... 

Before entering the detailed discussion of physical and chemical adsorption in the next two chapters, it is worthwhile to consider briefly and in relatively general terms what type of information can be obtained about the chemical and structural state of the solid-adsorbate complex. The term complex is used to avoid the common practice of discussing adsorption as though it occurred on an inert surface. Three types of effects are actually involved (1) the effect of the adsorbent on the molecular structure of the adsorbate, (2) the effect of the adsorbate on the structure of the adsorbent, and (3) the character of the direct bond or local interaction between an adsorption site and the adsorbate. 

As mentioned in Sect. I, even the simplest electrosorption systems are extremely complicated. This complexity means that a comprehensive theoretical description that enables predictions for phenomena on macroscopic scales of time and space is still generally impossible with present-day methods and technology. (Note that MD simulations, such as those presented in Sect. II, are only possible up to times of a few himdred nanoseconds.) Therefore, it is necessary to use a variety of analytical and computational methods and to study various simplified models of the solid-hquid interface. One such class of simpHfied models are LG models, in which chemisorbed particles (solutes or solvents) can only be located at specific adsorption sites, commensurate with the substrate s crystal structure. This can often be a very good approximation, for instance, for halides on the (100) surface of Ag, for which it can be shown that the adsorbates spend the vast majority of their time near the fourfold hollow surface sites. A LG approximation to such a continuum model, appropriate for chemisorption of small molecules or ions, ° is defined by the discrete, effective grand-canonical Hamiltonian,... 

Fig. 3.1 (Kapteijn et al., 1999) shows the model commonly u.sed to pre.sent a reversible reaction (A B) taking place on the surface of a solid catalyst. Three elementary steps are distinguished, i.e. adsorption of A on an active site, reaction of this adsorbed complex to adsorbed complex B, and desorption of B from the active site.

Of special interest in liquid dispersions are the surface-active agents that tend to accumulate at air/ liquid, liquid/liquid, and/or solid/liquid interfaces. Surfactants can arrange themselves to form a coherent film surrounding the dispersed droplets (in emulsions) or suspended particles (in suspensions). This process is an oriented physical adsorption. Adsorption at the interface tends to increase with increasing thermodynamic activity of the surfactant in solution until a complete monolayer is formed at the interface or until the active sites are saturated with surfactant molecules. Also, a multilayer of adsorbed surfactant molecules may occur, resulting in more complex adsorption isotherms. 

The NMR observable most commonly exploited in studies of solid acidity is the chemical shift. While some NMR observables (e.g., dipolar couplings) lend themselves to a more or less direct quantitative evaluation, the chemical shift must be interpreted. Changes in the 13C or 15N isotropic shifts of adsorbates are observed upon complexation with Brpnsted sites, and the same is true of the H shift of the Brpnsted site, but one is hard pressed to interpret such changes quantitatively in terms of a detailed structure of the adsorption complex or even the extent of proton transfer. 

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