Research On Preparation Of Molybdenum-based Nanomaterials And Their Electrocatalytic Nitrogen Reduction Property

Ammonia(NH₃)is an important chemical raw material,which can be used in many fields such as fertilizers.For agriculture,NH₃ is the main component of nitrogen fertilizer,and nitrogen fertilizer can increase the yield of crops.NH₃ can also be used to produce a variety of chemicals,including explosives,plastics,synthetic fibers,resins and industrial refrigerants.Among a variety of new energy sources,NH₃ has been considered to be an attractive,clean and efficient energy source.Nowadays,the Haber-Bosch process is the main process for industrial production of NH₃.This process generates a large amount of greenhouse gases and easily causes environmental pollution.Therefore,it is highly necessary to search for a clean and sustainable NH₃ production process.Electrocatalytic nitrogen reduction(NRR)has been considered to be an efficient,clean and sustainable method to produce NH₃ under ambient conditions.However,the development of this electrochemical method has been greatly hindered in practice.It is difficult for N₂ molecules to break the N?N triple bond and the existence of the competing hydrogen evolution reaction(HER)leads to the inefficiency of the NRR process.The biggest challenge at present is how to develop a catalyst with high NRR activity under ambient conditions.Many experiments and theoretical studies have shown that Mo possesses a nitrogenase-like catalytic mechanism and a high catalytic hydrogenation ability,thus Mo-based nanomaterials are the potential NRR catalysts.This thesis mainly studied two Mo-based catalysts:MoO₂/RGO(reduced graphene oxide)and Fe Mo₃S₄.The morphology and structure of the synthesized catalysts were fully characterized by XRD,TEM,SEM and XPS.The electrochemical performance of the catalysts was tested by LSV,CV and chronoamperometry.Density functional theory(DFT)calculations were used to systematically study the NRR catalytic mechanism of MoO₂/RGO and Fe Mo₃S₄ catalysts.(1)Using ammonium molybdate and graphene oxide as precursors,MoO₂/RGO nanocomposites were synthesized by a microwave method.The experimental results showed that MoO₂ nanoparticles were evenly and densely loaded on the surface of RGO.The electrocatalytic performance test showed that the NH₃ yield of the MoO₂/RGO nanocomposite was 37.4?g h^-1 mg^-1,and the Faraday efficiency(FE)was 6.6%,which were both better than that of RGO and MoO₂ alone.MoO₂/RGO nanocomposites also had a good electrocatalytic stability.DFT calculations showed that,compared with MoO₂ alone,MoO₂/RGO nanocomposite had a stronger electronic interaction with*N₂H,and contributed more electrons from the active Mo site to*N₂H,thereby greatly reducing energy barrier for the rate-determining step,leading to the accelerated progress of the NRR reaction.(2)FeCl₃,Na₂MoO₄ and thioacetamide were used as precursors to synthesize Fe Mo₃S₄nanorods by a two-step hydrothermal method.The experimental results showed that Fe Mo₃S₄nanorods with high crystallinity were synthesized.The electrocatalytic performance test showed that the NH₃ yield of Fe Mo₃S₄ nanorods was 65.3?g h^-1 mg^-1,and the FE was 19.2%.Fe Mo₃S₄ nanorods also had both good cycle stability and electrocatalytic stability.DFT calculations showed that Fe_3c sites exposed on the surface of Fe Mo₃S₄ were the main active sites for N₂ adsorption,activation,and hydrogenation reactions,and could effectively prevent the occurrence of the HER side reaction and improve the conversion efficiency of N₂?NH₃.