Preparation Of LaFeO₃ Perovskite And Its Catalytic Activity For Oxidation Of Carbon Monoxide And Rhodamine B

In this thesis,we studied the catalytic oxidation performance of lanthanum ferric perovskite catalysts with ABO₃ structure?LaFeO₃?,prepared by co-precipitation method and carbon microsphere template method,for CO and Rhodamine B oxidation,and the effects of metal substitution at A/B sites and precursors of carbon microsphere on the catalytic performances.Coprecipitation method:lanthanum nitrate and iron nitrate were used as precursors of LaFeO₃,macromolecular organicsincluding?citric acid,fructose and F127?were used as cpomplex agent and ammonia water was used as precipitant.Effects of macromolecular organics,pH value of the solution,thermal decomposition temperature and thermal decomposition time on the CO oxidation conversionat 300^oC were investigated.Results showed that the effect of them follows the order of thermal decomposition temperature>macromolecular organics>thermal decomposition time>pH.The optimum conditions are:pH value is 10,thermal decomposition time is 3h,macromolecular organics is citric acid,thermal decomposition temperature is 800^oC.the catalyst prepared under these conditions showed the best activity for CO oxidation,with CO conversion reaches to 99.1%at reaction temperature of 300^oC.In addition,effects of Ce substitution at the La site,i.e.,La_1-x-x A_xFe O₃?A=Ce,x=0.1,0.2,0.3,0.4?and transitional metals substitution at the Fe site,i.e.,LaFe_1-yB_yO₃?B=Co,Ni,Cu,Mn;y=0.1,0.2,0.3,0.4?,were further investigated.it was found that the optimum Ce content is 20%(i.e.,La_0.8Ce_0.2FeO₃),at which the sample showed the highest catalytic activity for CO oxidation.The reason is that the Ce addition not only increases the specific surface area?from4.08 m²/g to 17.28 m²/g?and the content of active oxygen,but also improves the catalyst's stability,explaining why La_0.8Ce_0.2FeO₃ shows higher CO oxidation activity than La FeO₃.In case the substitution of transitional metals at the Fe site,it was found that the specific surface area,as well as the ability of binding to oxygen,of all the substituted samples were improved.Compared to LaFeO₃ prepared under the same conditions,the Ni doped catalyst showed a decreased activity,due to its lower content of chemically adsorbed oxygen as detected by XPS spectra,and the Mn doped catalyst showed a lowered stability due to its strong reducibility at high temperature,as reflected by H₂-TPR characterizations.Carbon microspheres template method:Carbon microspheres were first synthesized by hydrothermal method with sucrose as precursor.Particle size and surface properties of the carbon microspheres were controlled by adjusting hydrothermal time?6-24 h?.The carbon microspheres were then used as template to prepare the LaFeO₃ perovskites.Characteristic results showed that all samples have perovskite structure and the crystal structure transforms from orthorhombic to cubic,depending on the hydrothermal time of carbon microspheres.Compared to LaFeO₃ prepared without template,the La FeO₃ prepared with carbon microspheres has higher surface area,surface chemical energy and dispersion.Catalytic activity of LaFeO₃ catalysts for H₂O₂ decomposition and rhodamine B oxidation with H₂O₂ as oxidant were tested at room temperature,which showed that LaFeO₃ catalyst prepared by template method had better catalytic performance for rhodamine B oxidation,with the highest activity obtained from that prepared using carbon microspheres hydrolyzed with 20 h?defined as La Fe O₃-C20?.By comparing the activity for H₂O₂ decomposition and rhodamine B oxidation,it can be inferred that the intermediates produced from H₂O₂ decomposition is the active species for Rhodamine B oxidation.Long-time stability tests on LaFeO₃-C20 for Rhodamine B oxidation showed that no appreciable loss of activity can be observed after running for 6 times,demonstrating that the sample has good stability in reaction.