Regulations of Spinel Metal Oxides and Their Photocatalytic Performance Investigation

Author:Li Jia Nan

Supervisor:li xin yong


Degree Year:2019





Photocatalysis as a green catalysis technology based on solar energy can accomplish the purpose of pollutant removal and resource utilization.It shows a promising future both in the area of environmental remediation and energy conversion.Spinel metal oxides have drawn much attention in the photocatalysis area around the world due to their stable structures,variable properties,cheap prices,and environmental friendly properties.Rational regulations of the micro-morphology and electronic structure of spinel metal oxides and comprehensive investigation of the structure-effect relationship between micro-nano properties and photocatalytic activity present great significance for the application in environmental pollution control.Therefore,the surface/interface modification of the material is conducted by us in this thesis from the aspects of crystal plane control,element doping,micro-morphology optimization and multi-component combination to prepare several functional spinel metal oxide photocatalysts.The effects of modification strategies on their photocatalytic performance was discussed,and combined with in situ FTIR and DFT calculations,the mechanism of catalytic reaction and intrinsic activity enhancement were also studied.The main findings are as follows:(1)A series of ZnFe2O4 nanoparticles were prepared by a hydrothermal method.The effects of different crystal facets exposed on the photocatalytic degradation of toluene under visible light were investigated.It was found that the truncated octahedral ZnFe2O4 nanoparticles disclosed by {001} and {111} crystal facets showed the best degradation performance on the gasous toluene,and the toluene degradation efficiency could reach 65%.Compared with the solo {001}or{111} crystal facet exposed ZnFe2O4 nanoparticles,the kinetic reaction rate was increased by 1.32 and 1.51 times,respectively.Theoretical calculations,photodeposition experiment and EPR capture of free radicals have shown that the surface facet junction could be formed between{001}and{111}facets of ZnFe2O4 photocatalysts,promoting the migration of photogenerated carriers.Acompanied with the catalytic conversion characteristics of{001}facets,the truncated octahedral ZnFe2O4 could produce abundant ·O2-and ·OH species in the reaction,improving the photocatalytic degradation activity.(2)Carbon-nitrogen co-doped yolk-shell structure ZnFe2O4 photocatalysts were prepared by an in situ doping solvothermal strategy.The spectral characterization and theoretical calculation results indicated that carbon and nitrogen doping effectively enhanced visible light absorption and promoted the migration of photogenerated carriers by inhibiting their recombination.Using o-DCB as the target pollutant,the effects of carbon and nitrogen source addition and calcination rates on photocatalytic performance were discussed.Under optimal conditions,the degradation efficiency of o-DCB could be 60%.In situ FTIR revealed that the degradation of o-DCB experienced nucleophilic substitution,benzene ring breakup,small molecules transformation and partial mineralization into H2O and CO2.EPR experiments showed that·O2-and ·OH were the reactive oxygen species in the reaction process.(3)Carbon dots modified yolk-shell structure ZnFe2O4 composite photocatalysts were prepared by a step-wise hydrothermal method.Carbon dots were modified on the outer surface of the yolk-shell structure ZnFe2O4,maintaining the interior multiple light scattering structure.It could effectively promote the absorption of catalysts in visible light region,improve the migration of photogenerated electrons,and reduce the recombination rate of electron-hole pairs.Theoretical calculation of differential charge density indicated that there was a strong charge interaction at the interface between carbon dots and ZnFe2O4,driving photogenerated electrons to migrate from ZnFe2O4 to carbon dots by a fast Imigration tunnel.Under optimal conditions,the degradation efficiency of o-DCB could be 74%.In situ FTIR was used to investigate transient species and their migration/transformation behavior during degradation.The results of EPR capture experiments showed that ·O2-and ·OH were involved in the photocatalytic degradation process,and the former was the major reactive oxygen specie.(4)2D nanosheet/3D microflower hierarchical structure CuCo2O4 spinel metal oxide was prepared by a solvothennal method.High temperature pressurized in situ topological transformation strategy was adopted to obtain a gradient sulfur doping from surface to interior.The smooth bending energy band structure effectively accelerated the separation and migration process of photogenerated electron-holes by a so-called "anti-quantum well" built-in field.The sulfur doping process did not destroy the 2D/3D hierarchical structure and low temperature EPR results indicated oxygen vacancy defects existed in the Grad-S-CuCo2O4 sample.Under optimal conditions,the CO product yield of Grad-S-CuCo2O4 photocatalysts could reach 181μmol,which was 5.0 times that of the undoped CuCo2O4 catalyst with CO/H2 selectivity improved 1.95 times.During a long time operation,the catalysts still maintained a good stability.(5)A series of CoMnOx hollow nanosphere spinel metal oxide catalysts were prepared by a carbon sphere soft template cation adsorption-calcination strategy.Theoretical calculations showed that Co doping could enhance J-T distortion of MnO6 unit,promote the delocalization of electrons and enhance the catalytic activity of materials.Combing photocatalysis with PMS activation process,the effects of different conditions and catalysts on the NOR degradation performance were studied.The experimental results showed that the NOR degradation efficiency in the coupling system is 18.8 times and 1.4 times higher than that in the single photocatalytic technology and PMS activation technology,respectively.Under optimal conditions,the NOR degradation effciency in the coupled system could be 94%,simultaneously with a good catalytic stability after several cycles.HPLC-MS results indicated that the NOR degradation went through several major processes such as piperazine ring conversion,defluorination process,decarboxylation reaction and hydroxylation reaction.The quenching experiment and EPR results confirmed that SO4·-was the dominant active species in the degradation process.