Anion chemisorption

At this point, we have reached the stage where we can describe the adatom-substrate system in terms of the ANG Hamiltonian (Muscat and Newns 1978, Grimley 1983). We consider the case of anionic chemisorption ( 1.2.2), where a j-spin electron in the substrate level e, below the Fermi level (FL) eF, hops over into the affinity level (A) of the adatom, whose j-spin electron resides in the lower ionization level (I), as in Fig. 4.1. Thus, the intra-atomic electron Coulomb repulsion energy on the adatom (a) is... 

Fig. 4.1. Anionic chemisorption energy-level diagram showing transfer of j-spin electron from substrate level ek to affinity level A on adatom, while experiencing Coulomb repulsion U from j-spin electron in ionization level I.

Certain regions where anionic chemisorption occurs on p-type semicon-... 

With the oxide semiconductors, anionic chemisorption would take place over the metal cations, and the interaction problem would be between the orbitals on the foreign atom and the cation band (the 3d band in CU2O, for example). The discussion in this section is relevant if this is the highest filled band. 

With the semiconducting oxides, we expect anionic chemisorption to occur over the lattice cations, and our simple molecular orbital theory will be adequate if the conduction band is associated mainly with the cation lattice. This is certainly the case with AI2O3, where there is direct evidence in the soft X-ray emission spectra that the highest filled band is the oxygen 2p band 16). 

Finally, we note that if the interaction problem is between the orbital on the foreign atom and the highest filled band of the solid, anionic chemisorption is found in all regions of the diagram in Fig. 7, provided only that the highest localized level falls below the impurity levels in the solid. 

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. 

As with metal cations, anion chemisorption occurs on soil minerals that possess surface hydroxyl groups. The most important minerals in this regard are noncrystalline aluminosilicates (allophanes) oxides and hydroxides of Fe, Al, and Mn and layer silicate clays (edge sites only). It is the H2O or valence-unsatisfied OH ligands bound to surface metal ions (usually Fe, Al, or Mn) that are the sites of chemisorption. In general terms, the surface reaction can be written... 

In the case of pseudocapacitance associated with anion chemisorption (so-called specific adsorption ), as referred to in Section 4.5.3.8, it can be fundamentally impossible to make a separation between the non-Faradaic, electrostatic double-layer capacitance and the parallel Faradaic pseudocapacitnce, so that an equivalent circuit such as that in Figure 4.5.37 would be inapplicable. This aspect of interfacial capacitance behavior was treated theoretically by Delahay [1966], as mentioned in Section 4.5.3.8 below. 

Pt surface, as studied by Pajkossy [1994], almost ideally capacitative behavior can, in fact, be observed this is obviously a critical result indicating that it is not inseparable coupling between solution resistance and capacitance at a roughened (Pt) electrode surface (Pajkossy [1994]) that is the origin of dispersion effects. This led (Pajkossy [1994]) to the conclusion that it is ion (anion) adsorption that plays a crucial role in capacitance dispersion, because of frequency-dependent adsorption pseudocapacitance associated with anion chemisorption and associated kinetics of that process (Pajkossy [1994], Pajkossy et al. [1996]). 

---