The syntheses of copper and silver delafossite-type oxides from their constituent binary metal oxides, oxide hydroxides and hydroxides, by low temperature (<210 °C) and low pressure (<20 atm) hydrothermal reactions are described. Particular emphasis is placed on how the acid−base character of a constituent oxide determines its solubility and therefore whether a particular delafossite-type oxide can be synthesized, a strategy utilized by geologists and mineralogists to understand the conditions necessary for the synthesis of various minerals. Thus, the geochemical and corrosion science literature are shown to be useful in understanding the reaction conditions required for the syntheses of delafossite-type oxides and the relationship between reactant metal oxide acid−base character, solubility, aqueous speciation, and product formation. Manipulation of the key parameters, temperature, pressure, pH, and reactant solubility, results in broad families of phase-pure delafossite-type oxides in moderate to high y...

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Hydrothermal synthesis of delafossite-type oxides

Electrochemical properties and growth mechanism of passive films on Alloy 690 in high-temperature alkaline environments

Typical materials for applications in high-temperature, high-pressure aqueous solutions are stainless steels, nickel-base alloys, titanium, tantalum, noble metals, or ceramics. Under these conditions, these materials are often severely attacked, especially in the presence of acids, salts, or gases. Corrosion phenomena in high-temperature aqueous solutions are mainly influenced by the following factors: dissociation of acids, salts, and bases, the solubility of gases, the solubilities of corrosion products, and the stability of the protecting oxide layers. The influences of these factors on the corrosion, together with literature data of dissociation constants and solubilities of oxides, are reviewed in this paper. A recommendation is given for the application of possible reactor materials for oxidizing high-temperature aqueous systems in the presence of different inorganic species.

Factors controlling corrosion in high-temperature aqueous solutions: a contribution to the dissociation and solubility data influencing corrosion processes

Bio-reduction of soluble chromate using a hydrogen-based membrane biofilm reactor.

Synthesis, characterization and stability of Cr(III) and Fe(III) hydroxides.

A platinum-lined, flowing autoclave facility is used to investigate the solubility behavior of Cr2O3 and FeCr2O4 in alkaline sodium phosphate, sodium hydroxide, and ammonium hydroxide solutions between 21 and 288°C. Baseline Cr(III) ion solubilities were found to be on the order of 0.1 nmolal, which were enhanced by the formation of anionic hydroxo and phosphato complexes. At temperatures below 51°C, the activity of Cr(III) ions in aqueous solution is controlled by a Cr(OH)3·3H2O solid phase rather than Cr2O3; above 51°C the saturating solid phase is γ-CrOOH. Measured chromium solubilities were interpreted via a Cr(III) ion hydrolysis/complexing model and thermodynamic functions for the hydrolysis/complexing reaction equilibria were obtained from least-squares analyses of the data. The existence of four new Cr(III) ion complexes is reported: Cr(OH)3(H2PO4)−, Cr(OH)3(HPO4)2−, Cr(OH)3(PO4)3−, and Cr(OH)4(HPO4)-(H2PO4)4−. The last species is the dominant Cr(III) ion complex in concentrated, alkaline phosphate solutions at elevated temperatures.

K. E. S. Combs

Papers

Solubility and phase behavior of Cr(III) oxides in alkaline media at elevated temperatures