Stabilization of Monodisperse, Phase-Pure MgFe₂O₄ Nanoparticles in Aqueous and Nonaqueous Media and Their Photocatalytic Behavior

Monodisperse, monocrystalline magnesium ferrite (MgFe₂O₄) nanoparticles were synthesized phase purely by fast nonaqueous microwave-assisted solution-phase synthesis. Colloidal stabilization of the nanocrystals in nonaqueous media was realized either in-situ during synthesis or postsynthetically by surface capping with oleylamine and oleic acid. Phase transfer to aqueous media was performed employing citric acid and betaine hydrochloride, resulting in agglomerate-free dispersions of citrate- or betaine-functionalized MgFe₂O₄ nanocrystals. Furthermore, a one-step synthesis of highly stable, water-dispersible colloids of MgFe₂O₄ was achieved using polyvinylpyrrolidone as stabilizer. Characterization of the as-synthesized and functionalized nanoparticles was performed employing X-ray diffraction, UV–vis and infrared spectroscopy, thermogravimetry, dynamic light scattering, and transmission electron microscopy. Special focus was laid on phase purity, which was thoroughly monitored using Raman microscopy/spectroscopy. Photocatalytic reactions were performed to evaluate the use of such highly stable ferrite colloids for solar energy conversion

Shape-Controlled CeO₂ Nanoparticles: Stability and Activity in the Catalyzed HCl Oxidation Reaction

CeO₂ is a promising catalyst for the HCl oxidation (Deacon process) in order to recover Cl₂. Employing shape-controlled CeO₂ nanoparticles (cubes, octahedrons, rods) with facets of preferential orientations ((100), (111), (110)), we studied the activity and stability under two reaction conditions (harsh: Ar:HCl:O₂ = 6:2:2 and mild: Ar:HCl:O₂ = 7:1:2). It turns out that both activity and stability are structure-sensitive. In terms of space time yield (STY), the rods are the most active particles, followed by the cubes and finally the octahedrons. This very same trend is reconciled with the complete oxygen storage capacity (OSCc), indicating a correlation between the observed activity STY and the OSCc. The apparent activation energies are about 50 kJ/mol for cubes and rods, while the octahedrons reveal an apparent activation energy of 65 kJ/mol. The reaction order in O₂ is positive (0.26–0.32). Under mild reaction conditions, all three morphologies are stable, consistent with corresponding studies of CeO₂ powders and CeO₂ nanofibers. Under harsh reaction conditions, however, cubes and octahedrons are both instable, forming hydrated CeCl₃, while rods are still stable. The present stability and activity experiments in the catalytic HCl oxidation reaction over shape-controlled CeO₂ nanoparticles may serve as benchmarks for future ab initio studies of the catalyzed HCl oxidation reaction over well-defined CeO₂ surfaces

Aqueous Sol–Gel Route toward Selected Quaternary Metal Oxides with Single and Double Perovskite-Type Structure Containing Tellurium

Highly crystalline SrFe_2/3Te_1/3O₃, Ba₃Fe₂TeO₉, and Ba₂NiTeO₆ have been synthesized by using a specially developed sol–gel route methodology, reducing the time needed employing solid-state routes and resulting in high reaction yield up to 75%. These materials have been studied by X-ray powder diffraction (XRPD), scanning and transmission electron microscopy, Raman spectroscopy, and dielectric and magnetic measurements. At room temperature, the crystal structure of SrFe_2/3Te_1/3O₃ is cubic, space group <i>Pm</i>3̅<i>m</i>, with <i>a</i> = 3.9373(2) Å, whereas Ba₃Fe₂TeO₉ crystallizes in the hexagonal crystal system, space group <i>P</i>6₃/<i>mmc</i>, <i>a</i> = 5.7691(4) Å, and <i>c</i> = 14.208(1) Å. The third studied perovskite Ba₂NiTeO₆ crystallizes in the trigonal <i>R</i>3̅<i>m</i> space group with <i>a</i> = 5.7974(4) Å and <i>c</i> = 28.599(2) Å. Based on structural characterization results, the obtained single and double perovskite crystallites are nearly in nanometer regime, ranging from 45 to 164 nm, building micrometer-sized particles with visible well-faceted hexagonal morphology. Magnetic measurements show the onset of ferrimagnetic ordering at relatively high temperature of 667 K for the SrFe_2/3Te_1/3O₃, whereas Ba₃Fe₂TeO₉ and Ba₂NiTeO₆ show antiferromagnetic ordering below 80 and 8.6 K, respectively. The measured room temperature dielectric constants are in the range between 15 and 77