發(fā)布時間：2018-06-16 10:42

  本文選題：光催化 + 石墨相氮化碳�。� 參考：《華中師范大學》2015年博士論文

【摘要】：自從Fujishima和Honda在1972年發(fā)現(xiàn)被可見光照射的TiO2電極表面可以析出氧氣和氫氣以來,半導體光催化技術因為在環(huán)境和能源領域有很強的實用性而引起全世界科研工作者的極大興趣。但是,純的TiO2半導體光催化劑的可見光光催化活性很弱,原因主要是純的TiO2只能吸收紫外光,而紫外光在太陽光中所占的比例不足4%。為了提高TiO2的光催化活性并使其在可見光照射下具有光催化性能,科學家們開發(fā)出了很多方法,比如陰陽離子的摻雜,與其它半導體的耦合,多孔化以及減小TiO2的顆粒尺寸引起量子尺寸效應等。盡管如此,TiO2的可見光催化性能依然很低。為了光催化事業(yè)能夠更好的發(fā)展,一些科研工作者轉(zhuǎn)向開發(fā)TiO2以外具有可見光催化性能的光催化劑。石墨相氮化碳(g-C3N4)是一種非金屬有機聚合物半導體。因為具有很好的化學穩(wěn)定性,熱穩(wěn)定性,半導體性能,合適的禁帶寬度(2.7 eV)以及合適的導帶(CB,-1.3 V)和價帶(VB,1.4V)位置,g-C3N4被認為在光催化領域有很大潛力。到目前為止,g-C3N4作為可見光催化劑已經(jīng)被廣泛應用于光催化生產(chǎn)新能源,光催化去除污染物以及光催化合成有機化合物等方面。眾所周知,光催化技術的核心目標是制備廉價、高效、穩(wěn)定的光催化劑。合成g-C3N4的原料和方法都比較簡單,因此g-C3N4滿足廉價的要求。但是,對于高效和穩(wěn)定這兩方面的要求,純的g-C3N4還沒有達到讓人們滿意的地步,這主要是因為純的g-C3N4存在多方面的缺點。這些缺點包括：(1)g-C3N4只能吸收450 nm處的藍光,對可見光的利用效率低；(2)光生電子和空穴很容易發(fā)生復合,導致有效光生電子或空穴的數(shù)量比較少；(3)g-C3N4容易被自身產(chǎn)生的光生空穴分解,導致g-C3N4的循環(huán)穩(wěn)定性不好。為了使g-C3N4成為一種廉價、高效、穩(wěn)定的光催化劑,本論文旨在通過對g-C3N4做出適當改性來提高g-C3N4光催化活性以及穩(wěn)定性,并研究清楚g-C3N4光催化活性以及穩(wěn)定性得到改善的機理。具體內(nèi)容如下：1、通過直接煅燒三聚氰胺合成純的石墨相氮化碳(g-C3N4),在此基礎上,將三聚氰胺換成三聚氰胺鹽酸鹽合成了多孔石墨相氮化碳(P-g-C3N4)。P-gC3N4的比表面積是g-C3N4比表面積的39倍,禁帶寬度比g-C3N4的禁帶寬度增大了0.13 eV。多孔化一方面可以使石墨相氮化碳光催化氧化羅丹明B的速率提高9.4倍,另一方面卻使石墨相氮化碳光催化還原二氧化碳的速率降低4.6倍。發(fā)現(xiàn)這些現(xiàn)象之后,我們設計了一些實驗詳細分析了多孔化導致石墨相氮化碳光催化氧化能力增強以及還原能力減弱的原因。2、通過理論計算結(jié)合實驗驗證的方法證明在不引入外來元素的情況下,通過用C元素均勻取代g-C3N4中的橋連N,引起的C自摻雜可以改變g-C3N4的電子結(jié)構(gòu)和價帶結(jié)構(gòu)。同時我們還發(fā)現(xiàn)C自摻雜可以在g-C3N4的結(jié)構(gòu)當中引入一些共軛大π鍵。這些共軛大π鍵可以增強g-C3N4吸收可見光以及傳導電子的能力,從而導致g-C3N4光催化氧化能力和光催化還原能力同時增強。3、通過溶劑熱處理g-C3N4的方法合成了甲酸根鑲嵌的g-C3N4,之后發(fā)現(xiàn)甲酸根鑲嵌不僅增強了g-C3N4可見光光催化還原Cr(Ⅵ)的活性,而且增強了g-C3N4的穩(wěn)定性。經(jīng)過一系列的研究,我們發(fā)現(xiàn)甲酸根鑲嵌導致g-C3N4穩(wěn)定性增強的原因是甲酸根可以捕獲光生空穴從而抑制g-C3N4的自分解；甲酸根鑲嵌導致g-C3N4可見光光催化還原Cr(Ⅵ)能力增強的原因則有兩個方面,一方面甲酸根捕獲光生空穴導致g-C3N4產(chǎn)生更多的光生電子,另一方面甲酸根使g-C3N4光催化還原Cr(Ⅵ)的機理從間接還原變成了直接還原。4、通過雙氧水氧化處理g-C3N4的方法合成了含氧官能團表面修飾的g-C3N4,發(fā)現(xiàn)這些含氧官能團賦予了g-C3N4可以在厭氧條件下氧化去除有機污染物的能力。在可見光的照射下,表面含氧官能團使g-C3N4光催化降解以及礦化污染物的速率常數(shù)分別提高了18倍和7倍。經(jīng)過系統(tǒng)研究,我們發(fā)現(xiàn)這些含氧官能團使g-C3N4可以在厭氧條件下光催化氧化去除有機污染物的原因是含氧官能團可以代替氧氣捕獲光生電子,抑制光生空穴與電子的復合。另外,含氧官能團還可以將捕獲的光生電子傳遞給水或質(zhì)子來產(chǎn)生氫氣,從而保證g-C3N4在厭氧條件下去除有機污染物的穩(wěn)定性。5、通過一步剝層的方法將g-C3N4的半導體類型從n型變成p型,并發(fā)現(xiàn)在可見光的照射下,p型g-C3N4可以高效率高選擇性地將CO2還原為CO。p型g-C3N4光催化還原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結(jié)構(gòu)以及剝離時形成的表面缺陷使得g-C3N4可以吸收更多的可見光,從而產(chǎn)生更多的電子空穴對；第二是超薄結(jié)構(gòu)以及表面缺陷有利于光生載流子的分離；第三是表面缺陷使CO2在g-C3N4表面的化學吸附性能得到提高。p型g-C3N4光催化還原CO2的產(chǎn)物選擇性比n型g-C3N4高的原因是表面缺陷將CO2在g-C3N4表面的化學吸附方式從C與橋連N相連形成N-CO2-基團變成O與橋連N相連形成N-O-C=O基團,這導致CO2在氮化碳表面的還原機理發(fā)生了變化。
[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.
【學位授予單位】：華中師范大學
【學位級別】：博士
【學位授予年份】：2015
【分類號】：O643.36
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本文編號：2026385