Boundary—Layer theory of chemisorption

A number of theoretical works have been devoted to the study of the hydrogen-deuterium exchange reaction. Hauffe (25) examined this reaction from the standpoint of the boundary layer theory of chemisorption. Dowden and co-workers (26) undertook a theoretical investigation of the hydrogen-deuterium exchange reaction from the viewpoint of the theory of crystalline fields. 

Any transfer of electrons giving rise to changes of semiconductivity during chemisorption must be controlled, inter alia, by the concentration of electrons or holes available in the semi-conductor. The boundary-layer theory of chemisorption 65) is built within the framework of this entirely physical model of the chemisorption act. The gas being adsorbed is represented solely as a donor or acceptor of electrons the adsorbent is represented as a conventional semiconductor with a given concentration of ionized donor or acceptor centers and whose ability to participate in chemisorption is otherwise uniquely determined by the height of the Fermi level. 

Hauffe has extended the boundary-layer theory of chemisorption to catalytic reactions and has shown the way in which the position of the Fermi level may be expected to influence reactions with well-defined ratedetermining steps. Wolkenstein s theory of catalysis on semiconductors,... 

The relationship of this type of space charge to that arising in the purely electronic boundary-layer theory of chemisorption is discussed elsewhere (8). 

In the boundary layer theory of chemisorption on semiconductors it is assumed that when a molecule of gas is adsorbed on a solid, charge transfer occurs resulting in the formation of a potential barrier at the surface. As adsorption proceeds, the potential barrier builds up, the chemical potential of the surface changes until at equilibrium the chemical potential at the surface of the solid is the same as the chemical potential of the adsorbate gas. 

The removal of an electron from an acceptor level or a hole from a donor level denotes, as we have seen, not the desorption of the chemisorbed particle but merely its transition from a state of strong to a state of weak bonding with the surface. The neglect of this weak form of chemisorption (i.e., electrically neutral form) which is characteristic of all papers on the boundary-layer theory of adsorption makes it quite impossible to depict the chemisorbed particle in terms of an energy level, i.e., to apply the energy band scheme depicted in Fig. 10 and used in these papers. ... 

Let us now consider another mechanism by which the imperfections affect the adsorptive and catalytic properties of the surface. This is based on their participation in the adsorption process as adsorption centers. The problem of chemisorption on an atomic imperfection has been treated quantum-mechanically by Bonch-Bruevich 98) it was discussed from the viewpoint of the boundary-layer theory by Hauffe 99) and has been investigated recently by Kogan and Sandomirsky 95). 

The similarity of the contact potential hypothesis and the theory of boundary layer chemisorption on semiconductors cannot easily be dismissed. The concepts also apply to organic materials and the interaction of oxygen with poly-N-vinylcarbazole, for example, has been well established. 

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