Photo-thermal Coupling Decomposition Of CO₂ Using Metal Oxide Semiconductors

With the continuous development of human society,the demand for energy is growing.Fossil fuels are still the main energy source for hu man beings.On the one hand,the reserves are limited and the energy crisis is an urgent problem to be solved;on the other hand,numerous emissions of CO₂ and other pollutants have triggered a series of environmental and climate problems.Solar energy is considered to be theoretically unlimited and an important part of the clean energy revolution in the fourth scientific and technological revolution.To produce solar fuels from CO₂ and H₂O is to store solar energy in the form of chemical energy,which is more stable and can well match the modern energy consumption system,and provides one method to realize carbon circulation as well as solve energy and environmental problems simultaneously.Photo-thermochemical cycle proposed by our research group is a method based on the idea of photothermal synergy to produce solar fuels.The design and synthesis of photo-thermochemical cycle materials is the key to improve system efficiency.This thesis is aimed to research the properties of various oxide materials and try to find suitable catalytic materials.TiO₂ and Mn doped TiO₂ samples were produced by sol-gel method.Among different doping amount,1.0wt%Mn doped samples performed more CO₂ reduction than 0.5wt%and1.5wt%,and all Mn-doped samples produced more CO than undoped TiO₂.Mn ions enlarged the absorption range of samples due to the impurity energy levels introduced by Mn ions to the bandgap,which would generate more photo induced electrons and holes.During the photoreaction process,Mn cations became the electron trapping agent and inhibit the recombination of photogenerated electron hole pairs at low doping level,while more doping Mn cations would become introduce more interstitial defects and reduce the separation efficiency of photogenerated charges instead.DFT calculations confirmed that the impurity energy levels existed in the bandgap and doping Mn ions lowered the oxygen vacancy formation energy.Synthesize multi-component Zinc based oxides using hydrothermal method and ion exchange method,including ZnO,ZnGa₂O₄ and Zn₂GeO₄.Experiment results show that the average CO yield on ZnO at 623K is 2.2mmol·g^-1,similar to that of TiO₂?P25?,while ZnGa₂O₄ and Zn₂GeO₄ both produce more CO than ZnO and TiO₂,and the average production at 623K is 7.49mmol·g^-1 and 9.52mmol·g^-1 respectively.XPS for Zn₂GeO₄indicates that a number of oxygen vacancis formed during illumination,and the vacancies recovered after thermal reaction,which is similar to the mechanism of TiO₂ for cycle.ZnO/Zn₂GeO₄ heterojunction structure was constructed for photo-thermal coupling cycle.The composite material enhanced the ability to photo-thermal coupling decompose CO₂ with an average CO production of 12.40mmol·g^-1.XRD result showed that there were two different crystal compounds in the complex,and HRTEM patterns as well as line-scan result confirmed the formation of heterojunction.The nanocomposite system increases the light response range and enhances the photoreaction activity.