Mixed oxides, phase equilibria

Stabilization of Ru based oxides by valve metal oxides has not been studied in such detail using photoelectron spectroscopy. The most common compositions, however, with relatively high valve metal content, are not in favor of formation of a solid solution. Studies of the phase formation in Ru/Ti mixed oxides has shown [49] that homogeneous solutions are formed for compositions with Ru < 2% or Ru > 98% (see Section 3.1.1). Therefore electrodes with other compositions are better described as physical mixtures and the electrochemical behaviour is most likely that of a linear superposition of the single components. It has to be considered, however, that the investigations performed by Triggs [49] concern thermodynamic equilibrium conditions. If, by means of the preparation procedure, thermodynamic equilibrium is... 

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-...

The simplest models for the composition of the planets presume that the differences between them can be explained in terms of an equilibrium condensation. At the highest temperatures a sequence of mixed oxides of calcium, titanium, and aluminum would be found (>1,400 K). This would be followed, at lower temperatures, by metal and silicate fractions. At temperatures somewhat greater than 600 K alkali metals enter the silicate phase along with sulfur, which combines with iron at 650 K to form triolite... 

The interpretation of the viscosity-temperature behavior of these complex systems is difficult since many aspects of the melt conditions must be simultaneously considered. These include the chemical composition of the melt to establish the nature of the polymeric network including the amphoteric behavior of species like AI2O3 and Fe2O3> as well as the acid/base behavior of mixed valence constituents such as iron oxides, and the formation of immiscible liquid phases sometimes associated with the existence of several types of stable anions of significantly different size or charge in the system the nature of the container since some of it may dissolve and affect the composition of the melt the existence of a solid phase to establish the effect on the composition of the residual liquid phase (the solid phase may not be the one expected from related phase equilibrium studies) the relative amount of the liquid and solid phases to establish the composition of the liquid phase (this composition changes as the solid crystallizes out of... 

Catalysts considered in the present discussion cover a wide spectrum of solids reducible multivalent metal oxides as well as non reducible basic compounds Reducible metal oxides possess some inherent problems whereas these problems are less for the alkali ions promoted alkaline earth oxides. Alkaline earth oxides seem to be more suitable for working at low partial pressure of oxygen. By doping alkaline earth oxides with alkali metal compounds it is conceivable that O species can be stabilized for dissociative absorption of methane. Reducible metal oxides will tend to transform into lower valent oxides or even upto metallic state partly under applied reaction conditions specially at low partial pressure of O2. Both activity and selectivity will be deteriorated. But for the non reducible basic oxides structural changes will be quite different. They will tend to reach an equilibrium state in the surface level amongst the oxide, hydroxide and carbonate phases on reacting with evolved H2O and CO. Both the lattice distortion and the formation of O species can occur in the alkali earth oxides in doping with alkali ions as they can not build a mixed oxide lattice. 

The steady-state microstructure described above is a simpUfication. The MgO layer is far from uniform, both with respect to position and time [50,56,66], and thicknesses <0.1 pm has occasionally been observed [66]. In some samples the interface between aluminum and MgO was found to contain spinel [47], while the solidified metallic phase frequently contained nanocrystals of MgO [66]. Even less consistent with the steady state hypothesis was the apparent equilibrium between aluminum and MgO in the presence of a magnesium concentration that was necessarily, and experimentally, close to the three-phase equilibrium with spinel and alumina [50]. Cation de-mixing of spinel, as a source of MgO (periclase), seems unlikely when spinel is the minor phase or apparently absent. An alternative hypothesis assumes that oxidation of magnesium in the vapor phase is responsible for the presence of MgO, an assumption that is supported by the demonstration of... 

In the pseudo-binary mixed-oxides the structure assumed will obviously depend on the structures of the pure oxide components, the mole ratio of these, and (most importantly) on the difference between the radii of the two cations. The first thorough study of these systems was carried out by Schneider and Roth (1960). Their specimens were prepared by solid state reaction to equilibrium of individual oxides at 1650°C or sometimes 1900°C. The phase-fields were then determined from room-temperature X-ray diffraction data, and the results correlated with the average cation radius . Full details of the observed and predicted phase relationships are best obtained from the original publication, but a general summary can be given here. 

Wlchmann, U., 1974, Phase conversion and phase equilibrium o uranlun-rare earth mixed oxides, KFK -1924, Kernfiorschungszent Karlsruhe, Karlsruhe Germany,... 

This PUCI3 also acts as a salt-phase buffer to prevent dissolution of trace impurities in the metal feed by forcing the anode equilibrium to favor production (retention) of trace impurities as metals, instead of permitting oxidation of the impurities to ions. Metallic impurities in the feed fall into two classes, those more electropositive and those less electropositive than plutonium. Since the cell is operated at temperatures above the melting point of all the feed components, and both the liquid anode and salt are well mixed by a mechanical stirrer, chemical equlibrium is established between all impurities and the plutonium in the salt even before current is applied to the cell. Thus, impurities more electropositive than the liquid plutonium anode will be oxidized by Pu+3 and be taken up by the salt phase, while impurities in the electrolyte salt less electropositive than plutonium will be reduced by plutonium metal and be collected in the anode. 

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