Preparation And Photoelectrocatalysis Performance Of Carbon Nitride-based Catalyst For Ammonia Synthesis

Ammonia（NH₃）is a vital chemical that has a wide range of applications in human production and life.At present,the Haber-Bosch process is the most important ammonia synthesis technology in industry,which not only requires severe high-temperature and high-pressure reaction conditions,but also releases large amounts of CO₂ and other greenhouse gases.In order to solve the energy shortage problem and achieve the goal of“carbon neutrality”and“carbon peaking”as soon as possible,it is very relevant to develop green and sustainable ammonia synthesis technology.Photocatalysis,electrocatalysis,photoelectrocatalysis,plasma,and biological synthesis technologies have been developed by scientists.Photoelectrocatalysis under environmental conditions has attracted much attention in ammonia production because of the preponderances of both electrocatalysis and photocatalysis,and the electric field caused by the applied bias can accelerate the charge separation of semiconductor materials,thus improving the performance of photoelectrocatalysis.However,due to the very strong inertness of N₂and the competition of hydrogen precipitation reaction（HER）,the ammonia yield and Faraday efficiency still cannot reach the ideal level for practical applications.Carbon nitride（g-C₃N₄）as a nonmetallic catalyst with the advantages of stable properties and low cost has attracted research interest in various fields,but the rapid recombination of photogenerated carriers limits its catalytic performance.In this thesis,different g-C₃N₄based composites were prepared for photoelectrocatalysis ammonia synthesis using g-C₃N₄ as the substrate material from three modification means:constructing oxygen vacancies,plasma effect and layered bimetallic hydroxide modification.Through a series of tests and characterization,the conformational relationships among the g-C₃N₄semiconductor composites and their effects on the catalytic performance of ammonia synthesis were investigated.The main research of this paper are as follows.1.The g-C₃N₄ bulk was exfoliated into nanosheets by thermal oxidation,followed by the synthesis of Bi OBr/g-C₃N₄ catalysts containing oxygen vacancies by hydrothermal method,and an efficient and stable photocathode was successfully prepared for the catalytic conversion of N₂ to NH₃ under ambient conditions.among them,the Bi OBr/g-C₃N₄（15%）composite at a bias voltage of-0.5 V had the best ammonia The best ammonia synthesis performance was achieved with an ammonia yield of 227.3μmol?h^-1?g^-1 and an FE of 25.1%.The theoretical calculations show that this ammonia synthesis process follows the conjoined distal mechanism.Due to the electronic coupling and synergistic effect between g-C₃N₄ and Bi OBr,the rapid separation of electron-hole pairs were promoted,which increases the carrier migration rate and results in excellent catalytic activity and selectivity of the catalyst for photocatalytic ammonia synthesis.2.Cu:Fe OOH/g-C₃N₄ composites were prepared by a one-step in situ precipitation method.The 0.1Cu:Fe OOH/g-C₃N₄ composite catalyst had the best photocatalytic ammonia synthesis performance at-0.7 V in a 0.1 M Na₂SO₄ electrolyte under ambient conditions.The ammonia yields and FE were 904.94μmol?h^-1?g^-1 and 5.5%,respectively.The loading of Cu:Fe OOH enhanced the light absorption capacity of the g-C₃N₄composite and accelerated the electron-hole separation efficiency.The plasma light-captured Cu was also integrated with Fe catalytic sites,and the surface Fe atoms were considered as the active sites for effective N₂ adsorption and activation,while Cu generated hot electrons through the plasma to further promote the rate of photo-catalytic ammonia synthesis reaction.3.First,the Ni Fe-LDH/g-C₃N₄ photocathode materials were successfully prepared by preparing g-C₃N₄ thin films by electrophoretic deposition followed by deposition of Ni Fe-LDH arrays on g-C₃N₄ thin films by light-assisted electrodeposition technique.The catalyst deposited at-0.6 V for 400 s had the best photocatalytic ammonia synthesis performance with ammonia yield and FE of 7.35 mmol?h^-1?m^-2(1838.89μmol?h^-1?g^-1)and15.62%,respectively,by testing different samples.Due to the large surface area of g-C₃N₄/Ni Fe-LDH,this catalyst possesses abundant active sites,and the interfacial coupling between Ni Fe-LDH and g-C₃N₄ nanosheets provides a new channel for charge transfer.Meanwhile,the presence of Ni inhibits the occurrence of hydrogen precipitation reaction,and the excellent ammonia synthesis performance of this catalyst can be attributed to the synergistic effect of Ni and Fe atoms,which promotes the ammonia synthesis reaction to a certain extent.