Surface desorption coefficient

It is seen, from equation (5), that a graph relating the reciprocal of the corrected retention volume to the concentration of the moderator can provide values for the adsorption/desorption coefficient and the surface area of the stationary phase. Scott and Simpson [1] used this technique to measure the surface area of a reversed phase and the curves relating the reciprocal of the corrected retention volume to moderator concentration are those shown in Figure 2. 

Equation 17.59 has been confirmed experimentally, suggesting that molecules move over a surface by hopping to adjacent adsorption sites. It may be assumed that this process involves a lower energy of activation than that required for complete desorption. The molecule will continue to hop until it finds a vacant adsorption site, thus explaining the increase of surface diffusion coefficient with coverage. 

For this estimate, values for the surface diffusion coefficient (D) and the surface exchange coefficient (i) in eq 2 were obtained by linearizing Mitterdorfer s rate expressions for surface transport and adsorption/desorption (ref 84) and re-expressing in terms of the driving forces in eq 2. 

Proportionality of and t Is often (but not always) an indication of a diffusion-controlled process, but such a proportionality does not have to extend over the entire time domain considered. It may happen that diffusion control is realized but that the computed D, is lower than the corresponding value in the gas phase. One possible explanation for this may be that the supply is followed by a slower surface diffusion process, which Is rate-determining. Surface diffusion coefficients D° tend to be lower than the corresponding bulk values. Such diffusion has been briefly discussed In sec. I.6.5g, under (1). When surface diffusion Is zero, the adsorbate is localized. In that case equilibration between covered and empty parts of the surface can only take place by desorption and readsorption. For D° 0 the adsorbate is mobile it then resembles a two-dimensional gas and we have already given the partition functions for one adsorbed mobile atom in sec. I.3.5d. In sec. 1.5d we shall briefly discuss the transition between localized and mobile adsorption. 

Kinetic coefficients. The kinetic adsorption and desorption coefficients can be estimated [82, 219, 412] if the form of the potential (Z) of interaction between a particle (a surfactant molecule) and the solution surface is known here Z is the coordinate measured from the surface into the bulk of liquid. If the function (Z) has the form of a potential barrier with a potential well, then the saddle-point method [261] implies... 

If absorption measurements are made on samples, which initially contain no water, by immersing them in salt solutions of different concentrations, the effective absorption diffusion coefficient can be calculated. These measurements are followed by desorption experiments in which the samples are dried by blowing air across both major surfaces and the effective desorption coefficient is calculated. The diffusion coefficients obtained from such absorption and desorption measurements are shown in Table II. It is evident that the ratio of the diffusion coefficient obtained from desorption to that from absorption measurements increases as the concentration of water in the rubber, C, Increases. The diffusion coefficient obtained from a desorption experiment can only be expected to be greater than that from an absorption experiment if D decreases with increasing liquid concentration (3). 

The only gas chromatographic method used for the measurement of diffusion coefficients of gases on solid surfaces is the RF-GC technique validating a recent mathematical analysis, also permitting the estimation of adsorption and desorption rate constants, local adsorbed concentrations, local isotherms, local monolayer capacities, and energy distribution functions." The RF-GC technique has been successfully applied for the time-resolved determination of surface diffusion coefficients for physically adsorbed or chemisorbed species of O2, CO, and CO2 on heterogeneous surfaces of Pt/Rh catalysts supported on Si02." All calculations for the... 

Equation (3.58) quantifies the processes of absorption and desorption at the fibre surface with and the absorption and desorption coefficients, and a maximum saturation value of the fibre. This equation leads to the Langmuir absorption isotherm in the steady state. 

From the results of this kinetic study and from the values of the adsorption coefficients listed in Table IX, it can be judged that both reactions of crotonaldehyde as well as the reaction of butyraldehyde proceed on identical sites of the catalytic surface. The hydrogenation of crotyl alcohol and its isomerization, which follow different kinetics, most likely proceed on other sites of the surface. From the form of the integral experimental dependences in Fig. 9 it may be assumed, for similar reasons as in the hy-drodemethylation of xylenes (p. 31) or in the hydrogenation of phenol, that the adsorption or desorption of the reaction components are most likely faster processes than surface reactions. 

A and E refer to the desorption, dissociation, decomposition or other surface reactions by which the reactant or reactants represented by M are converted into products. If [M] is constant within the temperature interval studied, then the values of A and E measured refer to this process. Alternatively, if the effective magnitude of [M] varies with temperature, the apparent Arrhenius parameters do not specifically refer to the product evolution step. This is demonstrated quantitatively by the following example [36]. When E = 100 kJmole-1 andA [M] = 3.2 X 1030 molecules sec-1, then rate coefficients at 400 and 500 K are 2.4 X 1017 and 1.0 X 1020 molecules sec-1, respectively. If, however, E is again 100 kJ mole-1 and A [M] varies between 3.2 X 1030 molecules sec-1 at 500 K and z X 3.2 X 1030 molecules sec-1 at 400 K, the measured values of A and E vary significantly, as shown in Fig. 7, when z ranges from 10-3 to 103. Thus, the measured value of E is not necessarily identifiable with the rate-limiting step if a concentration of a participant is temperature-dependent. This... 

The reaction of Si02 with SiC [1229] approximately obeyed the zero-order rate equation with E = 548—405 kJ mole 1 between 1543 and 1703 K. The proposed mechanism involved volatilized SiO and CO and the rate-limiting step was identified as product desorption from the SiC surface. The interaction of U02 + SiC above 1650 K [1230] obeyed the contracting area rate equation [eqn. (7), n = 2] with E = 525 and 350 kJ mole 1 for the evolution of CO and SiO, respectively. Kinetic control is identified as gas phase diffusion from the reaction site but E values were largely determined by equilibrium thermodynamics rather than by diffusion coefficients. 

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