Preparation Of Transition Metal (Mo, Nb) Oxide Nanomaterials And Their Application In Electrochemical Nitrogen Reduction Reactions

NH₃ is of great significance in agriculture,industry,national defense fields.In biological system,N₂ fixation is usually catalyzed by nitrogenase from specific bacteria under environmental conditions,but in general,nitrogenase is relatively fragile and shows low efficiency.The traditional artificial nitrogen fixation is predominantly produced on the basis of the Haber-Bosch process,but the harsh reaction conditions（typically operated at 500°C and 300 ATM）and the use of natural gas as the hydrogen source leads to large energy consumption and CO₂ emission.Therefore,a sustainable and less energy-intensive approach to produce NH₃ is urgently demanded.Electrochemical reduction as an environmentally-benign and sustainable approach under environmental condition has received extensive attention.However,this process involves difficulty in breaking the strong N≡N bond of inert N₂ and challenged by N₂ activation,underlining the need of abundant,good dispersion and high-performance electrocatalysts for the N₂ reduction reaction（NRR）.In view of this,we have carried out a series of research work,including the following parts:1.Inspired by Mo element in natural nitrogenase,we prepared that MoO₃ nanosheet acts as an efficient non-noble-metal catalyst for electrochemical N₂ fixation to NH₃ at room temperature and atmosphere.In 0.1 M HCl,this catalyst exhibits remarkable NRR activity with a NH₃ yield of 4.80×10^–10 mol s^–1 cm^–2(29.43μg h^–1 mg^–1_cat.)and a Faradaic efficiency（FE）of 1.9%.Moreover,this catalyst also shows high electrochemical stability and durability.2.Nb compounds have been widely used in the fields of multiphase catalysis and photocatalysis,which indicates their potential application in electrocatalytic reduction of nitrogen.Nb₂O₅ nanofibers were prepared by electrospinning as efficient catalysts for NRR.The catalyst has high catalytic activity and selectivity.In 0.1 M HCl,such Nb₂O₅ nanofiber is capable of achieving an average NH₃ yield of 43.6μg h^-1 mg^-1_cat.and a FE of 9.26%at-0.55 V vs.reversible hydrogen electrode（RHE）.Notably,this catalyst also shows high electrochemical stability.The catalytic mechanism of NRR on Nb₂O₅（181）surface is further discussed by density functional theory（DFT）calculations.3.The corrosiveness of acid-base to equipment is also an important factor that puzzles industrial production.In addition,powdered catalysts need to be grafted onto the surface of electrodes through film-forming agents,which also hinders their catalytic activity.We report in-situ growing Nb₂O₅ nanowires array on carbon cloth（Nb₂O₅/CC）as a self-standing NRR catalyst electrode by hydrothermal.In 0.1 M Na₂SO₄,such Nb₂O₅/CC achieves a NH₃ formation rate of 1.58×10^–10 mol s^–1 cm^–2and a high FE of 2.26%at–0.60 V,rivaling the performances of most reported aqueous-based NRR electrocatalysts at ambient conditions.Notably,Nb₂O₅/CC also shows high electrochemical stability during electrolysis and recycling test.