Oxygen species types

Activated carbons contain chemisorbed oxygen in varying amounts unless special cate is taken to eliminate it. Desired adsorption properties often depend upon the amount and type of chemisorbed oxygen species on the surface. Therefore, the adsorption properties of an activated carbon adsorbent depend on its prior temperature and oxygen-exposure history. In contrast, molecular sieve 2eohtes and other oxide adsorbents are not affected by oxidi2ing or reducing conditions. 

Oxygen species are now thought to play an important role in many types of cellular injury (eg, resulting from administration of various toxic chemicals or from ischemia), some of which can result in cell death. Indirect evidence supporting a role for these species in gen-... 

A third way to increase both the active surface area and the number of oxygenated species at the electrode surface is to prepare alloy particles or deposits and then to dissolve the non-noble metal component. This technique, which is similar to that used to prepare Raney-type catalysts, yields very high surface area electrodes and hence some improvements in the electrocatalytic activities compared with those of pure platinum. However, it is always difficult to be sure whether the mechanism of enhancment of the activities is due to this effect or the possible presence of remaining traces of the dissolved metal. Results with PtyCr and PtSFe were encouraging, although the effect of iron is still under discussion. From studies in a recent work on the behavior of R-Fe particles for methanol electrooxidation, it was concluded that the electrocatalytic effect is due to the Fe alloyed to platinum. ... 

In order to have more infoimation on the nature of the oxygen species active in partial and total oxidation we investigated the interaction of the hydrocarbons with the pre-oxidized surfaces of oxides where different types of surface oxygen species are formed. In particular we investigated p-type semiconductors like chromia, chromites and cobalt oxide C03O4. Moreover, we studied n-type metal oxides like FezOs, metal ferrites and CuObased catalysts. 

Results discussed above show in several lines a distinct biomimetic-type activity of iron complexes stabilized in the ZSM-S matrix. The most important feature is their unique ability to coordinate a very reactive a-oxygen form which is similar to the active oxygen species of MMO. At room temperature a-oxygen provides various oxidation reactions including selective hydroxylation of methane to methanol. Like in biological oxidation, the rate determining step of this reaction involves the cleavage of C-H bond. 

Historically, leukocyte NADPH oxidase was the first discovered enzyme of this type the existence of this enzyme in neutrophils was reported in 1962-1966 [59-61]. (As the evidence of superoxide production in biological systems has been obtained several years later, at that time the nature of oxygen species produced by leukocytes was of course unknown.) And only about 10 years later, Babior et al. [62] have shown that the activation of human neutrophils resulted in the production of superoxide. The structure of leukocyte NADPH has been widely discussed and well-established [57]. Superoxide production catalyzed by NADPH oxidase is described by Reaction (5) ... 

From the above results, one can conclude the existence of the two types of adsorbed oxygen species which may be speculated as monoatomic and diatomic oxygen species. The monoatomic oxygen species can produce (In) and the diatomic oxygen species can produce C2Hi,0 and decompose (In). 

Especially for the low temperature water gas shift reaction the mechanistic scheme, proposed here, seems to correspond to the three different adsorbed oxygen species, proposed by Kobaya-shi (13) for the ethylene oxidation on silver, whereas the importance of some surface complexes of CO - 1 0 type has been revealed (14) by analysing steady state data. 

Although pathway 2 in the oxidation process (Scheme 2) may be considered analogous to mechanisms proposed for carbon hydroxylations catalyzed by cytochrome P-450, abstraction of an electron from the lone pair on nitrogen (pathway 1) would be a more likely first step in these types of reactions. It is reasonable to assume that the nature of substituents R, R2, and R3 would greatly influence the rate and path of reaction. The mechanistic possibilities in Scheme 2 are undoubtedly simplistic in their representation of the active oxygen species of cytochrome P-450 and are by no means comprehensive. However, these pathways do serve to illustrate.the role of radical substrate intermediates in cytochrome P-450-catalyzed reactions. More detailed analyses of mechanistic studies on these and other cytochrome P-450-mediated reactions can be found in recent reviews on the subject 49, 50, 60). 

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