Redox mechanism intermediates

Selective oxidation and ammoxldatlon of propylene over bismuth molybdate catalysts occur by a redox mechanism whereby lattice oxygen (or Isoelectronlc NH) Is Inserted Into an allyllc Intermediate, formed via or-H abstraction from the olefin. The resulting anion vacancies are eventually filled by lattice oxygen which originates from gaseous oxygen dlssoclatlvely chemisorbed at surface sites which are spatially and structurally distinct from the sites of olefin oxidation. Mechanistic details about the... 

Eisenberg et ah—the redox mechanistic perspective over Rh and Pt/Sn systems. The term redox mechanism in catalysis is used to describe cases where the catalyst itself undergoes changes in oxidation state during the course of the mechanism. It does not refer to the oxidation state changes of reactants, products, or associated intermediates, although associated intermediates are often involved in the course of the mechanism. 

Fiolitakis and Hofmann—wavefront analysis supports Eley-Rideal/redox mechanisms. In 1982 and 1983, Fiolitakis and Hofmann231,232 carried out wavefront analysis to analyze the dependence of the microkinetics of the water-gas shift reaction on the oxidation state of CuO/ZnO. They observed three important mechanisms after treatment of the catalyst surface with different H20/H2 ratios. These included two Eley-Rideal mechanisms which converted the reactants via adsorbed intermediates, and a redox mechanism that regulated the oxygen activity, as shown in Scheme 56. The authors indicated that different mechanisms could be dominating at different sections along the length of the fixed bed reactor. 

The authors use some arguments to rule out the formate intermediate before proceeding to the discussion regarding the redox mechanism. The basis was not the activation energy barrier for OCOH formation, as it is quite low relative to other possible pathways. They rule out the OCOH as an intermediate on the basis of its proposed decomposition selectivity, which they indicate will always preferentially decompose back to CO and OH. And the isomerization to a true formate species (i.e., with a C-H bond) they indicate cannot occur by reacting OH on Cu(110) with CO. Yet intense formate bands have been observed by infrared spectroscopy upon... 

In 2007, Meunier et al460 quantified reactive exchange rates of formates observed by DRIFTS over Au/Ce(La)02 relative to the C02 product exchange and identified that not all formates at the working temperature (155 °C and 220 °C) could be implicated as the main reaction intermediates. Those found at close to the rim of the metal particles were found to be more reactive than those further away. The authors concluded that fast formate intermediates or another kind of interfacial adduct too low in concentration and not observable by DRIFTS, or a redox mechanism, were still open possibilities. Formate surface concentrations observable by DRIFTS were estimated by a calibration curve developed using Na-formate as a reference. 

However, the competition between Srn 1 and polar abstraction mechanisms is complicated in certain reactions by the formation of disulfides which is inhibited by radical and radical anion traps, and requires photolysis [23, 24]. These results implicate a third possibility, the chain SET redox mechanism (Srt2, i.e. substitution, electron transfer, bimolecular), Scheme 10.34. This alternative mechanism occurs when the intermediate radical anion can be intercepted by the thiolate (Equation 10.23) prior to the dissociation required in the SrnI mechanism (Equation 10.17 in Scheme 10.29). It becomes possible when either... 

The general redox mechanism of metal-oxide catalyzed oxidation of hydrocarbons involves two major stages in the catalytic process, reduction of the surface layers by hydrocarbons and their reoxidation by interaction with oxygen. While these two stages occur simultaneously in a reactor with the catalyst working under steady-state conditions, they can be carried out in two separate reaction zones in a reactor with catalyst circulation [37]. A hydrocarbon is fed into the first zone where a desirable intermediate product of partial oxidation is formed after interaction with the oxidized catalyst. In the second zone, gas phase oxygen reoxidizes the catalyst. Obviously, the residence time of the catalyst in the first zone should be short enough to prevent formation of an inactive reduced state of the catalyst. If only surface layers participate in the interaction with hydrocarbons, the time of catalyst reduction is approximately several seconds. 

The extent of chemical reversibility of the ECE electron transfers depends either on the type of enol or on the solvent. In general, non-coordinating dichlomethane favours the chemical reversibility as opposed to the coordinating acetonitrile. Furthermore, the ECE mechanism can be in some cases enriched or complicated by further intermediates, thus making in some cases the voltammetric profiles more intriguing. Such a complication can be observed in the redox mechanism involved in the oxidation pathway of the dimesityl... 

Some degree of controversy exists in the literature with respect to the reaction mechanism and the rate-determining steps during WGS catalyzed by the commercial catalysts listed above, especially for the CuZn catalyst. In the case of the HTS fer-rochrome catalyst, Rhodes et al.9 concluded that the most convincing published evidence favors mechanism that involves water dissociation in a redox step on Fe2+/ Fe3+. However, for the LTS CuZn catalyst type, evidence suggests that copper active sites are responsible for both dissociative adsorption of water required by a redox mechanism and the formation of a COOH (formate) intermediate that is subsequently decomposed by similar sites.9... 

The redox mechanism operates with late transition metals, as exemplified by Mn, Fe, and Ru, which have easily accessible multiple oxidation states. Theoretical studies and trapping experiments suggest that in porphyrins and salen compounds, the mechanism involves the formation of a radical intermediate. 

It has been established in literature that the formation of the propylene occurs on the add centers (1-7). On the contrary, the formation of acetone and MAA occurs via a redox mechanism, even if it was proposed that possibly different kinds of molybdenum sites can be involved in the two reactions (4,7). A common reaction intermediate has been proposed for MAA and acetone, but disagreement exists about the nature of this intermediate, either cationic (underlying the importance of surface addity) (9,12) or radicalic (1). 

We have previously shown that the kinetics of the reverse water gas shift (RWGS) reaction can be modelled on the basis of a simple redox mechanism (1). This result contradicted an earlier assertion deriving from a kinetic analysis of the reaction which claimed that the reaction proceeded through a formate intermediate (2). The individual elementary reactions of the redox mechanism, which in combination constitute the overall reaction, are listed in table 1, together... 

The kinetics of the photoeatalytic reduction of NO by CO to CO2, N2O, and N2 over MoOa/Si02 catalysts at ambient temperature has been studied mass-spectroscopically using C labeled carbon monoxide. The kinetic data obtained for CO-NO mixtures of different compositions fit well the proposed redox mechanism, which suggest a paramagnetic complex (Mo ". ..NO ") as reaction intermediate. The formation of this complex is proven by EPR experiments. 

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