Synthesis Of Micro/Nano Structured CuMnO_x And VMnO_x For CO Oxidation

The catalytic oxidation of CO has been identified as an important reaction because of its wide applications in indoor air quality,gas sensors,CO₂ lasers,and automobile exhaust treatment.Although many studies have been devoted to the catalytic oxidation of CO using noble-metal catalysts such as Au/TiO₂,Au/ZrO₂ and Pt/SnO₂,it is still greatly desired to explore the use of less-expensive metal oxide catalysts with high activity and stability.This dissertation focuses on the syntheses,characterizations and catalytic performances of micro/nanostructured CuMnO_x and VMnO_x for CO oxidation,and is composed of three parts:In the first part,a series of CuMnO_x catalysts were synthesized by a pot pyrolysis method using mixed oxalate as the precursor and their catalytic performances were investigated for CO oxidation.The catalysts were characterized by X-ray diffraction?XRD?,N₂ physical adsorption?BET?,scanning electron microscopy?SEM?,X-ray photoelectron spectroscopy?XPS?,and temperature-programmed reduction?TPR?.The effects of aging time and calcination temperature on the structure and catalytic activity of CuMn₂O₄ catalyst are also discussed in this study.The results suggested that the CuMnO_x catalyst synthesized by a pot pyrolysis method can obtain a single spinel CuMn₂O₄ phase with a larger specific surface area and pore volume,which can increase the amount of Cu-O-Mn interface required for CO oxidation reaction,therefore,a higher catalytic activity can be obtained.Although a high calcination temperature could improve the crystallinity of CuMn₂O₄,the specific surface area and pore volume as well as the exposed Cu-O-Mn interface of the catalyst decreased remarkably,which resulted in a low CO conversion.In the second part,OMS-2 supports were prepared by three different method and the supported copper oxide catalysts?CuO/OMS-2?were prepared by an incipient wetness impregnation method.The catalysts were characterized by X-ray diffraction?XRD?,N₂ physical adsorption?BET?,transmission electron microscopy?TEM?,X-ray photoelectron spectroscopy?XPS?,and temperature-programmed reduction?TPR?for the purpose to explore the "structure-performance" relationships.The results suggested that the OMS-2 support prepared by the grinding method had a high surface area and a large amount of adsorption sites,which could promote the dispersion of supported Cu species and increase the amount of exposed Cu-O-Mn interface required for CO oxidation,therefore,a high catalytic activity can be achieved.However,the OMS-2 support with a needle-like or fibrous feature prepared by the refluxing method or the hydrothermal method had a smaller specific surface area,and a large amount of aggregated copper oxide species were found on the surface of OMS-2,which resulted in a low catalytic activity for CO oxidation.In the third part,V-OMS-2 catalysts with different V:Mn molar ratios were prepared by the refluxing method using different vanadium precursors and their catalytic performances were investigated for CO oxidation.The catalysts were characterized by X-ray diffraction?XRD?,N₂ physical adsorption?BET?,Raman spectroscopy,X-ray photoelectron spectroscopy?XPS?,and temperature-programmed reduction?TPR?.The results suggested that the V-OMS-2-O prepared by using V₂O₅ as the precursor had a rod-like feature with high surface area,whereas the V-OMS-2 samples prepared by using other vanadium precursor showed lower surface area.The high catalytic activity of V-OMS-2-O using V₂O₅ as the precursor also ascribed to its high mobility and reactivity of surface lattice oxygen.Doping a small amount of vanadium ions?V:Mn molar ratio less than 3%?greatly increased the surface area,and the doped V5+ ions were isomorphously substituted Mn⁴⁺ into the octahedral framework of OMS-2,which significantly changed the O coordination environment of octahedral manganese oxide and enhanced the mobility and reactivity of surface lattice oxygen,thus greatly improved the catalytic activity of CO oxidation.We found that the 3%V-OMS-2-O catalyst showed the best catalytic performance for the CO oxidation reaction,which decreased the reaction temperature about to 50 ? for 100% CO conversion.However,the more vanadium dopants?V:Mn molar ratio more than 6%?led to a damage of the pore structure in OMS-2,and enriched the manganese oxide surface with vanadium species,which exerted a serious influence on the mobility and reactivity of surface lattice oxygen,and thus resulted in a decrease of catalytic activity.