Synthesis Of Bionic-Porous TiO₂ For Photocatalytic Of CO₂

With the growth of population and the acceleration of modernization and industrialization process, the demand of energy becomes larger and larger. What’s worse, after hundreds of years of over exploitation and consumption, fossil energy such as coal, oil and natural gas, as the main source of energy of human society, has irreversible geostrophic to run out. And fossil fuels combustion caused by a sharp increase in atmospheric CO2, one of the most important greenhouse gases, destroying the balance of carbon cycle in nature, and further leading to global warming. In recent years, photocatalytic reduction of CO2 into hydrocarbon fuels, inspired from the photosynthesis of green plants, is one of the most attractive solutions to to achieve a win-win situation between the energy and the environment, which is based on the semiconductor photocatalysis and couples the reductive half-reaction of CO2 fixation with a matched oxidative half-reaction such as water oxidation to achieve a carbon neutral cycle.Titanium dioxide （TiO2） is widely used in the photocatalytic fields including water splitting, degradation of organic contaminants, and CO2 reduction, because it is cheap, non-toxic, and light stability. However, the poor selective adsortion for CO2, the recombination of electron-hole pairs and the limited light absorption limit its photocatalytic reduction of CO2 performance. In this dissertation, inspired by the nature photosyhthesis of leaves, and based on the approachs to obtain pore geometry, a 3D interconnectively macro/mesoporous TiO2 sponges and MgO-TiO2 compouds were fabricated and the photocatalytic activity of CO2 reduction were studied. The details are summarized as follows:（1）A particular TiO2 sponge, consisting of macroporous framework with interconnected mesoporous channels, was fabricated through a co-gelation of lotus root starch （LRS） with TiO2 precursor, followed by lyophilization and subsequent calcination. This strategy advantageously inherits both the traditional hard-templating technique for well-defined 3D predesigned macroporous architecture and softtemplating techniques for interpore connectivity. The resulting TiO2 sponge exhibits about a 2.60 fold improvement in CO2 photoconversion rate （CH4:5.13 ppm h₁） compared to the referred TiO2 （1.97ppm h₁） formed in the absence of the LRS. The higher photocatalytic activity of the macro/mesoporous TiO2 sponge can be attributed to the following three reasons:（1） macroporous architecture favors gas diffusion of the reactants and the products; （2） macroporous architecture also promotes the multiple-reflection effect occurring inside the interior macrocavities, which enables trapping （or harvest） the incident light in the photocatalyst for a longer duration and bring forth more opportunities for light absorption; and （3） the mesoporous structure enhances gas capture/adsorption of the reactants and provides more reaction sites.（2） The stem of water convolvulus was employed as biotemplate and applied the sol-gel coating procedure for the replication of its optimization 3D hierarchical architecture to synthesize MgO-TiO2 compounds for photoconversion of CO2 into methane. It revealed that the MgO played an important role in improving CO2 adsorption owing to MgO on activation of CO2, compared the efficiency of different MgO content of MgO-TiO2 compounds in photocatalytic reduction of CO2 into CH4, and combined with the characterization of the physical and chemical properties of the catalysts. Although excess amounts of MgO can further enhance the activation of CO2, the insulating MgO, when overlapping the surface of TiO2, would possibly block the migration of photo-generated charge carriers to the catalyst surface and therefore lower the catalytic activity. Hence, it is of great significance to load an optimum amount of MgO and 0.2% in our work.