Platinum ethylidyne

Figure 4.10 Secondary ion intensities of ethylidyne, =CCH3, on platinum(l 11) during reaction with D2 at 383 K. Curves a-d are the measured SIMS intensities of CH + fragments at 15-18 amu, respectively. Curves e-h represent a kinetic simulation for a consecutive reaction via two intermediates (adapted from Creighton et al. [30]).

Figure 8.10 shows the application of SFG on adsorbed hydrocarbons [35], Ethylene was adsorbed on the (111) surface of platinum at 240 K, and subsequently heated to different temperatures. The spectra monitor the conversion of di-G bonded ethylene to ethylidyne (=C-CH3), via an intermediate characterized by a frequency of 2957 cm-1 attributed to the asymmetic C-H stretch of a CH3 group in the ethylidene (=CH-CH3) fragment. Somoijai and coworkers have demonstrated the usefulness of the SFG technique for in situ work with studies of ethylene hydrogenation and CO oxidation at atmospheric pressure [36]. 

Kinetics of Ethylidyne Formation on Platinum(lll) Using Near-Edge X-ray Absorption Fine Structure... 

Species D is most likely an ethylidyne complex which forms from self-hydrogenation on the palladium surface. Such species along with species E have been suggested to be part of the compounds formed from platinum and acetylene. 

With the advent of sophisticated experimental techniques for studying surfaces, it is becoming apparent that the structure of chemisorbed species may be very different from our intuitive expectations.10 For example, ethylene (ethene, H2C=CH-2) chemisorbs on platinum, palladium, or rhodium as the ethylidyne radical, CH3—C= (Fig. 6.2). The carbon with no hydrogens is bound symmetrically to a triangle of three metal atoms of a close-packed layer [known as the (111) plane of the metal crystal] the three carbon-metal bonds form angles close to the tetrahedral value that is typical of aliphatic hydrocarbons. The missing H atom is chemisorbed separately. Further H atoms can be provided by chemisorption of H2, and facile reaction of the metal-bound C atom with three chemisorbed H atoms dif-... 

Another example of this type of investigation is the SIMS study of the H-D isotope exchange reaction in ethylidyne on platinum, as reported by Creighton et al. 

N.M.R. STUDIES of ADSORBED ETHYLENE We have also investigated the reaction of C ethylene with colloidal palladium. Our initial intent was to attempt to observe the formation of ethylidyne from ethylene on the surface of the colloidal palladium particles, a reaction which is known to occur readily on the surface of supported palladium and on palladium single crystals (17). Such a reaction has been identified for ethylene on supported platinum by magnetic resonance experiments in which spin echo double resonance techniques were used to characterize the organic species (18,19), but direct observation of resonances for adsorbed ethylene or ethylidyne was not possible in the highly inhomogeneous solid samples used. The chemical shift differences... 

These conditions must be satisfied in order to correctly apply the steady-state approximation to a reaction sequence. Consider the H-D exchange with ethylidyne (CCH3 from the chemisorption of ethylene) on a platinum surface. If the reaction proceeds in an excess of deuterium the backward reactions can be ignored. The concentrations of the adsorbed ethylidyne species have been monitored by a technique called secondary ion mass spectroscopy (SIMS). The concentrations of the various species are determined through mass spectroscopy since each of the species on the surface are different by one mass unit. Creighton et al. [Surf. Sci., 138 (1984) L137] monitored the concentration of the reactive intermediates for the first 300 s, and the data are consistent with what are expected from three consecutive reactions. The results are shown in Figure 4.2.2. 

Figure 10-3 Ethylidyne as chemisorbed on platinum. (Adapted from G. A.

O3BF4RU2C15H13, Ruthenium(l+), jLCarbonic acid cobalt complexes, 21 120, 23 107, 112 platinum chain complex, 21 153, 154... 

Table 7.9 INS spectra ( a/cm )of ethene and ethylidyne on platinum black [64].

Figure i 0-3 shows the bonding from the adsorption of ethylene on a platinum surface to form chemisorbed ethylidyne. Like physical adsorption, chemisorption is an exothermic process, but the heat.s of adsorption are generally of the same magnitude as the heat of a chemical reaction fi.e., 40 to 400 kj/mol). If a catalytic reaction involves chemisorption, it must be carried out within the temperature range where chemisorption of the reactants is appreciable. 

During ethylene hydrogenation over Pt(lll) the reaction intermediate appears to be weakly bound 7t-bonded ethylene which produces most of the ethane, while ethylidyne and di-o bonded ethylene are spectators during the catalytic process. The surface concentration of k-bonded ethylene is 4% of a monolayer during the turnover, which yields an absolute turnover rate 25 times higher than the turnover rate per platinum atom. 

D. Godbey, F. Zaera, R. Yeates, and G.A. Somoijai. Hydrogenation of Chemisorbed Ethylene on Clean, Hydrogen, and Ethylidyne Covered Platinum (111) Crystal Surfaces. Surf. Sci. 167 150 (1986). 

Figure 7.42. Turnover rates for ethylene hydrogenation, the rehydrogenation of ethylidyne, and the deuteration of the methyl- group of ethylidyne on platinum and rhodium crystal surfaces [190]. Note that ethylene hydrogenation rates are orders of magnitude faster than the rate of removal of chemisorbed ethylidyne.

Another reaction model involves the compression of the ethylidyne overlayer at high pressure of ethylene. Because of repulsive adsorbate-adsorbate (ethylene-ethy-lidyne) interaction, and the expected small activation energy of ethylidyne surface diffusion, ethylene could adsorb on the metal in the small hole created near the compressed ethylidyne. Compression of this type has been detected by STM upon the adsorption of hydrocarbons on platinum and the coadsorption of CO and sulfur on both platinum and rhenium surfaces [218]. 

Ethylidyne (8) has been recognised on Pt/SiOa at 300 K using the SEDOR NMR technique applied to heavily C-labelled ethene the C—C bond length was 149 pm. This seemed to occur on large platinum particles, where areas of (111) face are most likely it was also seen by SIMS on platinum black, but on small particles vinylidene (17) predominated. Similar SEDOR experiments with ethyne showed 75% vinylidene and 25% ethyne as 12A or 15. " Adsorbed benzene was shown to rotate freely at 300 K, and cyclopropane was adsorbed, but not strongly, i.e. without loss of hydrogen. ... 

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