Synthesis, Characterization And Study Of Catalytic Properties For Perovskite Nanocrystals

The properties of multicomponent perovskites are good in magnetism, electricity, optics, catalysis,sensor for gas detection and resistance to poisoning etal. They also have been entitled for almighty materials. The perovskite structure materials made of ABO₃(LaCoO₃,LaMnO₃,LaFeO₃) have attracted considerable attention in the field of material and catalysis for their good catalytic oxidation activities for catalytic oxidation little molecule(CH₄,CO,NO_x). SrTiO₃, LaCoO₃, LaFeO₃, BiFeO₃ are also of good photo-catalytic activities. The bandgap energy(eg) values of LaCoO₃, LaFeO₃ and BiFeO₃ are lower and this results in their absorbance to visible-light. So these perovskite materials are catalysts to be friendly for environment. In this paper, the improved sol-gel method and the pyrogenation of oxalate combined to melt salt method are used to prepare the Mg-doped LaCoO₃ and BiFeO₃ catalysts. The properties of materials lie on the method of preparation. The syntheses for multi-component perovskite include mainly the high temperature solid state reactions between oxides and/or carbonate precursors, hydrothermal method, sol-gel method, pyrogenation method, co- precipitation and high energy mechanical mill method(HEM). Generally, the High activities catalysts should be of high BET, well dispersivity and good surface crystalline etc.In above mentioned methods, the relatively high temperature results in the decreasing in BET, the severe agglomerate and the bad inhomogeneous in size in methods of high temperature solid state reactions, pyrogenation method and co- precipitation.The samples with high dispersivity obtained by hydrothermal method and high energy mechanical mill method because of the relatively low temperatures which favors the formation of low agglomeration and homogeneity. But the thermal stability obtained by the high energy mechanical mill method is poor that in the application in high temperature as a catalyst will decrease rapidly in BET. The particles obtained from the hydrothermal method possess of well dispersivity and good surface crystalline. But the sizes are very large to lead to the low in BET. The sol-gel method possess of many advantages in preparation catalysts of high activities. In its initial stage, precursors are controlled by adding chelating agent to slow the hydrolytic rate of ion and mix well even in the molecule level. The temperature is low for form the final product .The size is little and the sample obtained by sol-gel method is pure and well in dispersivity. So the sol-gel process is favorable for synthesize the perovskite materials with catalytic property.Semiconductors with heterogeneous photocatalytic behavior have attracted considerable attention due to the possible applications in environmental amelioration and solar energy utilization. LaCoO₃ solid with perovskite structure is a good material in electrics and catalysis. LaCoO₃ is a semiconductor at room temperature and its bandgap is about 0.6eg. So its absorbance corresponds well with visible–light .In this study, we focus on the improving in synthetic method to enhance the its BET and activities. In synthesis the perovskite structure substance, the conventional sol-gel process actually has many advantages ,for example,the reaction carries out in lower temperature (about 600-700℃) and the purity of sample is high . but the agglomeration still exists arise from the calcination process in this method and the BET of sample is lower. Tnvestigators have made great efforts for increasing the BET and the reported specific surface area for LaCoO₃ is usually still less than 40 m²/g. Dispersing nanoparticles in certain matrix during the calcination process is applied in preparing monodispersed nanoparticles with large surface area. Adding a small amount of a second inert phase such as Al2O₃ or SiO2 during the sol-gel manufacturing process is used to restrict the growth of nanocrystals, which is believed to result in tiny discrete particles of this phase located in the interface regions between the nanocrystals, but it is difficult to remove the inert phase. In this paper, we report the preparation of well-dispersed Mg-doped LaCoO₃ nanocrystals by a modified sol-gel method. Controllable particle size can be obtained by introduction excess magnesium nitrate into the reactant mixture. During the calcination process, a MgO phase serves as pinning particles to inhibit the growth and agglomeration of Mg-doped LaCoO₃ nanocrystals. After removing MgO by dissolving in diluted acetic acid, well dispersed Mg-doped LaCoO₃ nanocrystals with large surface is obtained and with the increasing of MgO content, the size of nanocrystals gradually decreases from 31.6 to 13.2nm and the BET value increases from 13.9 to 64.5m2/g. The photocatalytic activity, which is evaluated by measuring the decolorization ratios of Reactive Brilliant Red X-3B, is significantly enhanced with the well-dispersed Mg-doped LaCoO₃ nanocrystals. The photocatalytic activity is related to the crystal size and specific surface area.BiFeO₃ is of simple perovskite-structure. It is typical ferroelectric materials and antiferromagnetic materials. Curie temperature(Tc)is 1110K and Nier temperature (Tn)is 650K. At present, the reports on the BiFeO₃ locus on the study of ferroelectric film applying in semiconductor memorizer. The reports on optical property of this materials is few. JunHua Luo reported the SrTiO₃/BiFeO₃–Fe2O₃ photo-catalytic materials with shell-core structure. The BiFeO₃ is a visible-light sensitizer and the bandgap is 2.1eV corresponding to the absorbance of 590nm photons.The reports on low temperature synthetic methods of BiFeO₃ mainly include the pyrogenation of oxalate and tartaric acid complex. The carrying temperature in two methods is all about 650℃.The photocatalytic-activities of samples obtained from the above methods is almost consistent. In order to enhance the photocatalytic-activities of samples, BiFeO₃ and LiNO₃ are mixed well by milling the mixture. Then the mixure is heated at the melting point of the LiNO₃ for some time ,following by washing the mixture to remove the LiNO₃. The study show that the photo-catalytic activities of BiFeO₃ samples enhance greatly after crystallization in melting salt. Experiment shows that the photo-catalytic activities of BiFeO₃ samples are affected by ratios of LiNO₃ to BiFeO₃ and crystallization time in melting salt. There is not any report on the photo-catalytic activities of BiFeO₃ which is developed in melting salt up to now.