Open-circuit Behaviour

Duncan and Frankenthal report on the effect of pH on the corrosion rate of gold in sulphate solutions in terms of the polarization curves. It was found that the rate of anodic dissolution is independent of pH in such solutions and that the rate controlling mechanism for anodic film formation and oxygen evolution are the same. For the open circuit behaviour of ferric oxide films on a gold substrate in sodium chloride solutions containing low iron concentration it is found that the film oxide is readily transformed to a lower oxidation state with a Fe /Fe ratio corresponding to that of magnetite . 

In the case of ZrNi and ZrCo intermetallics, the preformed hydrides are seen to loose hydrogen at open circuit in acid solutions, after which the intermetallic matrix dissolves [554]. A relatively higher stability of the hydride of ZrCo has been attributed to the presence of higher Co oxides which are more resistant to acids. Studies of this kind have relevance to the possible open circuit behaviour of cathodes based on intermetallic compounds. 

Electrochemical aspects of the stress-corrosion behaviour have been investigated, mainly in neutral solutions. The open-circuit potential of Ti-8Al-lMo-l V is —800mV (v5. S.C.E.). The crack initiation load reaches... 

PCT diagrams of AB2 (electrode alloys) /H systems reflect multiphase or nonideal behaviour [54], This is illustrated in Fig. 19, in which both the equilibrium pressure and the open—circuit equilibrium voltage, Er are plotted for Zr()5Ti05 Vo.sNij Fe02Mn02. 

It is a recent discovery, presented in Chapter 6, that behind the apparent chaos of Table 4.2 there are some simple regularities which enable one to predict the r vs local and global behaviour on the basis of the open-circuit... 

Table 4.2 lists the same catalytic systems but now grouped in terms of different reaction types (oxidations, hydrogenations, reductions and others). In this table and in subsequent chapters the subscript D denotes and electron donor reactant while the subscript A denotes an electron acceptor reactant. The table also lists the temperature and gas composition range of each investigation in terms of the parameter Pa/Pd which as subsequently shown plays an important role on the observed r vs O global behaviour. Table 4.3 is the same as Table 4.2 but also provides additional information regarding the open-circuit catalytic kinetics, whenever available. Table 4.3 is useful for extracting the promotional rules discussed Chapter 6. 

In Table 4.3 we had classified all published electrochemical promotion studies on the basis of the catalytic reaction and had provided the observed global r vs behaviour together with the observed r vs po and r vs pA open-circuit kinetic behaviour. We had then invited the reader to use Table 4.3 in order to derive the rules of promotion. As a further step we present here in Table 6.1 the same information given in Table 4.3 with only one difference In Table 6.1 the 58 catalytic reactions are grouped in terms of their global r vs [Pg.285]

Figure 6.8. Example of rule G3 (volcano-type behaviour) Effect of Ph2(=Pd) (a), Po2 (=Pa) (b) and of potential UWR and AO (c) on the rate of H2 oxidation on Pt /graphite (a and b) and Pt/black (c) in aqueous 0.1 M KOH solutions.72,73 Note that under the pH2, Po2 conditions of Fig. 6.7c the open-circuit rate is positive order in H2 (Fig. 6.8a) and negative order in 02 (Fig. 6,8b) and that the orders are reversed with the applied positive potential (Uwr=1 -2 V). At this potential the rate passes through its maximum (volcano) value (Fig. 6.8c). Reprinted with permission from McMillan Magazines Ltd (ref. 72) and from the American Chemical Society (ref. 73).

The opposite effect is depicted on Fig. 8.32 where the catalyst under open-circuit conditions exhibits stable limit cycle behaviour with a period of 184 s. Imposition of a negative current of -400 pA leads to a steady state. Upon current interruption the catalyst returns to its initial oscillatory state. Application of positive currents leads to higher frequency oscillatory states. 

A striking feature of the effect of current on the CO oxidation oscillations is shown in Fig. 8.33. It can be seen that the frequency of oscillations is a linear function of the applied current. This holds not only for intrinsically oscillatory states but also for those which do not exhibit oscillations under open-circuit conditions, such as the ones shown on Fig. 8.31. This behaviour is consistent with earlier models developed to describe the oscillatory behaviour of Pt-catalyzed oxidations under atmospheric pressure conditions which are due to surface Pt02 formation35 as analyzed in detail elsewhere.33... 

It was found that both the catalytic rates and the selectivity to the various products can be altered significantly (rate changes up to 250% were observed) and reversibly under NEMCA conditions. Depending on the product, electrophobic or electrophilic behaviour is observed as shown in Fig. 8.57. In addition to the selectivity modification due to the different effect on the rate of formation of each product, acetaldehyde, which is not produced under open circuit conditions is formed at negative overpotentials (Fig. 8.58). Enhancement factor A values up to 10 were observed in this complex system.59... 

Ethylene oxidation on Pt/p"-Al203 was studied1 at temperatures 150° to 300°C, but most of the NEMCA experiments were carried out at 290°C. Experimental details about the cell can be found in ref. 1. The open-circuit kinetic behaviour was found to be similar to the case of C2H4 oxidation on Pt/YSZ (Chapter 8), i.e. the rate expression ... 

It must be emphasized, however, that since the Faradaic efficiency A is on the order of 2Fr0/I0, one anticipates to observe NEMCA behaviour only for those systems where there is a measurable open-circuit catalytic activity r0. Consequently the low operating temperatures of aqueous electrochemistry may severely limit the number of reactions where Non-Faradaic A values can be obtained. 

The corrosion of stainless steel in 0.1 mol-1 NaCl solutions at open circuit potential was studied in detail by Bruesch et al. [106] using XPS in combination with a controlled sample transfer system [38]. It was verified by XPS analysis that the passivating film contains chromium oxide. The position and the height of the Cr concentration maximum depends critically on the bulk chromium content of the steel. Significant variations in the electrode passivation properties were observed at a Cr concentration of 12%, while the film behaviour was found to be rather independent of the other components like Mo, Ni, Cu. From the fact that the film structures and... 

In this section both the open-circuit and closed circuit behaviour of electrochemical cells will be briefly discussed. The mechanism of the charge-transfer process for oxygen-ion conducting systems will also be discussed. 

It is assumed here that a large surface Ti02 photoelectrode mimics closely the behaviour of the Ti02 particles of not too small size, and, in particular, that the surface species, formed during irradiation of the Ti02 dispersion, respectively, the TiOz electrode at open circuit, are identical. 

Fig. 4b demonstrates that under UHV conditions, electro-pumped Na is identical in behaviour and in chemical state with Na supplied by vacuum deposition firom a Na evaporation source. Spectrum (1) shows the XPS of the catalyst film when Na was vacuum-deposited. Heating to 400 K under open circuit conditions caused no change- spectrum (2). 

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