Study On Electrochemical Ammonia Synthesis Of Low Temperature Plasma Enhanced Copper-based Catalysts

As one of the largest chemicals in the world,ammonia is the basic raw material of various chemical industries.It is especially important for the production of nitrogen fertilizers such as ammonium nitrate and urea,as well as various nitrogen-containing compound fertilizers.At present,the Haber process based on the reaction of nitrogen and hydrogen is still the only ammonia synthesis technology on an industrial scale,but this reaction requires severe catalytic synthesis conditions of high temperature and high pressure.At the same time,hydrogen,as a raw material,relies on the reforming of fossil fuels,and emits a large amount of CO₂,which aggravates the global energy consumption and greenhouse effect,and does not conform to the sustainable development of“low-carbon economy”under the dual carbon background.Therefore,it is urgent to find a sustainable method for synthesizing NH₃ with low energy consumption and green development.Nitrate（NO₃^-）is a promising nitrogen source because the N=O bond has a relatively low dissociation energy and is abundantly present in nitrogen-contaminated soils from bacterial decomposition of fertilizers and industrially polluted wastewater.Selective reduction of NO₃^-to NH₃ with high added value through the electrochemical method compatible with new energy can not only provide a green and sustainable synthesis of NH₃ technology,but also alleviate the energy crisis and environmental pollution problems faced by the world.However,the efficient and selective conversion of NO₃^-to NH₃ still faces great challenges,especially the serious competitive hydrogen evolution reaction in its water reaction system.Therefore,this dissertation focuses on the research and development of catalytic materials with high activity,high selectivity and high stability,and studies the performance and mechanism of its reduction of NO₃^-to NH₃.The main contents of this dissertation are as follows:（1）Two new copper-based bimetallic catalysts were prepared by electrodeposition,and the prepared catalysts were characterized by SEM,TEM,XRD,XPS and other means.The experimental results confirm that the introduced cobalt and nickel atoms can be well dispersed on the catalyst surface,which can adjust the microstructure of the electrocatalyst and enhance its catalytic performance;（2）The nitrate conversion to ammonia synthesis reaction of two copper-based bimetallic catalysts with different ratios was studied,and the reactivity of Cu/Co and Cu/Ni interfaces was analyzed.The experimental results demonstrate that the ammonia synthesis performance of any bimetallic catalyst is better than that of the single metal catalyst,and the doping of heteroatoms can improve the catalytic performance of the electrocatalyst.Under the condition of 1 M KOH+0.1 M KNO₃（-0.27V vs.RHE）,the NH₃ yield rate of 2732.4μg cm^-2 h^-1 and faradaic efficiency of 77.6%was achieved using Cu₃₀Co₇₀catalyst,while the NH₃ yield rate of 2725.1μg cm^-2 h^-1 and faradaic efficiency of 77.2%was achieved using the Cu₅₀Ni₅₀ catalyst;（3）The modification effect of different types of low temperature plasma on carbon paper substrates was studied,and it was found that the modification effect of argon low temperature plasma was the best.The experimental results show that the carbon substrate modified by argon plasma changes from hydrophobicity to hydrophilicity,and the electrochemically active area of the catalyst increases greatly.The Cu₃₀Co₇₀catalyst increased from 6.07 m F cm^-2 to 14.7 m F cm^-2,while the current density increased from 56 m A cm^-2 to 114 m A cm^-2（at-0.4V vs.RHE）.It is demonstrated that Ar-plasma modification could boost the activity of the electrocatalyst by enhancing the adhesion force between catalyst and carbon substrate,the highest NH₃ yield rate of 4180.7μg cm^-2 h^-1 and faradaic efficiency of 78.1%was achieved at-0.27 V（vs.RHE）using the Ar-plasma modified Cu₃₀Co₇₀ electrocatalyst;（4）In situ infrared spectrum characterization coupled with DFT calculation was used to explore the mechanism of catalyst with high catalytic performance,and to reveal the reaction path of NO₃^-deoxygenation and hydrogenation to synthesize NH₃in an alkaline environment.The DFT revealed that the activity of Cu₃₀Cu₇₀ was due to the lowest energy difference（0.775e V）in the rate-determining step and higher partial density of states,while the introduction of Co atoms is beneficial to optimize the desorption of NH₃ and to further improve the catalytic performance of the catalyst.