Dissociative oxygen adsorption

Figure 2.34. The effect of Cl coverage on the rate of oxygen dissociative adsorption on Ag(l 10).96 Reprinted with permission from Elsevier Science.

Despite the poisoning action of Cl for oxygen dissociative adsorption on Ag, it is used as moderator in the ethylene epoxidation reaction in order to attain high selectivity to ethylene oxide. The presence of Cl adatoms in this... 

The oxygen dissociative adsorption energy on the island (relative to free O2) with and without support Xs, are computed as... 

Oxygen separation from the atmosphere requires membrane catalysis interfacial requirements compatible with promoting initial oxygen dissociative adsorption at the reducing surface and subsequent electron transfer leading to formation of oxygen anions (O -). This may be represented by Eq. (7.23) ... 

The EXAPS results were obtained in situ but not under operando conditions, and thus it can only give information about the initial state of the surface. The most important result obtained from these experiments is that no Pt-S signal fit the results in the SOj poisoned catalysts. This suggest that, in this case, S is bonded to O species adsorbed on Pt, which agrees well with the conclusion that S is inhibiting oxygen dissociative adsorption instead of CO adsorption. 

Activation by adsorbed oxygen Dissociative adsorption of oxygen CaO, La203 La203... 

The adsorption of C02 on metal surfaces is rather weak, with the exception of Fe, and no molecular or dissociative adsorption takes place at room temperature on clean metal surfaces. At low temperatures, lower than 180 to 300 K, a chemisorbed COf" species has been observed by UPS6 on Fe(lll) and Ni(110) surfaces, which acts as a precursor for further dissociation to CO and adsorbed atomic oxygen. A further step of CO dissociation takes place on Fe(l 11) above 300 to 390 K. 

Equations (3.16) and (3.17) describe the dissociative adsorption and, recombination of oxygen on a donor D. The transfer between the donor D and acceptor A is described by eq. (3.18). The spillover oxygen (O) is a mobile species which is present on the acceptor surface without being associated with a particular surface site. The mobile spillover species can interact with a particular surface site B forming an active site C (eq. 3.19). Eq. (3.20) represents the deactivation of the active site C by interaction with a reactant E. 

H2 adsorption is weak on the anatase surfaces [8], No dissociative adsorption of H2 takes place over the smooth surfaces of Au at temperatures below 473 K [9,10]. On small Au particles, adsorption is possible at low temperature. Dissociative adsorption of H2 can be accelerated by the negatively charged molecular oxygen species at steps, edges, comers of Au particles [5]. 

The reaction mechanism is fairly straightforward. It starts with the dissociative adsorption of oxygen on silver ... 

Therefore, a bifUnctional mechanistic scheme, including the participation of both the metal (via the adsorption of CO) and the support (via the formation of "oxygen vacancies" which are active sites for the H2O dissociative adsorption) seems quite relevant to explain the specific behavior, for the NO r uction in the presence of water, of samples containing Zr02 Such active sites would be located at the met -support interface and are linked to the redox properties of the support... 

Methane reforming with carbon dioxide proceeds in a complex sequence of reaction steps involving the dissociative adsorption/reaction of methane and COj at metal sites. Hydrogen is generated during methane dissociation In the second set of reactions CO2 dissociates into CO and adsorbed oxygen. The reaction between the surface bound carbon (from methane dissociation) and the adsorbed oxygen (from CO2 dissociation ) yields carbon monoxide. A stable catalyst can only be achieved if the two sets of reactions are balanced. 

In our previous work [11], it has been shown that the reduction of NO with CH4 on Ga and ln/H-ZSM-5 catalysts proceeds through the reactions (1) and (2), and that CH4 was hardly activated by NO in the absence of oxygen on these catalysts. Therefore, NO2 plays an important role and the formation of NO2 is a necessary step for the reduction of NO with CH4. In the works of Li and Armor [17] and Cowan et al. [18], the rate-determining step in NO reduction with CH4 on Co-ferrierite and Co-ZSM-5 catalysts is involved in the dissociative adsorption of CH4, and the adsorbed NO2 facilitates the step to break the carbon-hydrogen bond in CH4. It is suggested that NO reduction by use of CH4 needs the formation of the adsorbed NO2, which can activate CH4. 

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