Homogeneous charge-transfer catalyst

S.3.3 Electrocatalytic Modified Electrodes Often the desired redox reaction at the bare electrode involves slow electron-transfer kinetics and therefore occurs at an appreciable rate only at potentials substantially higher than its thermodynamic redox potential. Such reactions can be catalyzed by attaching to the surface a suitable electron transfer mediator (45,46). Knowledge of homogeneous solution kinetics is often used to select the surface-bound catalyst. The function of the mediator is to facilitate the charge transfer between the analyte and the electrode. In most cases the mediated reaction sequence (e.g., for a reduction process) can be described by... 

The interconversion of the various oxidation states of Mn in natural waters is influenced by UVR through its effects on reactions involving ROS [Chapter 8] and natural phenols, photoinduced charge transfer reactions, and microbial processes. The oxidation of Mn + is slow at pH < 8.5 in the absence of a catalyst. The oxidation of Mn(ii) is faster on metal oxide surfaces than in homogeneous solution in the pH range of 8 to 9 [217], and its oxidation also can be biologically mediated in the environment [153]. In comparison to bacteria-free waters, the oxidation rate of Mn(ii) in seawater is increased dramatically by catalysis on bacterial surfaces. However, even with such catalysis, its half-life still is of the order of weeks to months in open ocean waters [153]. 

When catalysts are recycled as solid residues, it is important to exclude impurities that may piggyback —such as metal particles—as the active species. This was probed in two ways. First, the tape was removed after a first cycle, rinsed, and transferred to a new vessel. A second charge of 17 and dibutyl ether was added, but not the PhMe2SiH. The sample was warmed to 55 °C, the now off-white tape was fished out , and PhMe2SiH was added. The rate profile was similar to the first cycle (ca. 20% slower at higher conversions), consistent with predominant homogeneous catalysis by desorbed fiuorous species. Second, the second cycle of a sequence was conducted in the presence of elemental mercury, which inhibits catalysis by metal particles [57]. However, the rate profile was the same as a sequence in the absence of mercury. 

Clusters, as possible catalytic reactors, are perfectly dispersed in solutions. They are thus suitable systems for observing, under quasi-homogeneous conditions by time-resolved techniques, the kinetics of catalyzed electron transfer, which would be inaccessible on a solid catalyst. It was demonstrated that the reaction of radiation-induced free radicals COT and (CH3)2COH catalyzed by metal clusters started by the storage of electrons on clusters as charge pools and that electrons were then transferred pairwise to water-producing molecular hydrogen [22,75]. 

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