尖晶石钴氧化物(Co3O4)已成为一类有前途的催化剂,用于电化学硝酸盐还原反应(eNO3RR)到氨,提供低成本等优点,高活性,和选择性。然而,晶体学方面在确定催化剂性能方面的具体作用仍然难以捉摸,阻碍了高效催化剂的发展。在这项研究中,我们已经合成了各种Co3O4纳米结构,其暴露面{100},{111},{110},和{112},旨在研究eNO3RR活性对晶体学方面的依赖性。在测试的催化剂中,Co3O4{111}显示出最佳性能,在-0.6V与RHE下,氨的法拉第效率为99.1±1.8%,产率为35.2±0.6mgh-1cm-2。实验和理论结果揭示了一个转变过程,其中活性相从Co3O4演变为具有氧空位(Ov)的Co3O4-x,其次是Co3O4-x-Ov/Co(OH)2杂化物,最后是Co(OH)2。这个过程在所有方面都被观察到,但Ov和Co(OH)2的形成在(111)面上最为迅速。Ov的存在将*NH2中间体形成的自由能从1.81降至-0.53eV,密集重建的Co(OH)2上的大量活性位点使Co3O4{111}成为通过eNO3RR合成氨的理想催化剂。这项工作提供了对现实活动组件的理解的见解,通过调整暴露的方面,为开发用于氨合成的高效Co基尖晶石催化剂提供了一种策略,并有助于进一步推进eNO3RR领域催化剂的设计和优化。
Spinel cobalt oxides (Co3O4) have emerged as a promising class of catalysts for the electrochemical nitrate reduction reaction (eNO3RR) to ammonia, offering advantages such as low cost, high activity, and selectivity. However, the specific role of crystallographic facets in determining the catalysts\' performance remains elusive, impeding the development of efficient catalysts. In this study, we have synthesized various Co3O4 nanostructures with exposed facets of {100}, {111}, {110}, and {112}, aiming to investigate the dependence of the eNO3RR activity on the crystallographic facets. Among the catalysts tested, Co3O4 {111} shows the best performance, achieving an ammonia Faradaic efficiency of 99.1 ± 1.8% with a yield rate of 35.2 ± 0.6 mg h-1 cm-2 at -0.6 V vs RHE. Experimental and theoretical results reveal a transformation process in which the active phases evolve from Co3O4 to Co3O4-x with oxygen vacancy (Ov), followed by a Co3O4-x-Ov/Co(OH)2 hybrid, and finally Co(OH)2. This process is observed for all facets, but the formation of Ov and Co(OH)2 is the most rapid on the (111) surface. The presence of Ov significantly reduces the free energy of the *NH2 intermediate formation from 1.81 to -0.53 eV, and plentiful active sites on the densely reconstructed Co(OH)2 make Co3O4 {111} an ideal catalyst for ammonia synthesis via eNO3RR. This work provides insights into the understanding of the realistic active components, offers a strategy for developing highly efficient Co-based spinel catalysts for ammonia synthesis through tuning the exposed facets, and helps further advance the design and optimization of catalysts in the field of eNO3RR.