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 ...

The formation of a multiple interface consisting of a platinum-base electrode covered by a chemically produced Ni (OH)2 layer is investigated in alkaline electrolytes. The potentiodynamic response of the multiple interface is relatively complex but can be interpreted as the sum of two main contributions, namely, the processes related to the electrosorption and electrodesorption of hydrogen and oxygen on platinum and the conjugate redox processes related to the nickel hydroxide electrode. The mutual interference of the oxygen electrosorption and electrodesorption reactions with those of the nickel hydroxide electrode is inferred from the change of the aging characteristics of the oxygen electrodesorption process. The overall electrochemical reaction is interpreted through a reaction model involving two limiting planes dividing the electrochemical interface in two main regions associated with the two processes referred to above.In a recent publication the concept of the multiple electrochemical interface was applied to explain the potentiodynamic behavior of iridium in different acid electrolytes (1, 2). A multiple electrochemical interface involves at least three different phases, namely, the base metal, the conducting or semiconducting film which covers the base metal either partially or totally, and the electrolyte. The cations or atoms of the metal in the film forming chemical species are different from those of the base metal electrode. Various electrochemical processes take place at the multiple electrochemical interface. The sites of each reaction within such an interface are differently located so that a reaction plane proper to each reaction can be as-* Electrochemical Society Active Member.

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