Molecular oxygen, kinetically stabilized

With regard to the susceptibility of many organometallics to aerial oxidation, it must be remembered that most compounds containing C-H bonds are thermodynamically unstable with respect to oxidation by molecular oxygen to produce water and carbon dioxide. The mechanisms whereby (for example) many alkyls are spontaneously flammable in air are necessarily difficult to determine the mechanisms involved in the combustion of hydrocarbons are still incompletely understood. Suffice it to say that the kinetic stability of ER to air seems to be low where E is of low electronegativity. 

Within our discussion of the stabilities of fluorinated radicals we have, of course, been referring to thermodynamic stabilities. In fact, most fluorinated radicals will be seen to have enhanced kinetic reactivity in reactions with closed shell molecules. However, appropriate fluorine substitution can also give rise to long-lived, or persistent radicals, the most dramatic example being Scherer s radical, 3, which persists at room temperature, even in the presence of molecular oxygen [39] ... 

Platinum, which belongs to the second group of materials, " is considered to have the best catalytic activity for the ORR in an acid as well as in an alkaline medium. This metal possesses surface properties which facilitate the adsorption of molecular oxygen, and thus it is able to catalyse the direct reduction of oxygen into water. Moreover, it has a greater stability in an 02-saturated acid medium, a very oxidant medium, than less noble materials and its surface properties are less altered, or with a lower kinetics. However, despite these properties, the ORR on the platinum surface... 

The relation in eq. (4) is nicely reflected in the abilities of metals to dissociate O2, NO and CO (the dissociation energies being around 500, 630 and 1100 kJ mol-1, respectively). Since the dissociation energy of the oxygen molecular bond is low it is dissociated by all close packed surfaces of metals, including Ag, whereas CO is not easily dissociated on Rh(lll). On the other hand, Rh(lll) will readily dissociate NO, whereas Pt(111) will not (Root et al., 1983). Of course, there is an interplay between thermodynamics and kinetics here, but nevertheless there is a correlation between ease of dissociation and thermodynamic stability. Generally, the weaker the adsorption heal, the higher the barrier to dissociative adsorption. 

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