g-C_3N_4光催化氧化还原性能调控及其环境催化性能增强

发布时间：2018-06-16 10:42

  本文选题：光催化 + 石墨相氮化碳　； 参考：《华中师范大学》2015年博士论文

【摘要】：自从Fujishima和Honda在1972年发现被可见光照射的TiO2电极表面可以析出氧气和氢气以来,半导体光催化技术因为在环境和能源领域有很强的实用性而引起全世界科研工作者的极大兴趣。但是,纯的TiO2半导体光催化剂的可见光光催化活性很弱,原因主要是纯的TiO2只能吸收紫外光,而紫外光在太阳光中所占的比例不足4%。为了提高TiO2的光催化活性并使其在可见光照射下具有光催化性能,科学家们开发出了很多方法,比如阴阳离子的掺杂,与其它半导体的耦合,多孔化以及减小TiO2的颗粒尺寸引起量子尺寸效应等。尽管如此,TiO2的可见光催化性能依然很低。为了光催化事业能够更好的发展,一些科研工作者转向开发TiO2以外具有可见光催化性能的光催化剂。石墨相氮化碳(g-C3N4)是一种非金属有机聚合物半导体。因为具有很好的化学稳定性,热稳定性,半导体性能,合适的禁带宽度(2.7 eV)以及合适的导带(CB,-1.3 V)和价带(VB,1.4V)位置,g-C3N4被认为在光催化领域有很大潜力。到目前为止,g-C3N4作为可见光催化剂已经被广泛应用于光催化生产新能源,光催化去除污染物以及光催化合成有机化合物等方面。众所周知,光催化技术的核心目标是制备廉价、高效、稳定的光催化剂。合成g-C3N4的原料和方法都比较简单,因此g-C3N4满足廉价的要求。但是,对于高效和稳定这两方面的要求,纯的g-C3N4还没有达到让人们满意的地步,这主要是因为纯的g-C3N4存在多方面的缺点。这些缺点包括：(1)g-C3N4只能吸收450 nm处的蓝光,对可见光的利用效率低；(2)光生电子和空穴很容易发生复合,导致有效光生电子或空穴的数量比较少；(3)g-C3N4容易被自身产生的光生空穴分解,导致g-C3N4的循环稳定性不好。为了使g-C3N4成为一种廉价、高效、稳定的光催化剂,本论文旨在通过对g-C3N4做出适当改性来提高g-C3N4光催化活性以及稳定性,并研究清楚g-C3N4光催化活性以及稳定性得到改善的机理。具体内容如下：1、通过直接煅烧三聚氰胺合成纯的石墨相氮化碳(g-C3N4),在此基础上,将三聚氰胺换成三聚氰胺盐酸盐合成了多孔石墨相氮化碳(P-g-C3N4)。P-gC3N4的比表面积是g-C3N4比表面积的39倍,禁带宽度比g-C3N4的禁带宽度增大了0.13 eV。多孔化一方面可以使石墨相氮化碳光催化氧化罗丹明B的速率提高9.4倍,另一方面却使石墨相氮化碳光催化还原二氧化碳的速率降低4.6倍。发现这些现象之后,我们设计了一些实验详细分析了多孔化导致石墨相氮化碳光催化氧化能力增强以及还原能力减弱的原因。2、通过理论计算结合实验验证的方法证明在不引入外来元素的情况下,通过用C元素均匀取代g-C3N4中的桥连N,引起的C自掺杂可以改变g-C3N4的电子结构和价带结构。同时我们还发现C自掺杂可以在g-C3N4的结构当中引入一些共轭大π键。这些共轭大π键可以增强g-C3N4吸收可见光以及传导电子的能力,从而导致g-C3N4光催化氧化能力和光催化还原能力同时增强。3、通过溶剂热处理g-C3N4的方法合成了甲酸根镶嵌的g-C3N4,之后发现甲酸根镶嵌不仅增强了g-C3N4可见光光催化还原Cr(Ⅵ)的活性,而且增强了g-C3N4的稳定性。经过一系列的研究,我们发现甲酸根镶嵌导致g-C3N4稳定性增强的原因是甲酸根可以捕获光生空穴从而抑制g-C3N4的自分解；甲酸根镶嵌导致g-C3N4可见光光催化还原Cr(Ⅵ)能力增强的原因则有两个方面,一方面甲酸根捕获光生空穴导致g-C3N4产生更多的光生电子,另一方面甲酸根使g-C3N4光催化还原Cr(Ⅵ)的机理从间接还原变成了直接还原。4、通过双氧水氧化处理g-C3N4的方法合成了含氧官能团表面修饰的g-C3N4,发现这些含氧官能团赋予了g-C3N4可以在厌氧条件下氧化去除有机污染物的能力。在可见光的照射下,表面含氧官能团使g-C3N4光催化降解以及矿化污染物的速率常数分别提高了18倍和7倍。经过系统研究,我们发现这些含氧官能团使g-C3N4可以在厌氧条件下光催化氧化去除有机污染物的原因是含氧官能团可以代替氧气捕获光生电子,抑制光生空穴与电子的复合。另外,含氧官能团还可以将捕获的光生电子传递给水或质子来产生氢气,从而保证g-C3N4在厌氧条件下去除有机污染物的稳定性。5、通过一步剥层的方法将g-C3N4的半导体类型从n型变成p型,并发现在可见光的照射下,p型g-C3N4可以高效率高选择性地将CO2还原为CO。p型g-C3N4光催化还原CO2的活性比n型g-C3N4好的原因有三方面,第一是超薄的氮化碳结构以及剥离时形成的表面缺陷使得g-C3N4可以吸收更多的可见光,从而产生更多的电子空穴对；第二是超薄结构以及表面缺陷有利于光生载流子的分离；第三是表面缺陷使CO2在g-C3N4表面的化学吸附性能得到提高。p型g-C3N4光催化还原CO2的产物选择性比n型g-C3N4高的原因是表面缺陷将CO2在g-C3N4表面的化学吸附方式从C与桥连N相连形成N-CO2-基团变成O与桥连N相连形成N-O-C=O基团,这导致CO2在氮化碳表面的还原机理发生了变化。
[Abstract]:Since Fujishima and Honda found that oxygen and hydrogen can precipitate oxygen and hydrogen on the surface of TiO2 electrodes irradiated by visible light in 1972, semiconductor photocatalytic technology has attracted great interest from researchers all over the world because of its strong practicability in the field of environment and energy. However, the visible light photocatalytic activity of pure TiO2 semiconductor photocatalyst has been found. The reason is that the pure TiO2 only absorbs ultraviolet light, and the proportion of ultraviolet light in the solar light is less than 4%.. In order to improve the photocatalytic activity of TiO2 and make it photocatalytic performance under visible light, scientists have developed many methods, such as the doping of yin and Yang ion, coupling with other semiconductors, porous and porous. Reducing the particle size of TiO2 causes quantum size effect, etc., although the visible photocatalytic performance of TiO2 is still very low. In order to develop better photocatalytic activity, some researchers have turned to the development of photocatalysts with visible photocatalytic properties outside TiO2. Graphite phase carbon nitride (g-C3N4) is a nonmetallic organic polymer half. Conductors, because of their good chemical stability, thermal stability, semiconductor properties, appropriate band width (2.7 eV) and appropriate conduction band (CB, -1.3 V) and valence band (VB, 1.4V) positions, g-C3N4 has been considered to have great potential in the field of photocatalysis. So far, g-C3N4 as a visible light catalyst has been widely used in the production of new energy for photocatalytic production. Source, photocatalytic removal of pollutants and photocatalytic synthesis of organic compounds. It is well known that the core goal of photocatalytic technology is to prepare cheap, efficient and stable photocatalysts. The raw materials and methods of synthesizing g-C3N4 are simple, so g-C3N4 meets the requirements of cheap. But the requirements for the two aspects of high efficiency and stability are pure G -C3N4 has not yet reached satisfaction, mainly because the pure g-C3N4 has many shortcomings. These shortcomings include: (1) g-C3N4 can only absorb blue light at 450 nm and use low efficiency for visible light; (2) the number of photogenerated electrons and holes is easily complex, resulting in less effective photoelectrons or holes; (3) g- In order to make g-C3N4 a cheap, efficient and stable photocatalyst, the purpose of this paper is to improve the photocatalytic activity and stability of the g-C3N4 by modifying the g-C3N4 to improve the photocatalytic activity and stability of g-C3N4. In order to make g-C3N4 a cheap, efficient and stable photocatalyst, the purpose of this paper is to study the photocatalytic activity and stability of the g-C3N4. The main contents are as follows: 1, on the basis of the synthesis of pure graphite phase carbon nitride (g-C3N4) by directly calcining melamine, the specific surface area of the porous graphite phase carbon nitride (P-g-C3N4).P-gC3N4 is 39 times the specific surface area of the porous graphite phase carbon nitride (.P-gC3N4), and the band gap width of the band gap is more than the band gap of g-C3N4. The increase of 0.13 eV. porosity can increase the rate of Shi Moxiang's carbon nitride to catalyze the oxidation of rhodamine B by 9.4 times. On the other hand, the rate of carbon dioxide reduction by carbon nitride by Shi Moxiang is reduced by 4.6 times. .2, the reason for the enhancement of catalytic oxidation capacity and the weakening of reducing capacity, is proved by theoretical calculation and experimental verification. It is proved that the C self doping can change the electronic structure and valence band structure of g-C3N4 by using the C element to replace the bridged N in g-C3N4 without introducing the foreign elements. Meanwhile, we also found that C is self doped. Some conjugated large PI bonds are introduced in the structure of g-C3N4. These conjugated large pion bonds can enhance the ability of g-C3N4 to absorb visible light and conduction electrons, resulting in the g-C3N4 photocatalytic oxidation capacity and the photocatalytic reduction ability to enhance.3 simultaneously. The g-C3N4 of formic acid roots is synthesized by the method of solvent heat treatment g-C3N4. Acid root inlay not only enhanced the activity of g-C3N4 visible light photocatalytic reduction of Cr (VI), but also enhanced the stability of g-C3N4. After a series of studies, we found that the reason that formate root inlay leads to the enhancement of the stability of g-C3N4 is that formate roots can capture photogenerated holes and inhibit the self decomposition of g-C3N4; formate root inlay leads to g-C3N4 visible. There are two reasons for the enhancement of photocatalytic reduction of Cr (VI). On the one hand, the photogenerated holes in the formate capture lead to more photogenerated electrons by g-C3N4, on the other hand, the mechanism of g-C3N4 photocatalytic reduction of Cr (VI) from the indirect reduction to the direct reduction of.4, and the synthesis of oxygen containing oxygen by oxidation of g-C3N4 by oxyhydrogen peroxide. The functional group surface modified g-C3N4 found that these oxygen functional groups give g-C3N4 the ability to oxidize organic pollutants under anaerobic conditions. Under visible light, the surface oxygen functional groups make g-C3N4 photocatalytic degradation and the rate constant of mineralized pollutants up to 18 times and 7 times respectively. After systematic research, we found that These oxygen functional groups enable g-C3N4 to remove organic pollutants by photocatalytic oxidation under anaerobic conditions. The oxygen containing functional groups can replace oxygen to capture photoelectrons and inhibit the recombination of photogenerated holes and electrons. In addition, oxygen functional groups can also pass the captured photoelectron transfer to water or proton to produce hydrogen, thus ensuring g-C3 N4 removal of the stability.5 of organic pollutants under anaerobic conditions, the semiconductor type of g-C3N4 is transformed from n type to p type through one step stripping method, and it is found that under the light of visible light, P g-C3N4 can be highly selective and selective to reduce CO2 to CO.p type g-C3N4. There are three aspects of the reason for the photocatalytic reduction of CO2 activity than that of N type. One is that the ultrathin carbon nitride structure and the surface defects formed during peeling make g-C3N4 absorb more visible light, thus producing more electron hole pairs; second, the ultrathin structure and surface defects are beneficial to the separation of optical carriers; third the surface defects make the chemical adsorption properties on the g-C3N4 surface improved. The selectivity of P type g-C3N4 photocatalytic reduction of CO2 is higher than that of N type g-C3N4. The surface defect causes the chemical adsorption of CO2 on the g-C3N4 surface to form the N-CO2- group from the C to the bridge to form the N-CO2- group and to the O and the bridging N to form the group, which leads to the change of the reduct on the carbon nitride surface.
【学位授予单位】：华中师范大学
【学位级别】：博士
【学位授予年份】：2015
【分类号】：O643.36
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本文编号：2026385