Ethene, adsorption

A similar effect was observed earlier in [5] for ethene adsorption by X zeolite modified with bivalent cations of Cd and Ca. The C-H stretching bands, which are intense for free ethene, are not detectable at low pressure, while the normally forbidden C-H deformation and C=C stretching bands are the strongest in the spectrum. Further, ethene is weakly adsorbed by monovalent cations such as K, Na or Li and the relative intensities of C-H stretching bands are very strong. 

The literature of the vibrational spectra of adsorbed alkynes (acetylene and alkyl-substituted acetylenes) is very much in favor of single-crystal studies, with fewer reported investigations of adsorption on oxide-supported metal catalysts. Fewer studies still have been made of the particulate metals under the more advantageous experimental conditions for spectral interpretation, namely, at low temperatures and on alumina as the support. (The latter has a wide transmittance range down to ca. 1100 cm-1.) A similar number of different single-crystal metal surfaces have been studied for ethyne as for ethene adsorption. We shall review in more detail the low-temperature work which usually leads to HCCH nondissociatively adsorbed surface structures. Only salient features will be discussed for higher temperature ethyne adsorption that often leads to dissociative chemisorption. Many of the latter species are those already identified in Part I from the decomposition of adsorbed ethene. 

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.]...

In frared Spectral Data for the Species Formed from Ethene Adsorption on a Range of Oxide-Supported Metals... 

On flat surfaces the residual carbon forms graphitic layers at high temperatures. C2 species without hydrogen derived from ethene adsorption have instead been observed on corrugated Pt surfaces, in the vicinity of 600 K, presumably bonded within the grooves. The same species clearly occurs from ethyne adsorbed on the nonflat Ni[5 (111) X (110)] surface after adsorption at the very low temperature of 150 K 195). 

I. Ethene Adsorption, C2H4-Hydrogen Coadsorption, and C2H4 Hydrogenation on Pd(l 1 1) and Pd/Al203... 

I.l. Ethene Adsorption and C2H4- Hydrogen Coadsorption on Pd( 111) under UHV... 

Ethene adsorption, in particular on platinum and palladium surfaces, has received much attention. The atomic structure of the various adsorbed ethene... 

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 non-activated adsorption rate coefficients and the preexponential factor of C2H4 desorption are within the ranges as predicted by transition state theory [28]. Since molecular ethene adsorption is not activated, the activation energy for ethene desorption is equal to... 

We have previously studied the adsorption of ethene to the (111) surface of Pt and Cu using the VASP simulation package. However more recently we have tested the alternative CASTEP code and a part of this work is to report a comparison of these two programs for calculations involving these types of molecule. Accordingly, in the methodology section we compare CASTEP calculations on ethene adsorption to Pt( 111)... 

Rg. 14.12 Catalytic hydrogenation of ethene (adsorption is shown by dotted lines). 

An alternative way to explain product distributions has been suggested (Figure 6.4). This involves two sites denoted A and B, on the first of which the ethyl radical is formed di-a ethene adsorption needs both. This then transforms to TT-ethene (there is independent evidence that removal of hydrogen from ethyl gives the di-a and not the TT-ethene ), thence to ethyl on the B site and finally to ethane. Assigning numerical values to the three rate constant quotients produces calculated distributions in fair agreement with those observed. There are supposed to be few type A sites and many type B. This scheme also necessitates three independent parameters, and one wonders whether all the niceties of Microscopic Reversibility are accommodated by it. 

For the ethylidyne species the question arises as to whether it is a simple spectator [52] or does it have a more active role in the hydrogenation of ethene [36]. The current opinion is [53], that it does not actively participate in the reaction, however blocks the sites available for ethene adsorption [54] or reacts with hydrogen and thus indirectly affects the reaction kinetics of hydrogenation [51, 55]. 

In particular, the adsorption behavior of TCE is studied on the surfaces MgO( 100), Mo(lOO), P/(lll) andMo(112) [4] and on MgO supported Ptx clusters of different sizes. Ethene adsorption is probed on MgO lOO), Pt l 11) as weU as supported Ptx clusters. As a function of cluster size, the reactivity of Ptx clusters towards ethene hydrogenation is probed by means of TPR. To further elucidate thereactivity of ethene on Pt clusters in contrast to the Pt single crystal surface, also initial experiments using AES and IRRAS are presented. 

For Pt (111) a decrease (A< —0.4 eV) until 0.15 tce/sa is visible, for additional coverage saturation at this value is observed (A —0.3 eV). These observations show the same trend as previously reported WF measurements for ethene adsorption... 

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