Ethylidyne formation

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

In this report we present NEXAFS results for the kinetics of ethylidyne formation. Previous data is scarce and comes mostly from thermal desorption (TDS) experiments (2). The only reported study of isothermal rates of reactions for this system was done by Ogle et. al. using secondary ion mass spectrometry (SIMS) (10). 

The rate of ethylidyne formation was measured by recording the partial electron yield signal as a function of time after setting the crystal temperature to a preestablished value. An example of the results is shown in Figure 2 for the conversion of at... 

K. A jump in the signal due to ethylidyne formation is clearly observed immediately after heating the sample (t - 0). The figure illustrates some typical characteristics of these experiments. 

Fig. 4. Temperature dependence of the rate constants for ethylidyne formation from (+) and from 02 (A).

We have measured the kinetics of ethylidyne formation from chemisorbed ethylene over Pt(lll) surfaces. The rates of reaction display a first order dependence on the ethylene coverage. There is an isotope effect, since the reaction for CjH is about twice as fast as for CjD. We obtain values for the activation energy of 15.0 and 16.7 Kcal/mole for the normal and deuterated ethylene, respectively. These values are lower than those obtained from TDS experiments, but the differences can be reconciled by taking into account the hydrogen recombination when analyzing the thermal desorption data. 

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

Rate = 0C2H4(a) A2a exp(-ElaIRT) and the rate of ethylidyne formation is given by... 

Fig. 15. Adsorbed ethene decomposition data (laser-induced thermal desorption mass 27 against time) and ethylidyne formation data (IR signal against time) for selected coverages and reaction temperatures. [Reprinted from Ref. 386, Surf. Sci. 301, W. Erley, Y. Li, D. P. Lang, and J. C. Hemminger, p. 177. Copyright 1994 with kind permission of Elsevier Science-NL, Sara Burgerhartstraat 25, 1055 KV Amsterdam, The Netherlands.]...

The most conspicuous absorptions in the spectra on Ni particles that were reduced in intensity as the ethylidyne absorptions grew were those of the (no) species. Lapinski and Ekerdt suggested that this, too, was adsorbed on (111) facets at the lower temperature. Perhaps this alternative to the expected di-cr species on (111) sites is also produced by high-coverage compression of the monolayer. However, quantitative measurements showed that species other than (no) must be involved in the early stages of ethylidyne formation, which occurred with limited loss of intensity from the (no) spectrum. It was suggested that the n species were the other ones contributing to ethylidyne formation. However, if the 2905 cm 1 absorption does correlate with the presence of di-cr species, this is an alternative and probable precursor, as on Pt or Pd. 

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. 

Chesters MA, McCash EM (1987) Ethylidyne formation on Pt(lll), studied by ET-RAIRS. Surf Sci 187 L639... 

Based on these considerations and experimental evidences a current model for the ethane formation on f t(lll) is depicted in Fig.2.1a, involving both itt-bonded and di - a bonded species [49]. The competitive interplay between ethylidyne formation and the hydrogenation to form ethane [49] is presented as both a reaction scheme and as an energy diagram. 

Kessmodel LL, Dubois LH, Somorjai GA. LED analysis of acetylene and ethylene chemisorption on the Pt(lll) surface evidence for ethylidyne formation. J Chem Phys. 1979 70 2180. 

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