Ethylidyne on Pt

This ethylidyne intermediate has recently been subject to two IRS investigations, by Chesters and McCash and in a more detailed study by Malik et alP Their spectra show three absorption peaks above 800cm which in light of the EELS work could be assigned to the C—C stretch mode at 1120cm , the symmetric CH3 bend mode at 1340cm and the symmetric CH3 stretch mode at 2885 cm In contrast to the surface methoxy discussed in the previous section, there is no peak associated with the asymmetric CH3 stretch mode at 2950cm This shows, in line with the discussion above, that the CCHj is oriented with the C—C axis normal to the surface (as indicated in Fig. 19) and verifies at the same time the validity of the infrared surface selection rule. 

Infrared spectra of the symmetric CHj bend mode of ethylidyne (CCHj) on Pt(l 11) at 82 K and 300 K. The dotted lines are calculated assuming a vibrational-rotational coupling. Inset shows the suggested orientation. (Reproduced by permission from Malik et 

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Fig. 7.3. Perspective view of ethylidyne on Pt(l 11), the stable structure reached after acetylene adsorption with hydrogen addition...

It is the di-cr species on Pt(lll) which is converted at higher temperature into ethylidyne. This conversion had also been investigated by infrared-visible sum-frequency generation (SFG) by Cremer et al. (371), a welcome first application of this new spectroscopic technique to hydrocarbon adsorption chemistry. They observed an absorption characteristic of an intermediate with a nCH3 band at 2957 cm-1 and suggested that this arises from an ethylidene M2CHCH3 (or possibly ethyl) species with its C-CH3 axis at an angle to the surface. It is very clear experimentally, as discussed in Part I, that ethylidyne on Pt(lll) is formed from di-cr-ethene and not directly from its 77-bonded isomer. 

Fig. 5. The adsorption geometry of ethylidyne on Pt(lll) as determined by LEED intensity analysis.

Fig. 47a shows SFG spectra characterizing room temperature adsorption of ethene on Pt(l 11) from UFIV to a pressure of about 130mbar, with the peak at 2880 cm clearly indicating the presence of ethylidyne. At the relatively high pressure, the ethylidyne peak decreases, which may indicate the coadsorption of di-CT-bonded ethene. Ohtani et al. 476) observed by IRAS that C2H4 at about 1 mbar reduced the formation of ethylidyne on Pt(l 1 1), which the authors attributed to the reversible adsorption of di-a-bonded ethene. Di-a-bonded ethene was converted to ethylidyne at temperatures of 260-300 K in the presence of ethene at 1 mbar, whereas it was already converted at 240-260 K in vacuum. Vacant sites adjacent to di-a-bonded ethene seem to be necessary for ethylidyne formation, which are occupied by di-a-bonded ethene if the surface is equilibrated with gaseous ethene. 

Cremer P, Stanners C, Neimantsverdreit J, Shen Y, Somorjai GA (1995) The conversion of di-sigma bonded ethylene to ethylidyne on Pt(lll) monitored with sum-frequency generation - evidence for an ethyUdene (or ethyl) intermediate. Surf Sci 328 111... 

Vinyl species have not been detected on metal surfaces but there is kinetic evidence in the context of the dehydrogenation of ( H-labeled) ethylene to chemisorbed ethylidyne on Pt(lll) sufaces, cf. eq. (9) [22]. 

Horsley JA, Stohr J, Koestner RJ (1985a) Structure and bonding of chemisoibed ethylene and ethylidyne on Pt(lll) from near edge X-ray absorption fine structure spectroscopy and multiple scattering calculations. J Chem Phys 83 3146-3153... 

The entire energy profile for conversion of ethane through ethene to ethylidyne on Pt(l 11) is shown in Figure 4.11. 

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. 

Figure 8.10 Sum frequency generation spectra of ethylene adsorbed on Pt( 111) at 200 K after heating to the temperature indicated. The spectra indicate the conversion of di-o bonded ethylene to ethylidyne via an intermediate attributed to ethylidenc (adapted from Cremer et al. [35].)...

Fig. 1. NEXAFS spectra of ethylene (T-90K) and ethylidyne (T-300K) chemisorbed on Pt(lll) for normal incidence. The difference between the spectra is also shown to indicate the maximum for ethylidyne at 285.8 eV photon energy.

Fig. 3. Time dependence of the conversion of normal ethylene adsorbed on Pt(lll) to ethylidyne at four different temperatures.

Kinetic Data for Ethene Desorption, Ethene Decomposition to give Ethylidyne, and Ethylidyne Formation from Ethene on Pt(Ill) Surfaces... 

The lack of any important effect of ethylidyne on the ethene hydrogenation has presented something of a dilemma because (CCH3)Pt(lll) models imply that there is little room left for the adsorption and reaction of ethene (400). Surface hydrogen (or deuterium) atoms which could be present despite the high coverage of ethylidyne would somehow have to be transferred to ethene weakly adsorbed on top of the ethylidyne adlayer (399), or, alternatively, the ethylidyne species would need to move apart from each other under reaction conditions to allow ethene to reach the surface for reaction (400). 

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