Morphology Control And Interfacial Catalysis Of Co 3 O 4 -CeO 2 Catalytic Materials

Cobalt oxide?C0₃O₄?is one of the most important transition metal oxides,which exhibits highly catalytic performance in the catalytic oxidation reaction of small-molecule gases,such as carbon monoxide and methane.What is more,the catalytic activity is closely related to the microscopic features of material,such as the morphology and size.Cerium dioxide?Ce02?is usually used as an oxygen storage materials,plays a significant role in the catalytic oxidation reaction syetem,because of the exquisite transformation characteristics of Ce4+/Ce3+ in its fluorite structure.In recent years,with the rapid development of the nanocatalysts and nanofabrication technology,the ingenious design of specific composition and morphological structure,has been one important path to improve the property of catalyst system.With this,the composite and/or the nano-structure catalyst could emerge the intrinsic superiority.In this thesis,we have prepared two different types of single oxide C0₃O₄ and Co₃O₄-CeO₂ mixed oxides with different structure by a hydrothermal method.The single oxide and Co₃O₄-CeO₂ mixed oxide were used as catalyst and oxygen carriers for chemical looping conversion of methane,CO catalytic oxidation and methane catalytic combustion in a fixed-bed reactor,respectively.For the chemical looping conversion of methane,the results showed that the Co₃O₄-CeO₂ complex oxides had a superior activity to single oxide Co₃O₄ and CeO₂.Due to a larger surface area and the formation of Ce-Co-O solid solution,the oxygen storage capacity of Ce1-yCoyO₂-? oxygen carrier was improved significantly.Moreover,the reaction of Ce1-yCoyO₂-? with methane mainly generated CO and H2 due to good matching between the lattice oxygen mobility and the methane activation rate.On the other hand,the strong interaction between CeO₂ and Co₃O₄ also strongly enhanced the oxygen storage capacity and activity of Co₃O₄/CeO₂?x?oxygen carriers.But they mainly concerted the methane to CO₂ and H2O owning to the relatively high concentration of active oxygen.In conclusion,the Co₃O₄/CeO₂?x?composite oxides could be used as oxygen carriers for chemical looping combustion of methane,while the Ce1-yCoyO₂-? oxides were more suitable for the chemical looping partial oxidation of methane to produce syngas.The sequential cyclic reaction experiments indicated that both the two types of oxygen carriers showed high stability during the successive redox testing.However,it was worth noting that the addition of the second active species in the CeO₂ and Co₃O₄ based oxygen carrier stimulates the reaction activity and stability,but played an inactive role in the product selectivity.In the CO catalytic oxidation reation,the influence of Co₃O₄ morphology and the CeO₂ loading molar ratio on the catalytic activity were detailly investigated and compared.The results indicated that the Co₃O₄ nanoparticles with special microstructure and morphology had remarkable CO and methane catalytic effect of morphology.Compared with nanorods and nanocubes,nanosheets exhibit higher CO catalytic activity,due to the higher specific surface area and concentration of Co3+active sites,more abundant adsorbed oxygen species,higher reducibility and superior oxygen desorption capacity in all three Co₃O₄ catalysts.For the Co₃O₄ nanosheet,the specific CO reaction rate was 6.05×l0-7 mol·g-1·s-1,and the TOF value achieved to 1.79×10-4 s-1.Moreover,it is found that Co₃O₄ nanoparticles have high catalytic stability and activity recovery ability in the CO catalytic stability tests.In the composite catalysts system,the T50and T90 decreased at least 50? over the rod-shaped Co₃O₄ catalyst after the high loading of nanosize CeO₂?Co/Ce =4/1?7/1?,suggesting the CO catalytic activity was significantly enhanced.Likely,after a small amount of CeO₂?Co/Ce = 10:1?loading,the sheet-like catalyst shows an decrease of more than 5? in the T10,T50 and T90 values.Unfortunately,CO catalytic activity of Co₃O₄-CeO₂ catalyst with cube morphology is inferior to that of cubic single oxide Co₃O₄.For the methane catalytic combustion,the activities of Co₃O₄-CeO₂ catalysts with peculiar morphology of Co₃O₄ were inferior to those of their nanosize Co₃O₄ counterparts.In detail,the T10,T50 and T90 values of methane catalytic combustion over Co₃O₄-CeO₂ samples were all higher than those over single oxide Co₃O₄ catalysts.The possible causes could be ascribed to the superimposed effect of the covered active sites and the scanty exposed active crystal.By comparison,nevertheless,the methane catalytic activity over various morphological Co₃O₄ and Co₃O₄-CeO₂ catalysts increased at large in the order of nanosheet>nanocube>nanorod.