Oxide galvanostatic

Figure 19-1 shows the experimental setup with the position of the steel test pieces and the anodes. The anodes were oxide-coated titanium wires and polymer cable anodes (see Sections 7.2.3 and 7.2.4). The mixed-metal experimental details are given in Table 19-1. The experiments were carried out galvanostatically with reference electrodes equipped to measure the potential once a day. Thus, contamination of the concrete by the electrolytes of the reference electrodes was excluded. The potentials of the protected steel test pieces are shown in Table 19-1. The potentials of the anodes were between U(2u-cuso4 = -1-15 and -1.35 V. 

Tin when made anodic shows passive behaviour as surface films are built up but slow dissolution of tin may persist in some solutions and transpassive dissolution may occur in strongly alkaline solutions. Some details have been published for phosphoric acid with readily obtained passivity, and sulphuric acid " for which activity is more persistent, but most interest has been shown in the effects in alkaline solutions. For galvanostatic polarisation in sodium borate and in sodium carbonate solutions at 1 x 10" -50 X 10" A/cm, simultaneous dissolution of tin as stannite ions and formation of a layer of SnO occurs until a critical potential is reached, at which a different oxide or hydroxide (possibly SnOj) is formed and dissolution ceases. Finally oxygen is evolved from the passive metal. The nature of the surface films formed in KOH solutions up to 7 m and other alkaline solutions has also been examined. 

Figure 4.14 shows a similar galvanostatic transient obtained during C2H4 oxidation on Rh deposited on YSZ.50 Upon application of a positive current 1=400 pA with a concomitant rate of O2 supply to the catalyst I/2F=2.M0 9 mol O/s the catalytic rate increases from its open-circuit value r0=1.8 10 8 mol O/s to a new value r= 1.62-1 O 6 mol O/s which is 88 times larger than the initial unpromoted rate value. The rate increase Ar is 770 times larger than the rate of supply of O2 ions to the Rh catalyst surface. 

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 8.15. Time dependence of the work function change, AO, the reaction rate, r, and the catalyst potential, Uwr, following galvanostatic steps during C2H4 oxidation on RuCVYSZ.20,21 Catalyst Ru02 (m=0.4 mg A=0.5 cm2), 1=50 pA, Pc2H4=1 14 Pa, po2=17.7 kPa, Fy=175 cm3 STP/min, T = 380°C.25...

Figure 8.68 shows a typical galvanostatic transient under oxidizing gaseous conditions. The reaction rate is enhanced by a factor of 20 (p=21) and the faradaic efficiency A (=Ar/(I/2F)) is 1880. The behaviour is clearly electrophobic (dr/dV Xi) and strongly reminiscent of the case of C2H4 oxidation on Pt/YSZ (Fig. 4.13) with some small but important differences ... 

Figure 11.5. Galvanostatic (constant current application) electrochemical promotion (NEMCA) transients during C2H4 oxidation on Ir02-Ti02 films deposited on YSZ T=380°C, Pc2H4=0.15 kPa, pO2=20 kPa.22,29...

Figure 12.5. Ethylene oxidation on Pt finely dispersed on Au supported on YSZ.7 Effect of the current 1 on x 1, where x is the time constant measured during a galvanostatic transient experiment with I as the applied current x is obtained by fitting either r/r0=exp(-t/x) or l-exp(-t/x) to the experimental data depending on the sign of the current and whether the reaction is electrophilic or electrophobic, (a) Positive values of I for electrophilic (squares, T=371°C, pO2=18.0 kPa, Pc2H4=0-6 kPa) and electrophobic behavior (circle, T=421°C, p02=l 4.8 kPa, Pc2H4 CU kPa) (b) negative currents, electrophilic behavior (T=421°C, p02=14.8 kPa, pC2H4=0.1 kPa. Reprints with permission from Academic Press.

Pyridin-, Chinolin- und N,N-Dimethyl-anilin-N-oxid werden elektrolytisch an Queck-silber in Methanol/Tetramethylammoniumchlorid zu Pyridin (81% d.Th.), Chinolin (78% d.Th.) bzw. N,N-Dimethyl-anilin (78% d.Th.) (galvanostat. bei 5 A/65°C) redu-ziert1. Analog verhalten sich 2- und 4-Methyl-pyridin-N-oxide (2-Methyl-pyridin 96% 4-Meihyl-pyridin 80% d.Th.)1. 

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. 

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

Anastasijevic NA, Baltruschat H, Heitbaum J. 1989. DBMS as a tool for the investigation of dynamic processes Galvanostatic formic acid oxidation on a Pt electrode. J Electroanal Chem272 89-100. 

Mishina F, Karantonis A, Yu Q-K, Nakabayashi S. 2002. Optical second harmoitic generation during the electrocatalytic oxidation of formaldehyde on Pt(lll) Potentiostatic regime versus galvanostatic potential oscillations. J Phys Chem B 106 10199-10204. 

Samjeske G, Miki A, Osawa M. 2007. Electrocatalytic oxidation of formaldehyde on platinum under galvanostatic and potential sweep conditions studied by time-resolved surface-enhanced infrared spectroscopy. J Phys Chem 111 15074-15083. 

The compact, nonporous anodic alumina film is the most suitable for fundamental investigations. It is grown by anodization, mostly under constant-current (galvanostatic) conditions, in neutral solutions of borates, tartrates, citrates, and phosphates, all of which possess significant buffering capacity and hence do not allow significant dissolution of the oxide. 

Dibasic and tribasic acids, such as sulfuric, oxalic, malonic, and phosphoric acids,96 cause the appearance and development of a very regular porous structure of the oxide (cf. Fig. 3). Here, the kinetics of galvanostatic anodization are characterized by an initial linear potential rise, followed, however, at relatively low anodic... 

In the case of a potentiostatic oxidation with different applied potentials, as shown in Fig. 14,98 there is always a dip in the current density corresponding to the potential maximum in galvanostatic... 

Data on anion incorporation into a growing porous oxide were obtained Fukuda and Fukushima.165,166 Their study was the first to demonstrate a correlation between the kinetics of accumulation of oxalate165 or sulfate166 anions and the change of porous oxide growth stages. The results of galvanostatic and potentiostatic... 

Figure 31. Correlation of the kinetics of oxide growth with kinetics of sulfate incorporation into the oxide during galvanostatic (a) and potentio-static (b) anodization of A1 in Ff2S04 solutions.166...

Analysis of experimental data shows that the dependence of the geometrical parameters of oxides on the temperature and concentration of electrolyte is different for galvanostatic and potentio-static conditions (Fig. 35).221 It appears that potentiostatic anodization is limited mainly by processes in the bulk of the oxide and thus is not influenced by temperature (Fig. 35b), whereas the galvanostatic anodization regime involves oxide dissolution processes at the O/S interface depending both on Tel and Cel. 

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