High-performance Electrochemical N₂ Reduction To NH₃ Based On Transition Metal Catalyst

Ammonia(NH₃),served as agricultural fertilizer,industrial raw material and energy fuel,is an essential chemical product for social development.The dependent Haber-Bosch process has caused intensive energy consumption and severe impact on the ecological environment,which urges to develop sustainable and green route for ammonia synthesis.Emerging electrochemical nitrogen reduction reaction(NRR)has been recognized as a potential candidate with its renewable electricity,abundant raw material of N₂ and H₂O,and mild reaction condition.However,the main challenges of the currently electrochemical ammonia synthesis are the low faradic efficiency and poor ammonia yield rate.It is imperative to explore high-performance and selective catalysts for electrochemical nitrogen reduction.Transition metals have been widely studied as electrocatalysts for their excellent conductivity and the binding force with N₂ molecules.In this thesis,a range of transition metal-based catalysts are prepared by different synthetic strategies for achieving highly efficient electrocatalytic nitrogen reduction in aqueous electrolyte.The specific contents are as follows:1.Inspired by the intrinsic NRR activity of Au,we systematically studied the synergistically enhanced NRR activity by incorporating other transition metals into Au.A NaBH₄ reduction general strategy was used to synthesize a series of Au-based alloy nanocatalysts(Au Cu,Au Ag,Au Pd and Au Ru),and the NRR catalytic performance of the as-obtained electrocatalysts was explored.The experimental results indicated that all prepared alloy catalysts significantly improved the intrinsic catalytic activity of Au,and the Au Cu alloy catalyst showed the most remarkable NRR response.The optimized nanocatalyst with the atom ratio of Au₁Cu₁ achieved the highest faradaic efficiency(54.96%)and NH₃ yield rate(154.91?g h^-1 mg^-1_cat)at-0.2 V vs.reversible hydrogen electrode(RHE),exceeding that of the previously reported Au nanocatalysts.The synergy between Au and Cu components tuned the electronic structure and coordination environment of Au,reduced the activation energy barrier of rate-determining step in the nitrogen reduction process,thus significantly enhanced the catalytic NRR activity of Au₁Cu₁ alloy.2.In this work,Fe-Bi₂WO₆ catalyst was fabricated by Fe doping strategy,and its electrochemical nitrogen reduction behavior was studied.The effect of different Fe doping amounts on the electrocatalytic nitrogen reduction response was discussed in detail.The _0.50Fe-Bi₂WO₆ catalyst with optimized Fe doping ratio achieved a ultrahigh NH₃ formation rate of 289?g h^-1 mg^-1_cat at-0.75 V vs.RHE in 0.05 mol L^-1 H₂SO₄electrolyte,which was far beyond the optimum response of reported Fe-and Bi-based catalysts,even single atom Ru catalyst.The keys of the outstanding NRR behaviors are that effectively suppressed HER,the promoted N₂ activation and hydrogenation by the potential synergy between Bi and Fe components,and the accelerated electron transfer rate and catalysis reaction kinetics by Fe doping.Rigorous control experiment,cautious electrolysis process,inert gas protection technology and the comparison of different quantitative methods,powerfully confirmed the accuracy and reliability of obtained experimental results.3.Amorphous Rh decorated SnO₂(Rh@SnO₂)catalyst was synthesized by one-step solvothermal method,in which the key to the formation of amorphous structure was the SnCl₂ reducing agent.The effects of different Rh dosage and amorphous structure on the electrochemical NRR performance on Rh@SnO₂ catalyst were systematically discussed.The composition-optimized amorphous Rh_0.5@SnO₂catalyst achieved the maximal NH₃ formation rate of 149?g h^-1 mg^-1_cat,along with faradaic efficiency of 11.69%at-0.35 V vs.RHE.This result was superior to the performance of previously reported SnO₂ or Rh-based catalysts.The remarkably enhanced NRR response was mainly ascribed to the effectively promoted electron transfer by Rh decoration,the increasing catalytic active sites exposure on the amorphous structure,and the reduced activation energy barrier of N₂ hydrogenation by the synergy between SnO₂ and Rh.This obtained results have been reliably confirmed by indophenol blue colorimetric and ¹H NMR methods.