Photocatalytic/electrocatalytic Nitrogen Reduction Properties Of N-doping TiO₂ And Mn-doping G-C₃N₄

At present,industrial large-scale ammonia synthesis relies on Haber-Bosch method,which leads to a large number of greenhouse gas emissions and is not conducive to carbon neutralization.Therefore,it is of great significance to find a green and economic ammonia production method.Due to the mild ambient temperature/pressure and low energy consumption,electrocatalytic and photocatalytic nitrogen fixation have attracted extensive attention.However,the extremely high bond energy of N≡N makes it difficult to adsorb and activate at room temperature,and serious hydrogen evolution reaction（HER）results in the very low efficiency of electrocatalytic photocatalytic ammonia synthesis.Therefore,the development of cheap and selective catalysts is the focus of nitrogen reduction.This paper puts forward strategies to improve nitrogen fixation activity from two aspects:（1）N-doping TiO₂ hollow microspheres（NTO-X,X is N-doping proportion）with abundant oxygen vacancies were synthesized by hydrothermal and high-temperature calcination for photocatalytic and electrocatalytic nitrogen fixation,respectively.Among them,NTO-0.5 catalyst showed the best photocatalytic nitrogen fixation activity(NH₃ production rate was 80.09μmol g_cat^-1 h^-1)and electrocatalytic nitrogen fixation activity(NH₃ production rate was 29.5μg mg_cat^-1 h^-1,Faradic efficiency was 1.21%),which was much higher than that of pure TiO₂ hollow microspheres.At the same time,the catalyst possessed excellent cycle stability and long-term stability.The N₂ adsorption-desorption curves indicated that the larger specific surface area of hollow microspheres exposed more active sites.The N₂-temperature programmed desorption curves stated clearly that the oxygen vacancies and the structure of hollow microspheres worked together to enhance the N₂ adsorption capacity of the NTO-0.5,and then enhance the photocatalytic and electrocatalytic nitrogen reduction activities.This part is of great significance for the development of hollow and defective catalysts for ammonia synthesis.（2）Based on the beneficial effect of Mn on nitrogenase,Mn-doping g-C₃N₄electrocatalysts with different Mn-doping amounts and calcination temperatures were synthesized.5-Mn-CN electrocatalyst（Mn doping amount was 5%,the calcination temperature was 550℃）had the best NH₃ production rate(15.20μg mg_cat^-1 h^-1)and Faradic efficiency（7.1%）at-0.4（V vs.RHE）in 0.1 mol L^-1 Na₂SO₄ solution,which were much higher than the NH₃ production rate and Faradic efficiency（4.3%）of pure g-C₃N₄(5.1μg mg_cat^-1 h^-1).At the same time,5-Mn-CN also had excellent nitrogen fixation selectivity and good electrocatalytic cycle stability.Mn-doping could change the morphology and increase the specific surface area of g-C₃N₄.In addition,Mn accepted electrons from g-C₃N₄ and applied them to the activation of N₂ molecules through the accept-donation mechanism,so Mn-doping improved the electrocatalytic nitrogen fixation activity of the catalyst.This paper studies the influence of oxygen vacancies and transition metal doping on NRR,analyzes the principle of oxygen vacancies and doping to improve ammonia production,explores the activity and mechanism of photocatalytic and electrocatalytic nitrogen fixation,and provides experimental data for improving ammonia production.