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The dynamic aging of the oxygen-containing species electrosorbed on platinum in sulfuric acid solutions in the region of the monolayer produces three energetically different Pt(O) species, which are revealed by running cathodic potentiodynamic scans. The dynamic aging process is described by a second-order rate equation. It depends on the perturbation frequency and on the number of potential scans related to the aging process. The results also show a possible penetration of oxygen into the metal to the depth of the first and second crystallographic layers. A reaction scheme is proposed to explain the existence of the different species. The kinetic behavior is interpreted tnrougr~ a model where all the reacting particles are dynamically coupled to the fast electrochemical perturbation.The potentiodynamic electroformation and electroreduction of the oxygen-containing monolayer on platinum ha~ been studied for a long time. Reviews on the subject have recently been published (1-3). Both processes are relatively complex since the electrochemical reactions are coupled to chemical reactions related to changes of the structural configuration of the film. The shapes of the resulting cathodic and anodic Eli profiles were quantitatively treated in terms of coverage of the electrode by oxygen species and time-dependent rearrangement effects (4-6).Another approach to the subject of the film irreversibility on platinum within the potential range of the monolayer electroformation and electrodesorption was given in terms of exponentially diminishing rate constants in both anodic and cathodic directions, resulting from surface rearrangement and film aging (7). The existence of two electrochemically distinguishable O-species on the surface was also demonstrated (8). More recently, the hysteresis phenomena related to the formation of oxygen-containing species at platinum anodes were explained through a growth model based on rate-determining nucleation (9, 10).The aging of the oxygen-containing monolayer occurs at any temperature ranging from ca. 300~ with a molten electrolyte (11, 12) down to low temperatures with aqueous solutions (4-9). The simplest way to study the aging effects is to form a fixed amount of oxygen-containing species by applying a linear anodic potential sweep to leave the circuit open for a certain lapse of time and finally to electroreduce the species with a linear potential sweep (5,6, 13). The aging effect produces a net shift of the characteristic electroreduction current peak toward more cathodic potential values. This can be denoted as the open-circuit (static) aging of the species. Another way of producing an equivalent effect is by means of three successive trains of triangular potential sweeps (14). The first train at the potential sweep rate, v, covers the complete potential range related to both the electrooxidation and electroreduction of the oxygen-containing monolayer. With the first train a stable and reproducible Eli profile is achieved. The second train is usually faster than the former and the ...

A study has been made of the temperature and rates at which molecular hydrogen is evolved and free radical decay occurs when semicrystalline polyethylene, irradiated at liquid-nitrogen temperature, is warmed to room temperature. Hydrogen evolution and free radical decay roughly parallel each other. The temperature of half reaction is the same as the temperature of the gamma-maximum in the mechanical dispersion temperature curve. At constant temperature over the temperature range —71° to 41°C, the free radical decay is first order and the free radical concentration seems to decrease to a limiting value at each temperature. The first-order decay constants are approximately constant with temperature. The data are interpreted in terms of free radicals decaying in spurs. If a constant activation energy of 15 kcal mole—1 is assumed, the frequency factors decrease with rise of temperature. This decrease is interpreted on the basis of a decreasing probability of reaction for those free radicals that escape reaction at low temperatures, but react at higher and higher temperatures. An alternative model based upon the release of free radicals from traps as the temperature is raised is also discussed. The reaction rate constant for free radical decay calculated from D—H exchange experiments agrees with that deduced from the ESR measurements. It is also concluded that the persistent free radical in polyethylene irradiated to moderate doses is the allyl free radical.

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