Single-crystal surfaces ethene hydrogenation

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. 

The decomposition of ethene under vacuum has been proven to occur over a family of single crystal surfaces, Pt, Rh, Pd, Pm as well as supported Pt, Prf and M particles. For Pt(l 11), but also for other rf-metals, in the absence of hydrogen, ethane is formed and can be detected when studying thermal desorption of ethene, thus the formation of ethane occurs via a self-hydrogenation [34]. 

In particular, the adsorption behavior of TCE is studied on the surfaces MgO( 100), Mo(lOO), P/(lll) andMo(112) [4] and on MgO supported Ptx clusters of different sizes. Ethene adsorption is probed on MgO lOO), Pt l 11) as weU as supported Ptx clusters. As a function of cluster size, the reactivity of Ptx clusters towards ethene hydrogenation is probed by means of TPR. To further elucidate thereactivity of ethene on Pt clusters in contrast to the Pt single crystal surface, also initial experiments using AES and IRRAS are presented. 

Taking the hydrogenation of ethyne as a case study, it is well established now that the interaction of ethyne and/or ethene with a metal surface leads to a wide variety of adsorbed species, the proportion of which depends on the hydrocarbon pressure, the temperature, and the nature of the metal [76]. Earlier identihcations of such species—in particular, the ethylidyne =C-CH3—have been reported hrst on Pt(l 11) single crystals [77,78], then on Pd/Al203 [79]. With all available spectroscopic and kinetic data at various conditions, a nearly complete reaction scheme for the hydrogenation of ethyne on Pd(lll) has recently been proposed [80] and illustrated in Figure 6. The choice between the two routes through ethylidyne or Ti-bonded adsorbed species obviously depends on the nature of the metal, but also on some... 

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