Preparation And Characterization Of Perovskite-type Metal Oxides Containing Iron Element

Due to consumption of fossil resources and environmental destruction,economic and environmentally friendly technology becomes more and more important.Natural gas as fuel has characteristics of high calorific value and low pollutant emission,but the traditional flame leads to low energy utilization.Therefore,efficient technology of catalytic combustion is required.In this dissertation,it is aimed to prepare perovskites and perform structural and compositional analyses including phase composition,surface composition,particle size and behaviors of competitive occupying of two kinds of transition metallic ions into the lattice B-site of perovskite.At last,catalytic activity for methane combustion was evaluated.The main results are as follows.（1）A series of La-Fe metal oxides with La³⁺deficiency in fed chemicals were prepared by citrate method,and characterized by XRD,TPR and XPS.The results indicate that with decreasing La/Fe atomic ratio,Fe₂O₃ phase was gradually formed.But,the content of Fe₂O₃ phase estimated by the Rietveld method is less than that calculated on mass balance,indicating presence of FeOx species which was not detectable by XRD.Furthermore,lattice parameters of perovskite phase were not changed with the La/Fe atomic ratios.This proved that there is no lattice vacancy of La³⁺in the perovskite crystals even in the case of the deficiency of La³⁺feeding.XPS analyses indicate that La³⁺ions are enriched on surface of the samples.Catalytic activity for methane oxidation per unit surface area of the samples is in the order of La_0.68Fe>La_0.76Fe>La_0.94Fe>La_1.02Fe both in the presence and in the absence of gaseous oxygen.（2）Under the condition of excess feeding of Fe³⁺and Co²⁺ions relative to La³⁺ions,formation of perovskite phase was investigated by using the Rietveld method.Standard curve of normalized cell volume（NCV）of perovskite LaFe_xCo_1-xO₃ with the x value was used to determine Fe³⁺occupancy at the lattice B-sites of the perovskite structure.The results showed that Fe³⁺ions were capable of occupying preferentially into the lattice B-sites.Excess Fe³⁺and Co²⁺ions formed spinel(Fe_yCo_1-y)₃O₄.Phase compositions calculated on mass balance are consistent with those estimated by the Rietveld method.Catalytic activity for methane combustion is in the order of LaCo_1.5>LaFe_0.5Co_1.0>LaFe_0.75Co_0.75>LaFe_1.0Co_0.5>LaFe_1.5.This order is in agreement with the order of surface comcentration of cobalt element.（3）Behavior of competitive occupying of Fe³⁺and Ni²⁺into the lattice B-sites of perovskite LaFe_yNi_1-yO₃ was studied under the condition of excess feeding of Fe³⁺and Ni²⁺relative to La³⁺.Standard curve of normalized cell volume（NCV）of perovskite LaFe_xNi_1-xO₃ with the x value was used to determine Fe³⁺occupancy at the lattice B-sites of the perovskite structure.The results showed that Fe³⁺ions were capable of occupying preferentially into the lattice B-sites.Excess Ni²⁺ions formed NiO phase.Phase compositions calculated on mass balance are consistent with those estimated by the Rietveld method.Catalytic activities of the samples for methane combustion are proportional to their specific surface areas.（4）Simple metal oxides M_xO_y（M=Fe,Co,Ni,Cu）were often present as impurity phases during preparation process of perovskites.Therefore,metal oxides MxOy were prepared by thermal decomposition of the corresponding metal nitrates,and crystallite sizes were calculated by the Rietveld method.It is found that NiO has the best catalytic activity for methane combustion,and order of catalytic activity for methane oxidation is in NiO>Co₃O₄>Fe₂O₃>CuO.It is consistent with the order of adsorption amount of oxygen.