Chemisorption, anionic bonds

Both cationic adsorption and anionic adsorption belong to what is called ionic adsorption. Covalent adsorption is due to the localized covalent bonding, and metallic adsorption is due to the delocalized covalent bonding. The distinction among these three modes of chemisorption, however, is not so definite that the transition from the covalent through the metallic to the ionic adsorption may not be discontinuous, but rather continuous, in the same way as the transition of the three-dimensional solid compounds between the covalent, metallic, and ionic bonding. 

On the homopolar line between the A(P and the C(P regions, for example, the usual anionic chemisorption of the last section and the unusual cationic chemisorption of this section coalesce, and a homopolar bond is formed between the foreign atom and the lattice. One electron is lost from an impurity level for each foreign atom adsorbed, and this homopolar chemisorption is depletive. 

We may summarise this sub-section by re-iterating the fact that the CO adlayers formed on oxidation of methanol clearly closely resemble those formed by adsorption of CO from aqueous solution, but they also show differences. Not only are coverages usually lower, but there is at least some indications that in addition to the terminal and bridge-bonded COads species expected, there are probably other carbon-containing species on the surface that affect the way in which methanol is oxidised. In addition, whilst there is some evidence for the formation of islands of adsorbed CO on chemisorption of methanol, these islands appear to be smaller than those that form on adsorption of CO itself, and particularly in the case of sulphuric acid, appear to be strongly associated with adsorbed anions. 

Considering that -O-H may be a weaker complex than -O-M, formation of the latter would be relatively independent of pH. The latter complex would involve a strong bond (e.g., chemisorption). The same explanation applies to anion adsorption. For example, phosphate (P04) adsorption by oxides may take place in an outer- or inner-sphere mode of the monodentate or bidentate type (Fig. 4.7). 

It has been suggested that virtually all samples of MgO (and probably silicates as well) contain small amounts of HjO and CO2 (Freund, 1981). These impurities lead to formation of O species. The species CO4 " has also been postulated to occur on the surface of MgO exposed to COj. This species has recently been studied by ab initio 8CF Hartree-Fock-Roothaan MO calculations, both in its anion form and as the protonated cluster C(0H)4 (Gupta et al., 1981). Its calculated equilibrium bond distance is intermediate between those observed for B emd N in tetrahedral coordination with oxygen, and there seems to be no intrinsic source of instability. Thus, such a species seems stable. However, the calculations indicate a charge on C in 04" " very similar to that in COj, arguing for the formulation O 04 as first approximation to the electronic structure, rather than the C°(04 ) formulation suggested by Freund (1981). Perhaps such a species is better formulated as a chemisorption complex of a anion (a bent, 18-valence-electron system) and an O. . . 0 ... 

Cations and anions that adsorb by forming short directional bonds with the surface cannot be considered to be indifferent in character. These ions actually alter the surface charge by the very process of adsorption, and their bonding is classified as chemical adsorption (or chemisorption) in this text. Examples of chemisorption include copper and phosphate adsorption on iron oxides ... 

Now an anion. A", is introduced into the system. For simplicity, we will assume that only the monodentate surface bonding reaction (4.30) is important. The equilibrium constant for chemisorption of this anion is then... 

Consider now adsorbed molecular or ionic species that are, practically speaking, immobilized in the soil. Unless the soil is extremely acid, metals such as Cu, Cr, and Pb fall into this category. Also, certain anions such as phosphate bond so strongly on minerals that they too behave as immobile elements. The property that all of these ions have in common is that their sorption isotherms are not reversible within a time scale relevant to soil processes the adsorption (forward) isotherm is usually approximated closely by a Langmuir function of the strong-affinity type, but the desorptioii (backward) isotherm deviates markedly from the adsorption isotherm. This kind of nonequilibrium behavior, depicted in Figure 9.6, is sometimes referred to as hysteresis. Possible reasons for hysteresis in chemisorption are discussed in Chapter 4. 

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