【作者】 张杰；

【导师】 李会鹏；

【作者基本信息】 辽宁石油化工大学 ， 化学工艺， 2019， 硕士

【摘要】 太阳能是一种可持续的新型能源,将太阳能转化成化学能进行储存一直以来都是研究的重点与难点。但是太阳能的利用多集中在紫外光区,这样就大大限制了光源的利用率。目前,石墨相氮化碳（g-C₃N₄）作为一种可见光光催化半导体催化剂在水解制氢、降解污染水、光催化固氮、有机合成等领域具有重要的应用前景,这得益于其具有适宜的禁带宽度（2.71eV）、二维层状的物理结构、独特的电子结构、较强的化学和热稳定性、绿色无污染等优势。但是其缺点在于比表面积较低、光生电子-空穴易复合、光能利用率较低。实现光生电子-空穴的有效分离、促进电荷的快速有效转移是提高光催化活性的关键。由此,以调控g-C₃N₄的能带结构,降低光生电子空穴的复合几率,增加光源的吸收利用率,促进光生载流子的表面迁移成为了研究的重心。本论文通过以下几个方面对g-C₃N₄进行改性,并通过光催化降解罗丹明B进行催化活性的考察。改性方案如下:（1）高比表面积g-C₃N₄的制备及光催化降解罗丹明B性能。g-C₃N₄由于比表面积较低（往往小于10m²/g）传质阻力较大,使得光生载流子未迁移至催化剂表面时已复合,这样就极大的增加了光生-电子空穴的复合几率,降低了催化性能。由此,通过盐酸预处理前驱体与硬模板剂相结合的方式制备高比表面积的g-C₃N₄,并通过光催化降解罗丹明B的方式对催化剂的光催化活性进行考察。研究结果表明,比表面积的增加能够有效的降低催化剂表面的传质阻力、促进光生载流子在催化剂表面的快速迁移、降低光生电子空穴的复合几率、增加光源利用率,且随着比表面积的增加效果越发明显。最佳催化降解罗丹明B（RhB）的效率是块状g-C₃N₄的13倍。（2）磷钨酸掺杂高比表面积g-C₃N₄催化剂的制备及其光催化性能研究。g-C₃N₄为致密的二维层状结构,表面活性位点较少,光生电子-空穴易复合。而磷钨酸具有强氧化性和酸性且具有独特的“假液相”行为,其缺点在于易溶于水等小分子溶剂。通过磷钨酸掺杂高比表面积g-C₃N₄,既能有效的增加载体表面的活性位点,降低光生-电子空穴的复合几率,又能增加HPW的选择性及氧化性。研究结果表明,HPW的掺杂,显著降低了带隙能和光生电子-空穴复合的几率,并且吸收波长发生红移,提高了对于光能的利用率。当HPW掺杂量为1.5%时,罗丹明B的降解率在两个小时的照射下达到96.10%,速率常数为0.0394min^-1,是纯g-C₃N₄的7倍。反应体系的主要活性物种为超氧自由基·O^2-,HPW掺杂后主要的反应活性物种没有改变。（3）磷钨酸修饰铁改性g-C₃N₄的制备、表征及光催化性能研究。g-C₃N₄的量子效率较低、光源利用率低以及晶界效应的存在严重限制了光生载流子的迁移。为此,通过铁（Fe）的掺杂以及HPW的表面修饰,来调控催化剂的能带结构,增加光源的吸收利用率,促进光生载流子的快速迁移。以三聚氰胺、Fe（NO₃）₃·9H₂O和HPW为原料,制备了不同Fe含量的HPW/xFe-C₃N₄催化剂。结果表明,Fe掺杂以及HPW的表面修饰,有效降低了带隙能,降低了光生电子-空穴复合的几率。吸收波长发生红移,提高了对于光能的利用率。并且当x=1.5时,由于磷钨酸盐与g-C₃N₄出现协同催化作用,使得光催化剂的活性进一步提高,降解率在两个小时的照射下达到96.10%,速率常数为0.0394min^-1,是纯g-C₃N₄的7倍。磷钨酸的修饰以及Fe的掺杂能够有效的增加催化剂表面的活性位点,调控了催化剂的能带结构,提高催化剂的表面动力学,促进RhB在催化剂表面的氧化还原反应。（4）Keggin型Fe单晶取代杂多酸盐掺杂g-C₃N₄对能带结构的调控及催化性能的影响。g-C₃N₄的禁带宽度为2.7eV,较宽的禁带宽度使得光响应范围较窄,对光源利用率较低。通过掺杂可以实现掺杂剂的电子轨道与g-C₃N₄分子轨道之间的杂化,调节导带与价带之间的位置,从而改变g-C₃N₄的电子结构与光学性质。由此,应用乙醚萃取法制备了Fe单晶取代的Keggin型杂多酸(PW₁₁Fe),并成功掺杂于石墨相氮化碳（g-C₃N₄）中,制备了不同掺杂量的复合催化剂。PW₁₁Fe的掺杂能够显著调节g-C₃N₄的能带结构,使得VB位置为1.60².19eV,CB位置为-1.11^-0.37eV,有效提高了光催化还原性能。此外,显著降低了光生-电子空穴的复合几率,使光响应范围从463nm拓宽至493nm。其中当PW₁₁Fe掺杂量为0.15时（0.15CN）表征结果最佳,但是当PW₁₁Fe的掺杂量过大时会成为电子-空穴的复合中心而减弱光学性能。且PW₁₁Fe掺杂后增加了活性位点及对能带结构的调控,显著提高了催化剂对于反应底物的吸附及氧化性能.其中0.15CN实验结果最为理想（与表征结果一致）,在反应90min后降解率就达到100%,反应速率常数为0.04266min^-1,是CN的21.9倍.当循环使用4次后,催化活性几乎不变。更多还原

【Abstract】 Solar energy is a new and sustainable energy source.Converting solar energy into chemical energy for storage has always been the focus and difficulty of research.However,the use of solar energy is concentrated in the ultraviolet region,which greatly limits the utilization of the light source.More recently,graphitic carbon nitride（g-C₃N₄）has been widely used in many photocatalytic application,such as hydrogen production by water-splitting,removal of organic pollutants,photocatalytic nitrogen fixation and synthesis of organic target compounds because of their appropriate bandgap（2.71eV）,two-dimensional layered physical structure,special electronic structure,excellent chemical stability and green pollution free.However,its low specific surface area,fast recombination of the photogenerated the electron-hole pairs and low light energy utilization limit the practical application of g-C₃N₄ in heterogeneous photocatalytic reactions.The effective separation of photogenerated the electron-holes and the efficient transfer of charges are the key to improving photocatalytic activity.Therefore,regulate the energy band structure of g-C₃N₄,reduce the recombination probability of photogenerated electron-holes,increase the absorption efficiency of the light source,and promote the surface migration of photogenerated carriers have become the focus of research.In this paper,g-C₃N₄ was modified by the following aspects,and the activity of the catalytic was tested in the photocatalytic degradation of rhodamine B.The modification schemes were as followed:Preparation of high specific surface area of g-C₃N₄ and photocatalytic degradation of rhodamine B.g-C₃N₄ has a low specific surface area（less than 10m²/g）,and the large mass transfer resistance,so that the photo-generated carriers are recombined when they do not migrate to the surface of the catalyst,which greatly increases the recombination probability of photogenerated the electron-hole pairs,reduced photocatalytic activity.Therefore,using SBA-15 as template,a series of high surface area porous g-C₃N₄ were successfully fabricated by pretreat melamine using hydrochloric acid.The photocatalytic activity of samples was tested by the degradation of rhodamine B（RhB）.The results indicated that the increase of specific surface area can effectively reduce the mass transfer resistance of the catalyst surface,promote the rapid migration of photogenerated carriers on the catalyst surface,reduce the recombination probability of photogenerated electron holes,and increase the utilization rate of the light source.The best catalytic degradation of rhodamine B（RhB）was 13 times as high as g-C₃N₄.Preparation of high specific surface area g-C₃N₄ doped by phosphotungstic acid and its photocatalytic properties.g-C₃N₄ is a dense two-dimensional layered structure with few surface active sites,and photogenerated electron-holes are easy to recombine.Phosphotungstic acid has strong oxidizing,acidity and unique pseudo-liquid phase,which has the disadvantage of being easily soluble in small molecular solvents such as water.High-specific surface area g-C₃N₄ doped by phosphotungstic acid can effectively increase the active site on the surface of the carrier,reduce the recombination probability of photo-electron-holes,and increase the selectivity and oxidation of HPW.The results indicated that the doping of HPW significantly reduces the probability of band gap energy and photogenerated electron-hole recombination,and the absorption wavelength was red-shifted,which improves the utilization of light energy.When the HPW doping amount is 1.5%,the degradation rate of Rhodamine B reaches96.10%under two hours of irradiation,and the rate constant is 0.0394 min^-1,which is 7 times than pure g-C₃N₄.The main active species in the reaction system is superoxide radicals O^2-,and the main reactive species after HPW doping have not changed.Preparation and characterization of iron doped graphitic carbon nitride modified by phosphotungstic acid and its photocatalytic properties.The lower quantum efficiency of g-C₃N₄,low light source utilization,and the presence of grain boundary effects severely limit the migration of photogenerated carriers.For this reason,the doping of iron（Fe）and the surface modification of HPW are used to regulate the energy band structure of the catalyst,increase the absorption efficiency of the light source,and promote the rapid migration of photogenerated carriers.A series of HPW/x%Fe-C₃N₄ photocatalytics were prepared by impregnation method with Melamine,Fe（NO₃）₃·9H₂O,phosphotungstic acid（HPW）as raw materials.The structure of HPW/x%Fe-C₃N₄ was characterized by FT-IR,XRD,UV-Vis,PL,SEM.The results showed that the doping of Fe and HPW changed the band structure of the catalysts and reduced the recombination probability of photogenerated electrons and holes.Besides,the doping of Fe ions inhibited the growth of the g-C₃N₄ crystals.When x=1.5,the activity of photocatalyst was further improved because the co-catalysis between g-C₃N₄ and phosphotungstate.The activities of the HPW/x%Fe-C₃N₄ catalysts were tested in the photocatalytic degradation of rhodamine B under xenon lamp.The rate constant was 7 times as high as that of pure g-C₃N₄ under optimized conditions when x=1.5 and degradation rate of rhodamine B reached 96.10%in 120 min with the rate constant of0.0394min^-1Keggin type Fe mono-substituted heteropolyacid salt doping on g-C₃N₄to tunable band gap and effect on catalytic performance.The band gap of g-C₃N₄ is 2.7eV,and the wider band gap makes the light response range narrower and the light source utilization rate is lower.By doping,hybridization between the electron orbital of the dopant and the molecular orbital of g-C₃N₄ can be achieved,and the position between the conduction band and the valence band can be adjusted,thereby changing the electronic structure and optical properties of g-C₃N₄.Fe substituted heteropoly acid potassium with Keggin type(PW₁₁Fe)was synthesized by extraction method with ether and loaded on g-C₃N₄ to composite catalysts with different amounts.The structure of catalysts was characterized by Fourier transform infrared（FTIR）spectroscopy,X-ray diffraction（XRD）,X-ray photoelectron spectroscopy（XPS）,ultraviolet-visible（UV-Vis）spectroscopy,electrochemical impedance（EIS）spectroscopy and photoluminescence（PL）spectrum.The results indicated that doping of PW₁₁Fe effectively tunable the band gap of g-C₃N₄,position of the valence band（VB）between 1.60 and2.19eV and conduction band（CB）between-1.11 and-0.37eV.In addition,it effectively reduced the recombination probability of the photogenerated electrons and holes and broadens the response range of light.Among them,when the doping amount was 15%（0.15CN）,the best result was obtained.The catalytic activity and stability of the catalysts were tested in the photocatalytic degradation of RhB under xenon lamp with 420nm filter.The results indicated that the doping of PW₁₁Fe could significantly improve the adsorption performance and reactivity of g-C₃N₄.0.15CN showed the best reactivity,and the degradation rate reached 100%after 90min illumination.The reaction rate constant of 0.15CN was 0.04266min^-1,which was 21.9times than pure g-C₃N₄.The catalytic activity was almost unchanged after four times used.更多还原