Role of the Oxidant

The influence of pH on the course of an oxidation is definite, affecting not only the velocity of the reaction, but the direction as well. The inhibiting effect of accumulated hydrobromic acid in bromine oxidations was shown by Bunzel and Matthews and others in this case, the velocity of the reaction was reduced. The influence on the direction of the reaction was shown by Lai and Mukherji in the electrolytic oxidation of D-glucose, too high an acidity led to the formation of saccharic acid. The production of the latter may have been either a secondary reaction starting from D-gluconic acid, or a primary reaction. In either case the desired primary reaction was obscured to a certain extent. 

The course of the reaction can be determined not only by the amount of product formed but also by the change in concentration of the actual oxidant. It is often difficult to determine the true concentration of the oxidant, because of its transient nature, or to determine in what form the oxidant is reacting with the sugar. Bunzel and Matthews studied the rate of reaction of bromine with D-glucose quite extensively and determined the specific reaction rates. The reaction rates were related to the bromine concentration and it was concluded that the free halogen, rather than hypobromous acid, was the oxidant. 

The study of the effect of pH on the reaction was not too satisfactory. As discussed previously (page 151), the oxidation is inhibited by acids, especially by hydrochloric and hydrobromic acids. It was concluded that the hydrobromic acid formed in the reaction, or added as an inhibitor, repressed the formation of the negative D-glucose ion, CaHuOd , so that the main oxidation was that of the cation CaHisOa after the initially neutral solution became acid. The increased hydrogen ion concentration drove reaction 27 to the left The cation was considered to be 

The considerably greater effect of hydrobromic and hydrochloric acids in inhibiting the oxidation appears to involve more than just acidity and might be considered as a repression of the available oxidant. This was considered by Bunzel and Matthews. If bromine is the oxidant, two reactions (29 and 30) might be involved. However, we are more 

For weakly acidic systems (pH 5-6) in which the accumulation of hydrobromic acid is prevented by buffering agents such as calcium carbonate or benzoic acid salts, more information is available. Isbell and Pigman have made an extensive study of such systems, including a thorough consideration of the effect of the concentration of total bromine, free bromine, hypobromous acid and bromide ion on the velocity of the reaction. The results very definitely showed a direct correlation between free bromine concentration and the velocity of the oxidation. No such correlation could be found with hypobromous acid. The results are shown in Tables VII and VIII. The velocity constants were determined for a- and for 8-D-glucose. In the table for /S-D-glucose, in experiments 2 and 5, the hypobromous acid concentration varied 1 10 but the reaction rate varied 1 3. The variations in free bromine concentration follow the variations in the reaction rate constants and the kf values are based on the assumption that free bromine is the oxidant. The concentration of the oxidant (a in equation 31) is therefore the concentration of free bromine. 

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Another contribution to die reaction involving steam is thought to be the role of the oxide support in the provision of hydrogen to the surface of adjacent catalyst particles. It is suggested drat die water molecule is adsorbed on the surface of oxides such as alumina, to form hydroxyl groups on the surface, thus... 

C20-0084. In superoxide dismutase it is the Cu center that oxidizes 0. Why is copper more suitable than the center for the role of the oxidizing agent in SOD ... 

The catalytic role of the oxide surface can be seen in terms of forming or providing oxygen in an activated state, which then permits a new reaction pathway characterized by a lower energy barrier, with the other reactants either in the gas phase or as an adsorbed species on the surface. Such reactions may modify both the electronic levels and the surface structure of the oxide, but it should be kept in mind that for a catalyst such modification will reach a dynamic equilibrium in which restoration of electrons and replenishment of vacancies by oxygen must balance their removal by reaction products. In this sense, many of the model systems studied are unrealistic since the changes to the surface are irreversible. 

In the preceding section, we explained that the bulk diffusion of oxide ion plays an important role in the enhancement of the catalytic activity of the multicomponent bismuth molybdate systems. Here, another important role of the oxide ion migration in increasing the stability of the catalyst system is introduced. 

The mechanistic role of the oxidant can be understood as follows. After addition of the amide ion to the azine (for example pyrimidine in Scheme 2) an anionic a-adduct is formed, which is quite stable in this polar medium, as shown by NMR spectroscopy (see Section IY.A). 

Since the hydride is a very poor leaving group, the role of the oxidant is to provide, in a redox-type reaction, two electrons to convert this anionic adduct into a cationic species, which aromatizes into the amino product by proton elimination (Scheme 2). It cannot be excluded that under these conditions a one-electron process occurs, yielding a pyrimidyl radical, that aromatizes by loss of a hydrogen atom. 

However, as data on the photocyclization of benzanilide-type enamides including a heteroaromatic ring have accumulated (39), the formation of the cyclized lactam by irradiation of this type of enamide seems to depend on the aromaticity of the ring involved. Although oxidative photocyclization can be applied to almost all types of enamides, the role of the oxidizing agent has not been precisely determined. 

Therefore, the formation of a dehydrolactam by irradiation of enamides under nonoxidative or unrestrictive conditions can be assumed to proceed via the route involving a 1,5-hydrogen shift of the Hb proton from the cyclic intermediate A to afford a thermally very unstable lactam 10 with a dihydrobenzene structure. This compound (10) would then undergo facile dehydrogenation even at room temperature to afford the dehydrolactam (9) as the final product, although the actual role of the oxidizing agent remains to be clarified. 

We further initiated theoretical studies to explore the role of the oxide species in the OAG process. The gas-phase composition of silicon oxide clusters evaporated by laser ablation or thermal treatment should be considered to be important in the SiNW synthesis. We first used density functional theory (DET) calculations to study the nature of the Si 0 n, m = 1-8) clusters formed in the gas phase during OAG... 

The uptake(pH) curves for the systems metal cation—activated carbon reported in the literature are very divergent, and they indicate significant role of the oxidation of the surface on the one hand and of the impurities on the other. These factors were often not controlled, thus it is difficult to quantify their effect on the adsorption. 

Thus, the surface serves to accumulate both the ester (by complex formation and electrostatic attraction) and the nucleophile (by electrostatic attraction alone), facilitating reaction. In fact, the overall first-order rate constant for hydrolysis (Ar ) reflects this concentration effect Atj, reaches its highest value at the pH where the product [>Al-MPT][OH"ld is at a maximum. No other role of the oxide surface in promoting hydrolysis need be postulated. 

The results shown in previous sections suggest the crucial role of the oxidant supply system, in particular the effect of the stoichiometric ratio (air flow rate) on FCS efficiency and dynamic performance. In order to give further information about this issue, in this paragraph different air management strategies are closely examined, with particular reference to their influence on cell voltage uniformity and air compressor parasitic losses. 

The role of the oxidation-reduction couple in the catalytic decomposition of hydrogen peroxide,... 

However, an alternative mechanism similar to that described in scheme 2, that considers the oxidative addition of aniline to the Rh° finely dispersed on the support, cannot be completely excluded. The evolution of carbamoyl intermediate to DPU should occur still via iodoformamide. The last mechanism could be also operative in the reductive carbonylation of nitrobenzene, when aniline is necessary for its conversion. In this case, the reaction could be better considered as an oxidative carbonylation process in which the nitrobenzene is playing the role of the oxidant in place of the oxygen. It has been ascertained that under these conditions the carbonylation occurs with the stoichiometry of reaction (11) [14], different from the one reported in reaction (4). 

The kinetics and mechanism of oxygen reduction and evolution reactions at oxide-covered platinum electrodes was studied by Damjanovic in the 1990s, as a continuation of their previous work. The role of the oxide film in these processes was analyzed and model assumptions were discussed. 

An illustration of the role of the oxidation state in the distribution of the metal particles across the CP layer was presented in single-step potential deposition experiments on platinum in sulfonated PANl that were carried out at two values of the potential (-0.2 and -1-0.2 V vs. Ag/AgCl reference electrode), corresponding to the reduced and oxidized states of PANI [ 101 ]. In the first case a homogeneous distribution of Pt across the entire CP layer up to the underlying carrying substrate was observed, whereas in the second case (oxidized PANI) the Pt content decreased steeply within a narrow region close to the polymer/solution interface [101]. 

For explaining the role of the oxide in Au-based WGS catalysts, one must understand well the catalytic process on pure gold systems. Figure 6.9 shows the calculated energy profile for the WGS on periodic Au(lOO) and Cu(lOO) surfaces. Copper surfaces are the typical benchmark for studies of the WGS on metal sur-... 

Ferrandon M, Krause T (2006) Role of the oxide support on the performance of Rh catalysts for the autothermal reforming of gasoline and gasoline surrogates to hydrogen. Appl Catal A 311 135-145... 

The effect of calcination versus reduction. In this case, catalyst activities were first obtained using the 10-port parallel microreactor to determine the main effects of particle size and pretreatment for several samples in a single run. Then, detailed kinetic information was obtained in the recycle microreactor. Because the activity was measured under oxidizing conditions, we were not sure if, under reaction conditions, the surface was oxidized or reduced, so this question was addressed first. To stabilize the state of the surface and to ascertain the role of the oxidized versus the reduced surface, prior to each run, the catalysts were pretreated either in air or... 

Recent work has been carried out by Conway and Novak " " on the role of the oxide film, plus coadsorbed CP ion at Pt electrodes in the chlorine evolution reaction optimum electrocatalysis appears to arise at a surface at which appreciable coverage of specifically adsorbed Cl" ion exists together with an incomplete film of OH and O species. Thick oxide films on Ir and Ru electrodes are also very effective electrocatalysts. 

The role of the oxide layer in this approach was limited to the modification of the frequency factor for electron tunneling. The probability of electron tunneling and hence the current density is then theoretically expected to be inversely exponentially dependent on the layer thickness. Schultze and Vetter showed experimentally that the current at constant overpotential indeed does decrease exponentially with the increase of the oxide layer thickness, calculated from the charge for the cathodic stripping of the oxide, assuming constant composition and density of this oxide. 

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