Preparation Of Non-noble Metal-based Catalysts And Their Nitrogen Reduction Performance

Ammonia is an important resource for human survival and development,which is the important raw material for the production of agricultural fertilizers and chemical products.The Haber-Bosch（H-B）process was used for the traditional industrial ammonia synthesis,which still contributes more than 90%of the ammonia production,but the nitrogen fixation reaction of H-B process at high temperature（350～550°C）and high pressure（150～350 atm）account for consumes 2%of global energy and emits nearly 2%of global CO₂ every year,causing it a non-green ammonia synthesis process with high equipment requirements.In recent years,synthesis of ammonia by electrocatalytic nitrogen reduction reaction（NRR）has been regarded as the most potential new strategy to replace the Haber-Bosch ammonia synthesis process due to its abundant raw materials,simple equipment,green environmental protection,and zero carbon emissions.However,the current widespread application of electrocatalytic NRR technology for ammonia synthesis faces the problems of low catalyst activity,poor ammonia selectivity,and low Faradaic efficiency（FE）.Therefore,the development of catalysts with high activity and high ammonia selectivity is the focus of current NRR research.Non-noble metals possess good catalytic performance,low cost and abundant reserves.In this study,Co and Bi were mainly used as active centers,and the performance of non-noble metal-based electrocatalysts can be further regulated by appropriately introducing two-dimensional MXene,to obtain high NRR activity and NH₃ selectivity non-precious metal catalysts.The specific research contents are as follows:（1）ZIF-67@MXene composites were synthesized by using two-dimensional Ti₃C₂MXene-supported Co-MOF（ZIF-67）.The introduction of 2D MXene can effectively solve the problem of poor conductivity of ZIF-67,and the synergistic effect of these two components makes it efficient to catalyze NRR synthesis of ammonia.The NRR test results showed that the highest NH₃ yield(110.8μg h^-1 cm^-2)and FE（20.2%）were obtained at the optimal potential of-0.4 V vs.RHE in 0.1 M KOH,superior to those of uncompounded ZIF-67 and Ti₃C₂ MXene catalyst.In addition,5 times repeated NRR experiments and 12 h continuous stability tests both prove that ZIF-67@MXene is a stable and efficient NRR electrocatalyst.（2）By subjecting the ZIF-67 precursor to high temperature pyrolysis,the synthesized Co@C material covers metallic Co nanoparticles with highly conductive carbon nanolayers,which provides abundant metal active sites and exhibits excellent electrocatalytic NRR activity under ambient conditions.The electrocatalytic performance results showed that the highest NH₃yield(79.2μg h^-1 cm^-2)and FE（11.5%）were obtained at the optimal potential（-0.4 V vs.RHE）.Moreover,the Co@C material also maintained good reproducibility,selectivity,and stability during the electrocatalytic NRR process.This work demonstrates the potential value of non-noble Co-based materials in the electrochemical synthesis of NH₃.（3）To further improve the NRR performance of the Co-based catalyst,Co@C was further composited with two-dimensional Ti₃C₂ MXene to prepare Co@C-MXene composites.The NRR test results showed that the highest NH₃ yield(22.44μg h^-1 cm^-2)and FE（31.57%）were obtained at the optimal relative low potential of-0.1 V vs.RHE.Moreover,the material exhibits excellent NRR reproducibility and electrochemical durability.The excellent electrical conductivity and fast electron transport were proved by DFT calculation,which is beneficial to improving its electrocatalytic NRR activity.Despite the unsatisfactory NH₃ yield exhibited by Co@C-MXene,its extremely low optimal reaction potential（-0.1 V）and excellent FE are of great significance for the development of a high NRR active catalyst,which is helpful to promote the research and development of catalytic NRR synthesis of NH₃.（4）Based on the N-philic and H-phobic properties of metallic Bi,it is expected to efficiently promote NRR and inhibit HER in electrochemical ammonia synthesis.Considering its poor electrical conductivity of Bi,highly conductive 2D Ti₃C₂ MXene was introduced,and Bi@MXene composites were synthesized by a liquid-phase reduction method.The test results show that the Bi@MXene catalyst can achieve a high NH₃ yield of 28.3μg h^-1 cm^-2,and a FE of 27.2%in 0.1 M KOH at-0.4 V vs.RHE,which is significantly better than most reported Bi-based catalysts.Meanwhile,5 times repeated experiments and 12 h continuous stability test proved that the Bi@MXene material had excellent NRR reproducibility and electrochemical durability.This work is expected to broaden the ideas for the application of non-precious metal Bi-based materials in the field of electrocatalytic NRR.DFT calculation proved that the performance of NRR can be significantly improved on the catalyst with excellent conductivity,strong adsorption and activation effect on N₂ and N₂H₄,as well as the rapid release of NH₃.