Construction Of High-Efficiency Catalysts For Electrocatalytic Conversion Of Nitrogen-Containing Molecules

Nitrogen is a significant element for biological activities and natural cycles.The process of interconversion of nitrogen-containing compounds plays a vital role in human life.The entire ecosystem is interconnected through the nitrogen cycle.Electrocatalysis,an eco-friendly approach to transforming nitrogen-containing compounds to high value-added chemicals is of importance for both scientific research and industrial applications.In recent years,although the electrocatalytic conversion of nitrogen-containing small molecules has made great progress,the electrocatalytic synthesis process of complex and valuable nitrogen-containing compounds is still exploring.Moreover,it is still great challenging to understand the reaction mechanism due to the multi-electron transfer process and coupling processes during the reaction process.In this thesis,we have synthesized highly performance catalysts for selective hydrogenation of N≡N and N-O bonds via these approaches of defect engineering,alloying,and interface engineering etc.from the perspective of the catalytic conversion of nitrogen-containing chemical bonds.Furthermore,the reaction mechanisms of catalytic hydrogenation have been deeply explored with the help of theoretical and experimental studies.The main research contents of this thesis are as follows:1.Nitrogen adsorption is a prerequisite for the subsequent nitrogen reduction reaction(NRR).Anion vacancies have unique advantages for nitrogen adsorption due to their high local electron concentration,thus exhibiting excellent electrocatalytic NRR activity.In this section,we employed a novel doped-oxygen induced strategy to successfully synthesize modified oxygen-doped cuprous phosphide nanosheet catalyst(RO-Cu3P)with rich surface phosphorus vacancies.The results indicated that the ammonia yield rate(RNH3)can reach 28.12 μg h-1 cm-2 with a Faradaic efficiency(FE)of 17.5%.Both the control experiments and density functional theory(DFT)calculations revealed that a suitable oxygen doping concentration is beneficial to high-efficiency NRR.This part of the work provides promising theoretical and experimental guidance for constructing high-performance NRR electrocatalysts with anionic vacancy defects.2.Lower solubility of nitrogen in aqueous electrolyte is one of the reasons for low NRR performance.Therefore,it is worthwhile to rationally design and construct the surface environment of heterogeneous catalysts to regulate the NRR activity and selectivity.In this section,the rhodium(I)-dodecanethiol complex followed by low-temperature pyrolysis to achieve dodecanethiol modified Rh(Rh@SC12H25/CFC-x,x represents the pyrolysis time)for electrocatalytic N2 reduction reaction(NRR).Experimental results demonstrated that the Rh@SC,2H25-0.5 exhibits excellent NRR catalytic activity(FE:55.4%;RNH3:121.2μg h-1 cm-2 or 137.7 μg h-1 mgRh-1).Importantly,the surface hydrogen(H*)coverage is obviously decreased after dodecanethiol modification,thus effectively inhibiting the competitive hydrogen evolution reaction and concurrently reducing the electrocatalytic NRR energy barrier.This part of the work sheds new light on the catalysts with hydrophobic properties for NRR,and broadens a new view for understanding the NRR mechanism.3.The hydrogen evolution reaction(HER)is a main competition reaction for NRR,which leads to the low Faradaic efficiency of ammonia synthesis.Therefore,to deeply understand the balancing mechanism between HER and NRR is critically important for efficient utilization of electric energy.In this section,we developed an iron covalent-doped WB2 catalyst(donated Fe-WB2)as a bifunctional electrocatalyst toward significantly simultaneous enhanced HER and NRR activities.Experimental results demonstrated the Fe-WB2 electrocatalyst not only exhibits superior HER activity with an overpotential of 68 mV at 10 mA cm-2 and 235 mV at 100 mA cm-2 but also decent electrochemical N2 fixation activity with an NH3 yield rate of 35.5 μg h-1 mg-1,respectively.This work paves a new way of understanding the reaction mechanism of competing reactions.Besides,we also broaden our understanding of the competitive relationship between HER and NRR.4.Compared with high N=N triple bond energy,the bond energy of N-O is much lower and easier to hydrogenate into ammonia.In this section,a cuprous phosphide(Cu3P)-nickel phosphide(Ni2P)heterostructure catalyst was synthesized by a vapor-phase hydrothermal method,and its electrocatalytic performance for the nitrate reduction reaction(NO3RR)was investigated in detail.The fundamental studies were explored using an H-type electrolytic cell,and it was found that at-0.6 V(vs.RHE),the ammonia yield rate is 3.02 mg h-1 cm-2.The kinetic mechanism of the reaction was further explored by the rotating disk electrode,and it was found that the Cu component was beneficial to decrease the onset potential of the reaction,while the Ni component was beneficial to increase the product’s selectivity.Importantly,MEA electrolyzer was futher utilized to achieve the industrial-scale ammonia yield rate of 1.9 mmol h-1 cm-2.DFT results demonstrated that the high NO3RR reaction performance is mainly due to the enhanced adsorption energy of nitrate at the heterointerface and suppressed HER activity at the same time.This part of the work explored the feasibility of MEA electrolyzer for NO3RR to NH3,providing strong support for the industrial applications of the related techniques.5.Electrocatalytic organic hydrogenation of N-O bond plays a crucial role in the synthesis of complex nitrogen-containing chemicals.Based on the understanding of N-O bond,we further explored the electrocatalytic hydrogenation process of complex nitrogen-containing organic compounds.For this,we synthesized Cu-Pt alloying catalysts with different atomic ratios by a successive solvothermal reduction method.In an alkaline solution,aminobenzene was obtained at a low applied potential.While at high applied potential,azoxybenzene was primarily generated.DFT calculations results demonstrated that the reported reaction pathway could not explain the relationship between the effect of applied potential regulation and the selectivity of the reaction products.Therefore,a new hydrogenation reaction route was proposed as follows:Ph-NO2*→PhNO*→PhN*→PhNH*→PhNH2*.Due to the energy barrier of the formation of the aminobenzene being different from the Ph-N=NO-Ph,the selectivity of the product can be effectively controlled by regulating the potential,which is basically consistent with the experimental phenomenon.