Preparation, Characterization, And Catalytic Performance Of La_2-xSr_xCuO₄ And YBa₂Cu₃O₇ For Methane Oxidation

Perovskite-like oxides attract much attention in catalysis due to their good catalytic activity, cheapness, and high thermal stability. In this thesis, perovskite-like oxides La_2-xSr_xCuO₄ (x = 0, 1) and YBa₂Cu₃O₇ with specific morphologies have been fabricated by adopting different synthesis methods, their physicochemical properties were characterized by means of the techniques such as X-ray diffraction (XRD), N2 adsorption-desorption (BET), scanning electron microscopy (SEM), transmission electron microscopy (TEM), selected-area electron diffraction (SAED), X-ray photoelectron spectroscopy (XPS), O2 temperature-programmed desorption (O2-TPD), and H2 temperature-programmed reduction (H2-TPR), and the catalytic performance of these nano/micro-sized materials were evaluated for the total oxidation of methane. From the results, the following conclusions can be drawn:1. Orthorhombic La₂CuO₄ and tetragonal LaSrCuO₄ with single-crystalline structure have been fabricated by using the hydrothermal method. The effects of alkali concentration, hydrothermal temperature, and hydrothermal time on the physicochemical properties of the La_2-xSr_xCuO₄ products were examined. It is shown that increasing the hydrothermal temperature was beneficial for the fabrication of single-phase catalysts, and no significant effects of alkali concentration and hydrothermal time on the physicochemical properties of the final products were observed. The La₂CuO₄ and LaSrCuO₄ derived from the hydrothermal process were of rod-like nano- and mircoparticles and uniform short rod-like nanoparticles, respectively, with the latter showing higher surface areas (11-18 m²/g). Using the self-synthesized CuO (or La₂O₃) as the metal source and adopting the hydrothermal strategy, one could generate the La₂CuO₄ microparticles with a spindle-like morphology (or the La₂CuO₄ nano- and microparticles with a short chain-like morphology); after Sr doping, the LaSrCuO₄ nano- and microparticles showed a platelet or short chain-like architecture. Compared to the La_2-xSr_xCuO₄ particles fabricated hydrothermally with cetyltrimethylammonium bromide (CTAB) as the surfactant, the ones fabricated hydrothermally with poly(ethylene glycol) (PEG) as the surfactant showed a more regular morphology.2. Single-phase perovskite-like oxides YBa₂Cu₃O₇ can be fabricated by using the citric acid complexing, hydrothermal, and citric acid complexing-hydrothermal coupled methods, in which the YBa₂Cu₃O₇ obtained by means of the latter two methods possessed the single-crystalline structure. With metal nitrates or commercial CuO as the metal precursors and adopting the citric acid complexing-hydrothermal coupled method, one could generate hollow microspherical YBa₂Cu₃O₇ particles or porous YBa₂Cu₃O₇ nanoparticles; using metal nitrates or self-fabricated Y2O3 as the metal precursors and the hydrothermal method, one could obtain polyhedral or rod-like YBa₂Cu₃O₇ microparticles.3. The characterization results showed that there was presence of adsorbed oxygen species and Cu²⁺ and Cu³⁺ species on the surfaces of La2?xSrxCuO₄ and YBa₂Cu₃O₇ catalysts, the doping of Sr could enhance the amounts of Cu³⁺ and oxygen adspecies as well as the redox ability.4. Under the conditions of space velocity = 50000 mL/(gcat h) and CH4/O2 molar ratio = 1/10, the LaSrCuO₄å’ŒYBa₂Cu₃O₇ catalysts fabricated hydrothermally with metal nitrates or self-fabricated Y2O3 as the metal precursors showed the best catalytic activities for the complete oxidation of methane. Their good catalytic performance might be associated with the amount of oxygen adspecies, the content of trivalent copper ions, and redox ability as well as the single crystallinity and specific particle morphology.