Chemisorption strong bond

Increasing catalyst potential and work function leads to a pronounced increase in total oxygen coverage (which approaches unity even at elevated temperatures) and causes the appearance of new chemisorption states. At least two such states are created on Pt/YSZ (Fig. 4.43) A strongly bonded one which, as discussed in Chapter 5, acts as a sacrificial promoter during catalytic oxidations, and a weakly bonded one which is highly reactive and causes the observed dramatic increase in catalytic rate. 

In case of the charged form of chemisorption a free lattice electron and chemisorbed particles get bound by exchange interaction resulting in localization of a free electron (or a hole) on the surface energy layer of adparticles which results in creation of a strong bond. Therefore, in case of adsorption of single valence atom the strong bond is formed by two electrons the valence electron of the atom and the free lattice electron. 

The ion-water interactions are very strong Coulomb forces. As the hydrated ion approaches the solution/metal interface, the ion could be adsorbed on the metal surface. This adsorption may be accompanied by a partial loss of coordination shell water molecules, or the ion could keep its coordination shell upon adsorption. The behavior will be determined by the competition between the ion-water interactions and the ion-metal interactions. In some cases, a partial eharge transfer between the ion and the metal results in a strong bond, and we term this process chemisorption, in contrast to physisorption, which is much weaker and does not result in substantial modification of the ion s electronic structure. In some cases, one of the coordination shell molecules may be an adsorbed water molecule. hi this case, the ion does not lose part of the coordination shell, but some reorganization of the coordination shell molecules may occur in order to satisfy the constraint imposed by the metal surface, especially when it is charged. 

This article is concerned with chemisorption, and our main problemis to discover how strong bonds might be formed between a solid and an adsorbate. This is a very difficult task. From the viewpoint of conventional valency theory, the solid is a giant molecule with free valence at its surface. This free valence is taken up by the adsorbate in forming the chemisorbed species, and the activity of the surface is thereby extinguished. This is, of course, an oversimplification. The surface may still have a residual activity, perhaps towards a different adsorbate, because the original free valence at the solid surface is partly transferred to the new surface composed of the chemisorbed species. 

In strong chemisorption, either a free electron or a free hole can participate in the bond hence, we distinguish two types of strong bonds which we shall call ... 

We see that the appearance of strong forms of chemisorption does not necessarily lead to the depletion of the electron or hole population of the crystal. On the contrary the concentration of the free carriers may be increased in chemisorption. The presence of an electron or a hole gas is therefore not a necessary condition for the formation of strong bonds in chemisorption. 

However, there are other possibilities. What if instead of an unsuccessful overlap of orbitals the overlap occurs in a suitable way, and instead of repulsion, attraction between the ion and the electrode atoms results In this case strong bonds may be formed between the ion and the electrode. These bonds are the result of donation or acceptance of electrons by the ion, and are responsible for the chemisorption of molecules. 

It has struck many workers in the field that the transition metals and near-transition metals are the best catalysts for many gas reactions (78), and it was suggested (79) that probably the covalent bonds between the chemisorbed atoms and the metal were formed by sharing of an electron of the atom with a d electron of the metal. It is, indeed, not improbable that d electrons of the metals play an important role in these chemisorption phenomena. It is known from their role in the bond strengths of complex chemical compounds that pairing of other electrons with d electrons leads to strong bonds. An incorporation of d elec-... 

The chemisorption studies of Parris and Klier (43) using the Cu/ZnO catalyst have been mentioned earlier. Carbon monoxide was irreversibly bonded at room temperature to the surface of the binary catalysts that were also active in methanol synthesis however, this irreversible adsorbate could be desorbed as CO, which indicates that it was not a surface carbonate but rather a strongly bonded carbonyl-type CO. Infrared studies of this chemi-sorbate are lacking and it would be very desirable to determine the structure of this surface species. 

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). 

The creation of two types of chemisorbed oxygen on Pt surfaces interfaced with YSZ and subject to NEMCA conditions is also manifest clearly by temperature-programmed-desorption (TPD) [26] as shown in Fig. 10. The strongly bonded backspillover oxygen species (peak desorption temperature Tp=750-780 K) displaces the normal chemisorption state of atomic oxygen obtained via gas phase adsorption (Tp=740 K) to a significantly more weakly bonded state (Tp=680 K). The pronounced rate enhancement in NEMCA studies of catalytic... 

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