Hydrogen-surface interactions spectroscopic studies

In addition to simultaneous in situ spectroscopic studies, accompanying ex situ investigations also provide valuable information about the specific interaction of the substrates (imines and respective hydrogenation products) with both the chiral modifier (P-acid) and the solid catalyst. Thus, FTIR spectroscopic analysis of the catalyst after adsorption of the imine points to a strong interaction of the latter with the catalyst surface, in particular with the support, which is reflected by marked band shifts. It could be shown that the surface of the catalyst is mainly covered by the imine after use in the hydrogenation reaction, besides some small quantities of the product [11]. 

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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