Redox mechanism reaction pathways

Radical ions, 33, 44 Raman spectroelectrochemistry, 45 Randles-Sevcik equation, 31 Rate constant, 12, 18 Rate determining step, 4, 14 Reaction mechanism, 33, 36, 113 Reaction pathway, 4, 33 Reaction rate, 12 Receptor-based sensors, 186 Redox recycling, 135... 

In the same way that we considered two limiting extremes for ligand substitution reactions, so may we distinguish two types of reaction pathway for electron transfer (or redox) reactions, as first put forth by Taube. For redox reactions, the distinction between the two mechanisms is more clearly defined, there being no continuum of reactions which follow pathways intermediate between the extremes. In one pathway, there is no covalently linked intermediate and the electron just hops from one center to the next. This is described as the outer-sphere mechanism (Fig. 9-4). 

The amount of the products formed over the studied catalysts, in the presence and absence of molecular O2, are listed in Table III. It is evident that the formation of the oxidation products is associated with the gas phase oxygen supply. Then, as the reaction rates in the mixture of reactant and in separate steps differ (19), these data exclude the participation of lattice oxygen in the partial oxidation of methane via a two step redox mechanism as main reaction pathway proving the occurrence of a "concerted mechanism". 

As discussed in Vol. 2, Chap. 4, experimental studies, mainly pioneered by Taube [11], revealed two different reaction pathways for redox reactions in solution (i) outer sphere mechanism characterized by weak interaction of the reactive species, with the inner coordination sphere remaining intact during the electron transfer, and reactions occurring through a common ligand shared by the metallic centers thus proceeding by an inner sphere mechanism. 

Mechanisms proposed for reactions envisage complex reaction pathways. Thus in the oxidation of methacrolein catalyzed by H3PM012O40, the chemisorption of the aldehyde precedes the redox step and is thought to occur through the acid-promoted formation of a gem-diol molybdate ester. The subsequent oxidation to methacrylic acid involves Mo(VI) undergoing reduction to Mo(V). Molecular oxygen only serves to reoxidize the reduced polyanion. 

The mechanisms by which manganese complexes and manganese superoxide dismutase react with superoxide radicals are of interest as knowledge of the kinetic parameters and the reaction pathways may allow the synthesis of model compounds with specific chemical features. These compounds may then have clinical application or may allow the control of specific redox chemistry in catalytic processes. 

The Pd-O bond also varies with the extent of oxidation of Pd. During the methane combustion reaction, the catalyst surface is a non-equilibrium, kineti-cally controlled structure. The oxygen concentration profile in the particle results from a combination of particle reconstruction, oxygen adsorption, bulk diffusion, and oxygen removal. This concentration profile varies as a function of time, and as the oxygen content increases, the Pd-O bond strength decreases. This increase is accompanied by an increase in the specific activity. The most widely accepted reaction pathway is the Mars and van Krevelen redox mechanism, which involves lattice oxygen and uneoordinated Pd centers as active species. Inhibition by products (H2O and CO2) and impurities (SO2) is a major drawback for low temperature combustion. The effect of sulfur is particularly important for catalytic converters for NGV applications because it drastically reduces the methane combustion activity. 

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