Effect of Adsorption

FIGURE 2.10 Schematic diagrams explaining the nature of the results shown in 

One attempts to generalize the above effect by rewriting Equation 2.1 as 

In this connection a special class of systems where Yl is close to the critical sinface tension of the solid needs to be mentioned. It is foimd that in snch cases the liqnid wets the dry solid bnt will not wet the solid surface, which has the vapor of the liquid already adsorbed on it (Hare and Zisman, 1955). This phenomenon is called autophobidty. The contact angles so formed are usually very small. 

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The effects of adsorption and desorption on the performance of fluidized beds are discussed elsewhere. Adsorption of carbon disulfide... 

The kinetics of this reaction, which can also be regarded as an erosion reaction, shows die effects of adsorption of the reaction product in retarding the reaction rate. The path of this reaction involves the adsorption of an oxygen atom donated by a carbon dioxide molecule on die surface of the coke to leave a carbon monoxide molecule in the gas phase. 

J. E. Lane, T. H. Spurling. Monte Carlo simulation of the effects of adsorption on interparticle forces. Aust J Chem 55 231-239, 1980. 

Anderson and Brown then suggested that the effects observed by McAlevy (M3, M4) of oxygen on the ignition characteristics of solid fuels might be the result of exothermic heterogeneous reactions, since the experimental observations of McAlevy as well as those of Shannon and Anderson (S3), can be correlated by Eq. (15). Shannon has also extended the original treatment of Anderson and Brown to include the effects of adsorption and desorption on the predicted results. 

Modification techniques for activated carhon were used to increase the removal capacity by surface adsorption and to improve the selectivity to volatile organic compounds (VOCs). Modified activated carbons (MACs) were prepared by modifying the purified activated carbon with various acids or bases. The effects of adsorption capacity and modified contents on the textural properties of the MACs were investigated. Furthermore, VOC adsorption and desorption experiments were carried out to determine the relationship between the adsorption capacity and the chemical properties of the adsorbents. High adsorption capacity for the selected VOCs was obtained over lwt%-H3P04/AC (lwt%-PA/AC). As a result, MAC was found to be very effective for VOC removal by adsorption with the potential for repeated use through desorption by simple heat treatment. 

The same Chapter contains results of studies of effects of adsorption of atom particles as well as simplest free radicals on electric conductivity of semiconductor zinc oxide films. 

We outline experimental results and provide theoretical interpretation of effect of adsorption of molecular oxygen and alkyl radicals in condensed media (water, proton-donor and aproton solvents) having different values of dielectric constant on electric conductivity of sensors. We have established that above parameter substantially affects the reversible changes of electric conductivity of a sensor in above media which are rigorously dependent on concentration of dissolved oxygen. 

The role of electronic theory of chemisorption in developing ideas on effects of adsorption on electrical and physical properties of semiconductor adsorbents. 

The necessity of the use of electronic notions to resolve several diemical and physical problems stemming hrom the studies of heterogeneous processes was realized by Pisarjevsky already in early twenties [1]. Several non-trivial ideas concerning the effect of adsorption on electrophysical properties of semiconductor adsorbents were formulated in classical studies of Yoffe [2], Roginsky [3] and others. These theoretical ideas were further developed by Volkenshtein and his colleagues (see book [4] and the reference list therein) as well as in studies by Hauffe [5, 6] and some other authors [7, 8]. 

The theory of the space-charge region in semiconductors was developed by Shottky [87, 88], Mott [89], Davidov [90, 91], Brattain [92] and several other authors. The idea concerning the effect of adsorption on characteristics of SCR of the semiconductor adsorbent was proposed by Hauffe [5, 6]. This theory was developed further by Volkenshtein and his group [4, 93, 94] as well as by Mark [95, 96], Morrison [21] and other scientists. Let us briefly dwell on general features of the model. 

The effect of adsorption charging of the surface on the value of conductivity of surface-adjacent layers and semiconductor adsorbent work function... 

We shall note that there are numerous experimental papers dealing with studies of effect of adsorption on other electrophysical characteristics of oxide adsorbents such as thermal electromotive force [107], Hall effect [108 - 110], volt-ampere [58, 111] and frequency [112] characteristics. The availability of results of these studies makes one expect that in near future the adequate theoretical model describing adsorption-induced changes in electrophysical characteristics of semiconductor adsorbent will be developed. 

So far, we have focused our attention on adsorption of donor particles on semiconductor oxides. As for the effect of adsorption of acceptor particles on electrophysical characteristics, in concurrence with conclusions made none of adsorption phenomenon involving such characteristic acceptor particles as molecular and atom oxygen on -semiconductor, atoms of nitrogen and simplest alkyl and amine radicals brought about a non-monotonous change in characteristics of adsorbents, despite the fact that experiments had been conducted at various conditions. 

The effect of adsorption surface charging on electrophysical characteristics of polycrystalline semiconductor adsorbents... 

In sections 1.7- 1.9 we have examined effects of surface charging in semiconductor adsorbent on electrophysical characteristics of the adsorbent. Although we did not go into details with respect to the crystalline origin of adsorbent, the consideration of effect of adsorption on electric conductivity of surface-adjacent layer led to conclusion that we considered monocrystalline samples. 

Now it is easy to trace the effect of adsorption of acceptors on degree of barrier disordering in polycrystalline adsorbent. 

The effect of adsorption on concentration of the surface and volume interstitial defects in semiconductor adsorbent and resultant change in electric conductivity... 

The effect of adsorption on electric conductivity of sintered polycrystalline adsorbents... 

In case of the use of polycrystalline adsorbent subject to high temperature oxidation and characterized by almost stoichiometric content of the surface its electric conductivity can be linked with penetration of the current carriers through high intercrystalline barriers. The effect of adsorption on electric conductivity of adsorbents of such type is mainly manifested through the change of heights in intercrystalline barriers controlled by the value of the surface charge. 

As it has been shown in Section 1.11 a comparison of experimental data on effects of adsorption of various particles on electric conductivity of oxide adsorbents results in conclusion that the dominant role in this... 

Thus, we have considered in detail various theoretical models of effect of adsorption of molecular, atom and radical particles on electric conductivity of semiconductor adsorbents of various crystalline types. Special attention has been paid to sintered and partially reduced oxide adsorbents characterized by the bridge type of intercrystalline contacts with the dominant content of bridges of open type because of wide domain of application of this very type of adsorbents as sensitive elements used in our physical and chemical studies. 

Experimental studies of the effect of adsorption of active particles on the conductivity of senuconduc--tor sensors... 

In conclusion to this part it seems noteworthy that in contrast to the effect of adsorption of molecular particles on electrophysical properties of oxide semiconductors, the major peculiarity of this effect for such chemically active particles as the simplest free radicals or atoms of simple gases (H2, O2, N2, CI2, etc.) is that they are considerably more chemically active concerning the impurity centres [47]. The latter are responsible for dope conductivity of oxide semiconductors. As for the influence of electric fields on their adsorption due to adsorption-induced surface charge distribution, they are of minor importance which is proved by results of the experiments on assessing field effect on adsorp-... 

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