Multisite adsorption

Koretsky, E. M., Sverjensky, D. A., and Sahai, N. (1998). A model of surface site types on oxide and silicate minerals based on crystal chemistry— implications for site types and densities, multisite adsorption, surface infrared-spectroscopy, and dissolution kinetics. Amer. J. Sci. 298, 349-438. 

If a surface reaction involving multisite adsorption exhibits a maximum with respect to concentration, slow reactant transport through the surface boundary layer can yield up to three steady states. The existence of a maximum is necessary but not sufficient for having multiplicity. The latter depends on the electrode potential, which can alter the shape and the position of the maximum, and on the magnitude of the mass transfer coefficient relative to the surface rate constant (418). Thus, as the potential becomes more negative for a reduction, the multiplicity region can be reached and oscillations may develop between two stable steady states. Oscillations could also arise from other simultaneous reactions such as oxide formation... 

Peacock, C.L. and Sherman, D.M., Surface complexation model for multisite adsorption of copper(n) onto kaolinite, Geochim. Cosmochim. Acta, 69, 3733, 2005. 

In addition to multisite adsorption, many gases and vapors adsorbed by solids do not produce a typical monolayer-type adsorption isotherm (Fig. 9.9a), but rather produce an isotherm indicating multilayer adsorption (Fig. 9.9c). An equation that treats multilayer adsorption is the BET equation, named after developers Brunauer, Emmett, and Teller. Multilayer adsorption is characteristic of physical or van der Waals attraction. It often proceeds with no apparent limit, since multilayer adsorption merges directly into capillary condensation as the vapor pressure of the adsorbate approaches its saturation value. 

Geometric effects can be subdivided into so-called primary and secondary ensemble effects. According to the primary ensemble effect a change in surface reactivity occurs, if a molecule adsorbs with several atoms to the metal surface (see Fig.(2.3)). Molecules adsorbed in this way, when partially stripped of hydrogen atoms, will hydrogenolyse. If the surface is diluted with metal atoms that are catalytically much less active (e.g. Cu), the probability decreases that active metal atoms (e.g. Ni) keep active metal neighbors. As a result the probability for multisite adsorption is reduced... 

This example illustrates the difficulty encountered when multisite adsorption occurs without dissociation. If molecular A requires n adjacent surface sites for adsorption, then the elementary steps that describe adsorption/desorption equilibria are... 

Figure 3.3. Different strategies for ssDNA probe immobilization in genosensing devices based on GEC composites, biocomposites, and nanocomposites. (A] Dry or wet multisite adsorption on GEC (B) avidin-biotin linkage on Av-GEB (C] [strept]avidin-biotin linkage on magnetic beads captured on m-GEC [D] chemisorption on nanoAu-GEC. See also Color Insert.

The local adsorption isotherm equations of the form Langmuir, Volmer, Fowler-Guggenheim and Hill-de Boer have been popularly used in the literature and are shown in the following Table 6.3-1. The first column shows the local adsorption equation in the case of patchwise topography, and the second column shows the corresponding equations in the case of random topography. Other form of the local isotherm can also be used, such as the Nitta equation presented in Chapter 2 allowing for the multisite adsorption. 

Adsorption of the reactant (A) follows a classical multisite adsorption behavior (step 1) with Am denoting the adsorbed substrate and m the number of sites required for adsorption. Adsorption of the modifier is described by steps 2 and 3 where Mp denotes the parallel adsorption mode of the (—)-cinchonidine involved in the enantiodifferentiation with the adsorbed reactant Am, while Mq denotes the tilted adsorption mode of the (—)-cinchonidine, which appears on the catalyst surface as a spectator. 

A more rigorous way of treating multisite adsorption is to consider that the reacting molecules are not able to completely cover the catalyst surface, and the competition between them is only for a fraction of the total surface sites. Fig. 7.20 displays adsorption of FAMEs (fatty acid methyl esters) in competition with much smaller H2, when a large organic molecule adsorbs on a catalytic sire ((g)), additionally covering a certain number of closely adjacent (gi-sites. The mass balance for sites should incorporate the coverage factor, which is lower than unity. 

An analytical expression for Eq. (7.239) can be obtained when the multisite adsorption is not taken into account or, for example, in case negligible adsorption of the modifier leading to the first-order dependence on it. In such case, Eq. (7.239) is simplified to... 

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