Synthesis, characterization and catalytic activity of La(Sr)Fe(Co)O3 perovskites (Doctoral thesis)

Μαργέλλου, Αντιγόνη

Perovskites possess the general form ΑΒΧ3 where A and B are metal cations connected with an anion X, usually oxygen. These materials exhibit interesting properties and are widely used in catalytic applications. Their properties and consequently their uses are determined by the procedure followed for their synthesis. The perovskites of this thesis were prepared by the sol gel auto combustion method in presence of glycine (-Gly) or anionic surfactant glycolic acid ethoxylate layryl ether (-AS) and La was substituted with Sr in the A-site of the structure (La1-xSrxFeO3-Gly and La1-xSrxFeO3-AS) and Fe with Co in the Bsite (LaFe1-xCoxO3-Gly and LaFe1-xCoxO3-AS). The materials were characterized by thermal analysis, X-Ray Powder Diffraction, X-Ray Fluorescence Spectroscopy, Mössbauer Spectroscopy, Fourier Transform Infrared Spectroscopy, Ν2 porosimetry, Diffuse Light Scattering, Ο2 Temperature programmed adsorption-desorption and Scanning Electron Microscopy. The catalytic activity of the perovskites was evaluated in the NO reduction by CO. La1-xSrxFeO3-Gly materials exhibit well crystalline perovskite phases and are mesoporous materials with surface areas 22-53 m2/g. Increase in the substitution led to decrease of Fe3+ and increase of Fe4+ in the perovskite phase. The perovskites exhibit the property of oxygen reverse uptake. La1-xSrxFeO3AS exhibit not only the classic perovskite phase but also SrCO3, Fe2O3 and perovskites homologous series. These materials are mesoporous with areas 6-13 m2/g, exhibits Fe3+ out of the perovskite structure and do not exhibit oxygen reverse uptake. The B-site substituted perovskites (LaFe1-xCoxO3Gly and LaFe1-xCoxO3-AS) exhibit well crystalline perovskite phases, surface areas 22-11 and 13-9 m2/g and do not exhibit the oxygen reverse uptake. The materials exhibit catalytic activity toward the reduction of NO by CO in the temperature range 100600oC. At low temperatures the reduction of NO yields to N2O production whereas at higher temperatures leads to N2. The activity of La1-xSrxFeO3-Gly exhibits a strong correlation with the oxygen reverse uptake and increase of oxygen reverse uptake leads to decrease of activity, attributed to the decomposition of perovskite phase into SrCO3, Fe2O3 and the homologous perovskite series during the NO reduction reaction. Moreover, N2O decomposition over La1-xSrxFeO3-Gly seems to be inhibited by oxygen reverse uptake, affecting the selectivity of NO reduction. The activity of La1-xSrxFeO3-AS do not exhibit a systematic change and the relative activity of La1-xSrxFeO3-AS with regard to La1-xSrxFeO3-Gly materials is attributed to the decomposition of the perovskite phase into SrCO3, Fe2O3 and the homologous perovskite series either during the synthesis of the materials or during the NO reduction. In the perovskites LaFe1-xCoxO3-Gly and LaFe1-xCoxO3-AS, increase in the substitution leads to increase of activity expressed as conversion per m2, due to the reduction of perovskite phase LaFe(Co)O3 into Ruddlesden-Popper phase La2CoO4 and the creation of La2CoO4-LaCoO3 systems which enhances the reduction of NO by CO.
Institution and School/Department of submitter: Πανεπιστήμιο Ιωαννίνων. Σχολή Θετικών Επιστημών. Τμήμα Χημείας
Subject classification: Περοβσκίτης
Keywords: Περοβσκίτες,Σύνθεση περοβσκιτών,Αντιστρεπτά προσλαμβανόμενο οξυγόνο,Αναγωγή ΝΟ/CO,Διάσπαση Ν2Ο,Perovskites,Synthesis of perovskites,Oxygen reverse uptake,Reduction NO/CO,N2O Decomposition
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