Preparation of Mixed Iron Oxides for Solar Hydrogen Production by Means of the Pechini Technique

A process of synthesis of mixed iron oxides for their application to solar hydrogen production is reported. To analyze the suitability of the selected technique, different compositions in the compound Mn_xNi_1−xFe₂O₄₋₅ were prepared by varying the Ni/Mn ratio between 0 and 3. The main objective was to identify the optimal amount of dopants for hydrogen production in such a magnetite. The powders were obtained from a solution of Ni, Mn and Fe nitrates by a polymeric method based on the Pechini process and were characterized by XRD and SEM. The characterization results indicated that the magnetite is fully developed at 1200°C by a multi-step solid state reaction between the mixed oxides produced after the resin heating (α- Fe₂O₃ and nickel-iron spinel). The particles have a polygonal morphology and are softly agglomerate. Their grain size vary with de manganese content and is about 1 micron for 0.25Mn in the mixed ferrite composition and 10 micron in the mixed ferrite without nickel. The activation endothermic step eventually resulting in an oxygen-deficient ferrite was carried out within a thermogravimetric balance. The TGA/DTA mixed magnetite analysis carried out with nitrogen as inert carrier gas showed a weight loss that can be attributed to the partial reduction of the magnetite. The weight losses and the activation temperature increases when the Ni/Mn ratio decreases, being 0.5% at 700°C and 2.57% at 900°C for Ni/Mn ratio 3 and 0 respectively. A series of experimental tests will follow at laboratory test facility with indirect and direct illumination, in order to select the most adequate operation conditions and to quantify the maximum cycle efficiency for a solarized process.