ON THE MECHANISM FOR THE CONVERSION OF ETHYLENE TO ETHYLIDYNE ON METAL-SURFACES - VINYL HALIDES ON PT(111)

The thermal chemistry of vinyl halides adsorbed on Pt(111) surfaces was studied by using thermal programmed desorption (TPD) and reflection-absorption infrared (RAIRS) spectroscopies, In the case of vinyl iodide some molecular desorption occurs at high coverages around 130 (multilayer) and 150 K (monolayer), and the remaining vinyl iodide then decomposes via an initial scission of the C-I bond, a step that presumably leads to the formation of surface vinyl groups. RAIRS experiments indicate that the subsequent conversion of this latter species may involve the formation of several intermediates, starting with a mixture of mu(3)-eta(2)-vinylidene and mu(3)-eta(2)-acetylene (both moieties coordinated to three metal atoms via both sigma and pi bonds). The vinylidene disappears around 170 K, while tile acetylene subsists to higher temperatures, but above 200 K no infrared signal can be seen in the IR experiments, a fact that we interpret as the result of the formation of a new flat-lying species on the surface, possibly pi-coordinated ethylene. Finally, heating above 300 K leads to the formation of ethylidyne. Based on these studies we conclude that since adsorbed vinyl groups eventually convert into ethylidyne but only by going through a series of intermediates that possibly includes ethylene, it is unlikely for the conversion of ethylene to ethylidyne to involve vinyl intermediates.