Synthesis Of Transition Metal Phosphides For Efficient Electrocatalytic Nitric Oxide Reduction

Nitric oxide（NO）is a major nitrogen-containing pollutant and greenhouse gas.Human activities have released a large amount of toxic and harmful substances into the atmosphere,which seriously disrupts the balance of ecological nitrogen cycle,leading to a series of environmental problems and endangering public health.Currently,selective catalytic reduction（SCR）using reductants is the most widely used technology for NO emission control in industry.However,the operation cost of SCR process is high and secondary pollution exists.Many SCR catalysts are susceptible to poisoning under operating conditions and exhibit good activity only at high temperatures.Electrochemical reduction of NO is a green and sustainable NO reduction technology under normal temperature and pressure,using water as a hydrogen source to produce a series of harmless or value-added nitrogen products.Among them,ammonia（NH₃）is the basic commodity that supports the global population and modern industry.However,the conversion reaction of NO reduction to NH₃ requires efficient catalysts to facilitate this process.Transition metal phosphides（TMPs）are efficient hydroprocessing catalysts with excellent electronic conductivity.Therefore,a series of TMPs were synthesized and investigated for electrocatalytic nitric oxide reduction（NORR）in this thesis.The main research contents are as follows:1.The properties of different phosphide nanoarrays as electrocatalysts for NO hydrogenation were investigated.Ni precursors were synthesized on carbon paper by hydrothermal method and Ni₂P nanosheet arrays were prepared by low-temperature topotactic phosphorization.The NORR activity of the free-standing electrode and the Faradaic efficiency（FE）of NH₃ synthesis was monitored by electrochemical tests.The results show that the FE of Ni₂P catalyst is 76.9%at the potential of-0.2 V vs.RHE in0.1 M HCl electrolyte,and the yield of NH₃ is 33.5μmol h^-1 cm^-2.The power density of1.53 m W cm^-2 and NH₃ yield of 62.05μg h^-1 mg^-1_cat.can be obtained by discharge tests using carbon paper loaded Ni₂P as the cathode of Zn-NO battery,which is better than many reported metal-N₂ cell systems.Density functional theory（DFT）calculations reveal that the Ni atoms form triple hollow sites on the Ni₂P（111）crystal plane,which provides a good coordination environment for NO molecules,and effectively promotes the adsorption and activation of NO through the"acceptance-donation"mechanism.2.Since Co has higher catalytic activity of denitration than Ni,the hydrothermally prepared Co precursor was phosphate into Co P nanoneedle arrays,and the dense and uniformly distributed Co P nanoneedle arrays on the metallic titanium mesh.In 0.2 M Na₂SO₄ electrolyte,the FE of Co P nanoarrays is 88.3%and NH₃ yield is 47.2μmol h^-1cm^-2 at the potential of-0.2 V vs.RHE.The maximum yield of NH₃ is 284.99μg h^-1mg^-1_cat.,and the peak power density of 0.496 m W cm^-2 is achieved by using titanium mesh-loaded Co P as the cathode of the Zn-NO battery.DFT calculation showed that there was an optimal reaction path for Co P（111）crystal plane.NO molecules start from the distal routes and then go through alternating paths until the N atoms obtain enough protons to turn into NH₃ to leave the catalyst surface,and then the final reaction continues the enzymatic pathway until the end of the reaction.