Oxygen evolution reaction intermediates

Platinum has also had its share of attention in recent years. The effect of phosphoric acid concentration on the oxygen evolution reaction kinetics at a platinum electrode using 0-7 m-17-5 m phosphoric acid at 25°C has been studied with a rotating disc electrode . The characteristics of the ORR are very dependent on phosphoric acid concentration and H2O2 is formed as an intermediate reaction. Also, platinum dissolution in concentrated phosphoric acid at 176 and 196°C at potentials up to 0-9 (SHE) has been reported . 

The polarization curves for the oxygen evolution reaction are more complex than those for hydrogen evolution. Usually, several Tafel sections with different slopes are present. At intermediate CD their slope b is very close to 0.12 V, but at low CD it sometimes falls to 0.06 V. At high CD higher slopes are found at potentials above 2.2 V (RHE) new phenomena and processes are possible, which are considered in Section 15.6. 

Ru losses can occur during electrolysis, as well as those due to shorting in mercury cell operations, from erosion, loss of Ru-based intermediates involved during the course of chlorine and oxygen evolution reactions and during shut-downs. 

Several oxides in the above family have been used for photocatalytic studies, especially for hydrogen and oxygen evolution reactions. Studies on the Dion-Jacobson series are presented first, followed by studies on the Ruddlesden- Popper series and finally on the intermediate series. Table 10 lists oxides employed for photocatalysis studies. 

The distinction between active and nonactive behavior and the underlying mechanistic explanation are supported by several experimental observations, including measurement of the concentration of reactive intermediates in the oxygen evolution reaction, such as hydroxyl radicals produced by discharge of water. In fact, hydroxyl radicals produced at nonactive electrodes can be intercepted by using p-nitrosodirnelhylaniline as a selective scavenger [170]. 

Fig.3 Reaction scheme for the oxidative mineralization of an organic toxin using an active-oxygen intermediate that is involved in the oxygen-evolution reaction.

The oxygen evolution reaction is, similar to the hydrogen evolution reaction, complicated by several intermediates. It is generally admitted that in an acid medium the overall reaction is ... 

The oxygen evolution reaction occurs at the anodic layer solution interface, whereby oxygen atoms are obtained as intermediate products. 

Recently, however [1-19], we have demonstrated that the electrochemical oxidation of organics can be achieved via the intermediates involved in the oxygen evolution reaction at potentials largely above the theimod3mamic potential of oxygen evolution (1.23V/SHE under standard conditions). Even if in this process electrical energy should be consumed, this system opens new possibilities for treatment at room temperature of very toxic organic pollutants present in industrial wastewater. 

The oxygen evolution reaction occurs at large positive potentials where most metal electrodes are covered with thin (or rather thick) native oxide layers. This implies that studies of the reaction mechanism on metal oxide electrodes are very important. In this respect, the photooxidation reaction of water on an n-Ti02 electrode (2) is very interesting. A number of studies have been made on the mechanism of this reaction (i-77), but the details still remain unclear because the water oxidation is a four-electron process and will proceed via many reaction intermediates at the electrode surface. 

The oxygen evolution reaction at the octane/water interface occurs upon illumination at the wavelength 670 nm, so that the thermodynamic possibility of the reactions following the mechanism of Eqs. (1-3) is excluded. For a three-electron reaction (4) a stronger oxidant than the cation radical of chlorophyll, bivalent cation of chlorophyll and the cations of a hydrated oligomer of chlorophyll is necessary. It should be noted that Eqs. (1-3) are thermodynamically possible if the intermediate particles formed are in the adsorbed state. 

The potentials for the onsets of anodic currents vary significantly (1.4 to 4.1 V vs. Ag/Ag+) for the non-aqueous electrolytes examined. The anodic currents appear to be caused by the decomposition of the organic solvent. As mentioned above, there are no differences of the potentials for the onsets of anodic currents between the diamond and the other carbon-based electrodes. Therefore, this decomposition does not appear to involve adsorbed intermediates, which are involved in the hydrogen and oxygen evolution reactions from aqueous electrolytes, but instead may involve outer-sphere one-electron transfers. 

In the reaction following the second pathway, the 0-0 bond is not broken while the first two electrons are added it is preserved in the HjOj produced as an intermediate, and breaks in a later step, when the hydrogen peroxide is reduced or cat-alytically decomposed. An analog for this pathway does not exist in anodic oxygen evolution. 

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