Lv Xingshuai, Kou Liangzhi, Frauenheim Thomas
Shenzhen JL Computational Science and Applied Research Institute, 518110 Shenzhen, China.
Beijing Computational Science Research Center (CSRC), 100193 Beijing, China.
ACS Appl Mater Interfaces. 2021 Mar 31;13(12):14283-14290. doi: 10.1021/acsami.1c00871. Epub 2021 Mar 17.
MXenes, an emerging family of two-dimensional (2D) metal carbides and nitrides, have been demonstrated to be effective nitrogen reduction reaction (NRR) catalysts. So far, most of the theoretical studies toward NRR are based on bare MXenes; however, the structural stabilities are questionable. In this work, we studied the NRR process on several synthesized MXenes (TiC, VC, CrC, ZrC, NbC, MoC, HfC, and TaC) with hydroxyl (OH) termination since the structures are preferred under NRR operating conditions as per Pourbaix stability diagrams. It is found that OH plays an essential role in tuning the NRR chemistry, as a new surface-hydroxylation mechanism. Different from the widely accepted NRR mechanism where only protons are involved in the reaction, hydrogen (H) atoms from surface hydroxyl could be captured by the intermediate and participate into the NRR, while the remaining H vacancy can subsequently be self-repaired by the protons under the applied potential. The cooperative effect of surface hydroxylation can effectively boost the NRR, while MoC(OH) stands out with the most favorable limiting potential of -0.62 V and highest selectivity. Moreover, new scaling relationships based on the H vacancy energy are established, elucidating the possibility for structure-activity tuning. This study not only elaborates the essential role of surface OH functionalization in evaluating NRR performance but also affords new insights into advance sustainable NH production.
MXenes是一类新兴的二维金属碳化物和氮化物，已被证明是有效的氮还原反应（NRR）催化剂。到目前为止，大多数关于NRR的理论研究都是基于裸露的MXenes；然而，其结构稳定性存在疑问。在这项工作中，我们研究了几种合成的带有羟基（OH）端基的MXenes（TiC、VC、CrC、ZrC、NbC、MoC、HfC和TaC）上的NRR过程，因为根据Pourbaix稳定性图，这些结构在NRR操作条件下是更优的。研究发现，OH在调节NRR化学过程中起着至关重要的作用，这是一种新的表面羟基化机制。与广泛接受的仅质子参与反应的NRR机制不同，表面羟基中的氢（H）原子可以被中间体捕获并参与到NRR中，而剩余的H空位随后可以在施加的电势下被质子自我修复。表面羟基化的协同效应可以有效地促进NRR，而MoC(OH)表现突出，具有最有利的极限电势-0.62 V和最高的选择性。此外，基于H空位能量建立了新的标度关系，阐明了结构-活性调节的可能性。这项研究不仅阐述了表面OH官能化在评估NRR性能中的重要作用，还为推进可持续氨生产提供了新的见解。
