Controlled Preparation And Catalytic Performance Of3DOM LaMnO₃, Au/3DOM La_0.6Sr_0.4MnO₃, Au/meso Co₃O₄and3DOM BiVO₄for The Oxidation Of Organic Pollutants

Due to the high surface areas, large pore volumes, and high-quality pore structures,porous transition-metal oxides have been widely applied in physics and chemistry,such as electronics, magnetics, adsorption, and catalysis. The mesopores in porousmaterials can accommodate guest molecules selectively and the high surface areas ofmesoporous materials are beneficial for the adsorption of gaseous molecules. Theexistence of a macroporous structure can reduce the transfer resistance of guestmolecules and facilitate them to approach the active sites. In addition, macro-ormesoporous supports can also favor the dispersion of active components on theirsurface. It is well known that perovskite-type oxides （ABO₃） exhibit goodperformance in catalyzing the complete oxidation of volatile organic compounds（VOCs） and carbon monoxide, which is associated with the oxygen nonstoichometricamount, surface area, and redox ability. The monoclinic scheelite-type BiVO₄as avisible-light-responsive semiconductor photocatalyst is active photocatalyticallyunder visible-light illumination. In recent years, catalysis by gold has been one of thehot research topics. Catalysis over a supported gold material is related to the nature ofthe support. Usually, an inert support was used to load gold, and the metal-supportstrong interaction can influence the physicochemical properties of the supported metalcatalysts. The studies on the employment of reducible transition-metal oxides ormixed oxides as the support have been rarely reported. The loading of nano-sized goldon the surface of transition-metal oxides or mixed oxides with high surface areas isexpected to increase the loading and gold dispersion, thus effectively avoiding thepossible sintering phenomena during the reaction processes. In ordered to improve thephysicochemical properties of single transition-metal oxides or mixed oxides andhence to enhance their catalytic performance, we have in the present dissertation madeinvestigations on the controlled preparation of three-dimensionally ordered （3DOM）LaMnO₃,3DOM La_0.6Sr_0.43, chain-like macroporous LaMnO₃, and MnO_x/3DOMLaMnO₃with mesoporous skeletons, hollow spherical LaMO₃（M=Mn, Co）, solidspherical MOx（M=Mn, Co）,3D ordered mesoporous Co₃O₄, Au/3D orderedmesoporous Co₃O₄, and3DOM BiVO₄via the poly（methyl methacrylate）（PMMA） or3D ordered mesoporous silica （KIT-6） hard-templating routes. The physicochemicalproperties of these materials were characterized by means of a number of analyticaltechniques and the catalytic mechanisms were clarified. The catalytic activities of theas-prepared catalysts were evaluated for the oxidation of VOCs （benzene, toluene,xylene or methanol） and CO, so that the relationships between physicochemicalproperties and catalytic performance of the materials could be established. The maininvestigations and obtained results are as follows:1. The hollow spherical rhombohedral LaMO₃（M=Mn and Co） and solid spherical cubic MOx（M=Mn and Co） nanoparticles were prepared using thesurfactant-assisted PMMA-templating strategy. It is shown that the use ofpolyethylene glycol （PEG） and ethylene glycol （EG） was favorable for theformation of hollow and solid spherical mixed oxides and transition-metal oxides.Compared to their nano-sized counterparts, the spherical LaMO₃and MOxcatalysts possessed much higher surface areas （2133m²/g）, higher adsorbedoxygen species concentrations, and better low-temperature reducibility. For theoxidation of CO and toluene, the catalytic activities of the spherical catalystswere higher than those of the nano-sized catalysts, in which the solid sphericalCo3O4catalyst performed the best for CO oxidation (the T_90%（the temperaturerequired for achieving90%CO conversion）=109℃at a space velocity （SV）=10,000mL/（g h）), whereas the hollow spherical LaCoO₃catalyst showed thehighest catalytic activity for toluene oxidation （To90%=237C at SV=20,000mL/（g h））.2. The rhombohedral3DOM LaMnO₃catalysts with mesoporous skeletons wereprepared via the surfactant-assisted PMMA-templating route. P123or L-lysinemight play a critical role in the generation of mesopores on the macropore wallsof3DOM LaMnO₃. The LaMnO₃catalysts with dual pore structures showedhigher surface areas （3239m²/g）, higher surface oxygen species concentrations,and better low-temperature reducibility. Under the conditions of tolueneconcentration=1,000ppm, toluene/O₂molar ratio=1/400, and SV=20,000mL/（g h）, the porous LaMnO₃samples were superior to the bulk counterpart incatalytic performance, with the LaMnO₃-PP-2and LaMnO₃-PL-2samples withmacro-and mesoporous structures performing the best (T50%=222226℃andT_90%=243249℃). The apparent activation energies （5762kJ/mol） over theporous catalysts were much lower than that （97kJ/mol） over the bulk counterpart.3.3DOM-structured rhombohedral LaMnO₃and its supported MnO_xcatalysts（yMnO_x/3DOM LaMnO₃; y=5,8,12, and16wt%） were prepared using anin-situ tryptophan-assisted PMMA-templating strategy, and an one-pot methodfor the controlled preparation of rhombohedral LaMnO₃-supported MnO_xnano-sized catalysts has been established. It is shown that the surface areas ofyMnO_x/3DOM LaMnO₃were1931m²/g, and the MnO_xnanoparticles （diameter=435nm） were highly dispersed on the surface of3DOM LaMnO₃. Comparedto the12wt%Au/bulk LaMnO₃catalyst derived from the incipient wetnessimpregnation process, the yMnO_x/3DOM LaMnO₃catalysts possessed highersurface areas and adsorbed oxygen species concentrations, low-temperaturereducibility, and more uniform MnO_xnanoparticles （diameter=418nm）. Underthe conditions of toluene or methanol concentration=1000ppm, toluene or methanol/O₂molar ratio=1/400, and SV=20,000mL/（g h）, the12wt%MnO_x/3DOM LaMnO₃catalyst performed the best (T_90%=215and137℃for theoxidation of toluene and methanol, respectively).4. Chain-like macroporous rhombohedral LaMnO₃and rhombohedral3DOMLa_0.6Sr_0.4MnO₃and their supported gold (xAu/chain-like macroporous LaMnO₃（x=1.4,3.1, and4.9wt%） and xAu/3DOM La_0.6Sr_0.4MnO₃（x=3.4,6.4, and7.9wt%）) catalysts were prepared using the surfactant-assisted PMMA-templatingand gas bubble-assisted polyvinyl alcohol-protected reduction methods,respectively. It is shown that the sizes of gold particles were in the range of25nm, and the gold nanoparticles were uniformly distributed on the pore surface ofthe catalysts. xAu/chain-like macroporous LaMnO₃and xAu/3DOMLa_0.6Sr_0.4MnO₃catalysts possessed surface areas of3033and3133m²/g,respectively. The4.9wt%Au/chain-like macroporous LaMnO₃and6.4wt%Au/3DOM La_0.6Sr_0.4MnO₃catalysts showed higher surface oxygen speciesconcentrations and better low-temperature reducibility, and hence higher catalyticperformance. The T_90%values obtained over the4.9wt%Au/chain-likemacroporous LaMnO₃and6.4wt%Au/3DOM La_0.6Sr_0.4MnO₃catalysts were91℃at SV=20,000mL/（g h） and3℃at SV=10,000mL/（g h） for CO oxidation,and226and170℃at SV=20,000mL/（g h） for toluene oxidation, respectively.The apparent activation energies of xAu/chain-like macroporous LaMnO₃catalysts for CO and toluene oxidation were respectively2937and4752kJ/mol, much lower than those （63and97kJ/mol, respectively） of the bulkLaMnO₃catalyst; the apparent activation energies of xAu/3DOM La_0.6Sr_0.4MnO₃catalysts for CO and toluene oxidation were respectively3132and4448kJ/mol, much lower than that （57and74kJ/mol, respectively） of the bulkLa_0.6Sr_0.4MnO₃catalyst.5.3D ordered mesoporous cubic Co3O4（meso-Co3O4） and its supported Au（xAu/meso-Co3O4; x=3.7,6.5, and9.0wt%） nanocatalysts were prepared usingthe KIT-6nanocasting and PVA-protected reduction methods, respectively. Thesurface areas of xAu/meso-Co3O4were in the range of9194m²/g. The6.5wt%Au/meso-Co3O4catalyst exhibited higher surface oxygen species concentration,better low-temperature reducibility, and stronger Au meso-Co3O4interaction, andhence better catalytic activity and stability. Under the conditions of COconcentration=1vol%and SV=60,000mL/（g h） or VOC （benzene, toluene orxylene） concentration=1000ppm and SV=20,000mL/（g h）, the T_90%valuesover the6.5wt%Au/meso-Co3O4catalyst were45,189,138, and162℃for theoxidation of CO, benzene, toluene, and xylene, respectively.6. Monoclinically crystallized3DOM-structured BiVO4photocatalysts with high surface areas （1824m²/g） were prepared by using the ascorbic acid-or citricacid-assisted PMMA-templating strategy. Among the as-prepared BiVO₄photocatalysts, the one with a surface area of ca.24m²/g showed the bestvisible-light-driven photocatalytic performance for phenol degradation （phenolconversion=ca.94%at phenol concentration=0.1mmol/L and in the presenceof0.6mL H₂O₂）. The lower phenol concentration in aqueous solution and theaddition of an appropriate H₂O₂amount were beneficial for the enhancement inphotocatalytic performance.7. Among each series of the as-prepared catalysts, solid spherical Co₃O₄, hollowspherical LaCoO₃, LaMnO₃-PP-2and LaMnO₃-PL-2with macro-andmesoporous structures,12wt%MnOx/3DOM LaMnO₃,4.9wt%Au/chain-likemacroporous LaMnO₃,6.4wt%Au/3DOM La0.6Sr0.4MnO₃,6.5wt%Au/meso-Co₃O₄, and BiVO₄-AA-1catalysts showed the highest performance forthe oxidation of CO, VOC （benzene, toluene, xylene or methanol） or phenol.Based on the characterization results and catalytic activity data, it is concludedthat the excellent catalytic performance of these materials might be associatedwith their higher surface areas, higher adsorbed oxygen species concentrations,better low-temperature reducibility, strong gold-support interaction, better qualityporous structures or higher oxygen deficiency density.