二维材料负载型纳米复合催化剂的表界面设计及机理研究

发布时间：2018-05-15 07:51

本文选题：二维纳米材料 + 纳米复合材料　；参考：《中国科学技术大学》2015年博士论文

【摘要】：近十年来二维材料的高速发展为发展高活性、选择性和稳定性的催化剂提供了新的契机。二维材料负载型纳米复合催化剂被广泛开发而应用于光催化、电催化等解决能源和环境问题的绿色催化反应中。二维材料由于其特殊的结构优势而对复合催化剂的性能起到了很大的提升作用。同时,在复合催化剂的发展过程中,表界面的设计对于催化性能的优化作用开始受到重视。但是,合成技术和催化机理研究的不足往往制约着催化剂表界面设计的发展。本论文旨在将可控合成、先进表征技术和理论模拟“三位一体”相结合,用于二维材料负载型纳米复合催化剂的表面和界面的设计,以提高其催化活性、选择性和稳定性。同时,根据催化剂的性能和表界面之间的构效关系来进一步研究催化反应的机理。在本论文中,我们通过调节复合催化剂中特定组成单元的表面参数(包括表面暴露晶面、成分、面积和晶相)来提高催化剂对于某一特定光催化或者电催化反应的活化能力；通过调控组成单元间的界面结构来提高界面对于光生电荷的传输和分离能力,以改善复合光催化剂的活性；通过组成单元中表面和界面的协同控制将“表面极化效应”引入到电催化反应中。所取得的主要研究成果如下： 1.以二维C3N4纳米片负载的Pd共催化剂为体系,通过调节Pd的暴露晶面来研究共催化剂的表面晶面对于在水环境中光催化还原CO2反应选择性的影响。我们通过研究发现C3N4纳米片的独特结构保证了光生电子转移到不同Pd晶面上的效率是相同的。同时,转移到Pd上的电子在{111}晶面上容易发生光催化还原CO2反应,而在Pd{100}晶面上容易发生还原H2O产生H2的反应。通过第一性原理研究发现,这是由于H2O和CO2分子在不同Pd晶面上吸附和活化能力的差异导致的。 2.以二维MoS2纳米片负载的TiO2催化剂为体系,研究不同晶相的二维MoS2纳米片作为还原性共催化剂对于光催化分解水产氢活性的影响。我们通过研究发现1T相的MoS2(1T-MoS2)纳米片能有效地提高TiO2的光催化性能,而2H相的MoS2(2H-MoS2)却不能。这是由于1T-MoS2纳米片相对于2H-MoS2具有更高的电荷传输能力。同时,2H-MoS2纳米片产氢的活性位点只位于其边缘上,而1T-MoS2纳米片的基本面上也具有活性位点,这大大缩短了电子迁移的距离。高传输速度和短传输距离使得TiO2中产生的电子更容易到达1T-MoS2的活性位点参与产氢反应。 3.以Cu2O和Pd形成的肖特基结为体系,研究不同半导体-金属结构中影响Cu2O-Pd界面电荷转移的因素。我们发现界面缺陷和缺少电荷传递通道分别制约着Pd修饰Cu2O结构和Pd-Cu2O核壳结构中Pd对于Cu2O中光生电子-空穴的分离能力。为了同时解决这两个制约因素,我们设计了新型的半导体-金属-石墨烯叠层结构。该结构不仅继承了核壳结构中较低界面缺陷程度的优势,同时高导电性石墨烯纳米片的引入增加了Pd-石墨烯界面,使得抽取到Pd上的空穴可以通过石墨烯导出而与电子有效分离。从而所得的Cu2O-Pd-石墨烯叠层结构相比于其它基于Cu20的结构在光催化分解水制氢反应中表现出更高的催化活性。 4.以BiOCl半导体纳米片、在可见光范围内具有等离激元的金属Ag纳米立方体和不具有等离激元的金属Pd纳米立方体为体系,研究将肖特基结和等离激元效应协同应用于全谱光催化反应的方法。我们通过在BiOCl纳米片的上下两个(001)晶面选择性的负载Ag,同时在纳米片的周围四个(110)晶面选择性的负载Pd,使得Ag-(001)BiOCl和BiOCl(110)-Pd两个界面分别用于可见光激发下等离激元引发的热电荷注入和紫外光下肖特基结驱动的光生电荷导出。两者的协同导致所得到的Ag-(001)BiOCl(110)-Pd催化剂在全谱光催化分解水产氧反应中较其组成单元结构表现出更高的催化活性。 5.以石墨烯负载的Pd纳米立方体作为前驱体,通过分步对催化活性组成单元进行表面成分和表面暴露面积的调节以提高催化剂在电催化氧还原反应(ORR)中的活性。我们首先用Pt选择性包裹Pd使得催化剂的表面结构从Pd纳米立方体-石墨烯复合结构中低催化活性的Pd{100}晶面转变为Pt-Pd-石墨烯复合结构中高催化活性的Pt{100}晶面。然后,对于Pt-Pd-石墨烯复合结构中Pd的选择性刻蚀使得所得到的PtPd纳米笼-石墨烯复合结构中Pt{100}晶面的暴露面积接近翻倍。因此,所得各催化剂的活性以Pd纳米立方体-石墨烯复合结构Pt-Pd-石墨烯复合结构PtPd纳米笼-石墨烯复合结构的顺序递增。 6.以Pt-Pd-石墨烯叠层结构为体系,通过改变Pt的厚度来研究催化剂表面电子态对于电催化析氢反应(HER)活性的影响。我们通过将不同厚度Pt的Pt-Pd-石墨烯叠层结构应用于电催化反应中,发现随着Pt的厚度逐渐增加,催化剂的活性依次下降。通过第一性原理研究发现该变化趋势可以通过表面极化效应得以很好地解释。在该复合催化剂中,Pt与Pd之间不同的功函数导致Pt的表面极化,使得Pt表面电子态随着Pt的厚度不同而变化,导致不同的析氢活性。
[Abstract]:The rapid development of two dimensional materials has provided a new opportunity for the development of highly active, selective and stable catalysts for the last ten years. The two-dimensional material supported nanocomposite catalysts have been widely developed and applied to the reaction of green catalysis, such as photocatalysis, electrocatalysis, and so on. The two-dimensional material has its special structural advantages. Meanwhile, the performance of the composite catalyst has been greatly enhanced. At the same time, the design of the surface interface has been paid much attention to the optimization of the catalytic performance during the development of the composite catalyst. However, the lack of synthesis and catalytic mechanisms often restricts the development of the catalyst surface interface design.
The purpose of this thesis is to combine the controllable synthesis, the advanced characterization technology and the theoretical simulation "Trinity" to design the surface and interface of the 2-D supported nanocomposite catalyst, in order to improve its catalytic activity, selectivity and stability, and further study the relationship between the catalytic activity and the structure effect of the surface interface. In this paper, by adjusting the surface parameters of the specific components in the composite catalyst (including surface exposure, composition, area and crystalline phase), the activation ability of the catalyst for a specific photocatalytic or electrocatalytic reaction is improved, and the interface structure between the components is regulated to improve the interface. For the transmission and separation of photogenerated charges to improve the activity of the composite photocatalyst, the "surface polarization effect" is introduced into the electrocatalytic reaction through the synergistic control of the surface and interface in the components. The main achievements are as follows:
1. based on the Pd Co catalyst supported by two dimensional C3N4 nanoscale, the effect of the surface crystal surface of the co catalyst on the selectivity of the photocatalytic reduction of the CO2 reaction in the water environment was investigated by adjusting the exposed surface of the Pd. The unique structure of the C3N4 nanoscale was found to ensure the efficiency of the photoelectron transfer to the Pd crystal surface. The same. At the same time, the electrons transferred to Pd easily have a photocatalytic reduction of the CO2 reaction on the {111} surface, while the reaction of reducing H2O to H2 is easy to occur on the Pd{100} surface. It is found through the first principle that this is due to the difference in the adsorption and activation of the H2O and CO2 molecules on the different Pd crystal surfaces.
2. based on the TiO2 catalyst supported by two dimensional MoS2 nanoscale, the effect of the two dimensional MoS2 nanoscale on the photocatalytic decomposition of aquatic hydrogen was studied. We found that the MoS2 (1T-MoS2) nanoscale of the 1T phase could effectively improve the photocatalytic performance of TiO2, but the MoS2 (2H-MoS2) of the 2H phase was not. This is due to the higher charge transmission capacity of the 1T-MoS2 nanoscale than the 2H-MoS2. At the same time, the active site of the hydrogen production of the 2H-MoS2 nanoscale is only on its edge, while the base surface of the 1T-MoS2 nanoscale also has the active site, which greatly shortens the distance of the electron migration. The high transmission speed and the short transmission distance make the electricity produced in TiO2. It is easier to reach the active site of 1T-MoS2 to participate in hydrogen production.
3. based on the Schottky junction formed by Cu2O and Pd, the factors affecting the charge transfer of Cu2O-Pd interface in different semiconductor metal structures are studied. We find that the interface defect and the lack of charge transfer channel restrict the Pd modified Cu2O structure and the separation ability of Pd for the photoelectron hole in the Cu2O shell structure. To resolve these two constraints, we have designed a new type of semiconductor metal graphene stack structure. This structure not only inherits the advantages of the lower interface defects in the nuclear shell structure, but also increases the Pd- graphene interface with the introduction of high conductivity graphene nanoscale, making the holes decimating on Pd can be derived from graphene and electricity. The Cu2O-Pd- graphene laminated structure has a higher catalytic activity compared with other Cu20 based structures in the photocatalytic decomposition water hydrogen production.
4. using BiOCl semiconductor nanoscale, metal Ag nanometers with equal ionization excitations in the visible light range and metal Pd nanometers that do not have the plasmon polaritons, the method of applying the Schottky junction and the plasmon effect to the full spectrum photocatalytic reaction is studied. We pass the two (001) surface of the BiOCl nanoscale. Selective load Ag and four (110) surface selective load Pd around the nanoscale make Ag- (001) BiOCl and BiOCl (110) -Pd two interfaces used respectively for the thermal charge injection induced by visible light excite excitations and the output of light generated charge driven by the UV light driven by the Schottky junction. Both of them lead to the obtained Ag- (001) BiOC. L (110) -Pd catalyst showed higher catalytic activity in the whole spectrum photocatalytic decomposition of aquatic oxygen.
5. the Pd nano cubes loaded with graphene are used as precursors to regulate the surface composition and surface exposure area of the catalytic active components by step by step to improve the activity of the catalyst in the electrocatalytic oxygen reduction reaction (ORR). First, we use Pt to selectively package Pd to make the surface structure of the catalyst from Pd nano cube graphite. The low catalytic activity of the Pd{100} crystal surface in the complex structure of the alkene composite structure is transformed into a highly catalyzed Pt{100} crystal surface in the Pt-Pd- graphene composite structure. Then, the selective etching of Pd in the Pt-Pd- graphene composite structure makes the exposed area of the Pt{100} crystal surface of the obtained PtPd nanoscale composite structure nearly doubled. The activity of the catalyst is in the order of Pd nano cubic graphene composite structure Pt-Pd- graphene composite structure PtPd nano cage graphene composite structure.
6. the influence of the electronic state of the catalyst on the activity of electrocatalytic hydrogen evolution (HER) was studied by changing the thickness of the Pt-Pd- graphene layer by changing the thickness of the Pt. By applying the Pt-Pd- graphene laminated structure of different thickness of Pt to the electrocatalytic reaction, it was found that the activity of the catalyst decreased with the increase of the thickness of Pt. Through the first principle, it is found that the change trend can be well explained by the surface polarization effect. In the composite catalyst, the difference of work function between Pt and Pd leads to the surface polarization of Pt, which makes the electronic state of Pt change with the thickness of Pt, resulting in different hydrogen evolution activity.

【学位授予单位】：中国科学技术大学
【学位级别】：博士
【学位授予年份】：2015
【分类号】：O643.36;TB383.1

【共引文献】