Preparation And CO₂ Adsorption,Photocatalysis Performance Of Ti/Li/Al-LDHs/Coke Composite

CO₂ is the main greenhouse gas in the atmosphere,the greenhouse effect caused by CO₂ is now threatening human survival and social development,thus controlling the emissions of greenhouse gas has become a key issue for all mankind.The large-scale use of fossil fuels is the main reason for the rapid increase of CO-2 emissions.Amoung various emission source,coal fired power plant flue gas is the most important one,accounting for 37.5%of global emissions.Therefore,the effective capture and conversion of CO₂ in flue gas is an important breakthrough of CO₂ emission reduction.Physical adsorption of CO₂ and CO₂ photocatalytic reduction of hydrocarbon fuel technology are considered to be the most promising way to reduce the CO₂ emissions,and the development of high efficient adsorbent and photocatalyst is the key problem in breakthrough of this technology.In this thesis,the titanium lithium aluminum hydrotalcite?Ti/Li/Al-LDHs?and its carbon composites were prepared by co-pricipitation method due to their unique physical and chemical properties and adsorption catalytic properties.The characteristics of CO₂ adsorption and photocatalytic activity of CO₂ to methane were also investisted.The structure-activity relationship between the structure and the properties of the adsorbent and photocatalyst was established,which provided a new idea and theoretical basis for further improving the performance of the material:The effects of the ratio of metal element,pH value,water bath temperature and calcination temperature on the structure and morphology of Ti/Li/Al-LDHs and its carbon composites were investigated.The relation ship between their structure and properity were characterized by using atomic absorption spectroscopy?AAS?and X-ray diffraction?XRD?,The results showed that the crystallinity,of TiLi3Al4-LDHs hydrotalcite was the best when the pH value was 7?8 and the water bath time was 36 h;besides that,it was also found that the ratio of anion and cation between the layers of Ti₁Li₃Al₂-LDHs was the best,the content of anion and hydroxyl group was the highest,and the narrow band gap of the semiconductor was about 3.10 eV.Based on the above information,Ti/Li/Al-LDHs/xDC?X is the mass ratio of titanium lithium aluminum hydrotalcite and delayed coke?composite was prepared by using DC as the support.The results revealed that with the increase of DC content,the crystallinity of the material decreases,and the C atoms enter into the crystal lattice,resulting in the distortion of the crystal structure of hydrotalcite compounds;in addition,the Ti-O-C bond was formed at the interface of DC and hydrotalcite compounds.The effects of heat treatment temperature on the structure and properties of Ti₁Li₃Al₂-LDHs and its composites were investigated.After calcination,the removal of OH^-and CO₃^2-from Ti₁Li₃Al₂-LDOs300 leads to its structure changed from eight sides to equivalent tetrahedral hydrotalcite layered structure,then part of the layered structure of hydrotalcite destroyed and resule in the formation of pore structure.In addition,TiiLi3Al2-LDOs₇₀₀ changed from hydrotalcite to spinel leading to the formation of Li4Ti5O12 and Al2TiO5 mixed oxide,which eventually enhanced the amount of oxygen vacancies and reduced the band gap to about 2.90 eV.Moreover,Ti₁Li₃Al₂-LDOs/xDC was obtained by heat treatment Ti₁Li₃Al₂-LDOs/xAC at different temperatures.The results showed that the addition of AC can improve the dispersion of Ti₁Li₃Al₂-LDOs and decrease the grain size.The Ti₁Li₃Al₄-LDHs structure is the most stable and the crystallinity is the best in Ti/Li/Al-LDHs.Therefore,the structural model of Ti₁Li₃Al₄-LDHs hydrotalcite compounds was constructed by using Materials Studio molecular science simulation software.Compared with the traditional Mg/Al-LDHs,the charge density of the Ti/Li/Al-LDHs layer was not uniform,and the cell structure was deformed;what' more,a large number of hydrogen bonds was existed in the structure of Ti₁Li₃Al₄-LDHs.The energy calculation and structural optimization results revealed that Ti₁Li₃Al₄-LDHs were good semiconductor material with the band gap energy of Eg about 3.5 eV.Based on the most stable structure,the simulation of C doping in Ti₁Li₃Al₄-LDHs was performed.The results displayed that compared with Ti₁Li₃Al₄-LDHs,the band gap of the doped samples almost unchanged,while the symmetry and stability of crystal structure decreased greatly;besides that,the unit cell was deformation.Moreover,the lattice structure of H is absent resulting in lattice defects.The band gap energy of Eg is about 3.47 eV.The CO₂ adsorption performance of Ti/Li/Al-LDHs was investigated,and the maximum adsorption capacity of TiiLi3Al2-LDHs was found to be 39.30 mg/g.The CO₂ adsorption performance of Ti₁Li₃Al₂-LDOs after heat treatment was also investigated,and the adsorption capacity of Ti₁Li₃Al₂-LDOs300 was the highest,which could reach 53.5 mg/g.The removal of OH^-and CO₃^2-in Ti₁Li₃Al₂-LDOs30O increased the specific surface area and pore volume.The obtained Ti₁Li₃Al₂-LDOs300 sample has the highest total pore volume and micropore volume,which was 0.522 cm3/g and 0.008 cm3/g,respectivily.The CO₂ adsorption properties of Ti/Li/Al-LDHs/xDC calcined at different temperatures were investigated.The results showed that Ti/Li/Al-LDOs/xAC600 exhibited the best CO₂ adsorption properties duo to its highest surface area and pore volume;besides these,the more micropores and mesopores in the samples also provide favorable conditions for the adsorption and diffusion of CO₂.The adsorption behavior of Ti/Li/Al-LDOs/AC600 for CO₂ with different x values from large to small order as fllow:TiiLi3Al4-LDOs/3AC600>TiiLi3Al4-LDOs/2AC600>TiiLi3Al4-LDOs/AC600>Ti₁Li₃Al₄-LDOs/4AC600.It is clear that,when x=3,TiiLi3Al4-LDOs/3AC600 exhibited the best CO₂ adsorption properties amoung all the samples.The effect of the ratio of metal ions on the photocatalytic activity of Ti/Li/Al-LDHs for photocatalytic reduction of CO₂ to CH₄ was studied in a fixed bed reactor.The yield of CH₄ was higher over TiiLi3Al2-LDHs,which could reach 1.33 mmol/g^-1;the semiconductor band gap was about 3.10 eV.The calcination temperature has a great influence on the structure and photocatalytic activity of Ti₁Li₃Al₂-LDHs.Ti₁Li₃Al₂-LDOS700 displayed the highest CH₄ yield which was about 1.59 mmol/g^-1;while the semiconductor band gap of Ti₁Li₃Al₂-LDOs₇₀₀ is further reduced to about 2.90 eV after calcination.The photocatalytic performance of Ti₁Li₃Al₂-LDHs?LDOs?/xDC?AC?was investigated.The results showed that the photocatalytic activity CH₄ yield of Ti₁Li₃Al₂-LDHs?LDOs?was higher after supported by carbon.This phenomenon is duo to the fact that the DC?AC?load can improve the dispersion of hydrotalcite compounds,reduce the grain size and increase the contact surface between the catalyst and CO₂;meainwhile,the DC?AC?can enhance the photocatalytic reaction rate because of the adsorption and concentration of CO₂.It was also found that the adsorption of AC was enhanced by addition of Ti₁Li₃Al₂-LDOs,the adsorption saturation time of AC was prolonged,and the equilibrium adsorption capacity of AC was increased under the irradiation of ultraviolet lamp.Thus,it can be seen that the synergy between AC and LDHs is not a simple composite,but a synergistic effect between porous carbon materials and nanocrystalline semiconductors.