Oxygen reduction steps

Depending on the electrode material, the ORR can either proceed via a direct 4e" process transferring oxygen directly to water. The other option would be a sequential reaction mechanism involving two 2e" steps and H2O2 as an intermediate on metallic electrodes. The direct oxygen reduction step is catalyzed by... 

A striking example of the interaction of solution velocity and concentration is given by Zembura who found that for copper in aerated 0-1 N H2SO4, the controlling process was the oxygen reduction reaction and that up to 50°C, the slow step is the activation process for that reaction. At 75 C the process is now controlled by diffiision, and increasing solution velocity has a large effect on the corrosion rate (Fig. 2.5), but little effect at temperatures below 50 C. This study shows how unwise it is to separate these various... 

The half-wave potentials of these steps are approximately — 0.1 and — 0.9 V (versus the saturated calomel electrode). Hie exact stoichiometry of these steps is dependent on the medium. Hie large background current accruing from this stepwise oxygen reduction interferes with the measurement of many reducible analytes. In addition, the products of the oxygen reduction may affect the electrochemical process under investigation. 

For the additivity principle to hold, steps (8) and (9), the anodic processes taking place under nitrogen, would be coupled only with the cathodic reduction of oxygen in step (11). It is step (10), the attack by the constituents of the oxygen couple on the intermediate Cu, which lies outside the scope of the principle and which explains the observed findings. 

Other reachons involving oxygen are those reachon steps in oxygen reduction which are of importance in their own right, such as the formation of hydrogen peroxide as a relahvely stable intermediate ... 

As with the phase diagrams and Pourbaix diagrams, the theoretical standard hydrogen electrode also allows us to calculate the relative energies of intermediates in electrochemical reactions. As an example, we investigate the oxygen reduction reaction (ORR). We look at the four proton and electron transfer elementary steps ... 

Anderson AB, Cai Y, Sidik RA, Kang DB. 2005. Advancements in the local reaction center electron transfer theory and the transition state structure in the first step of oxygen reduction over platinum. J Electroanal Chem 580 17-22. 

Hartnig C, Koper MTM. 2002. Molecular dynamics simulation of the first electron transfer step in the oxygen reduction reaction. J Electroanal Chem 532 165-170. 

Kuzume A, Herrero E, Feliu JM. 2007. Oxygen reduction on stepped platinum surfaces in acidic media. J Electroanal Chem 599 333-343. 

Macia MD, Campina JM, Fehu JM. 2004. On the kinetics of oxygen reduction on platinum stepped surfaces in acidic media. J Electroanal Chem 564 141 -150. 

For mixed lanthanide-transition metal clusters, Yukawa et al. have synthesized an octahedral [SmNi6] cluster by the reaction of Sm3+ and [Ni(pro)2] in nonaque-ous medium [66-68]. The six [Ni(pro)2] ligands use 12 carboxylate oxygen atoms to coordinate to the Sm3+ ion, which is located at the center of an octahedral cage formed by six nickel atoms. The coordination polyhedron of the central Sm3+ ion may be best described as an icosahedron. The [SmNir, core is stable in solution but the crystal is unstable in air. The cyclic voltammogram shows one reduction step from Sm3+ to Sm2+ and six oxidation steps due to the Ni2+ ions. Later, similar [LaNis] and CjdNif> clusters were also prepared. 

Concerning the reduction step of the redox reaction, the heterotrophic microorganisms may use different electron acceptors. If oxygen is available, it is the terminal electron acceptor, and the process proceeds under aerobic conditions. In the absence of oxygen, and if nitrates are available, nitrate becomes the electron acceptor. The redox process then takes place under anoxic conditions. If neither oxygen nor nitrates are available, strictly anaerobic conditions occur, and sulfates or carbon dioxide (methane formation) are potential electron acceptors. Table 1.1 gives an overview of these process conditions related to sewer systems. 

---