The Unique Co₃O₄ Bases And Au-Co₃O₄ Interfacial Structures Significant For Low Temperature CO Oxidation And Benzene Combustion

Low-temperature CO oxidation,perhaps the most extensively studied reaction in the history of heterogeneous catalysis,has a great theoretical and realistic significance.With continuous development of the industry,CO emission has become one of the serious resources for environmental pollution issue.CO oxidation is also a valuable probe reaction to study the redox behavior in surface of catalyst,dispersion and chemical state of precious metal.Volatile organic compounds(VOCs)are the major type of air pollutants and their effective elimination is becoming one of the most important global goals.The most efficient approach to elminating VOCs is considered to be the catalytic oxidation,due to the features of low reaction temperature and no harmful by-products.Size and morphology of nanomaterials inherently determined materials performance.The specifically exposed crystal planes of nanomaterials showed notable advantages in activity and selectivity of catalyst.In this thesis,the Co3O4 bases of different texture and morphology were first synthesized,and then the uniform Au-loaded samples were successfully developed via a controllable deposition-precipitation process.The derived materials were applied for low temperature CO oxidation,focusing on the effect of pretreatment on catalytic performance of the related materials.The core-shell structured Co3O4@SiO2 was also prepared,and the Co3O4 size was regulated via controllable acid etching.The activity and stability of this type of material was evaluated for low/high temperature CO oxidation.Moreover,the morphologically uniform Co3O4 cubes(c-Co3O4),hexagonal plates(h-Co3O4),and tetrakaidecahedrons(t-Co3O4)were carefully synthesized and the unique Au-Co3O4 interfacial structures were obtained upon uniform Au deposition.The catalytic behaviors of these systems were further investigated with the pretreatments in inert and reactive atmospheres.By building the interfacial structure models,the Au-Co3O4 interaction and the origin of the pretreatment enhanced CO oxidation was discussed in detail.Besides,the derived catalysts with the specific exposed crystal planes and the Au-loaded systems were applied for the benzene combustion reaction,while the Au size effect on catalytic performance was also explored at the moderate temperature range.Furthermore,the role of Au-Co3O4 interface was elucidated by comparisons made between the morphologically uniform SnO2 and Co3O4 bases as well as the corresponding Au-Co3O4 and Au-SnO2 interfaces in the target reaction.The major conclusions can be derived as follows:1.Spherically shaped Co3O4 particles were synthesized by one-pot solvothermal treatment that is free of structure-directing agents or pore formers.Au nanoparticles(2 nm)dispersed on the Co3O4 substrates were fabricated using deposition-precipitation method.The as-synthesized Co3O4(without calcination)and the corresponding Au-containing catalyst achieved complete CO oxidation at 90?and 80 ?,respectively.These two types of catalysts were found to be extremely durable even when operated in a period beyond 70 h under certain conditions.2.The morphology of Au loaded in the calcined Co3O4 at 500? showed hemisphere and/or trapezoidal feature.The yolk-shell type Co3O4@SiO2 catalysts synthesized by controllable acid-etching of Co3O4 cores demonstrated an optimal Co3O4 core-SiO2 shell interaction and a suitable Co3O4 core particle size for CO oxidation.Both Co3O4 substrates and Au/C03O4 systems were found to encounter substantial activity enhancement by in situ pretreatment.The pretreatment resulted in(i)transformation of AuOx to AuO,(ii)higher fraction of surface Co3+,and(iii)suitably lower concentration of surface oxygen adspecies,accounting for the enhanced activities.3.Careful synthesis of the morphologically uniform c-Co3O4,h-Co3O4 and t-Co3O4 was accomplished;and the dominant crystalline orientations,namely,the(001)and(111)and(112)facets of these Co3O4 polyhedrons were identified by means of HRTEM and SAED.H2-TPR and XPS investigations revealed the important deviations in reactivity of surface and lattice oxygen,surface Co3+/Co2+ ratio,evolution of surface vacant sites,as well as the Au oxidation state upon Au loading and specific pretreatments.4.The experimental results are interpreted in view of the structural models associated with the(001),(111)and(112)facets of Co3O4substrates and the Au/(001)and Au/(001)and Au/(112)Co3O4 interfaces.The enhanced CO oxidation by Au deposition and particularly He-and in situ-pretreatments on these systems has been clarified in light of their structural specialties.Based on the TOR values,the consequence for the effect of pretreatment on activity enhancement over the Co3O4 substrates is(001)facet>(112)facet>(111)facet.In terms of the TOF together with the ?T50 values,one can find the following order for the effect of pretreatment on activity enhancement over Au-Co3O4 interfaces:Au/(112)>Au/(001)>Au/(001).The results of Au/h-Co3O4 clearly suggest both the size of Au entity and the structural feature of Co3O4 can affect profoundly on the character and catalytic behavior of the generated interface.5.The morphologically uniform Co3O4 crystallites with specific orientations,the octahedral SnO2 crystallites,and the corresponding Au-loaded samples were applied for the benzene combustion reaction.The Au particle size effect was found to be insignificant at the moderate temperature range for benzene combustion,obviously different from that observed in CO oxidation.By the comparison study between the Co-and Sn-based catalysts,one can deduce that the synergism of Au-Co3O4 as well as Au-SnO2 interface can directly promote the reaction,thus lowering reaction temperature and enhancing reaction activity as a result.