氨作为一种无碳能源载体，工业上采用Harbor-Bosch法合成，该法环保性较差。电化学合成氨是一种绿色环保、低能耗的方法，其关键在于提高电催化剂的选择性和抑制析氢性能。本文基于沸石咪唑酯骨架材料(ZIFs)，结合不同合成手段，开发了系列元素掺杂的限域碳基催化剂，并对催化剂的结构，形貌及其氮还原催化性能进行了研究，并对催化剂在碱性条件下的氮还原机理进行了探讨。具体内容如下：

       (1) 以ZIF-67为前驱体，采用高温热解法制备了钴-氮共掺杂的限域碳基催化剂。研究表明，ZIF-67拓扑结构在高温热解后得以保留，热解温度的改变可以调节催化剂中钴和氮的含量，进而调控催化剂活性。当热解温度为600℃时，催化剂中含有的氮还原催化活性位点最多，在0.10 M KOH碱性电解质中于-0.1 V 相对于可逆氢电极( vs. RHE)下，产氨速率为60.57 μg·h^-1·mg_cat.^-1，法拉第效率为23.30%。¹⁵N₂同位素标记实验表明得到产物氨的氮源来自于所提供的N₂。

      (2) 针对直接裂解ZIF-67获得的催化剂活性位点直接暴露，容易因腐蚀而影响催化剂的寿命问题，我们以ZIF-67和ZIF-8做复合前驱体，与三聚氰胺共裂解合成了氮掺杂碳纳米管包覆钴催化剂。研究表明，在0.10 M KOH碱性电解质中于-0.2 V ( vs. RHE)时产氨速率为59.11 µg·h^-1·mg_cat.^-1，而-0.1 V ( vs. RHE)时的法拉第效率达78.15%；持续催化合成氨10 h，产氨速率和法拉第效率保持稳定。催化剂中存在的Co-N和C-N键是氮还原催化反应的主要活性位点，起到与反应物充分接触和降低N₂还原活化能的作用；包覆结构有利于提高催化剂的稳定性。

      (3) 受天然固氮酶的启发，以ZIF-8为前驱体经高温碳化分别制备了硫-氮共掺杂和铁-氮共掺杂的碳基限域催化剂。1100℃制备的硫-氮共掺杂催化剂呈现高度无序的三维多孔结构，在0.10 M KOH电解质中于-0.4 V ( vs. RHE)时具有最佳的产氨速率和法拉第效率，分别为27.78 μg·h^-1·mg_cat.^-1和22.74%。铁-氮共掺杂的碳基催化剂在电解电位为-0.6 V( vs. RHE)时，产氨速率最大为40.86 μg·h^-1·mg_cat.^-1，电位移至-0.2 V ( vs. RHE)时，法拉第效率可达87.41%。催化剂表面积为696.54 m²·g^-1，孔容为0.48 cm³·g^-1，从而提升了电解质传质速率和气体扩散性能，Fe-N₄与C-N键的存在则为催化剂提供了更多的活性位点，从而有利于电催化氮还原反应 ( NRR )。

       本文设计制备了系列ZIFs衍生元素掺杂的限域碳基催化剂，并在碱性条件下对催化剂的氮还原催化性能进行了研究。掺杂后的催化剂具有丰富的活性位点和结构缺陷，为提高催化剂的氮还原性能提供了可能，同时将为开发具有高稳定性和高催化活性的N₂还原合成NH₃催化剂提供新的研究思路。

      Ammonia, as a carbon-free energy carrier, is traditionally synthesized by the Harbor-Bosch method, which is environmentally unfriendly. The electrochemical synthesis of ammonia is gaining attention because it offers the potential for a more sustainable and energy-efficient route to ammonia production. The key challenges in achieving efficient electrochemical ammonia synthesis are improving the selectivity of the electrocatalyst for the desired nitrogen reduction reaction (NRR), and inhibiting the hydrogen evolution which can reduce the overall efficiency of ammonia production. In this paper, a series of element-doped carbon-based confinement catalysts based on zeolitic imidazolate frameworks (ZIFs) were developed via various synthesis methods. The structure, morphology, and performance of the catalysts for the N₂-to-NH₃ conversion were investigated, as well as the mechanism of nitrogen reduction of the catalysts under alkaline conditions. The specific contents are as follows:

      (1) Cobalt-nitrogen codoped carbon-based catalysts were prepared by high temperature carbonization of ZIF-67. The results show that the catalysts preserve the topology of ZIF-67. The catalytic activity could be tuned by altering the carbonization temperature to adjust the cobalt and nitrogen contents in the catalyst. At a carbonization temperature of 600°C, the obtained catalysts contain the most catalytically active sites for nitrogen reduction, with an NH₃ yield rate of about 60.57 μg·h^-1·mg_cat.^-1 and a Faradaic efficiency of 23.30% at -0.1 V (vs. RHE) in 0.10 M KOH electrolyte. All ammonia production remain relatively stable, which suggests the excellent stability of the as-prepared catalysts. ¹⁵N₂ isotope labeling experiments indicated that the nitrogen source for the produced ammonia is from the supplied N₂ but not catalysts.

      (2) To address the problems that the active sites of the catalysts obtained by direct pyrolysis of ZIF-67 are exposed and the lifetime of the catalysts is easily affected due to corrosion, the catalysts with coating structure were prepared. The nitrogen doped carbon nanotube coated cobalt catalysts were synthesized by co-pyrolysis ZIF-67 and ZIF-8 complex precursors with melamine. Under the ambient condition, the catalysts can achieve an ammonia yield rate of about 59.11 μg·h^-1·mg_cat.^-1 at -0.2 V ( vs. RHE) and a Faradaic efficiency of 78.15% at -0.1 V ( vs. RHE) in 0.10 M KOH electrolyte. These two parameters remain stable under the continuous use of catalytic nitrogen reduction for 10 h. The Co-N and C-N bonds present in the catalyst may be the main active sites in the nitrogen reduction reaction, which play the roles of fully contacting with the reactants and lowering the activation energy of N₂ reduction. Also, the coating structure is beneficial for the improvement of the stability of the catalyst.

      (3) Inspired by natural nitrogen-fixing enzymes, carbon-based catalysts codoped with sulfur-nitrogen and iron-nitrogen were prepared by pyrolysis of ZIF-8. The sulfur-nitrogen codoped carbon-based catalysts were prepared at 1100°C, which exhibits a highly disordered three-dimensional porous structure and has the optimum ammonia yield rate of 27.78 μg·h^-1·mg_cat.^-1 and Faradaic efficiency of 22.74% at -0.4 V ( vs. RHE) in 0.10 M KOH electrolyte, respectively. The iron-nitrogen codoped carbon-based catalysts prepared at 900°C exhibit the maximum ammonia production rate of 40.86 μg·h^-1·mg_cat.^-1 at -0.6 V (vs. RHE) and the Faradaic efficiency of 87.41% at -0.2 V (vs. RHE). The surface area of 696.54 m²·g^-1 and the pore volume of 0.48 cm³·g^-1 provide great convenience for electrolyte mass transfer and gas diffusion, while the presence of Fe-N₄ and C-N bonds provides more active sites for the catalyst, thus favoring the NRR reaction.

      In this work, a series of elements doped carbon-based catalysts derived from ZIFs were prepared and the catalytic performance of the catalysts for nitrogen reduction under alkaline conditions was investigated. The as-prepared catalysts have abundant active sites and structural defects, which offer the possibility of improving the performance of NRR. In addition, this will provide new research ideas for the development of catalysts for the synthesis of NH₃ by N₂ reduction with high stability and high catalytic activity.

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