Guo Jianping, Chen Ping
Dalian National Laboratory for Clean Energy, Dalian Institute of Chemical Physics, Chinese Academy of Sciences, Dalian 116023, China.
University of Chinese Academy of Sciences, Beijing 100049, China.
Acc Chem Res. 2021 May 18;54(10):2434-2444. doi: 10.1021/acs.accounts.1c00076. Epub 2021 Apr 29.
ConspectusThe fixation of dinitrogen to ammonia is critically important for the biogeochemical cycle on earth. Ammonia also holds promise as a sustainable energy carrier. Tremendous effort has been devoted to the development of green processes and advanced materials for ammonia synthesis and decomposition under milder conditions, and encouraging progress has been made.The reduction of dinitrogen to ammonia needs electrons and protons, which hydridic hydrogen H could supply. Polarized, electron-rich NH intermediates, on the other hand, can be stabilized by alkali or alkaline earth metal cations to lower kinetic barriers in the transformation. The inherent properties of alkali/alkaline earth metal hydrides (denoted as AH) endow them with a unique function in ammonia synthesis.In this Account, recent efforts in the exploration of alkali or alkaline earth metal hydrides (denoted as AH), amides, and imides (denoted as ANH hereafter) for ammonia synthesis and decomposition reactions will be summarized and discussed. We begin with an introduction to the chemistry of A with N, NH, and H, highlighting the interconversion between AH and ANH that has profound implications on the formation and decomposition of NH. We then present our finding on the strong synergistic effect between ANH and transition metals (TM) in ammonia decomposition catalysis, which stimulated our subsequent research on AH for ammonia synthesis. We discuss the effect and function mechanism of AH in the thermocatalytic and chemical looping ammonia synthesis processes. In the thermocatalytic process, AH cooperates with both early and late TM forming either composite catalysts with two active centers or complex metal hydride catalysts with electron- and hydrogen-rich ionic centers facilitating ammonia synthesis with high activities at lower temperatures. Very interestingly, AH levels the catalytic performances of TMs and intervenes in the energy-scaling relations of TM-only catalysts. Moreover, ANH serves as a new type nitrogen carrier effectively mediating ammonia synthesis via a low-temperature chemical looping process, in which N is fixed by AH forming ANH. Subsequently, ANH is hydrogenated to ammonia and AH. Late TMs have a strong catalytic effect on the chemical looping process. The unique interplay of A, N, TM, and H offers plenty of opportunities for achieving dinitrogen conversion under mild conditions, while further efforts are needed to address the challenges in the fundamental understanding and practical application.
综述
将氮气固定为氨对地球上的生物地球化学循环至关重要。氨作为一种可持续的能量载体也具有潜力。人们已付出巨大努力来开发在更温和条件下进行氨合成和分解的绿色工艺及先进材料，并取得了令人鼓舞的进展。
将氮气还原为氨需要电子和质子，氢化氢H可以提供这些。另一方面，极化的、富电子的NH中间体可以通过碱金属或碱土金属阳离子来稳定，以降低转化过程中的动力学能垒。碱金属/碱土金属氢化物（表示为AH）的固有性质赋予它们在氨合成中独特的功能。
在本综述中，将总结和讨论最近在探索用于氨合成和分解反应的碱金属或碱土金属氢化物（表示为AH）、酰胺和亚胺（以下表示为ANH）方面所做的努力。我们首先介绍A与N、NH和H的化学性质，强调AH和ANH之间的相互转化对NH形成和分解的深远影响。然后我们展示了关于ANH与过渡金属（TM）在氨分解催化中强烈协同效应的发现，这激发了我们随后对用于氨合成的AH的研究。我们讨论了AH在热催化和化学链氨合成过程中的作用及作用机制。在热催化过程中，AH与早期和晚期TM协同作用，形成具有两个活性中心的复合催化剂或具有富电子和富氢离子中心的复合金属氢化物催化剂，从而在较低温度下以高活性促进氨合成。非常有趣的是，AH使TM的催化性能趋于平衡，并干预仅含TM催化剂的能量标度关系。此外，ANH作为一种新型氮载体，通过低温化学链过程有效地介导氨合成，其中N通过AH固定形成ANH。随后，ANH被氢化为氨和AH。晚期TM对化学链过程有很强的催化作用。A、N、TM和H之间独特的相互作用为在温和条件下实现氮气转化提供了大量机会，同时还需要进一步努力应对基础理解和实际应用方面的挑战。
