Hydrocarbons, adsorbed alkenes, cyclic

The next section will deal briefly with experimental techniques many of these have been introduced already, but the use of vibrational spectroscopy and of sum-frequency generation call for some further description. Section 4.4.1 describes the principal types of adsorbed hydrocarbon structure that have been found with alkenes and alkynes (aromatic hydrocarbons and cyclic Ce species will be considered in Chapters 10 and 12 respectively) Section 4.4.2 discusses the conditions under which the several chemisorbed forms of alkenes make their appearance. In Section 4.5 we look at detailed structural studies of a few adsorbed molecules, and Section 4.6 deals somewhat briefly with interconversions and decompositions of adsorbed alkenes, and structures of species formed. Finally there are sections on theoretical approaches (4.7), on the chemisorption of alkanes (4.8), and carbonaceous deposits that are the ultimate product of the decomposition process (4.9). 

In this article (Part I) we have comprehensively reviewed the structural implications of the vibrational spectroscopic results from the adsorption of ethene and the higher alkenes on different metal surfaces. Alkenes were chosen for first review because the spectra of their adsorbed species have been investigated in most detail. It was to be expected that principles elucidated during their analysis would be applicable elsewhere. The emphasis has been on an exploration of the structures of the temperature-dependent chemisorbed species on different metal surfaces. Particular attention has been directed to the spectra obtained on finely divided (oxide-supported) metal catalysts as these have not been the subject of review for a long time. An opportunity has, however, also been taken to update an earlier review of the single-crystal results from adsorbed hydrocarbons by one of us (N.S.) (7 7). Similar reviews of the fewer spectra from other families of adsorbed hydrocarbons, i.e., the alkynes, the alkanes (acyclic and cyclic), and aromatic hydrocarbons, will be presented in Part II. 

One important difference in activation energies, however, is apparent for Pi, which has been associated with "soft" coke probably adjacent to metal sites, where the activation energy of the cyclohexene-coked catalyst is ca. 15 kJ mok higher than for spent and 1-hexene coked catalysts. This difference may be related to the dynamics of hydrocarbon adsorption. For linear alkenes only one end of the chain may be attached to the catalyst surface, while for small cyclic alkenes, which have a more compact structure, the entire molecule may be adsorbed onto or electronically affected by the catalyst surface. That is, adsorbed cyclohexene which may lie... 

To conclude this section, we should note what has not been discussed -and why. The emphasis has been on trying to understand how the nature of the surface metal atoms influences the type of hydrocarbon structure formed, and although there is a great deal of information available on the structures formed by higher and cyclic alkenes, alkadienes, alkynes and benzene, some of which will be touched on below, it is only with ethene that a sufficiently wide range of surfaces has been investigated to make comparisons between them possible. A further omission, to be remedied in Section 4.7, is any attempt to provide a theoretical basis for the observations any more profound than that outlined above. The reason for this delay is that further useful information comes from studying the detailed structures of certain adsorbed molecules (Section 4.5) and the manner of their thermal decomposition (Section 4.6). 

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