Oxygen equilibrium with metal oxides

Figure 2.33. Ni-Co-O phase diagram (isothermal section at 1600 K). log p02 (oxygen partial pressure) is plotted against the molar fraction in the metallic alloy. The metallic alloy, (Ni, Co) solid solution is stable in (1) and the mixed oxide (Ni, Co)0 solid solution in (3). In the intermediate region (2) we have coexistence of alloy and oxide. For the value of the partial oxygen pressure corresponding to y, within the two-phase field, we will have the alloy of composition xt in equilibrium with an oxide containing the two metals in the ratio x2-...

Let s consider the simple case of a metal in equilibrium with its oxide and oxygen gas with no other species present. To represent the equilibrium, one may write ... 

In semiconducting oxides such as iron-doped SrTi03 (e.g., SrTio.8Feo.2O3) the conductivity depends thermodynamically on the oxygen partial pressure. In the thermodynamic equilibrium a metal oxide exchanges lattice oxygen Oq with the ambient gas phase. 

Table 3.1. Partial pressure of oxygen in equilibrium with the oxide of the metal at KMMPCand at the melting point of the metal TF.

The method of isothermal saturation consists of the addition of an excess quantity of oxide to the melt studied. Such an addition results in the formation of a saturated solution which is in equilibrium with the oxide precipitate. The fact that the equilibrium conditions have been achieved is detected in different ways among them we should mention potentiometric measurements using different oxygen electrodes [238, 326] and titrimetric determination of the concentration of metal ions in a sample of the melt [327, 328]. The sum of the concentrations of ionic and non-dissociated forms of the oxide according to equation (3.6.5) is the main result of these determinations. Taking into account the thermodynamic parameters which describe equilibria in the saturated solutions, equation (3.6.5) may be rewritten as... 

Hagaman et al. (2012) studied interaction of benzoic acid with metal oxides using solid-state O NMR spectroscopy. Complexes formed by dry benzoic acid with mesoporous silica and nonporous titania and alumina were analyzed. Chemical reactions with silica were not observed, but the behavior of benzoic acid on silica was a function of the water content. The acid was characterized by high mobility as evidenced by a liquid-like, Lorentzian NMR resonance. Excess benzoic acid remained as the crystalline hydrogen-bonded dimer. Benzoic acid reacted with titania and alumina surfaces in equilibrium with air to form the corresponding titanium and aluminum benzoates. In both materials, the oxygen of the O-labeled acid was bound to the metal, showing the bond... 

With the values shown in Figure 2.2, it is possible to calculate the partial pressure of oxygen in equilibrium with the oxide and the metal. Convention (I) applied to an ideal gas allows us to replace flo2 02 Convention (II) allows us to set the activities of the metal, and of the oxide, floxide to 1, because these are assumed to be pure. 

Correspondingly, a gradient in the partial pressure (activity) of oxygen exists across the scale from the partial pressure of oxygen in the ambient atmosphere at the outer surface to the partial pressure of oxygen at the metal/oxide interface. The latter partial pressure is the decomposition pressure of the oxide in equilibrium with its metal. 

An important aspect of any theory of the oxidation of a pure metal is that it enables us to see how the protective power of the oxide layer can be altered by the introduction of alloying constituents into the metal. According to Wagner s theory, the parabolic rate constant for the system Ni/NiO for example depends upon the concentration of cation vacancies in the oxide in equilibrium with oxygen gas. If this concentration can be reduced, the oxidation rate is reduced. Now this can be done if cations of lower valency than Ni can be got into the oxide (Fig. 1.77). Suppose, for example, that a little Li is added to the Ni. Each Li ion which replaces Ni is a negative... 

A metal-metal oxide mixture A with a low equilibrium partial pressure of oxygen is sealed in a vacuum and heated to a temperature T. A metal-metal oxide mixture B with a high equilibrium partial pressure of oxygen is treated similarly. If the two containers are linked so that there is a free interchange of gas, what will be the final solids present ... 

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