Rate constant adsorption

Mention was made in Section XVIII-2E of programmed desorption this technique gives specific information about both the adsorption and the desorption of specific molecular states, at least when applied to single-crystal surfaces. The kinetic theory involved is essentially that used in Section XVI-3A. It will be recalled that the adsorption rate was there taken to be simply the rate at which molecules from the gas phase would strike a site area times the fraction of unoccupied sites. If the adsorption is activated, the fraction of molecules hitting and sticking that can proceed to a chemisorbed state is given by exp(-E /RT). The adsorption rate constant of Eq. XVII-13 becomes... 

In general, a preparation of mixed monolayer can be realized by either a kinetic control or a thermodynamic control (Figure 1, left). Kinetic control is based on a suggestion that for an initial deposition step the desorption rate is ignorable in comparison with the adsorption rate. In this case, the concentration ratio of the adsorbed species A and B on the surface corresponds to the ratio of products of their adsorption rate constant ( a or b) and concentration (Ca or Cb) A aCa/A bC b. The validity of the initial assumption on low desorption rate means that the total surface coverage obtained under kinetic control is essentially lower than 100%. This non-complete coverage does not disturb most of optical applications of the... 

The dashed line in the complex in (4.21) and (4.22) indicates an outer-sphere (o.s.) surface complex, Kos stands for the outer-sphere complex formation constant and kads [M 1 s 1] refers to the intrinsic adsorption rate constant at zero surface charge (Wehrli et al., 1990). Kos can be calculated with the help of a relation from Gouy Chapman theory (Appendix Chapter 3). 

The term F2/CsRT is obtained from the constant capacitance model (Chapter 3.7). Fig. 4.6 gives a plot of the linear free energy relation between the rate constants for water exchange and the intrinsic adsorption rate constant, kads. 

Subscript (ads) denotes adsorption via a thiolate linkage, while (ps) stands for a physisorbed and/or adsorbed state via different interactions. However, large dimensions of the studied molecules and their amphiphilic nature make the surface reaction mechanism more complex than in case of cystine/cysteine. Interfacial microstructure plays an important role in the determination of the surface behavior of the adsorbed molecules. From the study on the charge-transfer kinetics, the transfer coefficient a was calculated as slightly less than 0.50, while the rate constant (based on Laviron s derivations [105]) was of the order of 10 s k The same authors [106] have shown earlier that the adsorption rate constant of porcine pancreatic phospholipase A2 at mercury via one of its disulfide groups is of the order of 10 s h... 

Here we concentrate our attention to the case when the adsorption rate constant k is infinitely large. 

The rate of H2Oz consumption and the OH production were directly related to total iron concentration. The concentrations of hydroxyl radical produced were controlled by the rate of reaction with dissolved constituents. Rate constants for adsorption (ka) and desorption (kd) of PCBs from particles were calculated by regression of data from 1.5 to 5 hr. Adsorption rate constants were estimated from Equation (6.130) assuming that the partitioning rate constants between 2 and 5 hr without OH could be used for calculation of equilibrium partition coefficients Ky)... 

Adsorption constants (K) were determined from slope/intercept relationships. It was observed that significantly lower K values resulted when the total number of adsorption sites was 4 x 10-4 mol/g TiOz therefore, the adsorption rate constants are observed to be dependent on the minimum cross-sectional area assigned per adsorbed solute molecule (Cunningham and Al-Sayyed, 1990). 

Aniline is a compound used in the synthesis of insecticides, chemical brighteners, and dyes and is a by-product of the petroleum, paper, and coal industries. The photocatalytic oxidation of aniline was studied by Sanchez et al. (1997). The reaction was found to follow Langmuir-Hinshelwood kinetics. The adsorption rate constant and the reaction rate constants were also reported. Higher yields were reported for acidic conditions and values near the pH at the point of zero charge (pHpZC) of Ti02. The rate of photocatalytic oxidation was also found to increase with the addition of small amounts of Fe. Hydroquinone is the main intermediate formed from the reaction. Photocatalytic reactions were carried out in a 130-cm3 cylindrical Pyrex cell. Medium-pressure mercury lamps provided UV light. Initial concentrations of 1.0 x 104 and 2 g/L were prepared. The pH of the solutions was adjusted and reactions were carried out for 15 min. Concentrations of aniline and byproducts formed were determined by HPLC. 

Ka Adsorption rate constant Sm Area per molecule of the absorbed... 

To look at the kinetics in heterogeneous systems, we consider the step of adsorbing a gaseous molecule A onto an active site s to form an adsorbed species As. The adsorption rate constant is ka. The process is reversible, with a desorption rate constant kd ... 

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