A Preliminary Study On The Preparation And Properties Of V-O Magnéli Oxide

Magnéli phase oxides,include Ti-O Magnéli phase,Mo-O Magnéli phase,W-O Magnéli phase and V-O Magnéli phase,are non-stoichiometric compounds.Because of the presence of oxygen defects and the special shear plane,Magnéli phase oxides are with the special properties such as optical,electrical,acoustic,magnetic,mechanical,and thermal properties,it could be as a kind of functional materials.Now a few reports are about V-O Magnéli phase.The present work investigated the thermodynamical stable conditions and the kinetic mechanism on preparing of V-O Magnéli phase preliminarily.Photocatalytic degradation in methylene blue using the prepared powder was simplely tested.The Gibbs free energies of oxidation step by step at different temperatures for parts of V-O Magnéli phase oxides were calculated,and the stability diagram of V-O system phases were plotted.Using Mixing gas (??H₂/CO₂?=1/300 in volume rate to kept 1423 K for 8 hours,V₂O₃ was oxided to V₄O₇ completely.The oxygen partial pressure was controlled by different proportions of CO/CO₂ in volume to check the accuracy of the plotted V-O system phase stability diagram.The XRD results showed that the product was V₅O₉ with V₄O₇ when volume rate was ??H₂/CO₂?=0.085/1;when ??H₂/CO₂?=0.200/1,product was V₄O₇,;when??H₂/CO₂?=0.295/1,V₄O₇ and V₃O₅ coexisted.Experiments results verified the plotted phase stability diagram.Mixing gas CO₂/H₂ was selected to adjust the oxygen partial pressure,V₂O₃ was oxidized at 1373 K,1423K and 1473 K repectively,the gained mass were recorded with the time changing,kinetic curves of V₂O₃ oxiding were drawn.According to the XRD and SEM analysis of the simples obtained in different reaction stages at 1423 K,it was found the oxygen content in product was increasing with the time incleasing,and the surface of the samples turned smooth.It could be a random growth and nucleation during oxidation.The convertion curves showed two-stage during the reaction.Based on thecomputer fitting of mathmatic models,the oxidetion is controlled by two stages of nucleation and growth,The first stage could be expressed by and the second stage by.A precursor,C₃H₄O₃V,with the flying saucer shape,was prepared by a hydrothermal process.The same shape V₄O₇ was produced by high temperature reduction.Their average diameter of particle was about 5.93 ?m.The V₄O₇ was used as the photocatalyst to degradation methylene blue.For test the light source was 300⁴00nm,when the concentration of catalyst was0.045g/L,the initial concentration of methylene blue was 15mg/L,after100 min irradiation,the degradation rate of methylene blue reached 45%.Finally,a novel developed quasi-gas phase decomposition apparatus was applied to prepare V-O Magnéli.At 1173 K,mixing gas ??H₂/CO₂?=0.200/1 in volume rate,nebulizing droplet of ammonium metavanadate aqueous solution was transport to high temperature zone for reduction to oxide V₄O₇.The reduction product was V₄O₇ and V₅O₉ mixture.With the spherical and cylindrical shapes,and their average particle size was about 240 nm.