Oxide potentiostatic

On both Pt and polyaniline-coated electrodes, diffusion-limited currents are observed at <250 mV for Fe(CN)500 mV for Fe(CN)g- oxidation. Potentiostatic EHD impedance was measured on both diffusion plateaux (50 mV and 550 mV), using Pt electrodes coated with polyaniline films of various thickness 50 and 130nm. As an example, the results obtained on the cathodic plateau are shown in Fig. 6-13 those observed on the anodic plateau were very similar [93]. 

The current-potential relationship ABCDE, as obtained potentiosta-tically, has allowed a study of the passive phenomena in greater detail and the operational definition of the passive state with greater preciseness. Bonhoeffer, Vetter and many others have made extensive potentiostatic studies of iron which indicate that the metal has a thin film, composed of one or more oxides of iron, on its surface when in the passive state . Similar studies have been made with stainless steel, nickel, chromium and other metals... 

Potentiometric titrations - continued EDTA titrations, 586 neutralisation reactions, 578, 580 non-aqueous titrations, 589, (T) 590 oxidation-reduction reactions, 579, 581, 584 precipitation reactions, 579, 582 Potentiometry 548 direct, 548, 567 fluoride, D. of, 570 Potentiostats 510, 607 Precipitants organic, 437 Precipitate ageing of, 423 digestion of, 423... 

Controlled-potential (potentiostatic) techniques deal with the study of charge-transfer processes at the electrode-solution interface, and are based on dynamic (no zero current) situations. Here, the electrode potential is being used to derive an electron-transfer reaction and the resultant current is measured. The role of the potential is analogous to that of the wavelength in optical measurements. Such a controllable parameter can be viewed as electron pressure, which forces the chemical species to gain or lose an electron (reduction or oxidation, respectively). 

Accordingly, the resulting current reflects the rate at which electrons move across the electrode-solution interface. Potentiostatic techniques can thus measure any chemical species that is electroactive, in other words, that can be made to reduce or oxidize. Knowledge of the reactivity of functional group in a given compound can be used to predict its electroactivity. Nonelectroactive compounds may also be detected in connection with indirect or derivatization procedures. 

Potentiometric stripping analysis (PSA), known also as stripping potenhometry, differs from ASV in the method used for stripping the amalgamated metals (22). hi this case, the potentiostatic control is disconnected following the preconcentration, and the concentrated metals are reoxidized by an oxidizing agent [such as O2 or Hg(II)] that is present in the solution ... 

Figure 13. Schematic diagram of the measurement of the ionic conductivity of a conducting polymer membrane as a function of oxidation state (potential), (a) Pt electrodes (b) potentiostat (c) gold minigrid (d) polymer film (e) electrolyte solution (0 dc or ac resistance measurement.133 (Reprinted with permission from J. Am Chem Soc. 104, 6139-6140, 1982. Copyright 1982, American Chemical Society.)...

Conclusion when using ionic conductors where the conducting, i.e. backspillover ion participates in the catalytic reaction under study (e.g. O2 ions in the case of catalytic oxidations) then both galvanostatic and potentiostatic operation lead to a steady-state and allow one to obtain steady-state r vs Uwr plots. 

Figure 8.10. Potentiostatic and galvanostatic transient during C2H4 oxidation on IrCVYSZ 17 Pc2h4=0.26 kPa pO2=20 kPa T=390°C A 100.

Figure 8.17. Potentiostatic transient of C2H4 oxidation on Pt/Ce02. Rate and current responses to step changes in catalyst potential, UWR) are plotted against time. T = 500°C, p02 = 5.5 kPa, PC2H4= 1 5 kPa.71 Reproduced by permission of The Electrochemical Society.

Figure 9.1. Rate and catalyst potential response to application of a negative current (Na supply to the catalyst) during C2H4 oxidation on Pt/p"-Al203, followed by potentiostatic restoration of the initial state1 T=291°C, pO2=5.0 kPa, pC2H4=2.1xlO 2 kPa. Reprinted with permission from Academic Press.

Figure 11.8. Effect of po2 on the rate (TOF) of C2H4 oxidation on Rh supported on five supports of increasing d>. Catalyst loading 0.5wt%.22,27 Inset Electrochemical promotion of a Rh catalyst film deposited on YSZ Effect of potentiostatically imposed catalyst potential Uwr on the rate and TOF dependence on po2 at fixed Pc2H4-22,33 Reprinted with permission from Elsevier Science (ref. 27) and Academic Press (ref. 33).

On the basis of experimental findings Heinze et al. propose the formation of a particularly stable, previously unknown tertiary structure between the charged chain segments and the solvated counterions in the polymer during galvanostatic or potentiostatic polymerization. During the discharging scan this structure is irreversibly altered. The absence of typical capacitive currents for the oxidized polymer film leads them to surmise that the postulated double layer effects are considerably smaller than previously assumed and that the broad current plateau is caused at least in part by faradaic redox processes. 

Meincke H, Ebling DG, Heinze J, Tacke M, Bbttner H (1998) Potentiostatic oxide formation on lead selenide single crystals in alkaline solutions. J Electrochem Soc 145 2806-2812... 

FIGURE 16.1 Charging curves recorded when an adsorbed layer of oxygen adatoms or a phase oxide layer are formed (a) galvanostatic (b) potentiostatic. 

In the following, after a brief description of the experimental setup and procedures (Section 13.2), we will first focus on the adsorption and on the coverage and composition of the adlayer resulting from adsorption of the respective Cj molecules at a potential in the Hup range as determined by adsorbate stripping experiments (Section 13.3.1). Section 13.3.2 deals with bulk oxidation of the respective reactants and the contribution of the different reaction products to the total reaction current under continuous electrolyte flow, first in potentiodynamic experiments and then in potentiostatic reaction transients, after stepping the potential from 0.16 to 0.6 V, which was chosen as a typical reaction potential. The results are discussed in terms of a mechanism in which, for methanol and formaldehyde oxidation, the commonly used dual-pathway mechanism is extended by the possibility that reaction intermediates can desorb as incomplete oxidation products and also re-adsorb for further oxidation (for the formic acid oxidation mechanism, see [Samjeske and Osawa, 2005 Chen et al., 2006a, b Miki et al., 2004]). 

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