Adsorption energies, differences

With the frozen density ansatz all terms in parenthesis in the last equation will be zero. The only contribution from F to the adsorption energy difference is therefore the non-local electrostatic energy,... 

In the present section we shall examine some contributions to sample adsorption energy and A" which are more conveniently related to the overall structure of the total sample molecule, rather than to the adsorption energies of individual groups within the molecule. In general it is not feasible to express these adsorption energy differences in terms of a simple change in the sample 5 value (i.e., in terms of values). Rather we reqtyre a term which is a complex function of sample structure, solvent, and adsorbent activity [as in Eq. (8-12)]. 

This equation expresses a linear relationship between the natural logarithm of the selectivity fiutor and the difference in adsoiptidn energy not only between different solutes and their interaction with a single adsorption site, but between similar solutes and different adsorption sites. This equation states that increased selectivity occurs under conditions where the adsorption energy difference ( - e f) decreases. Thus increased selectivity on heterogeneous stationary phases would theoretically be more prevalent on separations that typically would be more difficult for homogeneous stationary phases. 

Lateral interactions affect chemisorption on small clusters differently compared to surfaces. For CO at low-coverage adsorption, energy differences are relatively... 

The following derivation is modified from that of Fowler and Guggenheim [10,11]. The adsorbed molecules are considered to differ from gaseous ones in that their potential energy and local partition function (see Section XVI-4A) have been modified and that, instead of possessing normal translational motion, they are confined to localized sites without any interactions between adjacent molecules but with an adsorption energy Q. 

From these various examples, it is clear that the adsorption energy for a given kind of site can vary quite markedly from one crystal face of the adsorbent to another. For argon on solid xenon (Table 1.1), for example, the most favourable site has a o value of —1251 x 10" J on the (100) face but only -1072 on the (111) face. Such differences are in no way surprising, and they have been found also with ionic crystals. 

Physisorption occurs when, as a result of energy differences and/or electrical attractive forces (weak van der Waals forces), the adsorbate molecules become physically fastened to the adsorbent molecules. This type of adsorption is multilayered that is, each molecular layer forms on top of the previous layer with the number of layers being proportional to the contaminant concentration. More molecular layers form with higher concentrations of contaminant in solution. When a chemical compound is produced by the reaction between the adsorbed molecule and the adsorbent, chemisorption occurs. Unlike physisorption, this process is one molecule thick and irreversible... 

The main controlling parameters of that model are the size of homogeneous clusters M, and the difference between the adsorption energies AF= - V2. 

Fig. 5(a) contains the oxygen and hydrogen density profiles it demonstrates clearly the major differences between the water structure next to a metal surface and near a free or nonpolar surface (compare to Fig. 3). Due to the significant adsorption energy of water on transition metal surfaces (typically of the order of 20-50kJmoP see, e.g., [136]), strong density oscillations are observed next to the metal. Between three and four water layers have also been identified in most simulations near uncharged metal surfaces, depending on the model and on statistical accuracy. Beyond about...

The interfacial energy of the repulsive wall, for instance, should be completely independent of the adsorption energy e at the adsorbing wall one expects 7 to be a function of the bulk density only (and of temperature, of course, but we consider only k T = here). Since different choices of e in our geometry with finite thickness do lead to different pb, we get different results for for the various choices of e, albeit all data should be part... 

The surface of mercury is homogeneous and all sites (Hg atoms) are equivalent, whereas a solid metal will have a heterogeneous surface with sites that differ in adsorption energies. 

Let us consider a surface on which particles are adsorbed on sites with different activation energy of desorption, and the distribution of these energies over the surface is discrete so that ni0 particles are initially in a state with an activation energy of desorption Edt, n particles with an energy Ed/, etc. Such a model corresponds to a concept of adsorption on different crystal planes each of which is homogeneous, or to a concept of different adsorption states of the particles adsorbed on a single crystal (26, 88). 

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