Preparation And Structure-Activity Relationship Of Two Dimensional Nitrogen-Fixing Materials

Ammonia is widely used for agricultural fertilizer,precursor of nitrogenous compounds and carbon-free energy in daily life and industrial production,so the synthesis of ammonia(N2→NH3)is one of the essential processes for sustaining human life and healthy earth ecosystems.Nowadays,the global annual nitrogen fertilizer production is about 120 million tons,and 80%of the ammonia in the raw material is synthesized by the Haber-Bosch(H-B)process.However,the harsh reaction conditions(400-500℃,20-40 MPa)and the use of natural gas reforming as the hydrogen source lead to the consumption of 2%of the world’s annual energy supply and account for 1%of the global CO2 emissions.Therefore,as an environmentally friendly,clean and sustainable nitrogen fixation method,the electrochemical nitrogen reduction reaction(NRR)under environmental conditions provides a promising route to replace traditional thermochemical nitrogen fixation.Since this century,a large of works have been devoted to the development of efficient electrochemical N2 reduction catalysts.Due to the chemical inertness of N2 and the high energy for the breaking of N≡N triple bonds,the NH3 yield,Faradaic efficiency and catalyst stability need to be further improved.This dissertation,based on the strategy of strengthening N2 adsorption and hydrogenation,uses N2 and water from the electrolyte as reactants to develop several efficient two-dimensional(2D)based catalysts.The structure-activity-relationship of the NRR catalysts is further elaborated.The main research works are summarized as follows:(1)Developing an in-situ metal-doping method to synthesize mesoporous zirconium-doped graphitic carbon nitride(Zr-C3N4)with abundant carbon defects(C defects).Then,the ice-melting process was used to support RuAu as bimetallic catalysts for electrochemical N2 reduction under ambient conditions.The TEM,SEM,STEM,ICP,XPS,EA,TPSR and other characterization methods show that the template-free in-situ Zr doping strategy can spontaneously synthesize ZrC3N4 material with sponge-like mesopores(pore size 18 nm),regular layered structure(layer thickness 9 nm,interlayer spacing 11 nm)and high specific surface area(70.1 m2/g).In addition,the ZrOCl2 in raw material is prone to react with the carbonyl group on urea through nucleophilic reaction,so that the doped Zr is highly dispersed in the form of ZrOx sub-nanoclusters after the high temperature treatment,and the C contained in C3N4 intermediate is removed in the form of CO/CO2,which promotes the formation of a spongy mesoporous structure and induces a large number of C defects in the material.The optimal catalyst structure was screened out by comparing single and bimetal active sites,metal loadings,and the amounts of Zr doping.Under environmental conditions,N2-saturated 0.1 M KOH electrolyte and-0.1 V vs RHE potential,the 0.5%RuAu3/0.3Zr-C3N4 catalyst can achieve 5.28 μg h-1 mgcat.-1 NH3 yield and 11.54%Faradaic efficiency,which is nearly 10 times higher than that of RuAu3/C3N4.Finally,the relationship between structure-activity and N2 reduction selectivity was explored based on the regularity of NRR activity and catalyst characterization results.(2)Noble metal based catalysts have been proved to possess high reduction activities,however,these active sites are more prone to hydrogen evolution reaction(HER)than NRR.Thus,the high catalyst cost and low N2 reduction selectivity limit the development of efficient NRR catalysts.In this work,CH3I was used to iodide covalent triazine framework(CTF)to synthesize charge-modulated CTF-I structures,which was load with MoOx nanoparticles by ion exchange between non-noble metal precursors(MoO42-)and I-,and used as a catalyst for electrochemical N2 reduction at ambient temperature and pressure.TEM,SEM,STEM,XPS,XANES,EXAFS and other characterization methods showed that the nucleophilic reaction between CH3I and the pyridine nitrogen on CTF allowed I-ions to be grafted into the CTF microenvironment,protecting the CTF framework from collapsing by metal loading and high-temperature treatment.In addition,the MoO42-is directionally loaded on the CTF-I structure by ion exchange with the I-ions,and the metal-support interaction leads to a highly dispersed MoOx with a higher valence state(5+)in the form of nanoparticles.The optimal catalyst and reaction conditions were screened out by comparing different supports,metal loadings and potentials.5%MoOx/CTF-I showed a Faradaic efficiency of 27.3%and a NH3 yield of 7.23μg h-1 mgcat.-1 at-0.405 V vs RHE,and a high ammonia yield rate was highly repeatable after ten continuous electrolysis cycles without any obvious decay.Finally,the structure-activity relationship between the catalyst and NRR was further revealed by electrochemical impedance spectroscopy and double-layer capacitance characterization.(3)Metal supported catalysts have been widely investigated due to their unoccupied dorbitals,which can accept and back donate electrons to N2 to adsorb/activate ultrastable N≡N bonds.However,due to the scarcity of transition metal resources and their strong HER activity,nonmetallic materials are emerging as a more promising alternative for the NRR.Hightemperature calcination of a mixture of NaBH4,NaNH2,and MC under N2 atmosphere to synthesize boron carbon nitride(BCN)with high crystallinity and abundant unsaturated B/N sites and used as a catalyst for electrochemical N2 reduction at ambient conditions.TEM,SEM,STEM,TGA,XPS,CO2-TPD,NH3-TPD,EPR,Py-IR showed that the defect-rich BCN exposed a large number of unsaturated B(Lewis acid sites)and N(Lewis bases sites)atoms can form frustrated Lewis pair(FLPs),which can synergistically reduce N2 to NH3 with high efficiency.By comparing the NRR experiments with the combinations of the three elements and the element doping ratios,the optimal catalyst composition was explored.BCN-2 achieved a Faradaic efficiency of 18.9%and an NH3 yield of 20.9 μg h-1 mgcat.-1,and a high ammonia yield rate retention up to 90%was obtained after six continuous electrolysis cycles without any obvious decay.Finally,14N2/15N2 exchange experiments and density functional theory(DFT)calculations reveal that the interaction between the FLP structure and N2 in the catalyst can effectively activate the N≡N triple bond and reduce the energy barrier for the reduction of N2 to NH3.(4)For further research on non-metallic catalysts,boron carbon nitride/carbon(BCN/C)heterojunction composed of ultra-thin porous nanosheets were prepared under N2 high temperature calcination with NaBH4 and CTF as B,C and N sources,respectively.The prepared BCN/C was used for the electrochemical N2 reduction reaction under ambient conditions.TEM,SEM,STEM,XPS,EPR,XANES and other characterization methods showed that the BCN units spontaneously separated from amorphous C during catalyst formation,resulting in a heterojunction BCN/C catalyst with a homogeneous ternary BCN superstructure highly dispersed on the low-hybridized C support.The active hydrogen released from NaBH4 reacted with the nitrogen atom in CTF at a high temperature environment,thereby reducing the degree of hybridization of C atoms to improve the electron transport efficiency in the NRR process.By comparing the NRR experiments with different electrolytes and different element doping ratios,the effects of catalyst composition and electrolyte on the reaction activity were explored.In 0.1 M HCl electrolyte,the BCN/C-1 catalyst showed a Faradaic efficiency of 27.8%and a NH3 yield of 4.3μg h-1 mgcat.-1,and a high ammonia yield rate retention up to 90%was obtained after ten continuous electrolysis cycles.