Preparation And Photocatalysis Behaviors Of Supported Coupled-Semiconductors With Metal Modified For Synthesizing Methacrolein From C₃H₈ And CO₂

Photocatalytic reaction for synthesizing methacrolein from C3H8 and CO2 onsemiconductors has a great significance in utilization of alkanes and protection ofenvironment. In this thesis, we extended this study to the gas-solid reaction system byusing the photo-stimulated surface catalytic reaction technology, and designed a seriesof supported coupled-semiconductors with metal modified. The surface structures,energy band edge position, photon absorption property, chemiadsorption abilities tothe reactants and photocatalytic activities to the aim-reaction of the photocatalystswere systematically investigated.1. The three solid materials of supported coupled-semiconductors with metalmodified 1%Cu/11.6%V₂O₅-11.1%TiO₂/SiO₂, 1%Cu/17.2%MoO₃-9.4%TiO₂/SiO₂and 1%Cu/10.1%ZnO-10.1%TiO₂/SiO₂, used SiO₂ as the carrier, were prepared bythe impregnation-chemical modification method successfully. All of them get thehigh surface areas more than 200 m²/g, and their sizes are less than 10nm. The resultsof DTA-TG and TPR experiments show the preparation conditions of these solidmaterials.2. XRD, Raman, IR and UV-VIS DRS experiments were used to characterize thesurface structure. It was shown that the active species of three photocatalystsCu/V₂O₅-TiO₂/SiO₂, Cu/MoO₃-TiO₂/SiO₂ and Cu/ZnO-TiO₂/SiO₂ are dispersed wellon the surface of the carrier: the extremely small particles of V₂O₅, MoO₃ and TiO₂crystallites exist on the surface of SiO₂, while the metal Cu is then highly dispersed inthe surface of these semiconductors. Another distinct characterization of thesematerials on their surface structures is that the semiconductors V₂O₅, MoO₃ and ZnOare coupled with TiO₂ by V-O-Ti, Mo-O-Ti and Zn-O-Ti bonds, respectively. Theyalso form the chemical bonds with silica, which increase the combination of activespecies and the carrier. In addition, there exist abundant active sites on the surfaces ofthe three photocatalysts such as metallic site Cu, Lewis acid sites Tiⁿ⁺, Vⁿ⁺, Moⁿ⁺ orZn²⁺ and Lewis base sites M=O or M1-O-M2.3. The edge energy of the supported coupled-semiconductors and the photonabsorption properties of the three photcatalysts were investigated by the UV-VIS DRSexperiments and K-M functions. It shows that these supportedcoupled-semiconductors V₂O₅-TiO₂/SiO₂, MoO₃-TiO₂/SiO₂ and ZnO-TiO₂/SiO₂ havelarger edge energy than the corresponding bulk semiconductors because of their smallparticles. They also get some improvements in the photon absorption propertybecause of the coupling effects of semiconductors. When they are modified metal Cu,The photo absorption properties of them are reinforced in the intense and range of thelight, which assures the materials can fully absorb the irradiation light of the ultra highpressure Hg lamp and provides the experiment prerequisite for the application of thephotocatalysts in the photocatalytic reactions. Form these results, we described theenergy band structures of supported active species Cu-V2O5-TiO2, Cu-MoO3-TiO2 andCu-ZnO-TiO2 and the matching principles of the Eg, Ec and Ev in their positional.The different migration patterns and the redox abilities of the photoexcited electronsand holes show on their surfaces are also discussed.4. The experimental results studied by the chemiadsorption-IR and TPD-MStechniques show that CO2 could be mainly chemiadsorbed on the surfaces ofCn/V2O5-TiO2/SiO2, Cu/MoO3-TiO2/SiO2 and Cu/ZnO-TiO2/SiO2 to form the highactive horizontal state Cu-C(CO)→Tin+ with the synergistic effects of metallic site Cu,Lewis acid sites Tin+. The chemiadsorption performance of CO2 on thesephotocatalysts decreases in the order of Cu/V2O5-TiO2/SiO2≥Cu/MoO3-TiO2/SiO2>Cu/ZnO-TiO2/SiO2. C3H8 can be chemiadsorbed at the Lewis base sites O2-on thesurfaces of these materials to form two adsorption states. One is attributed to theadsorption of the hydrogen of propane's methylene on the catalysts' surface activesites. The other is then believed to the hydrogen's adsorption of the methyl andmethylene of propane on the sample surfaces. The chemiadsorption performance ofC3H8 decreases in the order Cu/MoO3-TiO2/SiO2 ≥ Cu/V2O5-TiO2/SiO2 >Cu/ZnO-TiO2/SiO2。5. The photocatalystic reaction results indicate that in the gas-solidphotocatalytic reactor, all of the three photocatalysts Cu/V2O5-TiO2/SiO2,Cu/MoO3-TiO2/SiO2 and Cu/ZnO-TiO2/SiO2 could promote the photochemicalsynthesizing of methacrolein from C3H8 and CO2 under the radiation of UV light(intensity 0.65mw/cm2, wavelength 365nm). Their photocatalytic activities to theaim-reaction are related to the reaction conditions, photon absorption property andchemiadsorption abilities of the solid materials. The photo quantum yield tomethacrolein of these photcatalysts decreases in the order of Cu/V2O5-TiO2/SiO2>Cu/MoO3-TiO2/SiO2 > Cu/ZnO-TiO2/SiO2. Especially to the photcatalystCu/V2O5-TiO2/SiO2, it has the highest photocatalytic activity to the aim reaction.Under the reaction conditions of temperature 120℃, space velocity 150h-1, n (C3H8):n (CO2)= 1: 1 and the radiation of UV light, the conversion of propane is 1.33%, theselectivity and photo quantum yield of methacrolein on Cu/V2O5-TiO2/SiO2 is 85.2%and 5.14%, respectively.6. According to the experimental results above, the PSSCR mechanism forsynthesizing methacrolein from C3H8 and CO2 on the supportedcoupled-semiconductors with metal modified was discussed. The photocatalyticreaction process of the active species on the surface of Cu/V2O5-TiO2/SiO2 and thereasons why there exist differences in the photocatalytic activities of the threephotocatalysts were also investigated. It showed that there are two main principles inthe design of photocatalysts. One hand, the active species of the solid materials shouldhave good chemicadsorption properties to the reactants. On the other hand, the energyband structure of the solid materials should be matched with the migrate patterns ofthe photoexcited electrons and holes. Furthermore, we point out the limitation ofthermal (temperature) in the "photo-surface-theraml" synergistic effects in thephotocatalytic reactions and the ways to promote the properties of the photocatayststhemselves and the conditions of photocatalytic reactions.