多Nb原子掺杂氮化碳与H离子协同作用促进氮高效还原

doi:10.1016/S1872-2067(21)63950-5

摘要/Abstract

摘要：

电化学合成氨因其可以低能耗产氨而备受关注. 目前, 迫切需要一种稳定、无污染、活性好和选择性高的催化剂来促进电化学合成氨. 石墨相氮化碳由于制备简单且具有较好的物理化学性质, 是一种有广阔应用前景的基底材料. 研究表明, 纯石墨相氮化碳不具有电化学合成氨的性能, 因此需要对其进行改性. 元素掺杂是一种常见的改性方式, 其中过渡金属元素因其具有活跃的d电子而被广泛用于掺杂改性. 目前实验上已经可以制备过渡金属单原子或者多原子掺杂的石墨相氮化碳基材料. 尽管在实验以及理论上都表明通过电催化可以实现还原氮气合成氨且具有较好的催化合成氨性能, 但是人们对该反应机理的理解还不够深入, 并且多数研究没有考虑到溶液中最常见的H离子对活性的影响. 因此, 本文以Nb原子掺杂的石墨相氮化碳为基础, 对多活性位点下H离子助力氮还原反应进行研究, 以揭示其内在机理.
本文研究所采用的密度泛函理论计算软件为VASP. 通过构建石墨相氮化碳的单层结构, 计算了中心空穴位置吸附1‒5个Nb原子以及不同Nb原子吸附结构的稳定性, 并在较稳定的模型上探索其氮气还原制备氨(NRR)的催化性能. 结果表明, 当金属原子数增加时, 活性位点附近可供吸附的空间变大, 导致反应过程中H原子更容易被金属原子捕获, 而非直接与N结合形成NH_x中间体, 从而加快了反应进程. 以Nb₃@g-C₃N₄结构为例, 详细探讨了额外的H原子吸附对整个NRR反应的影响. 结果表明, H的吸附可以把反应能垒较高的*NH₂→*NH₃步骤转变为低能垒的两步进行, 即*NH₂ + H → *NH₂ + *H和*NH₂ + *H + H → *NH₃ + *H, 从而降低反应需要的能量. 另一个有意义的发现是, 由于高的产氢能垒, 额外的H原子在此过程中不会形成H₂, 且在上述多步骤反应结束后, 又与N₂结合形成*NNH中间体, 从而持续进行生成氨反应. 同样, 在Zr₃@g-C₃N₄、Mo₃@g-C₃N₄结构中进行了验证. 通过对*NH₂ + *H中间体态密度等电子结构性质进行分析发现, 吸附在金属原子上的H原子可以有效地削弱金属原子对*NH₂中间体的吸附强度, 从而使其更容易进一步氢化. 上述研究结果表明, 在催化过程中H原子可以在催化剂的表面起到助催化作用.
本文通过密度泛函理论计算, 对石墨相氮化碳负载的多金属Nb原子的NRR催化过程进行了系统的研究, 阐明了H离子影响NRR过程的机理, 并且发现Nb₃@g-C₃N₄体系是一个催化性能良好且具有高选择性、低反应过电位的催化剂材料. 同时, 上述机理在Zr和Mo的体系也进一步得以验证.

关键词: 催化, N₂固定, 氮还原反应, H离子吸附, 选择性

Abstract:

Nowadays catalytic nitrogen reduction reaction (NRR) by electrochemistry has attracted much attention because of its key role in producing the basic chemical product ammonia with low energy consumption. A stable and environmentally-friendly single- or multi-atom catalyst with good performance in activity and selectivity is highly desired for NRR. From density functional theory calculations, the NRR mechanisms catalyzed by Nb monomer, dimer, trimer and tetramer anchored on graphitic carbon nitride (Nb_x@g-C₃N₄, x = 1, 2, 3, 4) have been deeply explored. It has been found that Nb₃@g-C₃N₄ exhibits the best catalytic ability among the four catalysts with the introduction of H⁺. A more stable intermediate (*NH₂+*H) can be found to reduce the huge free energy barrier of forming *NH₃ from *NH₂ directly in a multi-atom system. By analyzing the density of states and projected crystal orbital Hamilton population, a synergistic effect among Nb atoms and the adsorbed H⁺is responsible for reducing the overpotential of NRR. Furthermore, the competitive hydrogen evolution reaction is suppressed effectively. This work introduces a new insight in the reaction pathway in multi-atoms for developing high-efficiency NRR catalysts.

Key words: Catalysis, N₂ fixation, Nitrogen reaction, H⁺absorption, Selectivity

杨绍康, 张超楠, 饶德伟, 颜晓红. 多Nb原子掺杂氮化碳与H离子协同作用促进氮高效还原[J]. 催化学报, 2022, 43(4): 1139-1147.

Shaokang Yang, Chaonan Zhang, Dewei Rao, Xiaohong Yan. Synergistic interaction of Nb atoms anchored on g-C₃N₄ and H⁺promoting high-efficiency nitrogen reduction reaction[J]. Chinese Journal of Catalysis, 2022, 43(4): 1139-1147.

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链接本文: https://www.cjcatal.com/CN/10.1016/S1872-2067(21)63950-5

https://www.cjcatal.com/CN/Y2022/V43/I4/1139

图/表 5

Fig. 1. Optimized structure of Nb1@g-C3N4 (a), Nb2@g-C3N4 (b), Nb3@g-C3N4 (c), and Nb4@g-C3N4 (d); The computed PDOS of Nb1@g-C3N4 (e), Nb2@g-C3N4 (f), Nb3@g-C3N4 (g) and Nb4@g-C3N4 (h); The energy profiles of the formation and dissociation (i) and the AIMD result (j) at 500 K for Nb3@g-C3N4. The Fermi level was shifted to zero. The brown, silver and green balls represent C, N and Nb atoms, respectively.

Fig. 2. The computed charge density difference and projected density of states of Nb1@g-C3N4 (a), Nb2@g-C3N4 (b), Nb3@g-C3N4 (c) and Nb4@g-C3N4 (d) after N2 adsorbed on the surface while the bonding and antibonding states in COHP depicted by up and down in blue shaded areas. The contributions by the orbitals of N2 and the d orbitals of Nb atom are shown in blue and green, respectively. The brown, silver and green balls denote C, N and Nb atoms, respectively. The isosurface is set to 0.004 e/Å3and the red area represents the accumulation of electrons, the green area represents the loss of electrons.

Fig. 3. Gibbs free energy diagram and the reaction intermediates for NRR through the mixed mechanism on Nb3@g-C3N4. The brown, silver, green and white balls denote C, N, Nb and H atoms, respectively.

Fig. 4. (a) The structure of *NH2 + *H (up) and *NH2 (down); (b) the PDOS of *NH2 + *H (up) and *NH2 (down) species in the system of Nb3@g-C3N4. The green area represents the contribution of Nb atoms. The blue, sky blue, yellow and pink lines indicate the orbitals of N, H1, H2 and H3, respectively. (c,d) The COHP between two metal atoms (NbI, NbII) and the N in *NH2 species in the system of Nb3@g-C3N4; (e,f) The COHP between the two metal atoms (NbI, NbII) and the N in *NH2 + *H species in the system of Nb3@g-C3N4; (g) The correlation between the ICOHP of Nb?N and ΔE(*NH2).

Fig. 5. (a) Gibbs free energy diagrams of HER on Nb1@g-C3N4, Nb2@g-C3N4, Nb3@g-C3N4 and Nb4@g-C3N4; (b) Compare the selectivity of NRR versus HER.

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