Adsorption processes sites

The BET treatment is based on a kinetic model of the adsorption process put forward more than sixty years ago by Langmuir, in which the surface of the solid was regarded as an array of adsorption sites. A state of dynamic equilibrium was postulated in which the rate at which molecules arriving from the gas phrase and condensing on to bare sites is equal to the rate at which molecules evaporate from occupied sites. 

FIGURE 2.23 Schematic diagram showing the routes of possible removal of drug from the receptor compartment. Upon diffusion into the compartment, the drug may be removed by passive adsorption en route. This will cause a constant decrease in the steady-state concentration of the drag at the site of the receptor until the adsorption process is saturated. 

If the adsorption process is not saturable within the concentration range of the experiment, it becomes a sink claiming a portion of the drug added to the medium—the magnitude of which is dependent on the maximal capacity of the sink ([ 2]) and the affinity of the ligand for the site... 

As in a monolayer adsorption process, we consider that the rate of filling of sites by TCP molecules follows first-order kinetics. If No represents the total number of free sites per unit area at time t = 0, and N(t) is the number of sites available at time t, then dN(t)ldt = -kN(t), where k is the rate constant of the adsorption process. Therefore, N(t) decreases as No exp(-kt), and the number of sites occupied by TCP molecules at t becomes [No -N(t)], a quantity that determines directly the parameter (t) in Eq. (25). So Wo(t) can be written as... 

In the direct adsorption process, the ground state of the atom is in the three-dimensional space above the surface (we apologize for the somewhat counter-intuitive terminology of using ground state for a molecule above the surface). The transition state is the fully immobilized atom on the surface site, which is not allowed to move around... 

Clearly, the sticking coefficient for the direct adsorption process is small since a considerable amount of entropy is lost when the molecule is frozen in on an adsorption site. In fact, adsorption of most molecules occurs via a mobile precursor state. Nevertheless, direct adsorption does occur, but it is usually coupled with the activated dissociation of a highly stable molecule. An example is the dissociative adsorption of CH4, with sticking coefScients of the order 10 -10 . In this case the sticking coefficient not only contains the partition functions but also an exponential... 

In general, if an adsorbing molecule A occupies a single adsorption site, the adsorption process follows first-order kinetics ... 

Since in many cases it is not possible to unambiguously separate hydrogen from anion adsorption processes, it is convenient to write another equation similar to (7.7) for the charge density corresponding to anion adsorption on the free Pt sites ... 

Distribution of241 Am in a dialysis system containing sediment, phytoplankton, and detrital matter established that a substantial amount of americium accumulated in all three phases both in fresh and marine waters (NRC 1981). The adsorption process was not reversible and the longer the americium was adsorbed, the more difficult the chemical was to desorb. Appreciable amounts of americium have been shown to adsorb to bacterial cells such as those found in the Waste Isolation Pilot Plant in New Mexico (Francis et al. 1998). There is a potential that americium attached to biocolloids may facilitate its transport from the waste site. 

Data used for the design of adsorption processes are normally derived from experimental measurements. The capacity of an adsorbent to adsorb an adsorbate depends on the compound being adsorbed, the type and preparation of the adsorbate, inlet concentration, temperature and pressure. In addition, adsorption can be a competitive process in which different molecules can compete for the adsorption sites. For example, if a mixture of toluene and acetone vapor is being adsorbed from a gas stream onto activated carbon, then toluene will adsorbed preferentially, relative to acetone and will displace the acetone that has already been adsorbed. 

The FTIR data recorded upon N0/02 adsorption in the presence of C02 showed that bidentate carbonates were immediately formed along with nitrite species. By increasing the time of contact, carbonates were partially displaced, while nitrites evolved to nitrate species [52], Notably, the amounts of surface nitrites present at each contact time are lower in the presence of C02 than in its absence. This indicates that C02 competes for the surface oxygen sites able to give nitrites at the beginning of the adsorption process in fact, after several minutes of exposure to the N0/02/C02 mixture, the amount of nitrates stored was comparable to that obtained in the absence of C02. In conclusion, while in the presence of C02 the nitrite route is inhibited to some extent due to the competition between NO and C02 for the surface oxygen sites of the Ba phase, the nitrate route is only marginally affected by the C02 presence, if any. 

The degree of coverage, however, seems to influence the adsorbate composition at low methanol concentrations also. In particular, on smooth platinum, the dependence of surface composition on 6 is observed at concentrations as low as 5x 10-3 M (Fig. 2.7). In this case it could be possible that COH can be formed as long as adjacent sites are available. At high coverage (by all species involved in the adsorption process), the formation of COH should be geometerically prevented. 

For physical adsorption processes the third assumption is the poorest of these assumptions. For the case of chemical adsorption the worst of these assumptions is the second. There is a significant amount of experimental evidence that is contradictory to this assumption. Taylor (7) was the first to emphasize that adsorption sites may vary in energy. He noted that atoms at peaks on the surface and along crystal edges will be in high-energy states and will be the points at which adsorption first occurs. Other evidence for the lack of surface uniformity includes experimental data indicating that ... 

Poisoning is caused by chemisorption of compounds in the process stream these compounds block or modify active sites on the catalyst. The poison may cause changes in the surface morphology of the catalyst, either by surface reconstruction or surface relaxation, or may modify the bond between the metal catalyst and the support. The toxicity of a poison (P) depends upon the enthalpy of adsorption for the poison, and the free energy for the adsorption process, which controls the equilibrium constant for chemisorption of the poison (KP). The fraction of sites blocked by a reversibly adsorbed poison (0P) can be calculated using a Langmuir isotherm (equation 8.4-23a) ... 

Consider the adsorption of a species A with concentration ca in the bulk of the solution. The variation of the coverage 9 with ca, keeping all other variables fixed, is known as the adsorption isotherm. We regard the adsorption process as a reaction between the free sites on the electrode, whose number is proportional to (1 — 6), and the species A in the solution. Using absolute rate theory, we can write the rate of adsorption as ... 

When dissociation occurs, the mechanism of the adsorption process is, H2 + 2cr 2H cr, where cr is an active site on the surface. At adsorptive equilibrium the rates in terms of the fraction of occupied surface are r = k Cl-tf)2 = k2<32... 

In the most general case considered here, M is adsorbed on two kinds of sites, labelled V and 2 . Each adsorption process is assumed to be fast enough (when... 

Applications of local HSAB principle have been used for the determination of the softer regions in Si clusters by using Ga as probe atom [30a], or the site for H-atom adsorption on Si clusters. In the latter case, the isomer predicted by the Fukui function was found but it is not always the most stable one. The use of the reactivity indices is only valid when the adsorption process does not induce strong deformation of the cluster [30b]. 

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