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The Adsorption Coefficient

In most cases, however, b has to be estimated by a theoretical calculation, in order that k may be extracted from a measured velocity constant. This is important because only then is it possible to compare k with the kinetic or statistical theory and so to check numerically an assumed reaction mechanism. Accordingly, we shall present here some theoretical approaches to b. [Pg.256]

One could think about measuring b directly in a separate adsorption experiment. However, it has been found that the adsorption thus measured has no quantitative relation to the adsorption leading to a catalytic reaction. Not only the amounts adsorbed, but even the shape of the isotherm and the sensitivity to poisoning or surface alterations are totally different. The reason is that the adsorption experiment measures the total surface under favorable conditions (B.E.T. method), whereas the catalysis takes place on a quantitatively entirely different active  [Pg.256]

In a first order reaction k is equal to k b (see above), mental (apparent) activation energy (q ) is given by [Pg.257]

the temperature coefficient of k1 having been measured, for an absolute calculation of k only kt) and bo must be known, and not the heat of adsorption, X. At the moment we are concerned with b0. A simple statistical estimate can be based on the assumption that in the absence of adsorption energy the adsorption space is filled at a proportion given by the ratio of the molecular adsorption volume (liquid volume Fm) to the molecular gas volume [Pg.257]

The evaluation in pressure magnitudes (b is of the dimension of a reciprocal pressure) gives values for b0 of the order of 10-7 (mm. Hg.) 1, in agreement with the experience accrued from the studies of catalyzed reactions. [Pg.257]


The first terms in (99) and (100) say that adsorption can take place either on the remaining sites of the reconstructed surface or on those surface sites that are neither reconstructed nor occupied. The first term in (101) allows for reconstruction from the unreconstructed area, 1 — 9, but also says that this reconstruction may be hindered or helped if there is an adsorbate on the unreconstructed surface. A similar interpretation holds for the last term in (101) describing the lifting of the reconstruction. For the adsorption coefficients Wy, Ws, etc., one writes expressions analogous to (47). [Pg.476]

A further procedure will be described only for m-xylene, for which we obtained the following values of the constants fci = 173.7, fc2 = 84.2 mole hr-1 kg-1 atm-1 K — 20.6, Ko = 25.8 atm-1. The conclusions drawn from the study of consecutive hydrodemethylation were similar for all the three xylenes studied (100). The influencing of individual reactions by products and by the intermediate product was determined experimentally, by measuring their effect on the reaction of m-xylene and toluene. The adsorption coefficients, which express this effect, are listed in Table III. [Pg.29]

In contrast to consecutive reactions, with parallel competitive reactions it is possible to measure not only the initial rate of isolated reactions, but also the initial rate of reactions in a coupled system. This makes it possible to obtain not only the form of the rate equations and the values of the adsorption coefficients, but also the values of the rate constants in two independent ways. For this reason, the study of mutual influencing of the reactions of this type is centered on the analysis of initial rate data of the single and coupled reactions, rather than on the confrontation of data on single reactions with intergal curves, as is usual with consecutive reactions. [Pg.35]

The values of the rate constants and adsorption coefficients obtained by the study of isolated reactions agreed well with those obtained by the study of parallel reactions (Table V). The three values of the adsorption coefficient of each acid were obtained independently. In addition to one value from the study of isolated reactions, two additional values were determined by the study of the parallel system one from the kinetics of the consumption of the given acid by reaction (Vila) or (Vllb), and one from the kinetics of reaction (Vile). [Pg.36]

We have further attempted to suggest a procedure which would make use of the advantages of the method of competitive reactions, i.e. its simplicity and little time demand, and at the same time would yield separately the absolute values of rate constants and adsorption coefficients also for reactions with a more complicated kinetics. Using the values of relative reactivities S from the method of competitive reactions, the adsorption coefficients, for example, of the alcohols (Kb) in the reesterification reaction described by Eq. (26) can be evaluated from the relation... [Pg.41]

From the results of other authors should be mentioned the observation of a similar effect, e.g. in the oxidation of olefins on nickel oxide (118), where the retardation of the reaction of 1-butene by cis-2-butene was greater than the effect of 1-butene on the reaction of m-2-butene the ratio of the adsorption coefficients Kcia h/Kwas 1.45. In a study on hydrogenation over C03O4 it was reported (109) that the reactivities of ethylene and propylene were nearly the same (1.17 in favor of propylene), when measured separately, whereas the ratio of adsorption coefficients was 8.4 in favor of ethylene. This led in the competitive arrangement to preferential hydrogenation of ethylene. A similar phenomenon occurs in the catalytic reduction of nitric oxide and sulfur dioxide by carbon monoxide (120a). [Pg.43]

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. [Pg.45]

It is noteworthy that even a separate treatment of the initial data on branched reactions (1) and (2) (hydrogenation of crotonaldehyde to butyr-aldehyde and to crotyl alcohol) results in practically the same values of the adsorption coefficient of crotonaldehyde (17 and 19 atm-1)- This indicates that the adsorbed form of crotonaldehyde is the same in both reactions. From the kinetic viewpoint it means that the ratio of the initial rates of both branched reactions of crotonaldehyde is constant, as follows from Eq. (31) simplified for the initial rate, and that the selectivity of the formation of butyraldehyde and crotyl alcohol is therefore independent of the initial partial pressure of crotonaldehyde. This may be the consequence of a very similar chemical nature of both reaction branches. [Pg.46]

The values of the adsorption coefficient of hydrogen for both reactions were practically identical (1.9 and 2.1 atm-1). Here, the selectivity of the branched reactions depends on the partial pressure of methylcyclopentane. This difference may be accounted for by assuming that either the cleavage of the C—C bond of methylcyclopentane in the (3-position and in the 7-position with respect to the methyl group does not take place on the same sites of the surface of platinum (or on the sites of the same activity), or that the mechanism of hydrogenolysis is more complex than that ex-... [Pg.46]

A similar difference in the adsorption coefficients of the starting reactant of branched reactions was also found in the parallel dehydration and dehydrogenation of isopropyl alcohol on some oxide catalyst (123) here, of course, the chemical nature of both branches is clearly different. It is of interest, however, to note that for the series of catalysts with varying... [Pg.47]

In the cases under study the behavior of each of the compounds present could be expressed by a single value of the adsorption coefficient in all the reactions occurring on the given catalyst. This indicates that this coefficient has a more general meaning, since it is able to characterize a certain substance in different reacting systems [cf. (97) J. It is reasonable to... [Pg.48]

The relative reactivities obtained by the method of competitive reactions corresponded to the values of the separately obtained rate and adsorption constants. The reactivities obtained by the competitive method differ, of course, from the ratio of the rates of the separately studied single reactions this difference increases with the difference in the values of the adsorption coefficients of competing substances. [Pg.49]

An analogous law was established in 1803 by W. Henry for the solubilities of gases in water hence, this expression is called the Henry isotherm. The adsorption coefficient B (units dmVmol) depends on the heat of adsorption B = B° e,xp(q RT). The Henry isotherm is valid for low surface coverages (e.g., at 9 < 0.1). [Pg.158]

The adsorption coefficient or / is a function of the standard Gibbs adsorption energy alone when the molecules do not interact and the adsorption occurs without limitation, i.e. at very low electrode coverage. This dependence is expressed by the relationship... [Pg.238]

The order of the mobilities of alachlor, butylate, and metolachlor in columns of various soils was metolachlor > alachlor > butylate. This correlates directly with the water solubilities and inversely to the adsorption coefficients and octanol/water partition coefficients of these compounds. Diffusion of these compounds in soil thin-layers was as follows butylate > alachlor > metolachlor, which correlates directly with the vapor pressures of these compounds. Significant soil properties affecting diffusion appeared to be bulk density and temperature. Soil moisture is also probably important, but its effect on the diffusion of these compounds was not determined. [Pg.231]

The adsorption coefficients (K) were determined using the equation for the Freundlich adsorption isotherm ... [Pg.234]

Monkiedje et al. [10] investigated the fate of niclosamide in aquatic system both under laboratory and field conditions. The octanol/watcr partition coefficient (Kaw) of niclosamide was 5.880 x 10 4. Adsorption isotherm studies indicated that the Freundlich parameters (K, n) for niclosamide were 0.02 and 4.93, respectively, for powder activated carbon (PAC), and 9.85 x 10 5 and 2.81, respectively, for silt loam soil. The adsorption coefficient (Aoc) for the drug was 0.02 for PAC, and 4.34 x 10-3 for the same soil. Hydrolysis of niclosamide occurred in distilled water buffer at pH above 7. No photolysis of the drug was observed in water after exposure to long-wave UV light for 4 h. Similarly, neither chemically volatilized from water following 5 h of sample aeration. Under field conditions, niclosamide persisted in ponds for over 14 days. The half-life of niclosamide was 3.40 days. [Pg.70]

There may be problems from other adsorbing species in the house. Carbon-dioxide and water vapor have been found to have an adverse effect on the adsorption coefficient (Strong and Levins, 1978 Siegwarth et al., 1972). The likeliest place for indoor radon to accumulate in houses is in the basement or crawl space where a large surface area is in direct contact with the soil, and thus the most likely place to put an adsorption system is in these locations. However, these areas are also commonly used to store various household chemicals such as painting supplies, etc. These household items stored in basements can release contaminants that may be classified into 4 broad categories aromatics, paraffins,... [Pg.566]

Szabo, G., Prosser, S., Bulman, R. A. (1990) Determination of the adsorption coefficient (KoC) of some aromatics for soil by RP-HPLC on two immobilized humic acid phases. Chemosphere 21, 777-788. [Pg.57]

Hodson, J., Williams, N.A. (1988) The estimation of the adsorption coefficient (K( )C) for soils by high performance liquid chromatography. Chemosphere 17, 67-77. [Pg.609]

Szabo, G., Guczi, J., Bulman, R.A. (1995) Examination of silica-salicylic acid and silica-8-hydroxyquinoline HPLC stationary phases for estimation of the adsorption coefficient of soil for some aromatic hydrocarbons. Chemosphere 30, 1717-1727. [Pg.615]


See other pages where The Adsorption Coefficient is mentioned: [Pg.234]    [Pg.405]    [Pg.20]    [Pg.30]    [Pg.33]    [Pg.38]    [Pg.39]    [Pg.46]    [Pg.47]    [Pg.49]    [Pg.38]    [Pg.20]    [Pg.165]    [Pg.156]    [Pg.429]    [Pg.123]    [Pg.127]    [Pg.127]    [Pg.438]    [Pg.91]    [Pg.157]    [Pg.237]    [Pg.362]    [Pg.455]    [Pg.703]    [Pg.47]    [Pg.206]    [Pg.97]   


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