Adsorption mechanisms involved

Voltz et al. [117] postulated an adsorption mechanism involving the participation of atomic oxygen. On its basis they obtain the equation... 

It must be cautioned that isotherm features can never prove the adsorption mechanism involved. They can only point to reasonable mechanisms that must ultimately be confirmed by more direct methods such as molecular spectroscopy. 

Chlorpropham and propham were adsorbed in large amounts by charcoal 48, 180, 182), in small amounts by exchange resins 48, 51), nylon and cellulose triacetate 340), and montmorillonite clay 48, 51, 180), and in insignificant amounts by illite and kaolinite clays 180). Hydrogen bonding was suggested as the adsorption mechanism involved 180, 340). Scott and Weber 46) found that anion exchange resins... 

Since the thiocarbamate, carbothioate, and acetamide herbicides are nonionic and their adsorption by and volatilization from soils depends upon the temperature and moisture contents of the systems, adsorption mechanisms involved are probably physical in nature through dipole-dipole or ion—dipole interactions. 

This concept represents a new theory for the action of process corrosion inhibitors. In contrast to the old filming amine theory the new one is based on a specific adsorption mechanism involving an ion exchange step. It can correctly predict the decrease of protection efficiency with increasing pH. It also explains structural differences between inhibitors such as increased efficiency of propylenediamine derivatives over primary amines and decreased efficiency with decreasing molecular weight of the amine group compound. 

Spieker and Regalbuto [16] mention as one of the three adsorption mechanisms the complexation of the hexachloroplatinate ions with the dissolved oxide. The reaction will proceed where the acid solution contacts the alumina and, thus, immobilizes the platinum. The other two adsorption mechanisms involve chemical interactions between the metal complexes of the precursor and the surface of the solid support, and pure electrostatic interaction of charged complexes of the precursor with oppositely charged sites on the surface of the support. Results obtained by Roth and Reichard [17] demonstrate the immobilization by local neutralization of the hexachloroplatinum acid by reaction with the support. When an aqueous ammoniacal solution of platinum diamino dinitrite solution is impregnated, reaction with the surface of the support does not proceed, and a more uniform platinum profile results. 

It is at once evident that there is a remarkable degree of similarity between the shapes of L-type solute isotherms and Type I physisorption isotherms. However, this similarity is misleading since the adsorption mechanisms involved are likely to be quite different. We have seen already that Type I physisorption isotherms for gas-solid systems are normally associated with micropore filling. In contrast, the plateau of an L-type solute isotherm usually corresponds to monolayer completion. In this respect, solute adsorption appears to correspond more closely to the classical Langmuir mechanism. If this is indeed the case it would seem to be possible to calculate the surface area from nj by the application of a simple equation of the same form as Eq. (2). 

The physics and chemistry of these systems is complex, essentially because of the many possible adsorption mechanisms involved. It is also difficult to determine the internal structure of the adsorbed layers or what interactions might develop between them. There are a few excellent review papers on these topics (81-84). We shall only briefly describe here the main interaction mechanism between the surface of oxides and various macromolecules. 

In the case that the adsorption mechanism involves a pre-dissociation of O2 before the rate determining step we have... 

Wada and Ataka [1958] showed that both the cation and anion exchange capacities varied with both concentration and pH, using ammonium chloride as the leachant. From their experiments they concluded that there were two adsorption mechanisms involved in ion uptake ... 

Figure 16-9 depicts porous adsorbent particles in an adsorption bed with sufficient generality to illustrate the nature and location of individual transport and dispersion mechanisms. Each mechanism involves a different driving force and, in general, gives rise to a different form of mathematical result. 

Among all the low energy interactions, London dispersion forces are considered as the main contributors to the physical adsorption mechanism. They are ubiquitous and their range of interaction is in the order 2 molecular diameters. For this reason, this mechanism is always operative and effective only in the topmost surface layers of a material. It is this low level of adhesion energy combined with the viscoelastic properties of the silicone matrix that has been exploited in silicone release coatings and in silicone molds used to release 3-dimensional objects. However, most adhesive applications require much higher energies of adhesion and other mechanisms need to be involved. 

Mass Transfer Rale Consideralions - As discussed previously, the mass transfer mechanism involved in industrial adsorption processes is complex. Generally, basic physical data on the materials involved are insufficient for design. Experimental mass transfer rate data for the specific adsorbate-adsorbent system are usually required for good design. 

Fig. 8.4 Mechanisms involving embrittlement of the metal, (a) Crack-tip adsorption, (b) hydrogen adsorption, (c) decohesion by hydrogen influx to dilated lattice and (d) crack extension due to brittle hydride particle forming at crack tip...

The slope of the Tafel curve drj/d log / is only one of the criteria that are required to determine the mechanism of the h.e.r., since different mechanisms, involving different r.d.s. often have the same Tafel slope. Parameters that are diagnostic of mechanism are the transfer coefficient, the reaction order, the stoichiometric number, the hydrogen coverage, the exchange current density, the heat adsorption, etc. 

We have shown that the steam reforming of propane may be adequately described by LH mechanism involving different adsorption sites for steam and hydrocarbon. The associated model satisfied both statistical compliance and the BMV thermodynamic criterion. 

Xanthate compounds are widely used as collectors in flotation. Their function is to render the mineral surface hydrophobic and thus facilitate bubble attachment. The adsorption of xanthates onto sulfide minerals occurs via an electrochemical mechanism involving the reduction of oxygen and the anodic adsorption of xanthate. 

The adsorption action of activated carbon may be explained in terms of the surface tension (or energy per unit surface area) exhibited by the activated particles whose specific surface area is very large. The molecules on the surface of the particles are subjected to unbalanced forces due to unsatisfied bonds and this is responsible for the attachment of other molecules to the surface. The attractive forces are, however, relatively weak and short range, and are called Van der Waals forces, and the adsorption process under these conditions is termed as a physical adsorption (physisorption) process. In this case, the adsorbed molecules are readily desorbed from the surface. Adsorption resulting from chemical interaction with surface molecules is termed as chemisorption. In contrast to the physical process described for the adsorption on carbon, the chemisorption process is characterized by stronger forces and irreversibility. It may, however, be mentioned that many adsorption phenomena involve both physical and chemical processes. They are, therefore, not easily classified, and the general term, sorption, is used to designate the mechanism of the process. 

---