Vibrational spectroscopy ethene

Our article has concentrated on the relationships between vibrational spectra and the structures of hydrocarbon species adsorbed on metals. Some aspects of reactivities have also been covered, such as the thermal evolution of species on single-crystal surfaces under the UHV conditions necessary for VEELS, the most widely used technique. Wider aspects of reactivity include the important subject of catalytic activity. In catalytic studies, vibrational spectroscopy can also play an important role, but in smaller proportion than in the study of chemisorption. For this reason, it would not be appropriate for us to cover a large fraction of such work in this article. Furthermore, an excellent outline of this broader subject has recently been presented by Zaera (362). Instead, we present a summary account of the kinetic aspects of perhaps the most studied system, namely, the interreactions of ethene and related C2 species, and their hydrogenations, on platinum surfaces. We consider such reactions occurring on both single-crystal faces and metal oxide-supported finely divided catalysts. 

It is seen that vibrational spectroscopy, when the results are considered in conjunction with the results of other physical techniques, has already made useful contributions to the study of the kinetics of ethene-related reactions on platinum surfaces. 

The ligand-model vibrational spectroscopy approach has contributed strongly to fairly reliable identifications on metal surfaces of C2 species of the types 1, 2 (ethene type II spectra) (17), 3 (ethene type I spectra), 4 (ethene type I spectra), 8, and 13 (ethyne type B spectra) as well as to possible identifications of types 5, 7, 15 (ethyne type A spectra), 16, and 20. Approximate band positions and associated intensity distributions in the spectra from normal and perdeutero species should be considered together (/ 7). The correspondence of the infrared spectrum from 4 with type I spectra is less satisfactory for the C2D4 ligand than in most other cases. However an extra structural variable in this case is the degree of nonplanarity of the cyclic C2M2 skeleton, which may differ between the model compound and the surface species. 

Application of high-pressure vibrational spectroscopy in order to study and to monitor technically relevant fluid phase processes under extreme conditions is exemplified by high-pressure ethene polymerization. Several vibrational bands in the IR and the NIR may be used to detect concentrations directly in the ethene/polyethylene system (Buback, 1984). Some of these are plotted in Fig. 6.7-20. The conversion of unsaturated (ethylenic)... 

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). 

The catalytic hydrogenation of ethyne (acetylene, C2H2) to ethene proceeds through a surface reaction between adsorbed ethyne and hydrogen atoms. We use vibrational spectroscopy to learn about the interaction between adsorbed ethyne and the catalyst and to identify intermediates. 

The ethylidyne species, CH3C, is formed by the dissociative chemisorption of ethene on metals. Although present on a metal during catalytic hydrogenation of ethene, ethylidyne is not an intermediate it is a spectator [78]. Chemisorbed ethylidyne has been characterised by vibrational spectroscopy and low energy electron diffraction by comparison with the model compound... 

Information about propagation and termination ( t) rate coefficients during a polymerization reaction are obtained from pulse sequence (PS)-PLP and single pulse (SP)-PLP experiments [21-23]. In the latter technique, monomer conversion is induced by a single excimer laser pulse typically of 20 ns width and is recorded by on-line vibrational spectroscopy with time resolution in the microsecond range. A typical monomer conversion versus time profile obtained for an ethene polymerization [24] at 190 °C, 2550 bar, and at 9.5 wt% polyethylene (from preceding polymerization) is shown in Figure 4.6-3. 

The term microkinetic analysis has been applied " to attempts to synthesise information from a variety of sources into a coherent reaction model for the hydrogenation of ethene. The input includes steady-state kinetics (most importantly the temperature-dependence of reaction orders ), isotopic tracing, vibrational spectroscopy and TPD it uses deterministic methods, i.e. the solution of ordinary differential equations, for estimating kinetic parameters. It selects a somewhat eclectic set of elementary reactions, and in particular the model... 

Changes in a Cr-Al-CM-41 catalyst during exposure to ethene at 373 K were monitored by UV-vis-NIR spectroscopy by Weckhuysen et al. (2000), who used the Harrick equipment. Reduction of chromium (initially Cr6+, Cr5+, and Cr3+) was observed, and C-H vibrations of methylene groups that were detected in the NIR region indicated polymerization. 

Rotational constants and centrifugal distortion constants of the upper vibrational state 2 vg of H2B-NH2 have also been determined by microwave spectroscopy for details, see [3]. Also, the He(I) photoelectron spectrum of H2B-NH2 (produced by controlled thermal decomposition of H3N-BH3) has been measured [4]. The five ionization potentials observed up to 21.2 eV have been correlated with those of ethene. A good correspondence of the observed values was obtained with data from Koopmans theorem calculations for the ground state molecule (semiempirical MNDO and SCF ab initio calculations with 3-21G and 6-31G bases). Experimental ionization potentials (IP) and calculated orbital energies are given in Table 4/24, p. 222 [4]. A correlation of the IP data of H2B-NH2 and H2CCH2 is given for the five uppermost filled levels in Fig. 4-47, p. 222. 

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