Research On Preparation And Application In Nitrate Reduction Of Cobalt Based Nanomaterials

Ammonia（NH₃）is an important component of synthetic chemicals,and also considered as one of the most potential energy carriers due to its high energy density and easy liquefaction.At present,NH₃ is mainly produced by Haber-Bosch process in industry,but the conditions of the process are extremely harsh.Meanwhile,a large amount of greenhouse gases emitted in the process threaten the environment.Therefore,it is urgent to find a green way of NH₃ production.As one of the nitrogen-contained pollutants,nitrate（NO₃^-）widely exists in groundwater and industrial wastewater.The nitrate reduction reaction（NITRR）driven by clean energy can directly convert NO₃^-to NH₃,but the NITRR process involves multi-proton and electron transfer,with multiple pathways,reaction intermediates and reduction products,causing the low electrochemical performances and selectivity.Therefore,rational design of efficient and stable electrocatalysts is of great significance.Cobalt is a promising NITRR catalyst due to its high conductivity,unique electronic structure,high catalytic activity and stable chemical properties.In this paper,three cobalt-based nanomaterials have been designed and synthesized,and their properties and mechanisms for NO₃^-reduction to NH₃ have been studied.The main research contents and conclusions are as follows:（1）CoO nanowire array grown on titanium mesh was synthesizes by hydrothermal method,and its phase and morphology were analyzed by material characterization.Electrochemical tests show that the Faraday efficiency of the CoO NWA/TM in alkaline electrolyte is up to 95.1%,and the ammonia yield is 6.8 mg h^-1 cm^-2.And the catalytic activity of the catalyst keeps stability in the 24 h stability test,showing excellent electrochemical stability.Density functional theory（DFT）calculation shows that the NO₃^-adsorption energy of CoO（110）is-2.12 e V,and the desorption energy of NH₃ is as low as 0.54 e V.（2）Combining CoO with carbon nanotubes to further improve the conductivity of CoO.Graphite paper（GP）loaded CoO nanoparticle decorated N-doped carbon nanotubes（CoO@NCNT/GP）was synthesized by electrodeposition method.The electrochemical test results show that the ammonia yield of CoO@NCNT/GP increases to 9.0 mg h^-1 cm^-2,and has excellent structural and electrochemical stability.The DFT calculation shows that the N-doped carbon nanotube can improve the conductivity and maintain the catalytic activity of CoO nanoparticles.（3）In order to investigate the effect of different valence Co oxides on catalytic activity,Co₃O₄ nanoparticles supported by porous carbon nanofibers（Co₃O₄@PCNF）were synthesized by electrospinning method.The carbon substrate effectively enhanced the conductivity of the catalyst and increased the dispersion of Co₃O₄ nanoparticles,thus enhancing the reducing activity of nitrate.In the alkaline electrolyte,the ammonia yield of Co₃O₄@PCNF is further increased to 23 mg h^-1 mg_cat^-1,and the Faraday efficiency is92.4%.The DFT calculation shows that the adsorption energy of Co₃O₄ clusters for NO₃^-is 2.72 e V and the free energy of the potential determination step is only 0.28 e V.