Activation of oxygen

Even though the formation of an allylic intermediate has been relatively firmly established, the nature of the active oxygen species which interacts with the allylic intermediate is less clear. Nevertheless, much attention has been directed toward understanding the various oxygen species found on the surface of oxide catalysts. 

The adsorption of oxygen on oxide catalysts can be viewed as occurring according to a stepwise process, 

The existence of the molecular radical ion 02 , of atomic O-, and of the regular ions in the lattice O2- has been firmly established. A review by Lunsford (33) presents a summary of the experimental evidence which led to the discovery of 02 and O-. The participation of these various forms of oxygen in hydrocarbon oxidation is discussed in a review by Sachtler (11). It seems clear that both adsorbed and lattice oxygen species play an important role in the selective oxidation of hydrocarbons. 

A redox mechanism for oxidation catalysis was proposed by Mars and van Krevelen (34) for the oxidation of aromatics over V205. This mechanism introduced the concept that lattice oxygen of a reducible metal oxide could serve as a useful oxidizing agent for hydrocarbons. Moreover, it formed the basis for the early work at SOHIO which led to the development of the bismuth molybdate catalyst. Since that time there have been many reports which support the redox concept. 

Aykan (35) reported that ammoxidation of propylene occurred over a silica-supported bismuth molybdate catalyst in the absence of gas-phase oxygen, although the catalytic activity decreased rapidly with increasing catalyst reduction. The reduction process was followed by X-ray and it was found that phase changes which occurred in the catalyst and the decrease in catalytic activity corresponded quantitatively to the depletion of lattice oxygen. 

---

Fig. 11. The loss of carbon rapidly increases with the increase of temperature. Heating of the catalysts in open air for 30 minutes at 973 K leads to the total elimination of carbon from the surface. The gasification of amorphous carbon proceeds more rapidly than that of filaments. The tubules obtained after oxidation of carbon-deposited catalysts during 30 minutes at 873 K are almost free from amorphous carbon. The process of gasification of nanotubules on the surface of the catalyst is easier in comparison with the oxidation of nanotubes containing soot obtained by the arc-discharge method[28, 29]. This can be easily explained, in agreement with Ref [30], by the surface activation of oxygen of the gaseous phase on Co-Si02 catalyst.

Activation of oxygen by cytochrome P-450 and other hemoproteins. D. I. Metelitsa, Russ. Chem. Rev. (Engl. Transl), 1982, 51,1042-1059 (184). 

Wagner was first to propose the use of solid electrolytes to measure in situ the thermodynamic activity of oxygen on metal catalysts.17 This led to the technique of solid electrolyte potentiometry.18 Huggins, Mason and Giir were the first to use solid electrolyte cells to carry out electrocatalytic reactions such as NO decomposition.19,20 The use of solid electrolyte cells for chemical cogeneration , that is, for the simultaneous production of electrical power and industrial chemicals, was first demonstrated in 1980.21 The first non-Faradaic enhancement in heterogeneous catalysis was reported in 1981 for the case of ethylene epoxidation on Ag electrodes,2 3 but it was only... 

Wagner first proposed the use of such galvanic cells in heterogeneous catalysis, to measure in situ the thermodynamic activity of oxygen O(a) adsorbed on metal electrodes during catalytic reactions.21 This led to the technique of solid electrolyte potentiometry (SEP).22 26... 

Here, P represents the partial pressure, a the activity, and AG° the standard free energy of formation of the species indicated by the subscripts. The activity of oxygen in both above the relationships is the same. Hence, it follows that... 

Spectroscopic studies (XPS and HREELS) established first in 1980 that the activity of oxygen states in the oxidation of ammonia at copper-O surfaces was... 

When oxygen is pumped to the catalyst the activity of oxygen on the silver catalyst-electrode increases considerably because of the applied voltage. It thus becomes possible to at least partly oxidize the silver catalyst electrode. In a previous communication it has been shown that the phenomenon involves surface rather than bulk oxidation of the silver crystallites (17). The present results establish the direct dependence of the change in the rates of epoxidation and combustion Ari and Ar2 on the cell overvoltage (Equations 2,3, and 5) which is directly related to the surface oxygen activity. 

Work still continues on the way in which electron transfer occurs physically between the different prosthetic groups, the activation of oxygen as acceptor for the final stage in the oxidation, and the association between electron transfer and the generation of ATP (see below). 

Korzhinskii, D.S. 1963. Correlation between activity of oxygen, oxidity and reduction potential in endogenic mineral formation. Izvestiya of Academy of Sciences of USSR. Geological seria. No. 3. 54-61. (In Russian)... 

Table 3.27. NORADRENALINE-DEPLETING ACTIVITY OF OXYGEN-CONTAINING GUANIDINE DERIVATIVES...

Interaction of dioxygen species with Fe aq and with Fe " aq has been very briefly reviewed. In relation to 0x0-, peroxo-, and superoxo-complexes as models for intermediates in oxygenase activity, a brief report on a 2000 symposium on activation of oxygen summarizes the then-current situation in the search for a mechanism common to mono- and dinuclear iron sites, mono- and dinuclear copper sites, and copper-iron sites. The outline proposals comprise ... 

Fig. 14. Intermediates in the activation of oxygen by binuclear iron centers. Hr, Heme-rythrin RNR, ribonucleotide reductase MMO, methane monooxygenase.

Savy et al. 35> reach practically the same conclusion concerning the electron transitions during formation of the activated complex but consider an edge-on arrangement of the oxygen above the plane of the chelate molecule more probable. Their studies relate only to the catalytic properties of the phthalocyanines for 02 reduction. Their conclusion is that the conditions for optimal activation of oxygen... 

Substitutionally labile metal complexes often generate hydroperoxides by direct substitution with H202 (86 90) or in the reactions between 02 and the reduced metal (91). These mechanisms are commonly observed in naturally occurring molecules and their mimics in the processes of activation of oxygen and hydrogen peroxide (92-98). 

---