Dissociatively adsorbed oxygen

Several authors have studied the kinetics in detail and have proposed kinetic models. According to Trimm and Gabbay [329], the kinetics of the reaction over a Sn/Sb = 1/4 catalyst is best described by a Langmuir— Hinshelwood type of model, which reflects that hydrogen abstraction from butene, involving dissociatively adsorbed oxygen, is rate-controlling viz. 

With 0CO > 1/3 (i.e., for coverages beyond the completion of the y/3 x y/3 R 30° structure) a Pd(lll) surface is no longer able to dissociatively adsorb oxygen. Since this is a necessary prerequisite for C02 formation, the reaction is inhibited by CO if its coverage is too high. At lower CO concentrations on the surface oxygen can be co-adsorbed. Both components then form separate domains on the surface [competitive adsorption (182)] as becomes evident from LEED observations (172). The mean domain diameter is at least of the order of 100 A i.e., the coherence width of the electrons used with this technique. This indicates the existence of repulsive interactions between Oad and COad. As can be seen from the schematic sketch of Fig. 32b, eventual product formation can then only occur along the boundaries of these islands. 

Oxygen dissolved in the Ir can be identified it is unreactive towards CO or H2 even at 1100 K. Oxygen adsorbed on Ir is highly reactive to CO. Warming a surface which had NO adsorbed upon it at 170 K to 323 and 373 K resulted in loss of NO signal presumably due to NO dissociation. Adsorbed oxygen can block sites which would otherwise catalyse NO decomposition. 

It is well established that ethylene does not adsorb on a pre-reduced silver surface.On a pre-oxidized silver surface, frontal chromatography has demonstrated that ethylene can be adsorbed in both a reversible and an irreversible form. However, there is also strong evidence, obtained using a transient response technique, that the reversibly adsorbed ethylene plays no part in the oxidation reaction at least in the temperature range 353—433 The same work showed that a stable intermediate is present, which leads to carbon dioxide. Such an intermediate was not seen for the epoxidation reaction. The intermediate had the stoicheiometry C H O of 1 2 (1—2). Although this species is formed by the interaction of ethylene with dissociatively adsorbed oxygen it is apparently decomposed to carbon dioxide and water by adsorbed molecular oxygen. 

Epoxidation of butadiene occurs by addition of dissociatively-adsorbed oxygen to one of the localized C=C bonds to form epoxybutene. The addition of oxygen across the terminal carbon atoms does not occur to any measurable extent. The direct participation of molecular oxygen can be ruled out based both on selectivity arguments as well as the kinetic model for the reaction. The kinetics imply a dual site mechanism. One site, which is unpromoted, serves as the site for butadiene adsorption, while the second site, which is promoted, functions as the site for dissociative oxygen adsorption and epoxybutene formation. 

If we apply the "6/7" rule (see Sachtler (17) for explanation) typically cited as evidence for the role of molecular O2 in selective epoxidation of ethylene for the case of butadiene epoxidation, we would not expect selectivity for epoxybutene to exceed "11/12", or 91.7%. In fact, selectivities of 93-96% are typically seen at all reaction conditions. Selectivities of 97-98% are observed at differential conditions and lower reaction temperatures. Therefore, based only upon the observed selectivities to epoxybutene, dissociatively-adsorbed oxygen is clearly the active oxygen in butadiene epoxidation. Further, the kinetic model, which has been derived from the kinetic plots in Figure 5 has been used to very satisfactorily fit a wide variety of reaction data from several different reactor formats, assumes dissociatively-adsorbed oxygen at both promoted and unpromoted Ag sites. The oxygen incorporated into epoxybutene is dissociatively-adsorbed oxygen, not molecular oxygen. 

Such non-selective combustion of CH would likely occur on acidic sites while CHg surface species may be oxidized on dissociatively adsorbed oxygen atoms and/or molecular oxygen. In a similar manner the coupling products C2H and C2H4 may be non-selectively oxidized. 

In these expressions, a = ft3/(ft 3 + ft4), (3 = Kq KCi I is the intercept and S the slope of plots according to eqn. (41b). The fact that linear plots resulted forN = 0.5 provided support for the involvement of dissociatively adsorbed oxygen in the alkane photo-oxidation. In general, the experimental data of Formenti et al. could be adequately accounted for. 

At low temperature, the reaction rate can be expressed by a Rideal-Eley mechanism where dissociatively adsorbed oxygen is assumed to be in equilibrium with gas-phase O2 and to react with gaseous CH4. The kinetic rate expression is given by Arai et al. (1986)... 

At the very beginning of the reaction, hydrogen is stripped from ammonia by dissociatively adsorbed oxygen forming NHx and OH species. These... 

In the case of dense and regular crystalline lattice, the isotope transfer implies the exchange between neighboring oxygen atoms included into the crystalline lattice. In the defect-rich structure, dissociatively adsorbed oxygen can diffuse easily into the bulk via the defects and domain boundaries. The rate of oxygen diffusion is thus determined by the length and surface of boundaries and by the concentration of defects. 

Here denotes a Pt vacant site and the subscripts ad and s indicate, respectively, an adsorbed species and a species in solution close to the electrode surface. At variance with heterogeneous catalysis at solid/gas interface, where the oxidant is dissociatively adsorbed oxygen Oad ill electrocatalysis, oxygenated species... 

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