Metal oxidative coupling

Table II collects some metal oxide couples and their redox potentials together with the usual base metal.

Metal/Metal-Oxide Couples Capable of Acting as Heterogeneous Mediators for the Oxidation of Organic Substrates Dissolved in Aqueous Solution... 

Suppose a cell is constructed by using metal-metal oxide couples for both electrodes ... 

If a metal-metal oxide couple (e.g., Me-MeO) is used for the reference electrode ( die cathode in the present example), Pox(C) must he replaced with die equilibrium oxygen... 

Finally, the Aksay s model was used to explain the empirical correlation between reactivity and wettability observed in several metal/ceramic systems. It will be shown, for instance for metal/oxide couples (see Sections 6.4.1 and 6.5.2), that such a correlation can be explained by taking into account only the effect of reactions between liquid metals and oxides on interfacial energies. 

These considerations can explain the results of Figure 6.2 for reactive pure metal/oxide couples in which a new phase is formed at the interface. Wettability then depends on the bonding character of this interfacial layer and two cases can be distinguished ... 

To put this in perspective, the equilibrium Po, computed by using reliable Gibbs energy data for the biphasic coexistence of several metal/metal oxide couples of relevance to gas sensing is shown in Table 2.1 at a few representative temperatures. 

Indirect thermodynamic determinations of the free energy of formation of liquid water from the equilibrium potentials of a metal-insoluble metal oxide couple (i.e., Ag-Ag20, Hg-HgO) together with the free energy of formation of the metal oxide gave values inconsistent with these early... 

Given the logarithmic response of the probe, its best precision is obtained when the partial pressure to be measured is equal to the reference pressure. The oxygen pressure used as a reference in the electrochemical cell can be fixed, in various ways but two reference media are utilised in practice air and metal-oxide couples (19) ... 

Provided they reach equilibrium fast enough at measuring temperature (- 1600 C) and do not react with the solid electrolyte, metal-oxide couples allow fixation of oxygen pressure to desired value. The equilibrium can be written ... 

In actual practice, the most often used reference media in commercial probes are air with or without circulation and metal-oxide couples M0-M0O2 or Cr-Cr202 for which equivalent oxygen partial pressure at l600 C are respectively lO and 10 "13 atm, (22), Internal electrodes are either platinum or molybdenum. 

Figure 3.9. a) Possibilities of reactions between metal-oxide couples b) case where the Ellingham lines intersect one another... 

---