Ethylene, adsorption oxidation

It is clear from the kinetics that both ethylene and oxygen adsorption are important since both compounds appear in the rate equations with non-zero orders. Moreover, it is well known that ethylene is not adsorbed on pure silver, but that it does adsorb on a surface that is partially covered with oxygen. This implies that ethylene is either adsorbed on top of pre-adsorbed oxygen or on silver sites that are activated by the presence of oxygen (i.e. by formation of surface oxides, or another form of electron transfer or polarization). Consequently, two different mechanisms arise for the formation of ethylene oxide. The (direct) combustion of ethylene is another point of discussion. Although many favour the idea that different oxygen species are involved, others assume the same oxygen species, but different forms of ethylene adsorption. 

To summarize, our DFT calculations show that the experimental result corresponds to a specific orientation of ethylene on the oxide overlayer. The adsorption energy at this site is significantly greater than the ethylene adsorption energy on clean Ag lll. This, coupled with a non-negligible increase in the C-C bond length and some degree of sp3 hybridization in the adsorbed ethylene, indicates that the molecule is activated , to some extent, for subsequent surface reactions such as epoxidation. 

Reaction mechanisms with ethylene adsorbed at the second type of ethylene adsorption site identified (Ag-1/2) were also investigated. However after several unsuccessful attempts to identify a stable oxametallacy-cle associated with this ethylene adsorption site we concluded that ethylene was not reactive at this site. This is to be anticipated given that the ethylene molecule is a considerable distance from the O atoms in the oxide ring (around 4 A). 

Uranium oxides have been investigated as catalysts and catalyst components for selective oxidation. They are more commonly used as catalyst components, but there are also reports of uranium oxide alone as a selective oxidation catalyst The oxidation of ethylene over UO3 has been studied by Idriss and Madhavaram [40] using the technique of temperature programmed desorption (TPD). Table 13.3 shows the desorption products formed during TPD after ethylene adsorption at room temperature on UO3. The production of acetaldehyde from ethylene indicates... 

Halide ions can, sometimes, be promoters as in the use of chloride ion in the silver catalyzed epoxidation of ethylene. This oxidation takes place through a silver-oxygen surface complex that is charge deficient. On co-adsorption of the chloride ion, the presence of an oxychloride surface species optimizes the reaction between the electrophilic oxygen and the K electrons of the ethylene. ... 

Zimakov (120) suggested earlier that the impossibility of obtaining propene oxide from propene over silver was due to peculiarities of the propene oxide structure and the readiness of its further oxidation. However, Gorokhovatskii and Rubanik have shown that this is not so. Adsorption of propene on the silver surface seems to be different from ethylene adsorption. De Boer, Eischens, and Pliskin (121) suggest that ethylene sorbs on a silver surface covered with oxygen to form complexes... 

Faraday was the first to carry out experiments to explore why platinum facilitates the oxidation reactions of different molecules. He found that ethylene adsorption deactivates the platinum surface temporarily while the adsorption of sulfur deactivates platinum permanently. He measured the rate of hydrogen oxidation, suggested a mechanism, and observed its deactivation and regeneration. Thus, Faraday was the first scientist who studied catalytic reactions. In 1836 Berzelius (1, 2] defined the phenomenon and called it catalysis and suggested the existence of a catalytic force" associated with the action of catalysts. 

It was noted that Hg was oxidized more rapidly than C2H4 over Pd, and therefore it was concluded that adsorption of Oj is not limiting in the ethylene oxidation. The presumed mechanism is ethylene adsorption onto oxygen-covered Pd, a slow reaction of adsorbed ethylene with adsorbed 0 atoms, and further degradation steps. It may be that the C—H bond rupture is the slow step. Possible stages in the oxidation are ... 

The number of adsorbed forms of ethylene ascribed to the peaks in a TPO spectrum, increases with the adsorption temperature from 2 (or 3) observed at 250°C to 5 observed at 350°C. This implies the growing tendency towards ethylene dissociative adsorption with the reaction temperature. However, ethylene adsorption is affected not only by temperature but also by a redox state of the catalyst surface ethylene molecules are much more strongly bonded to the surface of the oxidized catalyst than of the active one, which is manifested by the temperature of the TPO maxima (compare curve a and b in Fig. 5). 

Taking n = 4, as discussed previously for ethylene adsorption, the reaction order for ethylene is predicted to be —0.25 as compared with the experimental value of —0.20. From Table 17, variation of the reaction order of the organic species for the anodic oxidation can be observed. With reference to last equation, it is evident that different reaction orders are expected if the organic species require different numbers of adjacent, free, metal sites for adsorption. For example, butadiene, which has two conjugated double bonds, should require approximately twice as many sites as ethylene for adsorption. This is in fact observed since... 

The aqueous solution layer that forms at the metal interface can ultimately provide a medium for the dissolution of Pd ions or oxidized Pd clusters into the supported liquid layer where they can then act as homogeneous catalysts. As was discussed earlier, the acetoxylation of ethylene can be carried out over various Pda,OAcj, clusters where alkali metal acetates are typically used as promoters. DFT calculations were carried out on both the Pd2(OAc)2 and Pd3(OAc)e clusters in order to examine the paths that control the solution-phase chemistry. The Pd3(OAc)e cluster is the most stable structure but is known experimentally to react to form the Pd2(OAc)2 dimer and monomer complexes in the presence of alkali metal acetates. The reaction proceeds by the dissociative adsorption of acetic acid to form acetate ligands. Elthylene subsequently inserts into a Pd-acetate bond. The cation is then reduced by the reaction to form the neutral Pd°. The reaction is analogous to the Wacker reaction in which ethylene is oxidized over Pd + to form acetaldehyde. Pd° is subsequently reoxidized by oxygen to form pd2+[35,36,44]... 

Figure 8.7. Deactivation by Ha and rejuvenation by oxidation. Ethylene adsorption isotherms at 120°C on different AgY samples, treated with Ha (at 0.5 atm for 1 h) and Oa for 0.5 h (Jayaraman et al., 2001, with permission).

Ethylene conversion to acetic acid can be increased in at least two ways. One is making the catalyst surface more acidic, which will strengthen ethylene adsorption on the surface to give more time for its oxidation to acetic acid. A more acidic surface will also facilitate desorption of acetic acid thus reducing its surface over-oxidation to carbon oxides. This approach has been realized by adding small amounts of P, B and Te to the MoVNb oxide catalyst. Table 11.2 presents the results of adding phosphorus. 

Dissociative Adsorption of Ethylene Glycol Oxidative Derivatives. 77... 

Biomaterials with Low Thrombogenicity. Poly(ethylene oxide) exhibits extraordinary inertness toward most proteins and biological macromolecules. The polymer is therefore used in bulk and surface modification of biomaterials to develop antithrombogenic surfaces for blood contacting materials. Such modified surfaces result in reduced concentrations of ceU adhesion and protein adsorption when compared to the nonmodifted surfaces. 

It is well known that pMMA and pSty in THF follow ideal GPC behavior on many common GPC columns. However, many commercially important acrylate polymers contain a wide array of other monomers. In general, acrylic polymers composed of monomers that do not contain polar groups will yield well-behaved polymers, giving ideal GPC separations. Monomers that contain polar groups should prompt the analyst to carefully evaluate the possibility of adsorption of the analyte onto the column. The most common functionalities of concern are hydroxyl groups, amine groups, ethylene oxide units, and carboxylic acids. In many cases, such monomers can be tolerated. However, the acceptable level can vary considerably with even apparently minor changes in... 

From the results of other authors should be mentioned the observation of a similar effect, e.g. in the oxidation of olefins on nickel oxide (118), where the retardation of the reaction of 1-butene by cis-2-butene was greater than the effect of 1-butene on the reaction of m-2-butene the ratio of the adsorption coefficients Kcia h/Kwas 1.45. In a study on hydrogenation over C03O4 it was reported (109) that the reactivities of ethylene and propylene were nearly the same (1.17 in favor of propylene), when measured separately, whereas the ratio of adsorption coefficients was 8.4 in favor of ethylene. This led in the competitive arrangement to preferential hydrogenation of ethylene. A similar phenomenon occurs in the catalytic reduction of nitric oxide and sulfur dioxide by carbon monoxide (120a). 

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