Preparation Of Photocatalysts With Different Microstructures And Their Photocatalytic CO₂Reduction Activities

The development of highly efficient, low-cost and practical photocatalytic reduction of CO2over semiconductor became one of the most hot topic in modern times. Many significant progresses have been made in the exploitation of semiconductor, improvement of reaction system and optimization of reaction conditions, but the conversion efficiency is low and it is difficult to control the selectivity of products, which are also critical issues for photocatalytic reduction of CO2. It is well known that photocatalytic efficiency and product selectivity closely depend on photocatalyst structure and reaction system. Therefore, the control and optimization of microstructure is the most effective means to improve the performance of the traditional ultraviolet response and novel visible response photocatalysts. And research of the relationship between the materials microstructure and their photocatalytic activity is a key and challenging issue in the field of photocatalytic reduction of CO2. In this case, we carried out the investigations into different microstructure synthesis and the activity of photocatalytic reduction of CO2over catalytic materials. The main contents and conclusion are as follows:1. Monoclinic lamellar BiVO4powers were prepared through a hydrothermal process by using cetyltrimethylammonium bromide (CTAB) as a template-directing reagent at200℃. The obtained lamellar BiVO4was used as photocatalyst for the reduction of CO2in the NaOH solution, and exhibited a selective methanol production and long-term stability under visible-light irradiation. The effects of the photoreaction conditions such as the amount of BiVO4and NaOH concentration on the photocatalytic activity were investigated. And it can be concluded that the microstructure of materials and reaction conditions are the main factors to product selectivity. It is found that adding NaOH solution in the BiVO4suspension can dissolve more CO2, and the OH-serves as a stronger hole-scavenger that can effectively improve charge separation and enhance photocatalytic activity. These results offer some good ideas for designing special structure photocatalytic materials, and research on the relationship between their microstructure and photocatalytic activity.2. Two kinds of graphitic carbon nitride (g-C3N4) were synthesized through a pyrolysis process of urea or melamine. The product derived from the urea (denoted as u-g-C3N4) shows mesoporous flake-like structure, and the pore sizes is～40nm, while m-g-C3N4(from melamine) only shows flaky morphology without obvious porous structures. The BET specific surface area of u-g-C3N4is ten times higher than that of m-g-C3N4. Compared with m-g-C3N4, it was found that the obtained u-g-C3N4showed better photocatalytic activity and different products selectivity under visible light. The u-g-C3N4as photocatalyst can result in the formation of a mixture containing CH3OH and C2H5OH, while m-g-C3N4only leads to the selective of C2H5OH. The adsorption of reactants and photogenerated carrier separation were investigated comparatively by using m-g-C3N4and u-g-C3N4, respectively. It was confirmed that the above different photocatalytic activity and selectivity for the formation of organic fuels were due to the differences in microstructure of u-g-C3N4and m-g-C3N4. And the distribution of active site and transfer of photogenerated carrier played a decisive role in photocatalytic reduction of CO2. Compared with m-g-C3N4, u-g-C3N4with smaller grain size and larger surface area was advantageous to the photogenerated carrier separation and adsorption of reactants, which showed better performance and different products selectivity in the present photocatalytic CO2reduction system. The present findings could offer some new ideas for the optimizing the microstructure of semiconductor, improving their photocatalytic activity, and controlling the selectivity of products.3. Anatase TiO2nanosheet with exposed{001} facets (denoted as TiO2-001) and nanorod with exposed{010} facets (denoted as TiO2-010) were synthesized by using modified hydrothermal methods. TiO2-001is composed of nanosheets with size of ca.200nm×10nm and ca.91%exposed percentage of {001} facets, while the TiO2-010is mainly composed of nanorods with particle size of ca.1000nm×100nm and ca.93%exposed percentage of {010} facets. Primary experiments showed that CH4was the main product from the gaseous CO2photoreduction system. It was found that TiO2-010without Pt-loading showed a higher photocatalytic CO2reduction activity than TiO2-001; while TiO2-010-1%Pt displayed a lower photoactivity than TiO2-001-1%Pt. This indicated that the Pt nanoparticles on TiO2-001and TiO2-010played a very important role in photocatalytic activity. The combination of catalyst surface and CO2, CO2adsorption capacity, the photoinduced carrier separation and the exist state of Pt nanoparticles were investigated comparatively before and after Pt-loading, respectively. It was confirmed that the better photocatalytic CO2reduction activity of TiO2-010might stem from its higher CO2adsorption capability and more efficient charge transfer property than TiO2-001. After Pt-loading, the highly uniform smaller Pt nanoparticles (4-5nm) on TiO2-001surfaces can effectively transfer the photogenerated electrons to restrain the carrier recombination, and then enhance the photoactivity, while Pt aggregates with larger size on the TiO2-010can not only trap electrons but also consume holes, thus serving as recombination centres reduce the life of photoelectron, and then show lower photoactivity. These results offer an important theoretical guidance for designing special micro and nanostructure photocatalytic materials with highly effective photocatalytic activity.