Oxygen, active Subject

The oxygen activity of various Co—N4 complexes in sulfuric acid can be considerably improved by subjecting the carbon/chelate sample to thermal pretreatment in an atmosphere of a protective gas 7>. Such an effect is also reported by other authors for CoTMPP 22>. Figure 21 shows the influence of pretreatment on CoTMPP, pCoPc, and CoTAA. The given loadings (in mA/g catalyst) are all related to chelate plus carbon. The activity of CoTPP and CoTPAP is so low that,... 

Although the silver-surface catalyzed epoxidation has been the subject of many investigations there still remain many unanswered questions (i) How is molecular oxygen activated on a silver surface (if) What is the nature of the oxygen that is transferred to ethylene and (hi) How does the oxygen transfer step take place ... 

The activation of aluminum with ultrasound or dispersion of liquid aluminum. The suspension of powder aluminum in petrol or n-geptane without oxygen is subjected to ultrasound the tough oxide film on the surface of aluminum is removed and aluminum becomes reactive. The second activation technique is the dispersion of liquid aluminum with argon or purified nitrogen flow into a finely dispersed state. It should be noted, however, that the most reactive aluminum powder for direct synthesis is the powder alloyed with transition metals (titanium, zirconium, niobium, tantalum) with the size of particles from 10 to 125 pm. 

By contrast, there is a wealth of mechanistic information available in biochemistry and homogenous catalysis concerning oxygen activation mechanisms on first-row transition metals. It is therefore appropriate to provide a brief overview of this field here. However, this is a very large and active research field hence, it is not possible to do full justice to this subject here. For more extensive overviews, see references [50-60] in Section 4.5.6. 

Why oxygen activation The widespread interest in oxygen activation, as is evidenced by the more than 200 publications a year devoted to the subject, stems from the enormous commercial potential that exists for selective catalysts for the dream reactions shown below (reactions 4-6). 

Metal-based oxidation catalysts have been described the chemistries involved are based on both C-H and oxygen activation, the latter category being conceptually subdivided into molecular oxygen (O2) activation and activation of molecules that already contain a reduced form of oxygen (superoxide, peroxide, oxide, etc.). These topics are the subjects of numerous reviews and books. ... 

For trace quantities of less than 100 ppm, the most successful method — and the most costly— is neutron activation. The sample is subjected to neutron bombardment in an accelerator where oxygen 16 is converted to unstable nitrogen 16 having a half-life of seven seconds. This is accompanied by emission of (J and 7 rays which are detected and measured. Oxygen concentrations as low as 10 ppm can be detected. At such levels, the problem is to find an acceptable blank sample. 

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