Chemisorption of olefins

Chemisorption [9] is an adsorptive interaction between a molecule and a surface in which electron density is shared by the adsorbed molecule and the surface. Electrochemical investigations of molecules that are chemisorbed to electrode surfaces have been conducted for at least three decades. Why is it, then, that the papers that are credited with starting the chemically modified electrode field (in 1973) describe chemisorption of olefinic substances on platinum electrodes [10,11] What is it about these papers that is different from the earlier work The answer to this question lies in the quote by Lane and Hubbard at the start of this chapter. Lane and Hubbard raised the possibility of using carefully designed adsorbate molecules to probe the fundamentals of electron-transfer reactions at electrode surfaces. It is this concept of specifically tailoring an electrode surface to achieve a particularly desired goal that distinguishes this work from the prior literature on chemisorption, and it is this concept that launched the chemically modified electrode field. 

A summary is made of the general classes of phenomena which can influence the reactivity of functional groups at heterogeneous interfaces, and potential pitfalls are pointed out in the reliance upon molecular analogy. Experimental results are reviewed pertaining to the thermodynamic and kinetic encounter frequencies of reagents on crosslinked polystyrenes and the chemisorption of olefins on oxide-free surfaces of elemental carbon. 

Extensive studies of hydrocarbon chemisorption have been made by Eischens and Pliskin (1). In a series of studies on olefins and paraffins chemisorbed on silica-supported nickel they were able to show that both associative and dissociative adsorption could occur, depending on catalyst pretreatment. Associative chemisorption of olefins is observed when hydrogen is left on the nickel surface dissociative absorption, when the hydrogen has been pumped off at an elevated temperature before chemisorption. The associative mechanism is deduced from the fact that the only absorption bands found when ethylene is added to a hydrogen-covered surface are in the C-H stretching region characteristic of saturated hydrocarbons, and that a C-H deformation band at 1447 cm-1 characteristic of two hydrogens on a carbon is also observed. 

The chemisorption of olefins on an aluminosilicate catalyst is also believed to proceed by a mechanism similar to that shown in Figure 5-33. As shown in Equation 5-69, the olefin couples with an acid/base pair, that is, a bridging hydroxyl group and a lattice oxygen center on the surface, probably as the result of a direct geometrical correspondence. 

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