【作者】 毛劲；

【导师】 彭天右；

【作者基本信息】 武汉大学 ， 纳米科学与技术， 2013， 博士

【摘要】 探索高效、廉价和实用的半导体光催化还原CO2技术成为了当前社会最关心的热点课题之一。虽然半导体的开发、反应体系的完善和反应条件的优化等各个方面都取得了重大的进展,但是目前光催化还原CO2技术面临转化效率低、产物选择性难以控制等问题。光催化效率和产物选择性主要是由光催化材料结构和反应体系条件决定的,因此,对传统紫外光响应半导体材料和新型可见光响应半导体材料进行微结构的调控和优化,增强其光电性能,研究其微结构与光催化活性之间的关系是光催化还原CO2领域的关键而具有挑战性的课题。为此,本论文开展了不同微结构的光催化材料的制备及其光催化还原CO2性能研究。获得的主要实验现象和研究结论归纳如下：(1)以十六烷基三甲基溴化胺(CTAB)为模板剂,200℃水热处理选择性合成了单斜相BiVO4微米片,并证实了其在碱性悬浮体系中可见光催化还原CO2选择性产CH3OH的性能和长效稳定性,对比研究了催化剂用量和NaOH浓度对光催化活性的影响。发现BiVO4微结构和反应条件是决定产物选择性的主要因素。另外,NaOH起到了两个关键的作用：能溶解更多的CO2促进反应底物和催化剂的接触；以及悬浮体系中OH-能捕获光生空穴,有效地分离光生电子-空穴,提高光催化活性。上述结果为具有特定微结构的半导体材料的设计和优化、并研究其微结构与光催化活性之间的构效关系等提供了一些新的思路。(2)采用尿素和三聚氰胺为原料高温热解合成了具有不同微结构的g-C3N4。由尿素热解所合成的u-g-C3N4呈现孔径约40nm的多孔薄片状结构,而由三聚氰胺热解所合成的m-g-C3N4呈大片的层状结构并只含少量的介孔,其比表面积仅为u-g-C3N4的1/10。与m-g-C3N4相比,u-g-C3N4在光催化还原CO2悬浮体系中表现出更好的催化活性和不同的产物选择性,即：u-g-C3N4的主要还原产物为CH3OH和C2H5OH,而m-g-C3N4的主要产物为C2H5OH。通过对比研究m-g-C3N4和(?)m-g-C3N4对反应物的吸附能力和光激发电荷转移性能,证实了g-C3N4的微结构不同是导致其光催化还原CO2的反应活性和还原产物的选择性不同的内在原因。光催化剂活性位点的分布和光生载流子的转移及分离效率在整个光催化还原CO2反应过程中起到决定性作用,即：比表面积大和晶粒尺寸小有利于反应底物的吸附和光生载流子的分离,其协同作用促进了m-g-C3N4表现出更好的光催化活性和不同的产物选择性。上述结果为今后优化半导体的微结构、改善光催化还原CO2性能以及调控产物的选择性提供了有益的借鉴和新的思路。(3)采用改进的水热法分别合成了暴露{001}晶面锐钛矿TiO2纳米片和{010}晶面锐钛矿TiO2纳米棒。纳米片宽度约200nm,厚度约10nm,其主导晶面{001}占到总表面积的91%；而纳米棒直径约100nm,长约1000nm,其主导晶面{010}所占到总表面积的93%。负载Pt前后的样品在光催化还原CO2气相体系中的主要还原产物为CH4。其中,TiO2-010的光催化活性高于TiO2-001,而在负载Pt纳米粒子后,TiO2-001-1%Pt的光催化还原CO2的反应活性则优于TiO2-010-1%Pt。即Pt纳米粒子对TiO2-001和TiO2-010的光催化活性显得尤为重要。通过对比研究了TiO2-001和TiO2-010及其负载Pt后产物的表面与CO2的结合方式、吸附性能、光激发载流子分离以及Pt纳米粒子存在状态等因素,发现较好的CO2吸附性能和光生载流子分离效率是TiO2-010表现出比TiO2-001更好的光催化活性的主要原因。负载Pt后,由于Pt纳米粒子在Ti02-001分散均匀且Pt颗粒大小均一(约4-5nm),致使它具有良好的电子转移性能,有效地抑制了光生电子-空穴的复合,进而表现出更好的光催化活性；而由于Pt纳米粒子在TiO2-010上团聚严重,且团聚体尺寸大,导致Pt团聚体成为电子和空穴复合中心,减少光电子寿命,因此TiO2-010-1%Pt表现出较低的光催化活性。上述结果对今后高效光催化材料的微纳结构的设计及其性能优化等研究具有重要的理论指导意义。更多还原

【Abstract】 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.更多还原