Ethylidyne

Fig. VIII-10. (a) Intensity versus energy of scattered electron (inset shows LEED pattern) for a Rh(lll) surface covered with a monolayer of ethylidyne (CCH3), the structure of chemisorbed ethylene, (b) Auger electron spectrum, (c) High-resolution electron energy loss spectrum. [Reprinted with permission from G. A. Somoijai and B. E. Bent, Prog. Colloid Polym. ScL, 70, 38 (1985) (Ref. 6). Copyright 1985, Pergamon Press.]...

Fig. XVin-1. (Continued) (d) Rh(lll) with a C(4 x 2) array of adsorbed CO and a layer of ethylidyne (C—CH3). (Courtesy of G. A. Somoijai see Ref. 21 for a similar example.)...

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. 

Comparison of the vibrational frequencies (cm ) of the ethylidyne surface species formed on Rh (111) with those of the ethyiidyne duster compound. 

P. Skinner, M. W. Howard, I. A. Oxton, S. F. A. Ketde, D. B. Powell, and N. Sheppard./ Chem. Soc., Faraday Trans. 2,1203, 1981. Vibrational spectroscopy (infrared) studies of an organometallic compound containing the ethylidyne ligand. 

At the same time, Ciajolo et al, [136] presented the crystal structure of azinobis(ethylidyne-p-phenylene)dipropionate. The crystal structure contains two crystallographically independent molecules with an almost planar conformation, the phenyl rings being slightly rotated with respect to the average molecular plane. The molecules are arranged in layers. 

Second, catalytic reactions do not necessarily proceed via the most stable adsorbates. In the ethylene case, hydrogenation of the weakly bound Jt-C2H4 proceeds much faster than that of the more stable di-cr bonded C2H4. In fact, on many metals, ethylene dehydrogenates to the highly stable ethylidyne species, =C-CH3, bound to three metal atoms. This species dominates at low coverages, but is not reactive in hydrogenation. It is therefore sometimes referred to as a spectator species. Hence, weakly bound adsorbates may dominate in catalytic reactions, and to observe them experimentally in situ spectroscopy is necessary. 

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]. 

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].)...

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

We suspect that the intermediate(s) involved in the conversion of ethylene into ethylidyne are closely related to those intervening in the mechanism for ethylene hydrogenation described above. 

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). 

However, due to the difficulties in calculating ion yields in SIMS, quantitation of the data is not very reliable, and their work was not conclusive. We have determined here that the reaction of chemisorbed ethylene to form ethylidyne is first order in ethylene coverage. A noticeable isotope effect was observed, with activation energies of 15.0 and 16.7 Kcal/mole for C H and 02 respectively. These values are smaller than those calculated from TDS, but the differences can be reconciled by including the recombination of hydrogen atoms on the surface in the interpretation of the thermal desorption experiments. 

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. 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.

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