Hydrocarbons, adsorbed vibrational spectroscopy

Our article has concentrated on the relationships between vibrational spectra and the structures of hydrocarbon species adsorbed on metals. Some aspects of reactivities have also been covered, such as the thermal evolution of species on single-crystal surfaces under the UHV conditions necessary for VEELS, the most widely used technique. Wider aspects of reactivity include the important subject of catalytic activity. In catalytic studies, vibrational spectroscopy can also play an important role, but in smaller proportion than in the study of chemisorption. For this reason, it would not be appropriate for us to cover a large fraction of such work in this article. Furthermore, an excellent outline of this broader subject has recently been presented by Zaera (362). Instead, we present a summary account of the kinetic aspects of perhaps the most studied system, namely, the interreactions of ethene and related C2 species, and their hydrogenations, on platinum surfaces. We consider such reactions occurring on both single-crystal faces and metal oxide-supported finely divided catalysts. 

Knowledge of the chemisorbed species present on a surface provides an essential database for the investigation of catalytic and other reaction mechanisms in which they are involved. The elucidation of reaction mechanisms in turn requires studies of chemical kinetics. An increasing number of such kinetic studies are now being made which involve vibrational spectroscopy and hydrocarbon adsorbates, and these will also be reviewed in Part II. 

The first chapter of this volume, by Sheppard and de la Cruz, addresses the application of vibrational spectroscopy for the characterization of adsorbed hydrocarbons. This chapter is a successor to the 1958 Advances in Catalysis chapter about infrared spectra of adsorbed species, authored by the pioneers Eischens and Pliskin. Vibrational spectroscopy continues to provide some of the most incisive techniques available for determination of adsorbate structures. The present chapter is concerned with introductory principles and spectra of adsorbed alkenes a sequel is scheduled to appear in a subsequent volume of Advances in Catalysis. 

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). 

Abstract Vibrational sum-frequency spectroscopy in conjunction with interfacial pressure measurements are used to provide the first direct spectroscopic information about the structure of amphiphillic molecules adsorbed to the interface between two immiscible liquids by total internal reflection sum-frequency vibrational spectroscopy (TIR SFVS). The effect of the ionic head group on the conformational order of sodium dodecyl sulfate (SDS), sodium dodecylsulfonate (DDS), dodecyltrimethylammonium chloride (DTAC), and dodecylamine hydrochloride (DAC) adsorbed at the D2O/CCI4 interface has been examined. In addition, the effect of the length of the alkyl chain on the conformation and orientation of sodium hexylsulfonate (HS), sodium undecylsulfonate (UDS), and sodium dodecylsulfonate (DDS) is also presented. SF vibrational spectra indicate the presence of gauche conformations in the hydrocarbon chains of all the surfactants examined. An increase in the surface coverage results in the reduction of gauche defects in the hydrocarbon chains as determined from the intensity ratio of the methyl to methylene symmetric... 

The present-day literature contains many more spectra obtained from singlecrystal metal surfaces by VEELS than by RAIRS. However, the much higher resolution available from the more recently developed RAIRS technique and its capability of operating in the presence of a gas phase suggest that it will contribute increasingly important information in the hydrocarbon adsorption field. The three spectroscopic techniques discussed above are much the most important ones in this area, with transmission infrared spectroscopy as the predominantly useful one for work with finely divided samples. A few other vibrational spectroscopic techniques (25) have provided information on adsorbed hydrocarbons, but are at present of more limited or specialized applications. Their principal characteristics are more briefly summarized below. 

The use of INS spectroscopy to study adsorbed hydrocarbons is complementary to the use of optical techniques. With INS we can exploit the frequencies and intensities of vibrational modes more readily. It is... 

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