H. Angerstein-Kozlowska

The real condition of electrochemically oxidized platinum surfaces: Part I. Resolution of component processes

Summary The problem that 3 peaks can be resolved in the initial stages of Pt surface oxidation below a monolayer of electrosorbed OH (1 e per Pt atom), even on single crystal faces, is examined. A high degree of solution purity is required for this resolution to be observed. Reversibility with respect to these stages in the formation and reduction of the surface oxide layer has been evaluated. The stages of surface oxidation below the OH monolayer limit are related to the geometries of various sub-lattices of OH on the Pt surface rather than to specific stoichiometries of platinum-oxygen surface compounds. It is shown that the cathodic reduction profile can also be resolved into reversible and irreversible components depending on end potential in the anodic direction of polarization and on temperature. Low temperature measurements enable complete resolution of a fast reversible component from an irreversible one, associated with the normal main cathodic peak in Pt surface oxide reduction. The irreversible component arises from a rearranged form of the surface oxide and is formed already to a significant extent well below a monolayer of “PtOH”. It is reduced at less positive potentials and gives rise to the characteristic hysteresis between oxide formation and reduction processes at Pt. The origin of the shape of the i-V profile at Pt and other noble metals is explained. Optical studies by means of ellipsometry show that in pure solutions and with sensitive instrumentation there is no distinguishability, with respect to potential dependence, between the change of optical properties and the change of the surface coverage determined by surface coulometry down to the lowest coverages that can be detected. The surface oxidation and reduction is shown, however, to be optically irreversible with respect to degree of surface oxidation so that a real change of condition of the surface arises between anodic and cathodic directions of polarization.

Elementary steps of electrochemical oxidation of single-crystal planes of Au Part II. A chemical and structural basis of oxidation of the (111) plane☆

A detailed experimental and mechanistic analysis of the elementary surface-chemical steps involved in the beginning stages of electrooxidation of Au is given with respect to the processes that occur on the (111) plane. At Au, specific adsorption of anions, even oxyanions such as ClO−4 and SO2−4 or HSO−4, plays a major role in the initial stages of oxidation of the metal. In the case of the (111) plane investigated here, the symmetry of the surface lattice has the strongest effect on the specific adsorption of the above anions compared with behaviour at other planes. This is connected with the matching symmetry of the tetrahedral ions with the trigonal symmetry of an unreconstructed (111) plane. The results indicate that the hydrated anions, which are found to be partially or fully discharged on the metal surface, form overlay-lattice arrays. This network of hydrated anions blocks the surface, preventing the formation of sublattices of OH discharged on the free surface of the metal but allows partial discharge of water molecules of the hydration shells of the ions in two energy states in a fast upd reaction. This provides the path for the initial stages of 2-dimensional oxidation of the surface. The resulting network of anion-OH(1−γ)− complexes stabilizes the surface, preventing the turn-over process which is possible only after, or coupled with, the desorption of the anions in an anion replacement, MOH turn-over process. This is possible when a potential is reached at which partly discharged OH(1−γ)− in complexes with anions can become fully discharged, forming free MOHs which are no longer part of the complex. In this way, the network is destroyed and the stabilization of the anions by the H-bonds of hydration H2O or partially discharged OH no longer remains, allowing the anions to be desorbed and processes of phase-oxide development to begin.

The role of ion adsorption in surface oxide formation and reduction at noble metals: General features of the surface process

Abstract Some important general features of the processes of electrochemical surface oxidation of noble metals are identified and related to effects of specific adsorption of anions. The first three stages in metal oxidation occur generally in successive overlay lattices which arise on account of repulsion between metal-oxygen dipoles; they are reversible on metals, such as Ru, Ir, having little tendency to adsorb anions or on Pt in alkaline solutions where anions other than OH − are absent. Irreversibility increases with oxide coverage, but the monolayer film always shows hysteresis due to place-exchange. The next stage of oxidation occurs over a single broad region, with hysteresis between its formation and reduction, decreased by anion adsorption. The anion effects arise on account of: (a) adsorption competitive with OH/O deposition which blocks initial stages of oxidation (Pt in acid or at Au); (b) change of inner-layer field which modifies the field-assisted place-exchange process and (c) lateral repulsion with metal-oxygen dipoles, which facilitates place-exchange (Au and Pt in acid). Cations, on the contrary, stabilize the unrearranged metal-oxygen dipoles, as is shown by the behaviour of Au in Ba(OH) 2 solution. Studies on the reduction kinetics at Au distinguish one reduction stage, which is a simple surface charge-transfer process, from a second, which is preceded by a slow chemical step, probably reverse place-exchange.

Computer simulation of the kinetic behaviour of surface reactions driven by a linear potential sweep: Part I. Model 1-electron reaction with a single adsorbed species*

Abstract The kinetic behaviour of a simple 1 e surface reaction is treated theoretically, especially with regard to (a) characteristic aspects of its behaviour when significant attractive or repulsive interactions arise in the electrodeposited film and (b) formulation of characteristic features of its kinetic and equilibrium behaviour which can provide a reference case for distinguishing, by comparison, the behaviour of other more complex sequential reactions treated in Part II.

A surface‐electrochemical basis for the direct logarithmic growth law for initial stages of extension of anodic oxide films formed at noble metals

Measurements on anodic surface oxidation of noble metals as a function of time and electrode potential show that the initial extension and subsequent thickening of such oxide films is directly logarithmic in time. A striking feature of this behavior is that the direct logarithmic extension law already applies to increase of coverage of Pt or Au electrodes with time well below the limit of formation of one monolayer of OH or O species on the metal surface. A direct logarithmic law of oxide film growth also applies to post‐monolayer growth involving early stages of quasi‐three‐dimensional film formation. Eventually, as the oxide film thickens, the Mott–Cabrera ‘‘high‐field’’ growth mechanism can apply. However, below the monolayer level of oxide film formation, electrochemisorption of two‐dimensional (2D) structures of OH or O arises so that the Mott–Cabrera mechanism cannot be applicable to that situation. It is shown that the kinetic relation for direct electrodeposition of OH or O species onto available ...

The real condition of oxidized pt electrodes: Part III. Kinetic theory of formation and reduction of surface oxides

Summary Various reaction mechanisms are considered which can quantitatively, or otherwise, explain the resolvable stages in the formation and reduction of Pt surface oxide demonstrated in Parts I and II. Two of the mechanisms are shown to be unacceptable as they cannot explain the main features of the experimental results. The resolution of three peaks below a monolayer may be quantitatively simulated by a three-stage mechanism of Pt sub-lattice occupation by OH species, followed by their oxidation to O species. Place-exchange rearrangement accounts for the hysteresis between the anodic- and cathodic-going i-V profiles and the experimentally observed single cathodic peak can be represented in terms of the kinetics of a 2-stage reduction of PtO to Pt via PtOH in rearranged states. The experimentally observed dependence of peak potential on log [sweep rate] of ca 40 to 55 mV can be accounted for, as well as other higher values of this slope which arise under certain conditions.

Evaluation of rate constants and reversibility parameters for surface reactions by the potential-sweep method

Abstract It is shown how the potentiodynamic sweep method, often used for qualitative evaluations of the behaviour of electrochemical processes, can provide a quantitative kinetic basis for evaluation of reversibility of electrochemical processes in a monolayer at an electrode surface. A characteristic quantity, s 0 , analogous to the exchange current density and related to the rate constant, k is defined. A method for evaluation of s 0 for simple surface reactions involving chemisorbed species obeying two kinds of isotherms is proposed. Applications of the treatment are made to evaluation of s 0 for two surface reactions of current interest: the adsorption of H and the formation of OH and O species on Pt, which were studied in aqueous HClO 4 , H 2 SO 4 and KOH solutions. Problems which arise in the evaluation of s 0 , i 0 and standard rate constants, k 0 , for a surface process involving multiple states of adsorption of the deposited species are discussed. It is shown that s 0 is a useful parameter for characterizing the kinetics of electrode surface processes since, unlike i 0 , it does not depend on real area of the electrode. Hence, electrochemical characterization of suitable electrocatalyst surfaces can be made without recourse to real area determinations.

Electrochemisorption and reactivity of nitriles at platinum electrodes and the anodic H desorption effect

Summary It has been shown that nitriles exhibit electroactivity at Pt electrodes in aqueous acid solutions. CH 3 CN was specially studied in 0.5 M aq. H 2 SO 4 by the potentiodynamic and anodic hydrogen displacement methods. In potentiodynamic experiments, a more or less reversible, 2-electron reduction-oxidation process occurs over the “double-layer” potential region at Pt and extends into the H-region. The reactive species is a chemisorbed one. The extent of adsorption can be determined from measurements of charge passed in the double-layer and H-region in potential sweeps and in potentiostatic current transients observed following adsorption of CH 3 CN. In the H-potential region, adsorption occurs by anodic H-displacement. CH 3 CN occupies 3 Pt sites per molecule at adsorption saturation leaving 33% of the surface available for co-adsorption of H. Electroactive CH 3 CN and H-species are co-adsorbed in the H-region at Pt but the coverage of H can be quantitatively distinguished from that of the nitrile by evaluating the kinetic relaxation characteristics of the two species. The stages of reduction and oxidation of the adsorbed species are considered in terms of a consecutive 2 e process which involves a change of shape of the molecule leading to steric hindrance for completion of the reduction of the ad-layer at the least positive potentials.

Computer simulation of the kinetic behaviour of surface reactions driven by a linear potential sweep: Part II. Sequential reactions of adsorbed species*

Abstract The kinetic behaviour under linear potential-sweep control of e.c. and c.e. type surface reactions involving chemisorbed species is investigated theoretically by computer simulation calculations. Characteristic features, which arise in comparison with those of the simple 1 e reaction involving deposition of a chemisorbed species, are emphasized. Especially the value of the Δ V 1/2 parameter for various conditions and the effect of holding the potential, at the end of a sweep, on the subsequent reverse sweep behaviour are of interest.

Stochastically-gated surface processes involving anions in oxidation of Au: time-resolution of processes down to 0.25% coverages and 50 μs time-scales

Abstract The mechanism of electrochemical surface oxidation of noble metals is compared with the oxidation of metals by gas-phase oxygen. In the presence of water, the role of preadsorbed anions of the electrolyte and of control of the potential across the interface, which is possible at electrodes, are discussed, including stochastically-gated oxide film growth. Time resolution of the initial stages of Au oxidation in which OH species are electrodeposited amongst chemisorbed anions in a surface array and become transformed, in time, to a quasi-2-dimensional phase oxide with anion desorption, can be made down to 0.25% coverage by OH and to times of ca 100 μs. The transformed species are reducible in two well resolved stages, both of which are substantially more stable, as measured by the Gibbs energies for their reduction, than the initial 2-D array from which they originated. The growth of the transformed species and the decrease of the coverage by the reversibly deposited OHs from which these species originate, are each logarithmic in time of growth. This behaviour is treated in terms of time-dependent changes of the surface-potential component of the metal—solution pd due to OH—M and O—M stochastically-gated place-exchange. The treatment is shown to lead to a direct logarithmic growth law having characteristic behaviour in agreement with experiment.