Surface adsorbed ethylene

The temperature regimes for the stability of intermediates is different for various transition metals. For example on Fe(lll) the adsorbed ethylene decomposes partially at 200 K, while the conversion to surface carbon is complete at 370 K. Similarly, on nickel faces molecular chemisorption of ethylene is restricted to temperatures below ambient. At temperatures between approximately 290 K and 450 K ethylene chemisorption on nickel... 

The electrode surface has heen modified with adsorbed ethylene. 

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]. 

We have developed a compact photocatalytic reactor [1], which enables efficient decomposition of organic carbons in a gas or a liquid phase, incorporating a flexible and light-dispersive wire-net coated with titanium dioxide. Ethylene was selected as a model compound which would rot plants in sealed space when emitted. Effects of the titanium dioxide loading, the ethylene concentration, and the humidity were examined in batches. Kinetic analysis elucidated that the surface reaction of adsorbed ethylene could be regarded as a controlling step under the experimental conditions studied, assuming the competitive adsorption of ethylene and water molecules on the same active site. 

Kinetic analysis based on the Langmuir-Hinshelwood model was performed on the assumption that ethylene and water vapor molecules were adsorbed on the same active site competitively [2]. We assumed then that overall photocatalytic decomposition rate was controlled by the surface reaction of adsorbed ethylene. Under the water vapor concentration from 10,200 to 28,300ppm, and the ethylene concentration from 30 to 100 ppm, the reaction rate equation can be represented by Eq.(l), based on the fitting procedure of 1/r vs. 1/ Ccm ... 

Calculated results are shovra as solid lines in Fig.4 under the respective experimental conditions, and the assumption that the rate was controlled by the surface reaction of adsorbed ethylene was reasonable under the experimental conditions studied. 

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 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. 

Figure 2.16 Temperature programmed reaction between O atoms and ethylene adsorbed on Rh(l 11). The majority of the adsorbed ethylene decomposes in several steps to H and C atoms, which react with the adsorbed O atoms to form H2, H20, CO and C02. Because there is insufficient oxygen, the surface still contains carbon at the end of the experiment (adapted from [36],...

The controlling mechanism is surface reaction between adsorbed ethylene and adsorbed HCl. Find the rate equation. 

Most of the ethylene that interacts with Ru(001) at 323 K produces a nondesorbable carbon layer. This result is similar to that for the interaction of C2H4 with Ni, which produces a surface carbide at temperatures between about 300-600 K (14). SIMS results suggest, however, the presence of small amounts of molecularly adsorbed ethylene, acetylenic and other hydrocarbon complexes in addition to the nondesorbable carbon layer. 

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. 

Adsorbed ethyl radicals are formed by the dissociative adsorption of ethane. Every ethyl radical may either leave the surface with a deuterium atom to form an ethane molecule or lose one of the three hydrogen atoms of the methyl group to form adsorbed ethylene. The chances of these two events are 1/(1 + P) and P/(l -f P), respectively, P being a constant for a given catalyst. Equal chance is assumed for the loss of each of the three hydrogen atoms of the methyl group in the second process. 

The hydrogen atoms thus liberated to the surface may then react with either gaseous ethylene [50,51], or associatively adsorbed ethylene [53] or with the surface C2H4. complex [52]. Volumetric [52,54] and magnetic susceptibility measurements [55] suggest that the extent of dissociation is dependent upon the temperature and varies from metal to metal. 

From the changes in magnetic susceptibility of nickel—silica catalysts during ethylene adsorption at room temperature, Selwood [55] has concluded that ethylene exists both as an associatively and a dissociatively adsorbed species. On increasing the temperature, the dissociative adsorption becomes more important. Thus at 100° C, the susceptibility changes are consistent with the formation of six bonds to the surface for each adsorbed ethylene molecule, suggesting the following process... 

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... 

Fig. 8. Hydrogen-deficient surface adsorbed states of ethylene.

Chemisorption measurements have shown that ethylene does not adsorb on pure silver, but only on a silver surface which has been preoxidized [339]. Complete coverage with an oxygen monolayer, however, also seems to destroy the capacity to adsorb ethylene, as was demonstrated by Force and Bell [114,116] (favouring the idea of adsorption on silver). Consequently, partial oxygen coverage seems to be a necessary condition for catalytic activity. 

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