Controlled Preparation And Catalytic Performance Of Porous Fe-and V-based Mixed Oxide Catalysts For The Removal Of Organic Pollutants

It is urgent to eliminate environment pollutants. Catalytic oxidation technology isacknowledged to be one of the most promising approaches. It is of significance inacademic and practical aspects to optimize the designing and fabrication of novel andefficient heterogeneous catalysts, evaluate their catalytic performance, and investigatethe relationship between physicochemical property and catalytic activity. It is wellknown that factors, such as chemical composition, surface composition, surface area,pore structure, and redox ability, exert effects on the catalytic performance ofperovskite-type oxides (ABO3) in the oxidation of volatile organic compounds(VOCs); and the photocatalytic efficiency of a visible-light-responsive photocatalystin the degradation of organic pollutants (dyes and VOCs) in water is associated withits chemical composition, crystal structure, bandgap structure, morphology, surfacearea, porous structure, and optical property. Porous metal oxides with high stability,large surface areas, and porous structures, have been widely applied in the fields ofelectronics, magnetics, adsorption, catalysis, and so on.Taking multiferroic Fe-based perovskite-type oxides and novel V-based oxides asthe research objects, the main contents in this thesis are the design, preparation, andmodification of porous materials that show high catalytic performance for theoxidative removal of organic pollutants. In Chapters3~5, porous AFeO3(A=Sr, La,Eu) catalysts with regular morphologies or three-dimensionally ordered macroporous(3DOM) structures and3DOM Eu0.6Sr0.4FeO3-supported cobalt oxide nanoparticleswere prepared using the glucose-assisted hydrothermal or polymethyl methacrylate(PMMA)-templating strategies, their physicochemical properties were characterizedby means of a number of analytical techniques, and their catalytic activities wereevaluated for the oxidation of typical VOCs (e.g. toluene), so that the relationshipbetween physicochemical property and catalytic performance could be established. InChapters6~8, a series of new-type and high-efficiency visible-light-driven3DOMvanadates (MVO4, M=In, Bi)-based composite photocatalysts were fabricated usingthe PMMA-templating, incipient wetness impregnation, and bubble-assisted reductionmethods, characterized by means of a variety of techniques, their photocatalyticactivities were evaluated under visible-light irradiation for the removal of organicpollutants (rhodamine B (RhB), methylene blue (MB), phenol, and para-chlorophenol(4-CP)), and their photocatalytic degradation mechanisms were discussed. The mainresults of this thesis are as follows:(1) A novel and facile one-pot hydrothermal route with the assistance of glucosewas developed to fabricate hollow spherical SrFeO3δwith nanoporous shells andLaFeO3with homogeneous mesopores.(i) The suitable preparation conditions for the porous SrFeO3δhollow spheres were: the pH value of the precursor solution was ca.4.2, glucose concentration was0.3mol/L, ethylenediamine volume was1.0mL, andhydrothermal treatment condition was at170oC for20h. The as-prepared sampleexhibited the best catalytic performance for toluene oxidation (with the temperature(T90%) required for achieving90%toluene conversion being298oC at a space velocity(SV) of20,000mL/(g h)). Such a good activity of the sample was associated with itslarger surface area (26.5m2/g), smaller grain size (42.5nm), higher surface oxygenadspecies concentration (the surface adsorbed oxygen and lattice oxygen speciesmolar ratio (Oads/Olatt)=1.94), and better low-temperature reducibility.(ii) Thesuitable preparation conditions for the homogeneously mesoporous LaFeO3were: thepH value of the precursor solution was ca.9.3, glucose concentration was0.3mol/L,ethylenediamine volume was1.0mL, and hydrothermal treatment condition was at170oC for14h. The surface area, grain size, Oads/Olattmolar ratio, T90%(at SV=20,000mL/(g h)), and apparent activation energy (Ea) of the mesoporous LaFeO3sample were25.8m2/g,32.8nm,2.42,275oC, and50.1kJ/mol, respectively. It isshown that the nature of the metal precursor used in the hydrothermal treatment couldinfluence the morphology, pore structure, and catalytic activity of the final catalyst;and the porous structure, high surface area, surface oxygen adspecies concentration,and good low-temperature reducibility of the catalyst could facilitate the oxidation oftoluene.(2) Well-defined3DOM Eu1xSrxFeO3(x=0,0.4,1.0) catalysts were prepared viathe PMMA-templating route. The3DOM SrFeO3δsample with a cubic structure, abigger crystallite size (37nm), a thicker pore wall, and a largest surface area (61.4m2/g) possessed the highest oxygen adspecies concentration and initial H2consumption; the3DOM EuFeO3sample with an orthorhombic structure and a lowestsurface area (20.6m2/g) exhibited a small amount of oxygen adspecies and lesslow-temperature reducibility; and the3DOM Eu0.6Sr0.4FeO3sample displayed asmallest crystal size (25nm), a medium surface area (31.1m2/g), oxygen adspeciesconcentration, and best low-temperature reducibility. At a SV of20,000mL/(g h), theT90%for toluene oxidation was310,347, and305oC over3DOM SrFeO3δ,3DOMEuFeO3, and3DOM Eu0.6Sr0.4FeO3, respectively. The apparent activation energy (Ea)was82.0kJ/mol over3DOM EuFeO3and81.1kJ/mol over3DOM Eu0.6Sr0.4FeO3. Itwas also found that the preparation conditions were different due to the discrepancy inphysicochemical property of the metal nitrates in precursor solutions, thus obtainingthe samples with different3DOM structures and chemical compositions. Thewell-defined3DOM structure and the A site-substitution in ABO3were beneficial forthe enhancement in catalytic performance for toluene oxidation and the decrease inapparent activation energy.(3)3DOM Eu0.6Sr0.4FeO3-supported cobalt oxide nanocatalysts (1~10wt% Co3O4/3DOM Eu0.6Sr0.4FeO3) with a surface area of22.6–27.8m2/g were preparedusing the incipient wetness impregnation method. By comparing to the bulkcounterparts for toluene oxidation, the3DOM-structured samples showed much bettercatalytic performance, with the best-performing samples being3–6wt%Co3O4/3DOM Eu0.6Sr0.4FeO3. Over3–6wt%Co3O4/3DOM Eu0.6Sr0.4FeO3at SV=20,000mL/(g h), the T50%and T90%were250and270oC, respectively, which werelowered by ca.30and10–20oC than those over3DOM Eu0.6Sr0.4FeO3and3DOMCo3O4, respectively. The Eavalues were ca.71.6,81.1, and73.9kJ/mol over6wt%Co3O4/3DOM Eu0.6Sr0.4FeO3,3DOM Eu0.6Sr0.4FeO3, and3DOM Co3O4, respectively.The enhanced catalytic performance of3–6wt%Co3O4/3DOM Eu0.6Sr0.4FeO3fortoluene oxidation was mainly related to their high-quality3DOM structures, largesurface areas, high oxygen adspecies concentrations, multivalent metal coexistence,and good low-temperature reducibility.(4) A structure design for high-efficiency photocatalysts was proposed bycombining the advantages of3DOM structure, heterojunctions, and noble metalnanocrystal plasmonic effect. Visible-light-driven photocatalysts3DOMInVO4–BiVO4(InBi-3D) and M/3DOM InVO4–BiVO4(M/InBi-3D, M=Au, Ag, Pd,Pt) with a M nanocrystal (3–4nm) loading of ca.0.2wt%were fabricated using thePMMA-templating and bubble-assisted reduction routes, respectively. The bandgapenergies (Eg) of these materials were similar (2.50–2.56eV). The loaded noble metalscould modify their surface compositions. The Au/InBi-3D sample exhibited thehighest photocatalytic activity for the degradation of MB and RhB, which was relatedto its featured structure that could dramatically increase the ability to capture andconvert light energy and the efficiency to separate and transfer photo-generatedcarriers of the photocatalyst. The high surface area (25.1m2/g), large pore volume(0.253cm3/g), and porosity (74%) of the3DOM-structured sample could not onlyfacilitate the adsorption and mass transfer of dye molecules, but also be beneficial foruniform dispersion of noble metals and exposure of more amounts of active sites inthe sample, thus enhancing dye degradation rates. In addition, a competitivedegradation mechanism was proposed according to the photocatalytic activities ofAu/InBi-3D for the removal of MB+RhB and the related kinetic analysis.(5) Visible-light-driven heterostructured photocatalysts p-Fe2O3/3DOM n-BiVO4and AgBr/3DOM BiVO4were prepared using the PMMA-templating, incipientwetness impregnation, and deposition methods. Both photocatalysts were low incrystallinity. The p-Fe2O3/3DOM n-BiVO4sample displayed two Egvalues (2.51and1.89eV), while the AgBr/3DOM BiVO4sample showed only one Egvalue (2.61eV).Using the polyvinyl alcohol-protected reduction route, M/p-Fe2O3/3DOM n-BiVO4and M/AgBr/3DOM BiVO4(M=Au, Pt, Pd) were prepared. Phenol or4-CP aqueoussolution with an initial concentration of0.4mmol/L or15mg/L was adopted to evaluate the photocatalytic activities of the samples under visible-light irradiation. Itis found that the Pd-loaded composite photocatalysts showed the best photocatalyticperformance in the degradation of phenol or4-CP. The time for phenol degradationover Pd/Fe2O3/3DOM BiVO4was about60min, and the time for4-CP degradationover Pd/AgBr/3DOM was about150min. The excellent photocatalytic performanceof Pd/Fe2O3/3DOM BiVO4and Pd/AgBr/3DOM was associated with the synergisticeffects among the3DOM structure, heterojunctions, and surface plasmon resonanceinduced by the noble metal nanoclusters. The phenolic compounds were step by stepdegraded on the low-crystallinity photocatalyst surface, indicating the important roleof stepped structure in the photocatalytic degradation of organic pollutants.