Kinetics of Aging of the Oxygen Monolayer on Platinum under a Complex Potentiodynamic Perturbation Program

Abstract

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 ...

Fast faradaic processes observed during the potentiodynamic polarization of polycrystalline palladium in acid electrolyte

“…Reaction (4) should be related to a place exchange mechanism as already discussed for other noble metals in acid [14,15]. On the basis of reactions (2)-(4) the average half-life time of the adsorbed OH on polycrystalline palladium can be estimated from the i QJQc ratio vs. f plot (Fig.…”

The electrooxidation of chemisorbed CO on polycrystalline platinum A mechanistic interpretation of the anodic current peak multiplicity

“…Likewise, by analogy with the results obtained with the platinum/CO(g) interphase, the irreversible CO adsorption on platinum is hindered, unless the degree of coverage by the O-containing species becomes sufficiently low [38]. Hence, when the platinum surface is partially covered by O-species there is competition between the reactions Pt + CO = Pt(CO) (10) and Pt(0) + CO = Pt + CO2 (11) where reaction (11) plays an important role in the whole CO electrooxidation potential range as revealed through the open-circuit potential decay from E > 1.2 V [39]. The CO electrodesorption can be regarded as a completely-irreversible electrochemical reaction, and on the assumption that two electrons per platinum site are involved in the electrodesorption of (CO)L and one electron per platinum site for (CO)B, linear relationships between the height of each current peak and its corresponding initial CO-coverage, (0~O),n,t,al (Fig.…”

“…On the other hand, the electrocatalytic oxidation of CO on polycrystalline platinum electrodes [8] reveals through the corresponding potentiodynamic Eli profile a complex electrooxidation reaction involving the participation of various surface species whose coverage distribution and electrochemical characteristics are very strongly dependent on the extent of oxidation and on the type of O-containing species remaining on the electrode surface. These processes are also influenced by the adsorption of anions in solution in two main d~rections namely through the inhibition effect which lags the initiation of OH-and O-submonolayer formation [9], and through the ageing of both the electrocatalyst [10] and the O-containing layer formed on it [11]. The latter alters the overall characteristics of the remaining O-layer on platinum.…”

Changes in real surface area, crystallographic orientation and morphology of platinum electrodes caused by periodic potential treatments: phenomenological approach

“…8c,d) is comparable to that produced by chemical etching . In this case, no oxide layer accumulation can be detected after the RSWPS treatment probably because ageing processes of the O-containing layer [40] formed at E" hinder further oxide growth . Under these conditions the situation already encountered in Case 1 is approached .…”