Mechanistic Changes in the Conversion of Ethylene to Ethylidyne on Transition Metals Induced by Changes in Surface Coverages

Abstract

The thermal chemistry of trideuterioethylene on Pt(111) surfaces was characterized by reflection−absorption infrared spectroscopy (RAIRS) and temperature-programmed desorption (TPD). The vibrational data indicate that thermal activation of a saturated layer of adsorbed ethylene by heating the surface to 350 K produces surface ethylidyne species with an isotopic composition of about 45% perdeuterioethylidyne and 55% dideuterioethylidyne. The hydrogen TPD data confirm that result, and also highlight the fact that the proportion of hydrogen-to-deuterium elimination from the original ethylene molecules changes with initial coverage. To explain these observations a previously proposed three-step mechanism is used where ethylene first isomerizes to ethylidene and ethylidene then either isomerizes back to ethylene or dehydrogenates to ethylidyne. The changes in hydrogen isotope removal probability with coverage are explained by changes in the relative rates between the latter two steps because of the fact that the outgoing hydrogen in the dehydrogenation step requires an empty surface site that may be blocked by ethylene molecules at high coverages.

Graphene Growth on Pt(111) by Ethylene Chemical Vapor Deposition at Surface Temperatures near 1000 K

“…Ethylene is believed to be converted into ethylidyne through a mechanism involving the transient formation of either a vinyl 46, 52 or an ethylidene 49 of ethylene that competes with desorption is a first order process that has been characterized by these techniques to have a rate constant pre-exponential factor, r ν , in the range from 3 × 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 To calculate the initial dissociative sticking coefficient of ethylene on Pt(111) relevant to chemical vapor deposition of carbon, we follow the lead of Ofner and Zaera 57 and consider simplified precursor-mediated thermalized trapping reaction kinetics,…”

Identification of surface intermediates during ethylidyne formation on Pt(111) by calculation of infrared intensities and deuterium isotope shifts

“…Nearly all of the C 2 species, including vinyl (CH 2 CH), vinylidene (CH 2 C), ethylidene (CH 3 CH), and ethyl (CH 3 CH 2 ), have been proposed as intermediates during this transformation. However, based on extensive kinetic and surface spectroscopic studies, a generally accepted mechanism involving an intermediate ethylidene species gradually emerged [9][10][11][12]. This mechanism involved a 1,2 Hshift reaction from ethylene to form ethylidene, followed by a dehydrogenation step to form ethylidyne.…”

Acetylenes as probes in the Fischer–Tropsch Reaction

“…Interestingly, alkylidyne species (RCBM, where R is an alkyl group, M is a transition metal) are formed by thermal activation of acetylenic or alkylidene (RC@M) species. Alkylidynes are also easily formed by the thermal activation of chemisorbed olefins [54][55][56]. While a thorough understanding of the mechanistic aspects of the olefin to alkylidyne conversion is far from complete, the finding that acetylenic compounds yield surface intermediates that closely resemble those obtained from terminal olefins fits well with the postulation that acetylenes initiate the FT via transformation to olefin In commercial practice, synthesis gas is passed over a cobalt catalyst at 200-250°C, no olefin is added to initiate the reaction.…”