The Preparation Of Layered Double Hydroxides Zn（Cu）/Al（Ti）-LDHs And Research On The Photo-convertion Of CO₂-H₂O With LDHs

The rapid development of human technologies still can not get rid of the greatdependence on fossil fuels, and the uncontrolled exploitation causes great energy crisesas well as huge environmental risks. In addition to the nitrogen oxides, sulfur oxides,dusts and other pollutants that have been paid attention to, carbon dioxide emitted byburning of fossil fuels also has attracted our attention. CCS（Carbon Capture andStorage） is a hot research recently, but the traditional methods is demanding, so theamount of CO₂emitted during CCS process is often more than the content that areconverted, which makes the result not worth the candle. With the use of solar energyand photocatalysts,we can try to simulate the natural plant photosynthesis to covertCO₂and H₂O to hydrocarbons, such as methnol, methane, dimethyl ether. In this way,the over-exploitation of fossil fuels and excessive CO₂emission can both be solved. Asa typical photocatalytic reaction, the emphasis and difficulty lie in finding theappropriate photocatalyst.LDHs is a kind of alkaline catalyst with typical layered structure. In this thesis, weprepared different kinds of LDHs with Zn²⁺, Cu²⁺, Al³⁺and Ti⁴⁺. By the means ofco-precipitation, we could flexibly combine these metal ions, so that produced Zn/Al,Zn-Cu/Al, Zn/Ti, Zn/Al-Ti-LDHs with layered structure. Physical and chemicalproperties of these samples were characterized by XRD, SEM, TG-DSC, DRS andFT-IR. In the self-designed reactor system, we tested the photoactivities of thesecatalysts. Since LDHs had thermal stability and memory effect, we also tested theinfluences of heat treatment at different temperatures and the recovery after heat treatment. We tried the method of sulfide treatment to modify these layered catalystsbecause of the exchangeability of interlayer anions. CdS was loaded with the method ofion-exchange, while Pt was loaded with photodeposition, both of them were effectiveto the catalytic activities.Na₂CO₃and urea were the precipitating agents, so the anions between the layerwere mainly CO₃^2-. The results showed that LDHs combined with M²⁺and M³⁺had abetter degree of crystallinity, lager interlayer spacing and more content of crystal water.Doping of Cu²⁺to Zn/Al-LDHs made the absorption edge redshift to about300nm,accordingly there was a better photoactivity. The Zn/Ti-LDHs and Zn/Al-Ti-LDHsexhibited greater absorption edges, smaller band gaps and better activities thanZn（Cu）/Al-LDHs. During the reactions, the main products were CO and CH₄, while COwas more than CH₄. After300℃retreatment, the layered structure of all LDHs weredestroyed, but LDHs combined of M²⁺and M³⁺showed good memory effect. Whenimmersed in deionized water or Na₂CO₃solution for a certain time, the structure andthe photoactivity could also recovered, while Zn/Ti-LDHs was negative.Using thiourea as the sulfur source to retreat Zn/Al-LDHs and Zn/Ti-LDHs, wefound that the structure of Zn/Ti-LDHs had been completely destroyed, and itscomposition was mainly ZnS, as the XRD showed. The products in the photoreactionon these samples were different from that of samples before vulcanization, whichcontained CO and H₂as the main products. The yeild of H₂was far higher than CO, themaxmium value could reach about35mol/（gcat h） for Zn/Al-S-LDHs, and the COyeild was also a little higher than LDHs samples before vulcanization. If added to asolution of Cd（CH₃COO）₂and stirred and heated for a certain time, the S-LDHs wouldchange from white to yellow, indicating that CdS could be loaded on the LDHs by anion exchange method. The product of CdS-LDHs in the photoreaction was mainly CO,and the CO yeild was greater than the initial LDHs without any retreatment. In theprocess of loading Pt with the method of photodeposition, if droped a trace amout ofNa2S and Na2SO3solution, the photocatalytic products would be CO and H₂, and H₂yeild was even twice as much as Zn/Al-S-LDHs, reaching about75mol/（gcat h）.Beside a series of LDHs metioned above, in this thesis, we also prepared andtested some other photocatalysts, namely TiO₂（anatase）, H-KLBT and Ti-MCM-41. The main products detected were still CO, CH₄and H₂, but the selectivity wasdiffferent from each other and Pt loaded on the catalysts also played different rolesduring the photoreaction. All the results of different catalysts would provide us thepossibility to reach a further understanding of the mechanism on the photocatalyticconversion of CO₂-H₂O.