Superoxide species

Figure 8.4. The FTIR spectra of adsorbed superoxide species during exchange of pre-oxidized Ceo63 ro 3702 with 1802 [14].

The specific role of OSC materials in NO activation and NO dissociation has largely been confirmed by many authors over Pt-Rh [87,88] and Pd catalysts [89,90] or even over bare OSC oxides [91]. By EPR, Lecomte et al. [87] evidence the presence of 02 superoxide species over a Pt—Rh/Al203 catalyst modified by ceria. The formation of these species could be closely related to the performance of the Pt—Rh/Ce02—A1203 catalyst in CO+NO reaction. 

The correlation between the concentration of the superoxide species, A and B, and catalytic activity is further illustrated in Tables LIII and LIV. A TS-1 sample (without any trace of anatase) as well as another one containing some anatase were prepared by the method of Thangaraj et al. (138) (with some minor modifications). A sample of TS-1 (fluoride) was prepared in a fluoride medium. 

Fig. 10.10 c60 is a very effective scavenger of superoxide species. The standard superoxide scavenger, superoxide dismutase, is not nearly as effective (See Color Plates)... 

Figure 6.15. Simplified schematic of the most important reaction pathways of the oxygen reduction reaction. The four-electron pathway results in the formation of water. The two-electron pathway forms hydrogen peroxide. Adsorption of molecular oxygen can form atomic oxygen (dissociative pathway) or form a superoxide species (associative pathway). The formation of Pt—OH and Pt— from water molecules represents the backward reactions of the later portion of the four-electron reduction pathway.

KIEs in excess of 1.010 for the rate-determining formation of an t]1-superoxide species. A significant decrease from the 180 EIEs determined for such structures (i.e., 1.005-1.015 in Table 9.1) is actually expected due to variations in the transition state structure, which arise from changes in the reaction driving force.47 The possibility that an rj -superoxide intermediate is formed reversibly, as a... 

Robertson [28] proposed that an additional possibility was that the destruction of the substrates may be mediated by hydroxyl radicals generated via the superoxide radical anion produced at the conduction band. This is subsequently hydrated or deuterated by the solvent. This may be rate determining since the O2 has to be generated at the conduction band prior to interaction with the solvent and subsequent formation of OH or OD" species. Therefore the kinetic isotope effect could be due to the interaction of the solvent with the superoxide species rather than the attack on the toxin. If this is the case it was suggested that a similar kinetic isotope effect would be observed no matter what substrate was being destroyed. Further kinetic isotope studies will help elucidate the potential of this proposed mechanism. 

O2 is able to abstract an electron from an Fsurface superoxide O2 species. This process has been investigated by EPR spectroscopy (238, 241-243) it may play an important role in many catalytic oxidation reactions (244-248). The formation of the superoxide species via the reaction F,f + O2 F + + 0-7 (146,243) is of course favored on metal-doped MgO surfaces. In a combined EPR and IR study, Giamello et al. (240) demonstrated that CO is able to abstract an electron from a surface F center to form novel dimeric species produced by C202. CO- radical anions have also been reported (237). 

A small number of surface sites are able to abstract hydrogen even from benzene and toluene. The occurrence of this surface process is revealed only by the subsequent interaction with oxygen to produce a small number of superoxidic species (257). 

Stohr, B., Boehm, H.P., and Schlogl, R. Enhancement of the catalytic activity of activated carbons in oxidation reactions by thermal treatment with ammonia or hydrogen cyanide and observation of a superoxide species as a possible intermediate. Carbon 29, 1991 707-720. 

Finally, the spontaneous liberation of free radicals via the unimolecular decay of CIII is feasible, since the peroxyl radical is not covalently bound to the porphyrin (pathway 2). This assumption is supported by experimental evidence, which demonstrates that in the presence of excess H202 and no reductant, CIII decays irreversibly into GS and superoxide species in lignin peroxidase [46], horseradish peroxidase [104], myeloperoxidase [51, 105, 106]. 

Transient quantities of 02 in aqueous solution can be generated by pulse radiolysis of 02 and by photolysis of H202 in aqueous media. In aprotic media stable solutions of OJ can be prepared by electrochemical reduction of molecular oxygen and by the base-induced decomposition of H202. Superoxide species can also be made from basic dioxygen-saturated solutions of aniline in dimethyl sulfoxide 34... 

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