Synthesis,Microstructure,CO Oxidation Behaviors Of Cobalt Doped MgFe₂O₄ Spinel Oxides

Spinel magnesium ferrite,Mg Fe₂O₄,has fascinated the science community over the years,owing to its merits of low cost,multiple mixed occupancy of metal cations,poisoning and sintering resistance,which shows many potential applications.According to the investigations about spinel oxides,the variation of particle sizes or the ion doping would change distribution of the Mg/Fe in tetrahedral site and octahedral site,thus tune the chemical and physical properties of Mg Fe₂O₄.However,there is lack in systematic research on the mechanism of Mg Fe₂O₄ grain growth and the influence of cobalt ion doping on its microstructure and the performance of CO oxidation.Herein,spinel Mg Fe₂O₄ is chosen as the target to study the grain growth of its nanocrystals,and the effect of grain size and cobalt ion substitution on the conversion of CO.The main results is as follows:（1）Growth kinetics of Mg Fe₂O₄ nanocrystals and optimization of Pt/Mg Fe₂O₄catalyst for CO oxidation:Mg Fe₂O₄ nanocrystals were controllably synthesized by a sol-gel route following a calcining process.By tuning the calcination temperature from400 ^oC to 1000 ^oC,the grain size of Mg Fe₂O₄ nanocrystals ranging from 8.1 nm to190.0 nm were obtained.The growth of Mg Fe₂O₄ nanocrystals obeys the Ostwald ripening mechanism with a quantitative equation of D⁵=2.08×10¹⁴exp（-122.13/）.The structure and Mo?ssbauer analysis demonstrated that the occupancy of Mg and Fe ions in octahedral and tetrahedral positions is highly dependent on the grain size of Mg Fe₂O₄nanocrystals.Further,the grain size of Mg Fe₂O₄nanocrystals as a support greatly affects to the surface chemical state,Pt-support interaction,and the ability to activate oxygen.Pt/Mg Fe₂O₄ catalyst with a support size of 20.6 nm is indicated to show the best catalytic activity with a broader operation temperature window of 70?400 ^oC.The results achieved for grain growth kinetic control over spinel Mg Fe₂O₄could provide new hints in comprehending growth mechanism and size-dependent properties of many other spinel supports for emerging applications.（2）Synthesis of Co doped Mg Fe₂O₄ nanoparticles and investigate the role of substitution in the structure and properties:Co_xMg_1-xFe₂O₄ spinel oxides were prepared by a combination of sol-gel and calcination process.Co doping leads to a peak shift in Raman and IR spectra.Mo?ssbauer spectra was utilized to determine the change of Fe occupancy at tetrahedral and octahedral sites.Systematic characteristics indicate that the redox behavior and surface chemistry of the obtained spinel oxides rely on the Co doping concentration.Compared with undoped Mg Fe₂O₄,the CO oxidation performance of Co doped Mg Fe₂O₄ has been greatly improved and the sample with x=0.2 gives the best catalytic performance among all the studied oxides.