Fabrication Of Carbon Nitride And Metal Oxide Photocatalysts And Their Ammonia Synthesis Performance

As one of the most valuable chemicals and carbon-free energy carriers,ammonia(NH3)has a global annual output of?150 million tons,and is mainly produced by the traditional Haber-Bosch process under high temperature and pressure.Under the ever-increasing pressure of the fossil energy crisis and anthropogenic global climate change with continuous CO2 emission in the 21st century,researches on the nitrogen reduction reaction(NRR)in a sustainable and environmentally friendly manner under ambient conditions are flourishing and thriving.Among many effective strategies,photo(electro)catalytic NRR to synthesize NH3 with the advantages of mild condition and direct utilization of solar energy and atmosphere nitrogen source has received tremendous concern.Meanwhile,in order to achieve low energy-consumption and high catalytic activity,photo(electro)catalyst must be reasonably designed to optimize their light-harvesting efficiency,protons and electrons transfer,and chemi(physi)sorption and activation of N2.Therefore,it is essential to understand the fundamentals of the nitrogen reduction reaction processes along with apprehending the key questions that limit the progress of photocatalysts.In this thesis,several high-efficiency NRR photo(electro)catalysts were developed and fabricated,which provided some new insight into NRR pathway with the help of controllable experiment and theoretical calculation simulation to achieve more efficient energy conversion and practical application.The main contents of this thesis work are as follows:1.The cyano group modification can be an effective strategy to improve the light-harvesting efficiency and photogenerated electron-holes separation efficiency of g-C3N4 photocatalyst.A K+ and cyano groups modified carbon nitride nanoribbons(mCNN)photocatalyst was fabricated via KOH-assisted high temperature calcination treatment on Dicyandiamide in this work.The obtained mCNN photocatalyst exhibited extended visible-light-harvesting capacity and inhibited photogenerated electron-holes recombination,leading to an obvious enhanced NH3 yield rata of 3.42 mmol g-1 h-1.A series of experimental and DFT calculation results demonstrated that the-C?N group participated in the NH3 synthesis and constructed the NRR catalytic cycle,and meanwhile stabilized the g-C3N4-based materials structure.2.Whether the surface active N atoms of N-contained catalyst would participate in the synthesis of NH3 has become a key issue that attracts wide attention.A porous boron doped carbon nitride nanosheet(BCN)photocatalyst with boron content of 13.8 wt%and specific surface area of 123 m2 g-1 was fabricated via a secondary calcination treatment on B2O3 and Dicyandiamide in this work.The result revealed that a certain amount of exposed active N atoms on the surface of g-C3N4 samples indeed were hydrogenated to form NH3 products during the photocatalytic reaction.While in the prepared BCN photocatalyst,the exposed surface active N atoms of g-C3N4 were stabilized by formation of B-N-C coordination.Meanwhile,B-doping can regulate the optical/electric properties of g-C3N4 photocatalyst and act as new reaction active sites of N2 adsorption and activation,resulting in increased light-harvesting efficiency,inhibited recombination of photogenerated charges and enhanced NH3 synthesis performance(reaching 313.9 ?mol g-1 h-1,nearly 10 times of g-C3N4 nanosheet).In this chapter,the research clarified the controversial application of nitride catalysts in the NH3 synthesis,and provided a feasible solution for stabilizing their surface active N atoms and constructing new NRR active sites.3.The recombination of photogenerated electron-holes can be suppressed by the photoelectrochemical system,leading to the increasing energy conversion efficiency and catalytic performance.A photoelectrochemical(PEC)system integrated of CoPi modified Ti doping Fe2O3 nanoarray photoanode and cathode Co single-atom catalyst was fabricated in this work to achieve PEC-NRR to synthesize NH3.This system can afford an NH3 yield rate of 1021.5 ?g mgco-1 h-1 and a faradic efficiency of 11.9%at an applied potential bias of 1.2 V(versus reversible hydrogen electrode)on photoanode in 0.2 mol/L NaOH electrolyte under simulated sunlight(AM 1.5 G)irradiation.The high NH3 synthesis performance via the solar-driven PEC-NRR system in this work can be attributed to the high-efficiency CoPi/Ti-Fe2O3 photoanode and Co-SAC cathode to overcome the slow reaction kinetics of water oxidation,provide abundant photoelectrons and N(O)-coordinated Co active sites for N2 adsorption and activation,respectively,leading to an efficient NRR to NH3.The work in this chapter provided a new idea and experimental evidence for the development of the photoelectrochemical NH3 synthesis.4.The development of photocathode materials that directly catalyze NH3 synthesis is of great scientific significance and social economic value.A Cu-based photocathode combined CuO,Cu2O and CuCl was fabricated via chemical-oxide,chemical bath and calcination treatment on Cu foil.This ternary-composited(CuFeOx/CuCl)photocathode exhibited brilliant visible-light harvesting capacity and photoelectrochemical stability.Under AM 1.5 G irradiation and applied bias of-0.55 V(vs.Ag/AgCl),the obtained CuFeOx/CuCl photocathode in Na2SO4 electrolyte(0.1 mol/L)reached a photocurrent density of 0.21 mA cm-2,and optimal NH3 yield of 9.2 ?g cm-2 h-1 and faradic efficiency of 11.9%.Moreover,the EPR result suggested that the density of oxygen vacancy can be tuned by chemical bath and calcination treatment,which contributes to the adsorption and activation of N2,resulting in improvement of photoelectrocatalytic NRR performance.In this chapter,the application of Cu-based photocathode in NH3 synthesis was explored,which is of great significance in developing high-efficiency nitrogen-fixation photoelectrocatalysts.