Chemisorption general aspects

Some General Aspects of Chemisorption and Catalysis Takao Kwan... 

Although the resonant level model successfully explains a few general aspects of chemisorption, it has nevertheless many shortcomings. The model gives no information on the electronic structure of the chemisorption bond it does not tell where the electrons are. Such information is obtained from a more refined model, called the density functional method. We will not explain how it works but merely give the results for the adsorption of Cl and Li on jellium, reported by Lang and Williams [20]. 

This picture of chemisorbed atoms on jellium, although much too simple, illustrates a few important aspects of chemisorption. First, the electron levels of adsorbed atoms broaden due to the interaction with the s-electron band of the metal. This is generally the case in chemisorption. Second, the relative position of the broadened adsorbate levels with respect to the Fermi level of the substrate metal determines whether charge transfer between metal and adatom takes place and in which direction. 

A feature common to both ir complex mechanisms is the nature of the second reagent in the exchange reaction [Eqs. (11), (12a), (12b)], namely heavy water or deuterium gas. Water is generally preferred in exchange reactions as it does not produce hydrogenated by-products. The important aspect concerning water and deuterium gas is the rapid exchange between these compounds on transition metal catalysts, which has been explained by dissociative chemisorption. 

This article consists of a review of certain specific effects that occur during the process of physical adsorption, in which the writer has been especially interested. While it is hoped that all work directly relevant to these aspects of adsorption has been covered in a comprehensive manner, this review does not aim to cover all the specific effects that are known to occur on adsorption. Some surprise may be occasioned by the title, as the statement is sometimes found (I) that physical adsorption is essentially nonspecific in nature, particularly when nonpolar gases are used as adsorbates. It will be shown, however, that this is an oversimplification of the process and that specificity is almost as prevalent, although not as obvious, in physical adsorption as in chemisorption. A general review of physical adsorption has been given in this series by de Boer 2), and other specialized topics have been discussed by Kemball (S), HiU (4), and Halsey (S) they may be consulted for the many aspects of the subject not considered in this article. 

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