Selective Electrocatalytic Denitrification By MoS₂

The development of denitrification catalysts which can reduce nitrate and nitrite to dinitrogen is critical for sustaining the nitrogen cycle.Electrochemical denitrification is considered as a green method,however,regulating the selectivity has proven to be a challenge for artificial denitrification,due to the difficulty of controlling complex multi-electron/proton reactions.Firstly?Chapter 2?,we investigate the possibility of selective nitrite reduction by bio-inspired oxo-molybdenum sulfide?oxo-MoS_x?catalyst under neutral conditions.It is demonstrated that utilizing sequential proton-electron transfer?SPET?pathways is a viable strategy to enhance the selectivity of electrochemical reactions.The selectivity of an oxo-MoS_x electrocatalyst towards nitrite reduction to nitrous oxide and dinitrogen exhibited a volcano-type pH dependence with a maximum at pH 5.The pH-dependent formation of the intermediate species?distorted Mo^V oxo species?with a pK_a of 5.5 was identified using operando electron paramagnetic resonance?EPR?and Raman spectroscopy.This was in accord with a mathematical model that the pK_a of the reaction intermediates determines the pH-dependence of the product,and the maximum reaction rate and Faradaic efficiency will be obtained when the pH is closed to the pK_a of the reaction intermediates.By utilizing this acute pH dependence and optimizing nitrite concentration,we achieved a Faradaic efficiency of 13.5%for nitrite reduction to dinitrogen,which is the highest value reported to date under neutral conditions.The utilization of SPET pathways will become an important concept for electrocatalyst design.Secondly?Chapter 3?,we probe the mechanism of SPET on the atomic scale.We select common transition metal dichalcogenide 1T and 2H MoS₂ as object of study.It is demonstrated that the phase transition from 2H to 1T causes the dramatic change of selectivity control towards catalytic reaction.The in situ Raman spectra,in situ EPR and electron-nuclear double resonance?ENDOR?spectra demonstrated that the protonation of S site of basal plane to form S-H bond with a distance of H to Mo 3.16?or its deprotonation generated distorted or isotropic species.Therefore,the dynamic structure change of 1T realized the selective N-N coupling from nitrite reduction by controlling the sequential electron and proton transfer.Meanwhile,the kinetic isotope effect?KIE?experiment resolved the rate-determining step of nitrite reduction catalyzed by metallic molybdenum sulfide at various pH.When pH was below 5.5,the charge transfer was the rate-determining step.However,when the pH was higher than 6,the proton transfer to form S-H was the rate-determining step.We also took the in situ spectra of 2H MoS₂and found that the 2H MoS₂ lacks the ability to induce the conformational changes to generate pH-dependent species and further confirming the effect of phase engineering of MoS₂ on the selectivity control of multi-electron/proton transfer reactions.Thirtly?Chapter 4?,we investigate how to regulate the SPET to enhance the selectivity of nitrite reduction.We synthesized molybdenum sulfide with different content of metallic phase?9%⁷6%?and pK_a values?5.5⁶.5?and the selectivity of different metallic molybdenum sulfide towards nitrite reduction to nitrous oxide exhibited a volcano-type pH dependence.The Faradaic efficiency and reaction rate of nitrous oxide reached maximum when the pH was closed to the pK_a of the each intermediate.Furthermore,the effect of overpotential on the formation of nitrous oxide was investigated under different intermediates with different p K_a values or under different pH conditions.It was shown that increasing the overpotential was benefit for the formation of nitrous oxide with maximum selectivity of 60%when increasing the pK_a of the catalyst or decreasing the solution pH.By optimizing the parameters of pH,pK_a,and Eh,we achieved the highest Faradaic efficiency of 35.5%for nitrite reduction to dinitrogen under neutral condition.Therefore,regulating the pH,potential?Eh?,and pK_a could control the transfer of proton and electron to enhance the selectivity of reactions possessing sequential proton and electron transfer.These results offered important principles of rational selection and design of catalyst for multi-electron/proton transfer reactions.Besides,it provided significant insight for the selective enzymatic reactions with different p K_a in nature.The results above clearly demonstrated that the metallic molybdenum sulfide had great potential in the realm of selective electrocatalytic denitrification.Finally?Chapter5?,we optimized the synthetic process of metallic molybdenum sulfide and explored the possibility of hydrothermal synthesis of molybdenum sulfide with high content metallic phase and crystallinity at elevated temperature.The chelating interaction of L-cysteine and MoO₃ was utilized to stabilize the octahedral coordination necessary for the 1T-MoS₂.The results demonstrated that 72%1T content was obtained at a hydrothermal synthesis temperature of 220°C when using L-cysteine.Notably,60%1T concentration could still be found even at 240°C,which is the highest purity at that temperature reported to date.As a proof-of-concept,the materials synthesized using L-cysteine at 220°C and 240°C exhibited excellent performance towards hydrogen evolution reaction?HER?with Tafel slope of 48 mV dec^-11 and 60 mV dec^-1,respectively,and are the best pure MoS₂ HER catalyst synthesized at high hydrothermal temperature reported to date.In addition,the synthesized 1T sample exhibited high catalytic stability after 3,000 cycles.Our phase-controlled bottom-up synthesis strategy paves the way for the preparation of high metallic content and crystalline 2D materials with high scalability.In summary,we demonstrated that the metallic phase molybdenum sulfide can be used for selective catalytic reduction with complex multielectron/proton transfer,and develop a synthetic strategy for metallic molybdenum sulfide under high hydrothermal temperature.Our work provides new insight into the preparation and application of transition metal dichalcogenides.