Study Of Efficient Chalcogenide Semiconductor Photocatalytic Nitrogen Fixation Catalysts

Ammonia is an essential chemical for biological synthesisand agriculture fertilizer.The Haber-Bosch process,as a typical industrial NH₃ synthesis,carried out at high temperature and pressures.That causes huge energy consumption and huge carbon emissions of N₂ fixation.In nature,nodule bacteria are very good at biological nitrogen fixation at ambient conditions.But that is not enough to meet the growing demand for ammonia.Photocatalytic N₂ fixation was a catalytic reaction at room temperature and pressure.Photocatalytic N₂ fixation was demonstrated by using wide bandgap TiO₂ about half a century ago,however,the progress is limited.Photocatalytic N₂ fixation reaction can be carried out in the presence of pure water and sacrificial agent system.High-activity photocatalysts were discovered by using Nessler’s reagent method and cation exchange chromatography as the detection technique of ammonia production.But the experimental results were vastly different for different methods.However,there is an open question on the rationality and accuracy of the ammonia production.There are still many serious problems in the determination and quantification of ammonia.The detection methods of ammonia in photocatalytic N₂ fixation products need to be further studied.Therefore,the detection method and synthetic routes in photocatalytic N₂ fixation were firstly corrected by using P25 TiO₂.The traditional wide bandgap semiconductor were stable and efficient,but they can only absorb ultraviolet light,such as TiO₂.Sulfide semiconductor has excellent visible light absorption and good visible light response.We took the sulfide for example in this doctoral thesis.The photocatalytic N₂ fixation catalyst was extended to the visible light by synthesizing Pd doped Zn-Cd-S solid solutionIn.And the Pd doped Zn-Cd-S composites have higher charge transfer efficiency.Toraise the efficiency of photocatalytic N₂ fixation,it requires（i）N₂ can well be chemisorbed on the surface of the catalyst;（ii）N₂ has a relatively low reduction potential on the catalyst surface;（iii）highly efficient charge separation process of the photocatalyst.Based on the above analyze,we propose a new strategy to construct composite photocatalyst by introducing the cocatalyst.Inspired by the photocatalytic water splitting we construct CdS nanorod loaded with NiS or MoS₂ nanosheets as photocatalyst and cocatalyst composite catalyst in this thesis.And realized the application of sulfide semiconductor in the field of photocatalytic nitrogen fixation.The details are as follows:（1）Herein,P25 TiO₂ was employed as a model photocatalyst and pure water or alcohol as sacrificial agent,and both nessler’s reagent and cation exchange chromatography were employed as ammonia detection methods.The different ammonia production amount was found by the different detection method.HPLC and¹H NMR results indicate that carbonyl compounds（formaldehyde,acetaldehyde,and acetone）are produced in the presence of sacrificial agent system.When the standard carbonyl compound was added to the ammonia standard solution,the interference effect on the detection of ammonia was found in the nessler’s reagent method.No interference effect was found in the cation exchange chromatography.Thus,the Nessler’s reagent is not suitable for ammonia detection in the presence of alcohol as the sacrificial agent.The results of nessler’s reagent are basically accordant to that of cation exchange chromatography in pure water.The nessler’s reagent is suitable for ammonia detection in the presence of pure water.（2）Sulfide with visible light absorption has a good photocatalytic water splitting function in visible light region.In this part,we developed the visible photocatalyst with nitrogen fixation performance.Herein,Pd doped ZnCdS was synthesized by one step solvothermal route.Transmission electron microscopy and scanning electron microscopy confirmed that the morphology of Zn-Cd-S solid solution did not change by doping Pd.SEM-EDX confirmed that Pd was dispersed uniformly in Zn-Cd-S solid solution.The transient photocurrent and PL indicate that the charge separation can be enhanced by introducing Pd.As a result,Zn-Cd-S sample achieves an ammonia production rate of 0.21 mg L^-1for the first hour under visible light（λ>420 nm）.And the 1%Pd doped Zn-Cd-S achieves an ammonia production rate of 0.98 mg L^-1for the first hour under visible light（λ>420 nm）.Moreover,it has a good stability under visible light irradiation.¹⁵N₂ isotope labeling result verified the ammonia is originated fromthe N₂.It can be confirmed that NiS/CdS is a highly efficient photocatalyst for visible light N₂ fixation（3）Photocatalytic N₂ fixation carries out nitrogen reduction reaction on the surface of the photocatalyst to convert the N₂ into ammonia,which strongly depends on the photocataly ticefficiency.N₂ can well be chemisorbed on the surface of the catalyst,N₂ has a relatively low reduction potential on the catalyst surface and highly efficient charge separation process of the photocatalyst.Inspiration by photocatalytic water splitting,the photocatalytic efficiency can be greatly enhanced by introducing a cocatalyst.We NiS,as a cocatalyst,is loaded on the CdS nanorods for the photocatalytic N₂ fixation.Theoretical calculation results imply that the N₂ can effectivelyadsorb on the NiS/CdS surface.Temperature programmed desorption confirms the N₂ molecule prefers adsorbing on the NiS/CdS surface.Furthermore,transient photocurrent and electrochemical impedance spectroscopy indicate that the charge separation can be enhanced by introducing NiS.Linear sweep voltammetry discloses that overpotential of N₂ reduction reaction can be reduced by loading NiS.The photocatalytic N₂ fixation is carried out in the catalyst water dispersion without any sacrificial agent.As a result,1.0 wt%NiS/CdS sample achieves an ammonia production rate of 2.8 mg L^-11 and 1.7 mg L^-11 for the first hour under full spectrum and visible light（λ>420 nm）.The catalyst demonstrates an apparent quantum efficiency of 0.76%,0.39%and 0.09%at 420,475 and 520 nm,respectively.（4）Non-precious metal MoS₂ is a good cocatalyst for hydrogen production.Here we introduce MoS₂ into photocatalytic nitrogen fixation.Herein,we used MoS₂as a cocatalyst,and MoS₂/CdS composite catalyst was synthesized by two step solvothermal technique.X-ray photoelectron spectroscopy（XPS）,transmission electron microscopy（TEM）,and energy spectrum scanning（EDAX）confirmed that the MoS₂ sheet was loaded on the CdS nanorods.Temperature programmed desorption confirms the N₂ molecule prefers adsorbing on the MoS₂/CdS surface.Linear sweep voltammetry discloses that overpotential of N₂ reduction reaction can be reduced by loading MoS₂.Furthermore,transient photocurrent and electrochemical impedance spectroscopy indicate that the charge separation can be enhanced by introducing MoS₂.The 2 wt%MoS₂/CdS showed the best photocatalytic N₂ fixation performance.2.0 wt%MoS₂/CdS sample achieves an ammonia production rate of 3.3mg L^-11 and 2.1 mg L^-11 for the first hour under full spectrum and visible light（λ>420nm）.The catalyst demonstrates an apparent quantum efficiency of 0.98%,0.43%and0.13%at 420,475 and 520 nm,respectively.