nature ARTICLES chemistry PUBLISHED ONLINE 21 OCTOBER 2012 I DOl:10.1038/NCHEM.1476 Ammonia synthesis using a stable electride as an electron donor and reversible hydrogen store Masaaki Kitano',Yasunori Inoue',Youhei Yamazaki',Fumitaka Hayashi2,Shinji Kanbara, Satoru Matsuishi,Toshiharu Yokoyama,Sung-Wng Kim2,Michikazu Hara*and Hideo Hosono alyst for i Highly efficient am oni orted Ru py reve of hydrogen adatoms can be suppressed effectively because of the ability of C12A7:e to store hydrogen reversibly. thes th be attributed solel which is u hough industria As出 he hah ed on the thermic (k]mol-)(ref.3). uobfor industrial ammonia pro 611,20.2 o)T e bond h-pr a e N results in the cleavage of ters is therefore key to ing the e caaytic activity of ma s be for a ting materia ture The ed am nia forms nd al ork structu in fou then the performance of these n6 。1 u-adceo industrial ngwo wit山 cavities counte represented by [(e)(ref.23). ATURE CHEMISTRY I ADVANCE ONLINE PUBUCATION I ww 2012 Macmillan Publishers Umited.All richts reservedAmmonia synthesis using a stable electride as an electron donor and reversible hydrogen store Masaaki Kitano1 , Yasunori Inoue1 , Youhei Yamazaki1 , Fumitaka Hayashi2, Shinji Kanbara1 , Satoru Matsuishi2, Toshiharu Yokoyama2, Sung-Wng Kim2†, Michikazu Hara1 * and Hideo Hosono1,2* Industrially, the artificial fixation of atmospheric nitrogen to ammonia is carried out using the Haber–Bosch process, but this process requires high temperatures and pressures, and consumes more than 1% of the world’s power production. Therefore the search is on for a more environmentally benign process that occurs under milder conditions. Here, we report that a Ru-loaded electride [Ca24Al28O64] 41(e2)4 (Ru/C12A7:e2), which has high electron-donating power and chemical stability, works as an efficient catalyst for ammonia synthesis. Highly efficient ammonia synthesis is achieved with a catalytic activity that is an order of magnitude greater than those of other previously reported Ru-loaded catalysts and with almost half the reaction activation energy. Kinetic analysis with infrared spectroscopy reveals that C12A7:e2 markedly enhances N2 dissociation on Ru by the back donation of electrons and that the poisoning of ruthenium surfaces by hydrogen adatoms can be suppressed effectively because of the ability of C12A7:e2 to store hydrogen reversibly. The commercial production of ammonia is greater than that of any other chemical, reaching 160 million tons per year. Most ammonia is consumed as ammonium sulfate, which is used as an essential fertilizer in crop production. Although industrial ammonia synthesis is conducted using the Haber–Bosch process with iron-based catalysts1,2 at 400–600 8C and 20–40 MPa, such high reaction temperatures are detrimental given that the reaction is exothermic (46.1 kJ mol21 ) (ref. 3). The rate-determining step of ammonia synthesis is cleavage of the N;N bond, because the bond energy is extremely large (945 kJ mol21 ) (refs 4,5). Transition metals, such as Fe or Ru, are indispensable for the promotion of N;N bond cleavage6–8, as are electron donors that provide electrons to the transition metals9–12. A N2 molecule is fixed to form a bond with a transition metal by donating electrons from its bonding orbitals and accepting elec￾trons to its antibonding p-orbitals (back donation)13. Effectively, this back donation is enhanced by electron donors, which further weakens the N;N bond and results in the cleavage of N2 (refs 14–16). Electron donation from appropriate promoters is therefore key to enhancing the efficiency of ammonia synthesis using Fe or Ru catalysts10,14. However, in general it is extremely difficult to produce a material with a low work function as well as chemical and thermal stability. Although the catalytic activity of Ru is enhanced drastically by adding alkali or alkaline earth metals with small work functions17, these metals are unstable for ammonia synthesis because they are so chemically active that reaction with the produced ammonia forms metal amides18. Alkali and alkaline earth oxides are used exclusively as promoters; however, the activation of metal catalysts is still not efficient and the microscopic mechanism remains unclear19. If an efficient promoter with a distinct electron￾donating ability could be found, then the performance of these catalysts would be increased significantly. Another obstacle for industrial ammonia synthesis with Ru-loaded catalysts is hydrogen poisoning under high hydrogen pressures. Siporin and Davis reported that promotion of Ru catalysts with basic compounds cannot be attributed solely to an effect on N2 dissociation20. Base promotion is a trade-off between sufficiently lowering the activation barrier for N2 dissociation without detri￾mentally increasing the competitive adsorption of H2. As the ammonia synthesis activity of Ru catalysts generally decays because of hydrogen adatoms formed on the Ru surface, the reaction order for H2 on Ru catalysts often approaches 21 (refs 11,20,21). Such H2 poisoning is a serious obstacle for industrial ammonia pro￾duction that requires high-pressure conditions. The chemical indus￾try is therefore currently searching for a supported Ru catalyst that promotes N2 dissociation, but suppresses H2 poisoning. Here we report a stable electride, C12A7:e2, that acts as an efficient electron donor for a Ru catalyst. The electride has a high electron-donating efficiency and chemical stability, and does not exhibit H2-poisoning because of its reversible hydrogen absorption/ desorption capability, which results from its crystal structure (Fig. 1a). Electrides are crystals with cavity-trapped electrons that serve as anions and were first synthesized by J. L. Dye in 1983 using crown ethers22. Although such materials are expected to have unique prop￾erties, no practical applications were reported because they were chemically and thermally unstable and decompose in an inert atmosphere or in air above approximately 230 8C. In 2003, an inorganic electride C12A7:e2 was synthesized by utilizing a stable inorganic oxide, 12CaO.7Al2O3 (C12A7), which is a constituent of commercial alumina cement. This material is able to form a complex with electrons and the resulting material became the first stable electride in air at and above room temperature23. The unit cell of C12A7 has a positively charged framework structure com￾posed of 12 subnanometre-sized cages that connect to each other by sharing a mono-oxide layer to embrace four O22 ions in four cages as counteranions to achieve electroneutrality. Chemical reduction processes are used to inject four electrons into four of the 12 cages by extracting two of the O22 ions accommodated in the cavities as counteranions to compensate for the positive charge on the cage wall. The resultant chemical formula is represented by [Ca24Al28O64] 4þ(e2)4 (ref. 23). 1 Materials and Structures Laboratory, Tokyo Institute of Technology, 4259 Nagatsuta, Midori-ku, Yokohama 226-8503, Japan, 2 Frontier Research Center, Tokyo Institute of Technology, 4259 Nagatsuta, Midori-ku, Yokohama 226-8503, Japan, † Present address: Department of Energy Science, SungKyunKwan University, Suwon, Korea. *e-mail: hosono@msl.titech.ac.jp; mhara@msl.titech.ac.jp ARTICLES PUBLISHED ONLINE: 21 OCTOBER 2012 | DOI: 10.1038/NCHEM.1476 NATURE CHEMISTRY | ADVANCE ONLINE PUBLICATION | www.nature.com/naturechemistry 1 © 2012 Macmillan Publishers Limited. All rights reserved