Barium Oxide Encapsulating Cobalt Nanoparticles Supported on Magnesium Oxide: Active Non-Noble Metal Catalysts for Ammonia Synthesis under Mild Reaction Conditions
Barium Oxide Encapsulating Cobalt Nanoparticles Supported on Magnesium Oxide: Active Non-Noble Metal Catalysts for Ammonia Synthesis under Mild Reaction Conditions
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氧化镁负载的氧化钡包封钴纳米粒子：温和反应条件下合成氨的活性非贵金属催化剂
DOI：
10.1021/acscatal.1c02887
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发表时间：
2021
期刊：
影响因子：
12.9
通讯作者：
Nagaoka Katsutoshi
中科院分区：
文献类型：
--
作者：
Sato Katsutoshi;Miyahara Shin-ichiro;Tsujimaru Kotoko;Wada Yuichiro;Toriyama Takaaki;Yamamoto Tomokazu;Matsumura Syo;Inazu Koji;Mohri Hirono;Iwasa Takeshi;Taketsugu Tetsuya;Nagaoka Katsutoshi
为了实现无碳社会，需要在温和的反应条件下（<400 °C，<10 MPa）使用可再生能源生产的氢气合成氨的催化剂。Ru基催化剂是目前最有前途的候选者;然而，Ru昂贵且丰度低。在这里，我们发现用BaO包覆Co纳米颗粒增强了Co的氨合成活性，并且在700 °C的异常高温下预还原的简单Ba掺杂的Co/MgO催化剂（Co@BaO/MgO-700 red）显示出突出的氨合成活性。在350 °C和1.0MPa下，该催化剂的氨合成速率（24.6mmolgcat-1h-1）和转换频率（0.246s-1）分别比未掺杂的母体催化剂高80倍和250倍。在相同的温度和较高的压力下（3.0 MPa），氨合成速率提高到48.4 mmol gcat-1h-1，高于活性钌基催化剂。此外，在1.0 MPa下，我们的催化剂即使在低至150 °C的温度下也能产生氨。扫描透射电子显微镜和能量色散X射线光谱研究表明，在700 °C下还原后，Co纳米颗粒已被BaO的纳米级分包裹。这种独特结构的形成机制被认为是包括氧化的Co还原为金属Co，BaCO 3分解为BaO，以及BaO和Co纳米颗粒的迁移。光谱和密度泛函理论研究表明，N2在催化剂表面的Co原子上的吸附，由于Ba 2+通过邻近的Co原子向BaO提供电子，使N2三键减弱为双键强度;这种减弱加速了三键的断裂，这是氨合成的速率决定步骤。
To realize a carbon-free society, catalysts are needed for the synthesis of ammonia under mild reaction conditions (<400 °C, <10 MPa) that use hydrogen produced from renewable energy. Ru-based catalysts are currently the most promising candidates; however, Ru is expensive and of low abundance. Here, we discovered that the encapsulation of Co nanoparticles with BaO enhanced the ammonia synthesis activity of Co and that a simple Ba-doped Co/MgO catalyst prereduced at an unusually high temperature of 700 °C (Co@BaO/MgO-700red) showed outstanding ammonia synthesis activity. The ammonia synthesis rate (24.6 mmol gcat–1h–1) and turnover frequency (0.246 s–1) of the catalyst at 350 °C and 1.0 MPa were about 80 and 250 times higher, respectively, than those of the nondoped parent catalyst. At the same temperature but higher pressure (3.0 MPa), the ammonia synthesis rate was increased to 48.4 mmol gcat–1h–1, which is higher than that of active Ru-based catalysts. In addition, at 1.0 MPa, our catalyst produced ammonia even at temperatures as low as 150 °C. Scanning transmission electron microscopy and energy-dispersive X-ray spectroscopy investigations revealed that after reduction at 700 °C, the Co nanoparticles had become encapsulated by a nanofraction of BaO. The mechanism underlying the formation of this unique structure was considered to comprise reduction of oxidic Co to metallic Co, decomposition of BaCO3to BaO, and migration of BaO and Co nanoparticles. Spectroscopic and density functional theory investigations revealed that adsorption of N2on the Co atoms at the catalyst surface weakened the N2triple bond to the strength of a double bond due to electron donation from Ba2+of BaO via adjacent Co atoms; this weakening accelerated the cleavage of the triple bond, which is the rate-determining step for ammonia synthesis.