Preparation Of Coupled Semiconductor Supported Metal Materials And Their Photocatalysis Behaviors For The Reaction Of CO₂ With C₃H₆

Methacrylic acid is one of important material in organic synthesis. Utilization of carbon dioxide and propylene as starting material for direct synthesis of methacrylic acid has an attractive alternative to organic synthesis, environmental protect and utilization of carbon. In this thesis, we extended this study to the gas-solid reaction system by using the photo-stimulated surface catalytic reaction (PSSCR) technology, and designed the coupled semiconductors supported metal. The preparation methodology, chemical component, surface structure, chemisorption, photo absorption property and photocatalytic activity of the coupled semiconductors supported metal were systematically investigated. 1.The solid material of coupled semiconductors MoO₃-TiO₂, WO₃-TiO₂, V₂O₅-TiO₂ and SnO₂-TiO₂ were prepared by sol-gel method. The coupled semiconductors MoO3-TiO₂, WO₃-TiO₂, V₂O₅-TiO₂ and SnO₂-TiO₂ supported metallic M(M=Pd,or Cu,or Ag) were prepared by an impregnation-reduction method. The surface structure, particle size, surface active species, photo adsorption property, chemisorption property and photocatalysis reaction with the "photo-surface-thermal"synergistic effects were characterized by TG-DTA, XRD, TEM, BET, LRS, TPR, IR, UV-VisDRS, TPD-MS and gas-solid photocatalysis reaction techniques. In the conditions of radiation of UV light(wavelength 365nm, intensity 0.65mW.cm^-2), temperature 110℃, space velocity 200 h^-1, and CO₂ : C₃H₆=1:1.5, the conversion of C₃H₆ reaches 8.4%, the selectivity to methacrylic acid is over 95%, and the photo-quantum efficiency is 22.8% on the Cu/MoO3-TiO₂ solid material. 2. The experiment results show that the M/MoO3-TiO₂ and M/WO₃-TiO₂ (M=Pd,or Cu,or Ag)are composed of anatase TiO₂ with particle sizes around 10-25 nm and the surface area over 70m2.g-1. The active composes are dispersed in a monolayer dispersing on the surface of TiO₂. The tetrahedrally coordinated Mo(or W) species transform to the octahedrally coordinated Mo(or W) species on TiO₂ with increasing of Molybdenum loadings, presence of metallic M(M=Pd,or Cu,or Ag) on the coupled semicondutors accelerates this progress. The monolayer phase threshold value of molybdenum loadings on TiO₂ is 10%,the Raman and XRD spectra of catalysts with Mo(or W) loadings of 10% MoO3 (or WO₃) or greater also show the presence of crystal MoO3 (or WO₃). The strong effect of Mo (or W) species with TiO₂ results in the formation of Mo-O-Ti (or W-O-Ti) bonds on the TiO2 surface. 3. The results investigated by the UV-Vis DRS technique reveal that TiO2 has favorable photo adsorption property at UV range, but 3.5% photo adsorption capacity can be adsorbed at visible light range . Compared with single TiO2, blue shifting in the absorption edge, increasing of the photo absorption capacity to visible light range, broading of band gap on MoO3-TiO2, all of theses promote to form the photogeneration electron-hole pairs in the semiconductor materials, which have stronger oxidation-reduction property. The octahedrally coordinated Mo species on MoO3-TiO2 have favorable photo adsorption property. Both increasing of the photo adsorption capacity at visible light range and blue shifting in the absorption edge on M/MoO3-TiO2 or M/WO3-TiO2(M=Pd,or Cu,or Ag) enhance photo adsorption property of the photocatalysts. Based on utilization ratio of photo energy of UV light, the photo absorption performance decreases in the order of Pd/MoO3-TiO2>Cu/MoO3 -TiO2 >Ag/MoO3-TiO2. The photo absorption performance on coupled semicondutors M/WO3-TiO2(M=Pd,or Cu,or Ag) is the same as coupled semicondutors M/MoO3-TiO2(M=Pd,or Cu,or Ag). 4. The experimental results studied by the chemisorption-infrared and chemisorption-TPD thechniques point out that CO2 can be chemisorbed on the surface of M/MoO3-TiO2 or M/WO3-TiO2 (M=Pd,or Cu) to form monodentate carbonate, linear state, shearing state and high active horizontal state Cu(or Pd)-(CO)-O→Ti4+(or Mo6+,or W6+) with the synesgistic effect of metallic M(M=Pd,or Cu) and Lewis acid sites Ti4+ (or Mo6+, or W6+), C3H6 can be chemisorbed on the surface of M/MoO3 -TiO2 and M/WO3-TiO2 (M=Pd,or Cu) to form molecular state Cu(or Pd)-C(CH2) (CH3)-H→O=Mo(or W) with synergistic effect of both adsorption of the β-site C-H band of C3H6 on Lewis base sites Mo=O(or W=O) and adsorption of β-site C of C3H6 on metallic M(M=Pd,or Cu). CO2 chemisorbed on the surface of Ag/MoO3-TiO2 or Ag/WO3-TiO2 has monodentate carbonate and linear state, while C3H6 chemisorbed on the surface of Ag/MoO3-TiO2 or Ag/WO3-TiO2 has double bond adsorption states. There exist competitive adsorption between CO2 and C3H6 on metallic M(M=Pd,or Cu), which are common adsorption sites of CO2 and C3H6. 5.The photocatalysis reaction results showed that the control tests did not give any products, it indicats that photochemical reaction without photocatalysts and thermal catalytic reactions can be ruled out. The chemisorption property of the solid materials and adsorption states of CO2 and C3H6 are the key to reaction. Thehorizontal states of CO2 and the β-site C-H band adsorption states of C3H6 on the surface of M/MoO3-TiO2 or M/WO3-TiO2(M=Pd,or Cu) is the activity of aim products, which can not form on Ag/MoO3-TiO2 or Ag/WO3-TiO2, these result in not aim products and low photo-quantum efficiency. The photo-quantum efficiency on MoO3-TiO2 or WO3-TiO2 is clear higher than single TiO2, it indicates that semicondutor coupled effect can inhibit the electro-hole pairs recombination and enhance photo-quantum efficiency. Compared with MoO3-TiO2 or WO3-TiO2, the photo-quantum efficiency on M/MoO3-TiO2 or M/WO3-TiO2(M=Pd,or Cu) increases obviously. The change of photo adsorption property on M/MoO3-TiO2 or M/WO3-TiO2(M=Pd,or Cu) is the same to the change of photocatalysis activity of the photocatalysts. The photo-quantum efficiency on Cu/MoO3-TiO2 is higher than on Cu/WO3-TiO2, because adsorption property of Cu/MoO3-TiO2 is better and the gap absorption edge of Cu/MoO3-TiO2 approaches strongest wavelength of the UV lamp, compared to Cu/WO3-TiO2. 6. According to the experimental results above, the PSSCR mechanism for direct-synthesis methacrylic acid from CO2 and C3H6 on M/MoO3-TiO2 or M/WO3-TiO2(M=Pd,or Cu) was discussed. CO2 which has horizontal state forms active intermediate CO2-on the effect that the photogeneration electrons on the surface of M(M=Pd,or Cu) enter into CO2, while C3H6 that has β-site C-H band adsorption states forms active intermediate C3H6+ on the effect that the photogeneration cavities on Lewis base sites Mo=O enter into C3H6. The effect result of the active intermediate CO2-and C3H6+ induces production of methacrylic acid, which desorbes from the surface of M/MoO3-TiO2 or M/WO3-TiO2(M=Pd,or Cu) at temperature 110℃.This is result of the "photo-surface-thermal"synergistic effects.