Adsorbate Sites

Still another type of adsorption system is that in which either a proton transfer occurs between the adsorbent site and the adsorbate or a Lewis acid-base type of reaction occurs. An important group of solids having acid sites is that of the various silica-aluminas, widely used as cracking catalysts. The sites center on surface aluminum ions but could be either proton donor (Brpnsted acid) or Lewis acid in type. The type of site can be distinguished by infrared spectroscopy, since an adsorbed base, such as ammonia or pyridine, should be either in the ammonium or pyridinium ion form or in coordinated form. The type of data obtainable is illustrated in Fig. XVIII-20, which shows a portion of the infrared spectrum of pyridine adsorbed on a Mo(IV)-Al203 catalyst. In the presence of some surface water both Lewis and Brpnsted types of adsorbed pyridine are seen, as marked in the figure. Thus the features at 1450 and 1620 cm are attributed to pyridine bound to Lewis acid sites, while those at 1540... 

Recently, in situ studies of catalytic surface chemical reactions at high pressures have been undertaken [46, 47]. These studies employed sum frequency generation (SFG) and STM in order to probe the surfaces as the reactions are occurring under conditions similar to those employed for industrial catalysis (SFG is a laser-based teclmique that is described in section A 1.7.5.5 and section BT22). These studies have shown that the highly stable adsorbate sites that are probed under vacuum conditions are not necessarily tlie same sites that are active in high-pressure catalysis. Instead, less stable sites that are only occupied at high pressures are often responsible for catalysis. Because the active... 

Displacement development is only really effective if the stationary phase is a solid and the solutes are adsorbed on its surface. The sample mixture is placed on the front of the distribution system, and the individual solutes compete for the immediately available adsorption sites. Initially, all the nearby adsorbent sites will be saturated with the most strongly held component. As the sample band moves through the system the next available adsorption sites will become saturated with the next most... 

Enantiomers were generated and tumbled at specific angles around each NH proton and the C=0 oxygen at the 2-, 3-, and 6-positions of the carbamoyl group of 23a and 23x, which are considered to be the most important adsorbing sites from chromatographic resolution and NMR studies,... 

In the case of Ns adsorbing sites, if P is the gas partial pressure held constant by some gas reservoir, and ka and kd are adsorption and desorption constants respectively, the following expression for adsorption and desorption rates holds ... 

This combined dependency of xD on pH and surface coverage has the net effect of decreasing the slope of the pH-fractional adsorption edge as the ratio of adsorbate to adsorbent sites increases, as in Figure 3a. 

With an S-type isotherm, the slope initially increases with adsorptive concentrations, but eventually decreases and becomes zero as vacant adsorbent sites are filled. This type of isotherm indicates that at low concentrations the surface has a low affinity for the adsorbate which increases at high concentrations. 

The foundation of equilibrium-selective adsorption is based on differences in the equilibrium selectivity of the various adsorbates with the adsorbent While all the adsorbates have access to the adsorbent sites, the specific adsorbate is selectively adsorbed based on differences in the adsorbate-adsorbent interaction. This in turn results in higher adsorbent selectivity for one component than the others. One important parameter that affects the equilibrium-selective adsorption mechanism is the interaction between the acidic sites of the zeolite and basic sites of the adsorbate. Specific physical properties of zeolites, such as framework structure, choice of exchanged metal cations, Si02/Al203 ratio and water content can be... 

A study of the effects of the adsorbent perturbation on the B.E.T. theory has been reported by Peticolas lOJf). The frequency of an adsorbent molecule, in an adsorbent site, is expected to increase by 10% when it interacts with an adsorbed molecule. Other consequences of the perturbation of the adsorbent have been discussed by Tykodi 106, 106) and Copeland (107). 

While the change in enthalpy tells us how strong the bonds are that have to be broken and formed din ing the adsorption process, the entropy of adsorption gives us other types of information, for example, how "mobile the ion is in its adsorbed site. Consider a nonlinear molecule composed of N atoms. It will have a total of 3 N-6 vibrational degrees of freedom.54 Consider now the case in which this molecule adsorbs on the surface without any possible movement—the molecule is immobilized [Fig. 6.95(a)]. Under this circumstance the molecule loses all the translational and rotational degrees of freedom that it had in solution. These degrees of freedom are transferred into vibrations in the adsoibed molecule. Each molecule adsoibed in this manner will have a total of 3 A vibrational degrees of freedom. 

The free energy of adsorption is formed by the enthalpy and the entropy of the adsorption process (Section 6.8.3). The enthalpy indicates the strength of the bonds that have to be broken and formed during the adsorption process [including those related to the different electrode sites (Fig. 6.94)]. Meanwhile, the entropy indicates how mobile the ions are in their adsorbed site (Fig. 6.95). 

Now consider the form of the BET adsorption isotherm written in Eq. 11.59. If multilayer adsorption were not possible, then Km would be zero. The adsorbed site fraction from Eq. 11.59 becomes... 

It has been stated that zeolites exhibit a distribution of adsorptive energies. This is due to the complex structure within the zeolitic micropores and the strong dependence of electrostatic energies upon structure geometry. A discrete number of types of adsorption sites can be considered the finite number is dependent upon electrostatic and steric considerations. The most strongly adsorbing sites correspond to locations near the cations (SII, Sill, etc.). After these positions are filled, the adsorbate molecules seek positions in the framework structure to minimize repulsive forces between them on the zeolite surface, and at the same time to maximize adsorptive forces with the framework. 

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