Adsorbed ethylene, observation

The addition of various Kolbe radicals generated from acetic acid, monochloro-acetic acid, trichloroacetic acid, oxalic acid, methyl adipate and methyl glutarate to acceptors such as ethylene, propylene, fluoroolefins and dimethyl maleate is reported in ref. [213]. Also the influence of reaction conditions (current density, olefin-type, olefin concentration) on the product yield and product ratios is individually discussed therein. The mechanism of the addition to ethylene is deduced from the results of adsorption and rotating ring disc studies. The findings demonstrate that the Kolbe radicals react in the surface layer with adsorbed ethylene [229]. In the oxidation of acetate in the presence of 1-octene at platinum and graphite anodes, products that originate from intermediate radicals and cations are observed [230]. 

The basis of the demonstration can be based on already published data on the surface reaction between NOz and adsorbed organic compounds. Yokoyama and Misono have shown that the rates of N02 reduction over zeolite or silica are proportional to the concentration of adsorbed propene [29], whereas Il ichev et al. have demonstrated that N02 reacts with pre-adsorbed ethylene and propylene on H-ZSM-5 and Cu-ZSL-5 to form nitro-compounds [30], Chen et al [2-4] have observed the same nitrogen-containing deposits on MFI-supported iron catalysts. The question on the pairing of nitrogen atoms is not considered here. 

The dependence of intensity of the observed bands on pressure is significant. Except for a relatively small intercept the intensities of bands at about 3130 and 3060 cm-1 are roughly proportional to pressure between 22 and 240 mm, whereas the intensities of bands at 1451, 1438, 1600, and 2984 cm-1 are insensitive to pressure in this region. (The band at 2993 cm-1 seems to behave similarly to the bands above 3000 cm-1, but its overlap with the band at 2984 cm-1 makes analysis difficult.) Thus, the bands above 3000 cm-1 (and perhaps the band at 2993 cm-1) are primarily due to physically adsorbed ethylene and only in part due to chemisorbed ethylene. By way of contrast the remaining bands stem primarily from chemisorbed ethylene. 

The shift in the C=C frequency, vi, for adsorbed ethylene relative to that in the gas phase is 23 cm-1. This is much greater than the 2 cm-1 shift that is observed on liquefaction (42) but is less than that found for complexes of silver salts (44) (about 40 cm-1) or platinum complexes (48) (105 cm-1). Often there is a correlation of the enthalpy of formation of complexes of ethylene to this frequency shift (44, 45). If we use the curve showing this correlation for heat of adsorption of ethylene on various molecular sieves (45), we find that a shift of 23 cm-1 should correspond to a heat of adsorption of 13.8 kcal. This value is in excellent agreement with the value of 14 kcal obtained for isosteric heats at low coverage. Thus, this comparison reinforces the conclusion that ethylene adsorbed on zinc oxide is best characterized as an olefin w-bonded to the surface, i.e., a surface w-complex. 

XPS studies of ethylene exposed to clean Au(lll) at 95 K were also carried out. The C(ls) peak occurred at 284.7 eV BE. Warm-up experiments show that the carbon peak was still present at 165 K, confirming that the long tail observed in the TPD spectra was due to adsorbed ethylene on the Au(lll) surface. Only trace amounts of carbon were observed at 200 K, and none was observed at 300 K. These results are consistent with the thermal desorption results. Also, LEED showed no new ordered structure was formed due to adsorbed ethylene. 

The co-existence of at least two modes of ethylene adsorption has been clearly demonstrated in studies of 14C-ethylene adsorption on nickel films [62] and various alumina- and silica-supported metals [53,63—65] at ambient temperature and above. When 14C-ethylene is adsorbed on to alumina-supported palladium, platinum, ruthenium, rhodium, nickel and iridium catalysts [63], it is observed that only a fraction of the initially adsorbed ethylene can be removed by molecular exchange with non-radioactive ethylene, by evacuation or during the subsequent hydrogenation of ethylene—hydrogen mixtures (Fig. 6). While the adsorptive capacity of the catalysts decreases in the order Ni > Rh > Ru > Ir > Pt > Pd, the percentage of the initially adsorbed ethylene retained by the surface which was the same for each of the processes, decreased in the order... 

Room temperature NMR spectra of adsorbed ethylene on Ru- exchanged Y zeolites are shown in Figure 1. These spectra are quite similar to those observed by... 

N.M.R. STUDIES of ADSORBED ETHYLENE We have also investigated the reaction of C ethylene with colloidal palladium. Our initial intent was to attempt to observe the formation of ethylidyne from ethylene on the surface of the colloidal palladium particles, a reaction which is known to occur readily on the surface of supported palladium and on palladium single crystals (17). Such a reaction has been identified for ethylene on supported platinum by magnetic resonance experiments in which spin echo double resonance techniques were used to characterize the organic species (18,19), but direct observation of resonances for adsorbed ethylene or ethylidyne was not possible in the highly inhomogeneous solid samples used. The chemical shift differences... 

A resonance for adsorbed ethylene was not observed. At this stage we may only speculate on the reasons for this, and two obvious factors come immediately to mind. 

Derive a rate expression for the hydrogenation of ethylene on Pt assuming steps 1, 2, and 3 are quasi-equilibrated, step 4 is virtually irreversible, and C2H5 is the most abundant reaction intermediate covering almost the entire surface ([ ]o [ C2H5]). Discuss why the rate expression cannot properly account for the experimentally observed half order dependence in H2 and zero-order dependence in ethylene. Could the observed reaction orders be explained if adsorbed ethylene ( C2H4 ) were the most abundant reaction intermediate Explain your answer. 

The Raman spectrum of ethylene chemisorbed on Ag(llO) is shown in figure 24 and provides an example of catalytic interest. The C=C stretch of adsorbed ethylene is observed at 1580 cm and the symmetric C-H in-plane deformation at 1320 cm ... 

A kinetic description of these reactions is difficult to give, due to the complicated decomposition pathways of the hydrocarbons on noble metal surfaces. The temperature programmed reaction between adsorbed ethylene and NO on rhodium in Fig. 5.16 illustrates some of the many reactions that may occur [58]. As seen before, the NO molecule starts to dissociate aroimd room temperature. Ethylene decomposes in several steps at different temperatures as evidenced by the release of H2 and H2O. The formation of CO and some CO2 between 500 and 600 K is well above the respective desorption temperatures of these gases, and suggests that the C-C bond of the hydrocarbon breaks in this temperature range and limits the rate of the oxidation on rhodium surfaces. Formation of HCN is observed as well. Note that a large reservoir of surface CN species forms at temperatures of 500 K and remains on the surface until 700-800 K, where it decomposes and is followed by the instantaneous desorption of N2. 

In contrast to the behavior of CO, the decomposition of ethylene is a facile process when performed on a nickel catalyst, but does not occur when the hydrocarbon is passed over iron. Based on these data we can rationalize the observed deactivation behavior observed in the present investigations according to the notion that at 725°C, the surface of the bimetallic particles become enriched in nickel, a condition that favors decomposition of adsorbed ethylene molecules, but is inert with regard to catalyzed disproportionation of CO. Subsequent lowering of the temperature to 600°C results in the restoration of the original surface composition and the concomitant attainment of the initial catalytic reactivity pattern. 

E. L. Force, A. T. Bell, The relationship of adsorbed species observed by infrared spectroscopy to the mechanism of ethylene oxidation over silver, /. Catal. 40 (1975) 356. 

In this case the catalyst has caused a radical reduction in overall activation energy, presumably by replacing a difficult homogeneous step by a more easily executed surface reaction involving adsorbed ethylene. The results lead to the kinetics observed by Wynkoop and Wilhelm," a reaction first order in H2 and zero order in strongly adsorbed ethylene. 

The oxidation of ethylene has been investigated on polycrystalline P UO3 surfaces. Two oxygen containing products were observed acetaldehyde, and furan. Furan desorption which requires a C-C bond formation, most likely is formed via dimerization of two adsorbed ethylene molecules followed by cyclization with available surface oxygen. Both the formation of acetaldehyde and furan from ethylene on UO3 are clear examples of the ease of removing oxygen atoms from UO3 surfaces. The reduction of acetaldehyde was also... 

Figure 22 shows that the rate of reaction is not directly proportional to ethylene pressure. When ethylene was adsorbed only on the active sites but its concentration in the gas phase was negligible, no polymerization was observed even after 68 hours at room temperature. Polymerization therefore does not occur by migration and interaction of the ethylene molecules adsorbed on active sites. It is possible that the physically adsorbed ethylene participates in the reaction. However, no simple rate expression could be found to fit the observed dependence on ethylene pressure. 

We consider first some experimental observations. In general, the initial heats of adsorption on metals tend to follow a common pattern, similar for such common adsorbates as hydrogen, nitrogen, ammonia, carbon monoxide, and ethylene. The usual order of decreasing Q values is Ta > W > Cr > Fe > Ni > Rh > Cu > Au a traditional illustration may be found in Refs. 81, 84, and 165. It appears, first, that transition metals are the most active ones in chemisorption and, second, that the activity correlates with the percent of d character in the metallic bond. What appears to be involved is the ability of a metal to use d orbitals in forming an adsorption bond. An old but still illustrative example is shown in Fig. XVIII-17, for the case of ethylene hydrogenation. 

Quite recently Yasumori el al. (43) have reported the results of their studies on the effect that adsorbed acetylene had on the reaction of ethylene hydrogenation on a palladium catalyst. The catalyst was in the form of foil, and the reaction was carried out at 0°C with a hydrogen pressure of 10 mm Hg. The velocity of the reaction studied was high and no poisoning effect was observed, though under the conditions of the experiment the hydride formation could not be excluded. The obstacles for this reaction to proceed could be particularly great, especially where the catalyst is a metal present in a massive form (as foil, wire etc.). The internal strains... 

Similar electrodes may be used for the cathodic hydrogenation of aromatic or olefinic systems (Danger and Dandi, 1963, 1964), and again the cell may be used as a battery if the anode reaction is the ionization of hydrogen. Typical substrates are ethylene and benzene which certainly will not undergo direct reduction at the potentials observed at the working electrode (approximately 0-0 V versus N.H.E.) so that it must be presumed that at these catalytic electrodes the mechanism involves adsorbed hydrogen radicals. 

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. 

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