Operando spectroscopic and isotopic-label-directed observation of LaN-promoted Ru/ZrH2 catalyst for ammonia synthesis via associative and chemical looping route
Operando spectroscopic and isotopic-label-directed observation of LaN-promoted Ru/ZrH2 catalyst for ammonia synthesis via associative and chemical looping route
复制标题
DOI：
10.1016/j.jcat.2020.05.039
复制
发表时间：
2020-09-01
影响因子：
7.3
通讯作者：
Jiang, Lilong
中科院分区：
文献类型：
--
作者：
Li, Lingling;Zhang, Tianhua;Jiang, Lilong
使用铁系催化剂的Haber-Bosch法合成氨需要苛刻的反应条件。因此，开发工业用低温合成氨催化剂具有重要意义。在此，我们首次报道了在Lan和Ru/ZrH_2之间的协同作用下，NH3可以通过双路径机制合成。该催化剂在350℃、1 Mpa下表现出极高的NH3合成速率(高达305 mmol(NH3)g(Ru)(-1)h(-1))和显著的耐久性(200h)。等温表面反应和N-15(2)和D-2同位素标记实验的结果表明，N-3与Hion反应生成NH3，留下N和H空位。在NH3合成气氛下补充N和H可以恢复Ru/xLaN/ZrH2的初始状态。此外，根据N-2反应级数、氮K边NEXAFS、原位XPS以及25%N-2-75%D-2气氛下的原位漂移分析的结果，推测在25%N-2-75%D-2气氛下，在LAN促进的Ru/ZrH_2催化剂上，N-2不发生直接解离，而N-2加氢在温和的条件下通过缔合途径进行。对于N-2的加氢反应，适量的LAN会在Ru活性中心上产生协同效应，使N-2更容易被活化和氢化为*N2H2。然而，过量的LAN会导致Ru位的封闭，从而阻碍*N2H2的形成，降低催化活性。通过缔合和化学环路，助剂促进的Ru/ZrH2催化剂绕过了N-2直接解离的瓶颈，使在温和条件下合成NH3成为可能。(C)2020 Elsevier Inc.保留所有权利。
The synthesis of ammonia via the Haber-Bosch process using Fe-derived catalysts requires harsh reaction conditions. It is hence meaningful to develop catalysts for low-temperature synthesis of ammonia for industrial application. Herein for the first time, we report that with the synergy between LaN and Ru/ZrH2, NH3 can be synthesized via a dual-path mechanism. The LaN-promoted Ru/ZrH2 catalyst shows exceptionally high NH3 synthesis rate (up to 305 mmol(NH3) g(Ru)(-1) h(-1)) at 350 degrees C under 1 MPa and remarkable durability (tested for 200 h). The outcomes of isothermal surface reaction and a suite of N-15(2) and D-2 isotopic labeling experiments reveal that the N-3 of LaN reacts with Hions to produce NH3, leaving behind N and H vacancies. The initial state of Ru/xLaN/ZrH2 can be restored by having the N and H entities replenished under the atmosphere adopted for NH3 synthesis. Moreover, based on the results of N-2 reaction order, nitrogen K-edge NEXAFS, in situ XPS as well as in situ DRIFTS analyses under a 25%N-2-75% D-2 atmosphere, it is reckoned that the direct dissociation of N-2 does not occur on the LaN-promoted Ru/ZrH2 catalyst while N-2 hydrogenation takes place via an associative pathway under mild conditions. For the hydrogenation of N-2, an appropriate amount of LaN would induce a synergic effect on the Ru active sites, leading to facile activation and hydrogenation of N-2 to *N2H2. Nonetheless, an excess amount of LaN would result in blocking of Ru sites, consequently hindering the formation of *N2H2 and decreasing the catalytic activity. Following the associative and chemical looping pathways, the LaN-promoted Ru/ZrH2 catalyst bypasses the bottleneck of N-2 direct dissociation, making the synthesis of NH3 at mild conditions possible. (c) 2020 Elsevier Inc. All rights reserved.