Microstructure Controlling Of Graphitic Carbon Nitride For Catalytic Application

As a green technology,semiconductor photocatalysis can convert solar energy into high-energy chemical energy and has shown great application prospects in the fields of renewable energy and ecological restoration.Besides,most catalysts for catalytic hydrogenation of p-NP are the size-and shape-controlled Au,Ag,Pd and Pt noble metal nanomaterials,or deposition of them on appropriate supports.Nevertheless,their high prices and low reserves severely limit the industrial application.In the last few years,two dimensional graphitic carbon nitride（g-C₃N₄）,as a promising metal-free（photo）catalyst,has received considerable interests and is expected to be widely applied in the fields of water splitting,organic pollutant degradation,carbon dioxide reduction and catalytic oxidation of alkane,benzene and alcohols.In this dissertation,a series of graphitic carbon nitride materials with controllable microstructure have been designed and prepared by soft chemistry method,and their photocatalytic and organic catalytic hydrogenation of p-nitrophenol have also been studied.The main contributions of this dissertation are described as follows:（1）A cobalt-doped porous spherical graphitic carbon nitride with enhanced photocatalytic degradation activityCobalt-doped porous spherical graphitic carbon nitride（COCN-2）is successfully prepared by solution-thermal method and its photocatalytic degradation performance of Rhodamine B（Rh B）is studied.X-ray powder diffraction（XRD）,X-ray photoelectron spectroscopy（XPS）,Fourier transform infrared spectroscopy（FT-IR）,transmission electron microscopy（TEM）and field emission scanning electron microscopy（FESEM）were used to confirm the morphology and structure of Co CN-2.Compared to the bulk graphitic carbon nitride,Co CN-2 shows excellent photocatalytic degradation of Rh B performance and good cycling stability,A possible mechanism for photocatalytic degradation of Rh B on the catalyst is proposed as:the dopant Co induced trapping of electrons facilitates quick transfer of the photogenerated electrons on the conduction band of the catalyst,thus inhibiting the recombination of photogenerated carriers.The transferred electrons can be trapped by dissolved O₂ to produce O₂^·-radicals.And then the as-generated O₂^·-oxidize Rh B molecules adsorbed on the surface of the catalyst,so as to achieve the photocatalytic degradation of Rh B.（2）Precursor-modified strategy to synthesize thin crimped porous ammonia-rich graphitic carbon nitride with enhanced catalytic performancesA precursor-modified strategy is designed to synthesize graphitic carbon nitride with highly efficient photocatalytic performance.The precursor dicyandiamide reformed by different acids undergo the basic structure change and transform into the diverse new precursors standby.Calcined by dicyandiamidine nitrate formed by concentrated nitric acid modified dicyandiamide,the obtaining thin crimped porous ammonia-rich HNO₃-CN（5H-CN）presents the best photocatalytic degradation rate of Rh B,more than 34 times of that of bulk graphitic carbon nitride（CN）.Moreover,the photocatalytic nitrogen fixation rate of 5H-CN is also much higher than that of the bulk CN.The TG-DSC-FTIR analyses indicate that the distinguishing thermal polymerization process of 5H-CN lead to its thin porous ammonia-rich structure,and the theoretical calculations reveal that the negative conduction band potential of 5H-CN is attributed to its ammonia-rich structure.It’s anticipated that the thin crimped porous structure and the negative conduction band position of 5H-CN play the important roles in the improvement of photocatalytic activity.In addition,5H-CN exhibits extremely high catalytic activity for catalytic hydrogenation of p-nitrophenol in the presence of Na BH₄,which can be equivalent to（or even higher than）those of the previously reported noble metal-based catalysts.Interestingly,the kinetics analysis of hydrogenation of p-nitrophenol to p-aminophenol shows that the reaction follows the zero-order kinetics,different from the first-order kinetics for the hydrogenation over the noble metal-based catalysts.In fact during reaction process,the catalytic oxidation of BH₄^-and the generation of hydride ion（H^-）occur on the catalyst surface,while the hydrogenation process of p-nitrophenol proceeds in the bulk solution.（3）General synthesis strategy for hollow porous prismatic graphitic carbon nitride:a high-performance photocatalyst for H₂ production and degradation of Rh BA general synthesis strategy for the hollow porous prismatic graphitic carbon nitride is obtained from a supramolecular precursor with prismatic morphology,which was synthesized via a one-pot hydrothermal process using melamine or other N-containing compounds as the starting materials,without any other assist-reagents.A detailed analysis of the formation process was conducted on melamine as a representative precursor,showing that the hydrothermal treatment offers a homogeneous environment for hydrolysis of melamine to produce cyanuric acid,and facilitates the in-situ reaction of self-assembing the new resultant and remained melamine to create the in-plane ordering and hydrogen bonded supramolecular complex.Besides,the in-situ reaction rate has a great influence on the uniform prismatic structure.Then a stable prismatic precursor can be prepared via the molecular self-assembly between melamine and cyanuric acid.The best g-C₃N₄photocatalyst in this work possesses a high hydrogen evolution rate and superior photocatalytic activity in degradation of rhodamine B dye,giving a rate constant,almost14.7 times than the bulk g-C₃N₄ under the visible-light irradiation.The excellent photocatalytic performance is mainly attributed to the large specific surface area,more negative conduction potential and the faster separation efficiency of the g-C₃N₄photocatalyst.（4）Hollow porous prismatic graphitic carbon nitride with nitrogen vacancies and oxygen doping:a high-performance visible light-driven catalyst for nitrogen fixationA hollow porous prismatic graphitic carbon nitride with nitrogen vacancies and oxygen doping is successfully constructed using dicyandiamidine as the only raw material via a facile two-step strategy of low-temperature hydrothermal method followed by subsequent calcination process.The as-obtained graphitic carbon nitride shows a hollow prismatic morphology with loose spongy-like walls,hierarchical pore structure,and an extremely large specific surface area of 220.16 m²g^-1.Such a graphitic carbon nitride exhibits an ultrahigh nitrogen fixation rate under visible light irradiation and shows excellent stability during the reactions.A possible mechanism for photocatalytic nitrogen fixation on the catalyst is proposed as:under the visible-light irradiation,graphitic carbon nitride with nitrogen vacancies and oxygen doping undergo charge separation to generate electron-hole pairs,then the photogenerated electrons on the conduction band are quickly transferred to the nitrogen vacancies induced mid-gap state,consequently,the trapped electrons react with the activated nitrogen on the nitrogen vacancies to produce ammonia.The significant enhancement in photocatalytic nitrogen fixation performance of the graphitic carbon nitride is mainly attributed to its unique hollow prismatic morphology with a loose porous structure,fully exposed active sites of nitrogen vacancies,more negative conduction band,suitable visible-light response and the efficient separation of photogenerated electron–hole pairs.