| Ammonia is one of the most significant industrial chemicals,and it can be used as fertilizer to safeguard food supply,as an excellent carbon-free hydrogen carrier which might play a crucial role in hydrogen economy in the near future.Up to now,ammonia is mainly produced through the well known Haber-Bosch process,which requires high energy consumption and causes huge CO2 emmissions.Renewable energy driven electro/photocatalytic nitrogen fixation for ammonia synthesis at ambient conditions is environmentally friendly without greehouse gas emissions,is an attractive alternative.The ammonia yiled in electrocatalytic/photocatalytic nitrogen reduction is relatively small,so the accurate detection of ammonia is very important.In this paper,the accuracy of electrocatalytic/photocatalytic nitrogen reduction to ammonia was studied by comparing the colorimetric method(Nessler’s reagent and indophenol blue metohd).And the influence of alcohols and their possible oxide products(carbonyl compounds)on the determination of ammonia by colorimetric method was also investigated.The results showm that both Nessler’s reagent method and indophenol blue method can be used to detect the concentration of ammonia after electrocatalytic/photocatalytic nitrogen fixation in the absence of alcohol additives.Alcohols and their corresponding carbonyl compounds have an effect on the accuracy of two colorimetric methods for the determination of ammonia concentration.In this paper,the mechanism and performance of electro/photocatalytic nitrogen fixation of nitride materials under ambient conditions were investigated after determing the detection method of ammonia concentration.We found that g-C3N4 can play dual roles in the electrosynthesis of ammonia under ambient conditions,acting as an electrocatalyst and a nitrogen source for the first time.It is evidenced from electrochemical operations,15N isotope labeling,and DFT calculations that the g-C3N4 catalyzed ammonia synthesis obeys a Mars-van Krevelen like mechanism.The N atoms in g-C3N4 can react with protons to form ammonia,and then the nitrogen vacancies generated after ammonia removal can be used for adsorption and activation of N2 molecules.To further improve the nitrogen fixation activity of g-C3N4,the glucose was used as the feedstock to introduce O and C atoms into the lattice of g-C3N4 for the synthesis of OC-GCN catalyst.The co-doping of O and C changed the band structure of g-C3N4 and increased the concentration of nitrogen vacancy on the surface,which enhanced the absorption of visible light and the adsorption and activation of N2 molecules.In addition,glucose can be used as bubble template to improve the specific surface area of the catalyst during the pyrolysis.The photocatalytic nitrogen fixation rate of OC-GCN is 0.659 μg h-1mgcat-1,which is 4.87-fold of g-C3N4.The Mo&S/g-C3N4 catalyst was prepared by in situ loading the Mo and S on g-C3N4 and applied to photo/electrocatalytic nitrogen fixation.The Mo&S/g-C3N4 catalyst has more surface defects and stronger adsorption capacity of N2 molecules.The results indicateteh photocatalytic nitrogen fixation performance of Mo&S/g-C3N4 is 12.40-fold of g-C3N4.When using the dilute sulfuric acid as the electrolyte,the electrocatalytic ammonia production rate of the catalyst is 27.96 μg h-1mgcat-1,which is 13.60-fold of g-C3N4 and 9.45-fold of MoS2,respectively.Mo&S/g-C3N4 catalyst has good electrocatalytic nitrogen fixation cycle stability.The rate of ammonia production and Faradaic efficiency of Mo&S/g-C3N4 catalyst do not decrease significantly after 6 cycles.The silicon nitride(Si3N4)was used for photocatalytic nitrogen fixation to synthesuze ammonia for the first time,and the feasibility of photocatalytic nitrogen was verified with the ammonia synthesis rate of Si3N4 is 0.128 μg h-1mgcat-1.Moreover,the Au nanoparticles were load on silicon nitride by sodium borohydride reduction method for the synthesis of Au/Si3N4 catalysts and applied to the photocatalytic nitrogen fixation for ammonia synthesis.Owing to the enhanced light absorption by the localized surface plasmon resonance(LSPR)of the Au nanoparticles,the xwt%Au/Si3N4 catalysts have the improved the nitrogen fixation performance.The ammonia synthesis rate of Au/Si3N4 is 0.545 μg h-1mgcat-1,which is 4.24-fold of Si3N4. |
PDF Full Text Request