Ethene, adsorbed

A recent study of ethene adsorbed on Fe(100) by Hung and Bernasek (48) showed adsorption as a di-cr species (type I spectrum) at 100 K, but with a soft-mode component at ca. 2720 cm-1 in addition to the stronger 2985-cm 1 absorption in the vCH2 region. When the temperature had been raised to 253 K, the spectrum largely changed to that probably indicative of a mixture of CH and a(CCH) species (see also Section IV.D). At 523 K, only adsorbed carbon remained on the surface. 

Ethene adsorbed on a c(2 X 2) Mn layer on Pd(100) gives a spectrum profile of type I , as on Pd(100) itself (373b). Ethene on Mo(110) at 80 K (373c) gives a spectrum resembling that of a type B, di-cr/77, ethyne species ... 

Fig. 14. TPD diagrams of 1.1 L of ethene adsorbed on Pt(lll) at 100 K. Inset plot of C(272 eV)/Pt(237 eV) AES ratios against temperature of ethene adsorption at 100 (- -) and 300 K (---). [Reprinted with permission from Ref. 379. Copyright 1988 American Chemical Society.]...

Particular identification problems arise when unanticipated surface species are present. A case in point is the ethylidyne surface species, M3(CCH3) (M = metal atom), which has turned out to be of common occurrence from ethene adsorbed on metals near room temperature ( II). 

The STM technique permits adsorbed species to be studied in situ at near atomic resolution (136), as demonstrated in a recent study of ethene adsorbed on Pt( 111) between 160 and 700 K (140). 

On-Specular VEEL Spectra Associated with the Structures of Adsorbed Species from Ethene Adsorbed on Single-Crystal Pt Surfaces" h... 

Fig. 6. Infrared spectra from ethene adsorbed on metal-particle samples of Pi at low temperatures (A) Pt/Si02 at 128 K (32) (B) Pt/Si02 at 195 K (96) (C) Pt/Al203 at 187 K (160) (D) Pt/AI203 at 180 K, reprinted with permission from (57), copyright 1988 American Chemical Society.

Ito and Suetaka (78, 173) obtained a spectrum from ethene adsorbed at room temperature on a Pt film evaporated on a quartz plate. They assigned absorptions at 3300 and 3200 cm"1 to adsorbed ethyne (acetylene) obtained by dissociative adsorption, and broad bands at ca. 2900 and 2725 cm "1 to saturated adsorbed species. The 2900-cm-1 band could be from di-cr or ethylidyne. The low wavenumber of 2725 cm 1 (a soft mode) probably implies an end-on interaction of CH bonds with surface metal atoms. 

It would be of interest to study spectra from ethene adsorbed on newly reduced but relatively unannealed catalysts of other metals. The marked changes in surface sites implied in going from Fig. 10C to Fig. 10G are analogous to those expected during catalyst break-in phenomena (194). 

Ito et al. (173) have observed weak spectra from ethene adsorbed on evaporated Ni films, including vCH soft-modes at 2700 cm-1 and what could be an absorption band from a n complex at ca. 1000 cm"1. Partial Raman spectra have been observed from ethene adsorbed on Ni/Si02 at 180 K and at room-temperature (26, 211). Krasser et al. (26) appear to have interpreted the spectrum taken at 180 K predominantly in terms of a di-unsaturated hydrocarbon groupings. Krasser et al. interpret the ca. 990 cm "1 band as from the vCC mode of the di-cr species, but the EELS data on Ni( 111) suggest that the vCC mode should be near 1100 cm-, possibly that shown in the Raman spectrum at ca. 1070 cm"1. Indeed, the set of Raman bands... 

Figure 15 shows infrared spectra from ethene adsorbed on a number of other oxide-supported metals [Co (217, 208, 56) Ru (50) Ir (58) Cu (52)]. 

Ethene adsorbed on Cu/ZnO at 300 K. (52), Fig. 15E, gives closely similar wavenumbers and relative intensities to the spectrum on Cu(100) with absorptions at 1550, 1290, and 920 cm-1—so much so that the spectrum may imply the dominance of (100) facets on the Cu particles. Ethene adsorbed on cold-deposited Cu films studied by SERS gives bands at 1544, 1278, and 896 cm 1 (227). A 3-monolayer covering of Cu on Ru(0001) gives a similar spectrum with somewhat shifted band positions at 1504, 1248, and 880 cm-1 (218). 

Ethene adsorption on Pd(l 11) was investigated by SFG spectroscopy (68,83,84,98,120). Figure 43 shows SFG spectra after adsorption of ethene at various temperatures. At 100 200 K, ethene adsorbed in a di-a configuration with a characteristic peak at 2910 cm (vs(CH2) Fig. 43a). The second, weak peak at... 

The various ethene adsorbate species can be identified by vibrational spectroscopy (cf. Fig. 43) (46,138,448,470 75). Calibration SFG spectra recorded under UHV include three vibrational features, at 2880, 2910, and 3000 cm (138), which are similar to those characterizing the adsorbates on Pd(l 11). The peak at 2880 cm is attributed to the Vs(CH3) stretch vibration of ethylidyne (MSC-CH3), the feature at 2910 cm results from the Vs(CH2) of chemisorbed di-a-bonded ethene, and the very weak peak at 3000 cm represents the Vs(CH2) of physisorbed 7i-bonded ethene. As has been stated, the Vs(CH2) signal characterizing 7i-bonded molecules on single-crystal surfaces is very weak and explained by the surface-dipole selection rule for metal surfaces (17). 

Figure 15 shows spectra obtained with unpolarised radiation in the v(CH) region of ethene adsorbed in HZSM-5 after heating to various temperatures. At low temperatures, the spectrum is dominated by a pair of bands at 2934 cm 1 and 2860 cm l due to asymmetric and symmetric stretching modes respectively of CH2 groups in... 

Figure 15. Infrared spectra of ethene adsorbed in a single crystal of HZSM-5 at room temperature (a) and after heating successively to 373K(b), 473K(c), 573K(d) and 673K(e) (unpolarised). Reproduced with permission from reference 50.

The foregoing generally supports therefore the model for hydrogenation of Dent and Kokes, namely that the 77-complex of ethene, adsorbed on O of the... 

The spectra of solid ethene at 4 K and 80 K consisted of weak and shifted peaks superimposed on a broad backgroimd [72], The spectrum was broadened through recoil from the low-mass ethene molecule ( 2.7.5) as observed also for dihydrogen. When ethene is bound in a complex or to a surface its effective Sachs-Teller mass ( 2.6.5.1) is increased the recoil spectrum is weaker and the peaks due to the vibrational modes are stronger and sharper. The vibrational peaks of gaseous and solid ethene and ethene adsorbed by carbon are listed in Table 7.8. 

In Fig. 15 is shown an SER spectrum [24] of ethene adsorbed at electro-chemically roughened gold at + 0.20 V (SCE) in ethene-saturated 1M H2S04. Red laser excitation (Kr ion, 647.1 nm) was used to obtain this spectrum, which shows strong bands centred at 1540 and 1283 cm h These have been assigned to C = C stretching and symmetric CH2 deformation modes, both shifted down from the ethene gas-phase values of 1623 and... 

Fig. 15. SER spectrum of ethene adsorbed at a gold electrode. (Reproduced with permission from ref. 24.)...

So for example we may ask, Ts ethene associatively adsorbed during its metal-catalysed hydrogenation and we may hope to obtain a straight yes or no answer but if the question is How is ethene adsorbed wehave to expect a more discursive reply, as we express our answer in terms of the many structural formula considered in Chapter 4. 

ICI catalyst showed that ethene adsorbed competitively with the ethyne, but the results required two types of site (see Table 9.6) (or two modes of chemisorption of the ethyne) to explain them. Their properties did not however match any of those proposed by Webb. Type X, in the majority, adsorbed both hydrocarbons, but ethene was favoured by a factor of 2200 Type Y adsorbed ethene only, perhaps because of its high concentration. The main source of the ethane was confirmed as ethene, since in the reaction with deuterium the main product was ethane-d2- When the pressure of ethyne was varied in the presence of excess ethene, its rate of removal (and that of formation of dimers) passed through a maximum, while that of ethane formation feU to zero at an ethyne pressure of 2 kPa (see Figure 9.7). The ethane rate was almost independent of the ethene pressure. Extensive work by Borodzinski and colleagues led " to detailed proposals for the identity of two types of site, designated A and E, that were thought to be created as the carbonaceous overlayer developed, and a third type (E ) that may play a role on certain supports. Type A sites, in the majority, were small, so that only ethyne and hydrogen could adsorb on them, the former perhaps as vinylidene (>C=CH2),... 

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