Study On Ammonia Synthesis Over Atomic Active Site Catalysts Under Mild Conditions

Ammonia is an important chemical product,which occupies an important position in agriculture and industrial production.At present,industrial ammonia synthesis is mainly carried out at high temperature（400～600°C）and high pressure（20～30MPa）through the Haber-Bosch process,which has high energy consumption and produces a large amount of carbon dioxide.If ammonia synthesis can be carried out under relatively mild conditions of atmospheric pressure and low temperature,it will be of great significance to the development of the ammonia industry.A suitable catalyst can reduce the activation energy of the ammonia synthesis reaction,accelerate the ammonia synthesis reaction rate,and make it possible to synthesize ammonia under mild conditions.In this paper,a nitrogen-doped carbon Ni/CN catalyst was prepared by the liquid phase one-pot method.In the experiment,the effects of different preparation conditions on the performance of the catalyst for ammonia synthesis were investigated.Subsequently,the effect of B doping on the performance of Ni/CN catalyst was further studied in the experiment.Combined with the characterization of the morphology,structure and surface chemical state of the catalyst,the structure-activity relationship of Ni/BCN is explained.The main research contents are as follows:Optimization of preparation conditions and structure-activity relationship about Ni/CN atomic-level active sites catalysts were explored.The catalyst prepared by liquid phase one-pot method with C/N of 2:1,supported metal of nickel,loading amount of0.8wt%and calcination temperature of 700°C has the maximum ammonia yield of 0.53μg·h^-1·mg_cat^-1 under one atmosphere pressure and 80°C mild conditions.The reasons for the synthesis of ammonia under mild conditions were discussed by characterizing the morphology,structure and surface chemical state of the catalyst.TEM,EDS,XRD indicate that Ni is highly dispersed on the surface of nitrogen-doped carbon support;Ni2p XPS spectra show that Ni mainly forms Ni-N coordination with N;Raman spectroscopy shows that there are many carbon defect sites in the catalyst,which is beneficial to N₂ activation.The possible reasons for the Ni-CN catalyst to synthesize ammonia under mild conditions are the larger specific surface area and pore volume of the catalyst,the fluffy and porous structure of the support,the atomic-level dispersion of Ni,the formation of local Ni-N coordination,and many defect sites for N₂ adsorption.The structure-activity relationship about the B-doped Ni/BCN atomic-level active sites catalysts were revealed.The mass percentage of element B in the catalyst is 2.3wt%,and the ammonia yield is 0.87μg·h^-1·mg_cat^-1 under one atmosphere pressure and80°C mild conditions.B-N,C-N,and B-C bonds appear in the FTIR spectrum,and B,C,and N form bonds with each other,forming a stable BCN structure.TEM,EDS and XRD characterizations indicate that Ni is dispersed at the atomic level on the surface of the BCN structure.The XPS Ni 2p spectra show that the Ni/BCN-X catalyst calcined at different temperatures retain the Ni-N coordination.The B 1s spectrum shows that the peak of the B-N bond increases with the increase of the calcination temperature,and the formation of the B-N bond contributes to the ammonia synthesis reaction.The improved performance of the Ni/BCN catalyst for ammonia synthesis after B modification is attributed to the uniform dispersion of metal Ni atoms on the catalyst surface and local Ni-N coordination,and also forms B-N bonds,which promotes the electron transfer on the BCN surface and facilitates the adsorption and activation of N₂.The EPR mechanism investigation found that the Ni-BCN catalyst showed a characteristic peak of nitrogen vacancies at g=2.002,indicating that the appearance of nitrogen vacancies was one of the reasons for the improvement of the performance of B-doped catalysts for ammonia synthesis under mild conditions.The peaks of NH intermediates and ammonia species appeared in the in-situ infrared spectrum,indicating the presence of NH intermediates in the ammonia synthesis reaction.