Dual sites mechanism

They indicated that both single- or dual-site mechanisms could be postulated as follows. 

For the dual-site mechanism assume (as Mathur and Thodos did) that the reaction step can be written as ... 

The authors proposed a dual-site mechanism whereby H20 is activated on the ceria at defects and CO adsorbs on Pt, the Pt-CO then reacts with the activated H20 to produce an intermediate on the metal, with decomposition of the intermediate occurring at the metal, as shown in Scheme 84. 

Sachtler [195] proposed a dual-site mechanism in which the hydrogen is dissociated on the Ni surface and then migrates to the substrate that is coordinated to the adsorbed dimeric nickel tartrate species. In their model, adsorption of modifier and reactants takes place on different surface atoms in contrast to Klabunovskii s proposal. Adsorbed modifier and reactant are presumed to interact through hydrogen bonding (Scheme 14.5). The unique orientation of adsorbed modifier molecules leads to a sterically favored adsorbed reactant configuration to achieve this bonding. 

Also, for both the one-molecule and the two-molecule cases the reaction might be catalyzed simultaneously by two different kinds of sites. Still another complexity arises when two kinds of surface sites are required for different steps of the reaction sequence. Such a dual-site mechanism can involve diffusion between solid phases. 

Probably, with ethyl substituent, a 7t-allylic adsorption [such as suggested by Paal and Dobrovolszky (97)] competes with the dual site mechanism. 

C5 cyclization requires stricter geometric conditions than aromatization. This is in favor of the dual-site mechanism of C5 cyclic reactions (25). All metals catalyzing it have an fee lattice, and their atomic diameter lies between 0.269 and 0.277 nm. These two criteria must be fulfilled simultaneously. With such a distance between the two sites, the screening of the C—C bond adjacent to the preferably adsorbed tertiary C atom becomes evident. Figure... 

If hydrogen occupies all sites, the dual-site mechanism may operate over two adjacent /2g sites 42). The importance of active site periodicity and the screening of the adjacent C—C bond is valid in this case, too. This (assumedly adsorbed) hydrogen does not participate in C5 cyclic reactions. There is some indication, however, that it might be mobilized for cyclobutane ring opening 97, 97a). 

Dual site mechanism for the dehydrogenation of propane over Ga-HZSM-5, Zn-HZSM-5... 

It is also possible to formulate these reactions as dual site mechanisms, instead of the single site type given above, viz. 

The authors prefer reaction (a) as the main route to ethylene oxide and reaction (d) to combustion products. Nevertheless, they find that selectivity is not a strong function of temperature, suggesting that there is a common rate-determining step. Recently, they studied the effect of dichloro-ethane as a moderator and concluded that such a compound decreases the activity because silver sites are occupied by chloride ions. But the selectivity is higher because the single site mechanism is first order in surface species (epoxidation) and the dual site mechanism second order (combustion). 

Sachtler proposes a "dual site" mechanism where the hydrogen is dissociated on the Ni surface and then migrates to the substrate which is coordinated to the adsorbed nickel-tartrate complex. In this context it is of interest that the well known Sharpless epoxidation probably takes place on a dimeric tartrate complex of Ti. Sachtler suggests that both the anion and the cation have a function which varies according to the conditions used. It is not clear whether the spillover mechanism is also proposed for the reaction in solution [55]. 

While complete agreement has not yet been reached with regard to the exact nature of the active sites, it does seem relatively well established that the selective oxidation reaction occurs via a dual-site mechanism. The hydrocarbon interacts with a reduction site the oxygen interacts with a reoxidation site. Moreover, these sites may be somewhat independent of one another, allowing a simple redox cycle to be used to describe the overall reaction. 

The etherification of two 1-octanol molecules to di-octyl-ether is more important. Nijhuis et al. [29] suggested a dual-site mechanism and a second-order reaction for the alcohol ... 

Results. The results demonstrated in an unambiguous manner that over the temperature range 500-1100°K the surface reaction proceeded through a dual site mechanism by reaction between adsorbed molecules of oxygen and ammonia in accordance with the rate equation ... 

This retardation of reaction rate with increase in reactant partial pressure is characteristic of catalytic reactions controlled by a surface reaction mechanism. Langmuir-Hinshelwood surface reaction rate mechanisms for single and dual site mechanisms are respectively ... 

In view of the uncertainty in determining whether single or dual site mechanisms controlled the dehydrogenation reaction, simple single site La ngmuir-Hinshel wood expressions for parallel and series coking were used to fit the experimental cokinjg data. For a parallel mechanism. 

Three types of elastomeric polypropylene are known (1) a large molecular weight atactic polymer [125], (2) polymers with alternating isotactic and atactic blocks [108,109,126-129], and (3) polymers formed by the dual-site mechanism [110]. The structure of the obtained elastomeric polypropylene was elucidated by comparison of the 13C-NMR spectra with those of an atactic oily polymer and isotactic polypropylene prepared by the zirconium complex 4. On the basis of the NMR data, for all the samples the statistical lengths of the isotactic blocks between two neighboring epimerization stereodefects were calculated [41]. It was found that for the isotactic polymer 90%), the... 

Another example of a dual-site mechanism is the reaction between two adsorbed species ... 

It is suspected that the reaction may involve a dual-site mechanism, but it is not known for certain. It is believed that the adsorptionequilibrium constant for cyclohexanol is around 1 and is roughly one or two orders of magnitude greater than the adsorptionequilibriumconstants for the other compounds. Using these data ... 

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. 

The first step is to develop equations for all possible controlling steps. They will include, among many possibilities, dissociation of one or more of the reactants, the common single-site mechanism for reactant decomposition, the dual-site mechanism where two sites are involved in the decomposition, and half-site mechanism where two molecules are adsorbed on a single site. Having formulated sufficient models (usually 15 to 20 are even excess of 100), various experimental and sophisticated statistical methods are available for selecting the most probable model, as described in many texts (e.g., Froment and Bischoff, 1990). A comprehensive stepwise procedure (not discussed here) is suggested by Doraiswamy (2001). 

A third dual-site mechanism is the reaction of two species adso on different types of sites S and S, such as reaction of CO with... 

We now consider a dual-site mechanism, which is a refomiing reaction found in petroleum refining to upgrade the octane number of gasoline. 

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