Transition ethylidyne

Studies to determine the nature of intermediate species have been made on a variety of transition metals, and especially on Pt, with emphasis on the Pt(lll) surface. Techniques such as TPD (temperature-programmed desorption), SIMS, NEXAFS (see Table VIII-1) and RAIRS (reflection absorption infrared spectroscopy) have been used, as well as all kinds of isotopic labeling (see Refs. 286 and 289). On Pt(III) the surface is covered with C2H3, ethylidyne, tightly bound to a three-fold hollow site, see Fig. XVIII-25, and Ref. 290. A current mechanism is that of the figure, in which ethylidyne acts as a kind of surface catalyst, allowing surface H atoms to add to a second, perhaps physically adsorbed layer of ethylene this is, in effect, a kind of Eley-Rideal mechanism. 

The complete conversion of C2H2 to ethylidyne requires the presence of surface hydrogen atoms and proceeds rapidly only at 350 K. By comparison with reported reaction mechanisms on related transition metal clusters it seems likely that vinylidene... 

These findings do not support the suggestion by Thomson and Webb (241) that the similarities in hydrogenation kinetic parameters during catalysis by a wide range of different transition metals could be understood if the reaction involved the participation of similar hydrocarbon residues in all cases. On the other hand, it is clear that at higher hydrocarbon/H2 ratios, relatively unreactive hydrocarbon species (mostly ethylidyne) do indeed accumulate on the surface in all cases. 

The chemisorption of organic molecules on transition metals usually reduces their work function. According to the available experimental data, the chemisorption of ethylene in the form of ethylidyne reduces the work function by about —1.2 eV ( — 1.9 X 10 J), corresponding to the formation of a surface dipole of -1-0.9 Debye (3 X 10 C m). Benzene chemisorption reduces the work function by — 1.4 eV... 

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