《基础化学》课程教学资源（文献资料）合成氨进展——科学家开发出氨合成节能技术

科学家开发出氨合成节能技术 东京工业大学教授细野秀雄领导的研究小组10月22日在新一期英国期刊《自然一化 学》网络版上报告说，他们开发出了一种高效合成氨的新技术，使用这种技术所消耗的能源 只有传统方法的十分之一。 氨对于地球上的生物相当重要，它是所有食物和肥料的重要成分，还会直接或间接参与药 物合成，并且有望在燃料电池领域得到应用。氨是目前世界上产量最多的无机化合物之一， 全球每年生产约1.7亿吨氨。 氨由氮和氢反应合成，但是破坏氮分子之间强有力的结合，使其与氢发生反应需消耗大量 能源。 细野秀雄等研究者向其开发的超导物质C12A7中加入现在合成氨时常用的钉微粒，制 成催化剂。C12A7是钙铝酸盐化合物，是高铝水泥的主要成分。 研究人员发现，在这种催化剂作用下，氮和氢能高效合成氨。他们认为，这是由于在化合 时相关电子变得容易移动，从而使氮分子容易成为原子。 东京工业大学的研究人员准备今后进一步提高上述催化剂的性能，争取在5年至10年后使 这项新技术达到实用水平。原文英文题目如下（原文见文献资料） Ammonia synthesis using a stable electride as an electron donor and reversible hydrogen store 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' 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 [Ca24Al28064]4+(e-)4 (Ru/C12A7:e-),which has high electron-donating power and chemical stability,works as an efficien 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:e-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:e-to store hydrogen reversibly

科学家开发出氨合成节能技术 东京工业大学教授细野秀雄领导的研究小组 10 月 22 日在新一期英国期刊《自然—化 学》网络版上报告说，他们开发出了一种高效合成氨的新技术，使用这种技术所消耗的能源 只有传统方法的十分之一。 氨对于地球上的生物相当重要，它是所有食物和肥料的重要成分，还会直接或间接参与药 物合成，并且有望在燃料电池领域得到应用。氨是目前世界上产量最多的无机化合物之一， 全球每年生产约 1.7 亿吨氨。 氨由氮和氢反应合成，但是破坏氮分子之间强有力的结合，使其与氢发生反应需消耗大量 能源。 细野秀雄等研究者向其开发的超导物质 C12A7 中加入现在合成氨时常用的钌微粒，制 成催化剂。C12A7 是钙铝酸盐化合物，是高铝水泥的主要成分。 研究人员发现，在这种催化剂作用下，氮和氢能高效合成氨。他们认为，这是由于在化合 时相关电子变得容易移动，从而使氮分子容易成为原子。 东京工业大学的研究人员准备今后进一步提高上述催化剂的性能，争取在 5 年至 10 年后使 这项新技术达到实用水平。原文英文题目如下（原文见文献资料）： Ammonia synthesis using a stable electride as an electron donor and reversible hydrogen store 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]4+(e−)4 (Ru/C12A7:e−), 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:e− 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:e− to store hydrogen reversibly

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 reserved

Ammonia 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

ARTICLES NATURE CHEMISTRY DOI:10.1038/NCHEM.1476 6 C12A7: Framework co .4e Cao C12A7: Figure 1 Ru- d C12A7 el ride catalyst for a a.sche odel of Ru-oaded C12A7eHigh-den y electrons (2.0x 10cm) n be e s are t teroshe7eem5oaomercoisnctgoheendb ages by sha ng one o and the inc by p 10n m the Results and discussion but the es with a positr g at room temper nd Mgo with F C1A, centres,are k NATURE CHEMISTRY ADVANCE ONLINE PUBLICATION wwwnature.com/naturechemistry 2012 Macmillan Publishers Limited All rights reserved

The injected electrons occupy a unique conduction band called the ‘cage conduction band’ (CCB)24, which is derived from the three￾dimensionally connected cages by sharing one oxide monolayer, and can migrate through the thin cage wall by tunnelling, which leads to metallic conduction (about 1,500 S cm21 at room temperature). This electron-trapped cage structure of the bulk is retained up to the top surface if the sample is heated appropriately25. The CCB in C12A7 was verified by photoemission spectroscopy and ab initio calculations and is derived from the very unique crystal structure of C12A7—three￾dimensionally connected subnanometre-sized cages with a positive charge (zeolite has a similar crystal structure, but the cage is negatively charged). No other stable electride has been realized since the first synthesis in 1983 by J. L. Dye, despite much interest in achieving an electride that is stable at room temperature. In addition, the electrons encapsulated in the cages of C12A7:e2 can be replaced readily with hydride ions (H2) by heating in H2 gas. The incorporated H2 ions desorb as H2 molecules at about 400 8C to leave electrons in the positively charged framework of C12A7 (ref. 26); the incorporation and release of hydrogen on C12A7:e2 is entirely reversible. The electride formation and reversible storage ability of hydrogen originates from the very unique crystal structure of C12A7 described above. Such a formation is, of course, impossible for other oxides, including Al2O3 and CaO (ref. 27). Results and discussion Structure and performance of Ru-loaded C12A7:e2. The electronic structure of C12A7:e2 is similar to that of an Fþ centre (see Fig. 1b), an electron trapped at the site of an O22 vacancy, in a CaO crystal. Basic oxides loaded with a noble metal, such as CaO and MgO with Fþ centres, are known to have catalytic activity for reactions that involve electron-transfer processes, because of electron transfer from the F centre on the substrate Valence band Valence band Cage conduction band 2.4 eV Conduction band 5.5 eV 7.4 eV CaO Evac Framework conduction band 4.7 eV Ru F+ centre Ef C12A7:e– 5 nm Electron H2 C12A7:e– Ca Al O Ru H– ion a b c d O Ca Ru CaO Electron 1.6 eV 4.1 eV Figure 1 | Ru-loaded C12A7 electride catalyst for ammonia synthesis. a, Schematic model of Ru-loaded C12A7:e2. High-density electrons (2.0 × 1021 cm23 ) are distributed statistically in the subnanometre-sized cages of C12A7 as counteranions and electrons encaged in C12A7 can be exchanged by H2 ions under an H2 atmosphere23,26. b, Fþ centres in the CaO crystal. Electrons are trapped at the oxygen-vacancy sites and octahedrally coordinated with six Ca2þ ions. The energy level of the Fþ centre in CaO is rather varied depending on the environment around the electron-trapping site30. c, Comparison of the energy levels for an Fþ centre in CaO, CCB in C12A7:e2 and the Fermi level (Ef ) in Ru. Evac denotes vacuum level. Here, CCB is the additional conduction band that originates from three-dimensionally connected nanocages in the fundamental band gap. Data for the relevant levels in CaO and C12A7:e2 are taken from previous reports30,31. The much higher energy level (that is, CCB) of the electrons trapped in the connected cages relative to that for the Fþ centre in CaO primarily results from a weaker Madelung potential caused by the larger separation between the electron and the nearest neighbour Ca2þ. d, TEM image of 0.3 wt% Ru-loaded C12A7:e2. ARTICLES NATURE CHEMISTRY DOI: 10.1038/NCHEM.1476 2 NATURE CHEMISTRY | ADVANCE ONLINE PUBLICATION | www.nature.com/naturechemistry © 2012 Macmillan Publishers Limited. All rights reserved

NATURE CHEMISTRY DOL:10.1038/NCHEM.1476 ARTICLES Emuent NH mole fraction(vo 0.1 0.2 0.3 0.4 406080100 120 Ru/C12A7:e RuC12A7 Ru/C12A7:O Ru/C12A7:0 Ru-Cs/MgC Ru-BaAC RU/Cao Rwy-Al.O. 10 1000 2.000 0050100150.20 -1h- TOE U -1 Figure 2 I Catalytic p e of Ru/C12A7:e at a n.A rate an uent mole gh士5 with rate of60 ml min】 centre er rea the than Ru/. Ru/ aO,Ru/Cao AO,(Ru/C Cs/Mgo re near-therm R. 4 e to th CCE nance on Ru-based cat s rate of Ru/C12A7:e increased with the Ru on at the s m that of the of Ru(6.0wt)s/ gC pro Ru.Ru has a higher turnover ted ru nanoparticles In addition. pect that the Ctalyshese TOFs do depend significantly an nt of Ru lo d Rdl,to re and the excess hyd zed cages of C12A7,to rve d by a solid-stat h Cand Ru-Cs/Mgo.In me 32 to e of other The mean par 62).As y CO chemis rements (Supplem ary Table SI)to lysts(Fig 2b and Supple mentary Table S1).The ely high NATURE CHEMISTRY IADVANCE ONUINE PUBLICATION I om/natu 3 2012 Publishers limited All rights reserved

surface to the deposited metal cluster28. However, there are three distinct differences between the Fþ centres and C12A7:e2. Both the electron-donating efficiency and the stability of the Fþ centres are low because of a high potential barrier that originates from a strong electron confinement, low surface concentration and ease of structural alteration29. C12A7:e2 resolves these two drawbacks, as explained in the following. Figure 1c shows the electronic structure of C12A7:e2, along with that of Fþ centres in CaO (ref. 30) and the Fermi level of Ru. Although the levels of valence maximum and conduction-band minimum for C12A7:e2 are close to those of the Fþ centres in CaO, the location of the CCB level of C12A7:e2 is higher than those of the Fþ centres in CaO by 1.6–4.1 eV (refs 30,31). Therefore, the electrons encaged in C12A7:e2 can be donated effectively to the metal Ru (work function 4.7 eV) because of its intrinsic low work function (2.4 eV), which is comparable to that of potassium metal32. The third difference is a much higher encaged electron concentration at the surface of C12A7:e2, 1014–1015 cm22 , several orders of magnitude greater than that of the conventionally produced Fþ centres25. It is therefore anticipated that C12A7:e2 functions as an efficient electron donor for Ru nanoparticles with respect to electron￾donating ability and the number of active sites for electron transfer to deposited Ru nanoparticles. In addition, we expect that the reversible hydrogen absorption/desorption capability of C12A7:e2 may prevent the poisoning of the Ru surface by hydrogen adatoms. Hydrogen adatoms on Ru would readily spill over onto C12A7:e2 and the excess hydrogen adatoms would be entrapped as H2 ions in the subnanometre-sized cages of C12A7, to reserve sufficient Ru sites to decompose N2. C12A7:e2 powders were prepared by a solid-state reaction according to a previous report33. The surface area of prepared C12A7:e2 was only 1 m2 g21 . Figure 1d shows a transmission elec￾tron microscopy (TEM) image of 0.3 wt% Ru/C12A7:e2 with Ru nanoparticles deposited on the C12A7:e2 surface. The mean par￾ticle size and the dispersion of Ru on C12A7:e2 were determined by CO chemisorption measurements (Supplementary Table S1) to be 30–40 nm and ,5%, respectively. Figure 2a shows the catalytic activities for ammonia synthesis over various 1 wt% Ru-loaded catalysts. Results for Ru-Cs/MgO and Ru-Ba/activated carbon (Ru-Ba/AC) are also shown for comparison; the former is one of the most active Ru catalysts for ammonia synthesis and the latter has been used for commercial ammonia production11,34. All the tested catalysts were reduced under reaction conditions before the reaction; nevertheless, Ru/C12A7:e2 exhibited a much higher cata￾lytic activity than Ru/Al2O3, Ru/CaO, Ru/CaO−Al2O3 (Ru/CA), Ru/C12A7:O2 and Ru-Ba/AC and the ammonia effluent mole frac￾tions for Ru/C12A7:e2 and Ru-Cs/MgO reached near-thermodyn￾amic equilibrium (about 0.5%). Deposition of a large amount of Ru is required to achieve a high catalytic performance on Ru-based cat￾alysts11,12,34–36. The relationship between the catalytic activity and the amount of deposited Ru was examined for each catalyst (Supplementary Table S1). The ammonia synthesis rate of Ru/C12A7:e2 increased with the Ru loading amount and reached a maximum at 1.2 wt% Ru deposition. Under optimal conditions, the catalytic activity of Ru/C12A7:e2 is comparable to those of Ru(6.0 wt%)-Cs/MgO and Ru(9.1 wt%)-Ba/AC, despite the lower surface area and lower amount of loaded Ru. Ru/C12A7:e2 has a higher turnover fre￾quency (TOF), at least more than 10 times those of the conventional catalysts (Fig. 2b and Supplementary Table S1). For Ru-Ba/AC and Ru-Cs/MgO, these TOFs do not depend significantly on the amount of Ru loading, but the TOF value of the Ru/C12A7:e2 cat￾alyst increases with a decrease in the amount of loaded Ru, to reach a maximum at 0.3 wt%. Supplementary Table S1 also shows that Ru on C12A7:e2 exhibits a much larger TOF than Ru-Ba/AC and Ru-Cs/MgO for Ru particles of similar size. There is no significant difference in Ru particle size (7–9 nm) between Ru(0.1 wt%)/C12A7:e2, Ru(9.1 wt%)-Ba/AC and Ru(6.0 wt%)-Cs/MgO, but nevertheless the TOF of Ru/C12A7:e2 is 20–50 times larger than those of Ru-Ba/AC and Ru-Cs/MgO. In addition, the catalytic activity of Ru/C12A7:e2 is superior to those of other catalysts at a reaction temperature of 320 8C, especially in terms of the TOF (Supplementary Table S2). As a consequence, the Ru/C12A7:e2 catalyst exhibits the smallest activation energy among the tested cat￾alysts (Fig. 2b and Supplementary Table S1). The extremely high TOF and small activation energy indicate that C12A7:e2 imparts a high catalytic activity for ammonia synthesis onto the Ru surface. 0 0.15 0.05 0.10 × 10 a b 0 0.3 0.1 0.2 0.4 0 20 40 60 80 100 120 140 Ru/C12A7:e– Ru/C12A7:O2– Ru-Cs/MgO Ru-Ba/AC Ru/CaO 0 1,000 2,000 3,000 Ru/CA Ru/γ-Al2O3 Ru/γ-Al2O3 Ammonia synthesis rate (μmol g–1 h–1) Effluent NH3 mole fraction (vol%) Ru/C12A7:e– Ru/C12A7:O2– Ru-Cs/MgO Ru-Ba/AC Ru/CaO 0.20 0.25 TOF (molecule site–1 s–1) Apparent activation energy (kJ mol–1) Figure 2 | Catalytic performance of Ru/C12A7:e2 at atmospheric pressure. a, Ammonia synthesis rate and ammonia effluent mole fraction at 400 8C over various 1 wt% Ru-loaded catalysts. b, Turnover frequencies (TOFs; black bars) and apparent activation energies (red bars) for ammonia synthesis at 400 8C over various 1 wt% Ru-loaded catalysts. AC, activated carbon. Reaction conditions: catalyst, 0.2 g; synthesis gas, H2/N2 ¼ 3 with a flow rate of 60 ml min21 ; pressure, 0.1 MPa; temperature, 400 8C. An error analysis was performed for these data and the error range found was +5%. NATURE CHEMISTRY DOI: 10.1038/NCHEM.1476 ARTICLES NATURE CHEMISTRY | ADVANCE ONLINE PUBLICATION | www.nature.com/naturechemistry 3 © 2012 Macmillan Publishers Limited. All rights reserved.

ARTICLES NATURE CHEMISTRY DOI:10.1038/NCHEM.1476 a0.30 0.25 Ru/C12A7: 30 02 0 0 0204 0.6 08 20 % 60 essure (MPa e (h Ru/C12A7:e +9%R/C12A7 talyst.0.2 g .H/N=3 e of 60 m min ure,360C.The rang d as the 1wt%Ru/C12A7 o (360C).The emror range was% s that the Ru/C12A7-e- Catav the Tor nia withou quired to have a duced dur ing the re ioreached 35 mm a p us for the e ilibrium of an synthesis and the reaction (that i en th which is 12A7 is almost indepe ntof-ade Table I summ the catalyt unt of for vol%) Und the Ru/C1 A7- Reaction mechanism.To understand the high nd TOF/total Ru The arious Ru-l Fourier transt s/MgO higher P tr ing vibr ol mode of ch N2 on the hat (2.245 of rbe on Ru/C ng to a alkali Table1 Catalytic performance of Ru catalysts on various supports at 400"C under a pressure of 1.0 MPa. Catalyst area Particle size TOF (0 RU/C12A7 oo 49 08 NATURE CHEMISTRY ADVANCE ONLINE PUBLICATION wwwnature.com/naturechemistr 2012 Macmillan Publishers Limited All rights reserved

Catalytic performance under high pressure. Figure 3a shows the variation of the TOF as a function of the total pressure on Ru/C12A7:e2 and Ru-Cs/MgO at 360 8C. Industrial catalysts for ammonia synthesis are required to have a high catalytic performance under high pressure, because high pressure is advantageous for the equilibrium of ammonia synthesis and the liquefied state of the resulting ammonia is much more convenient for utilization11. The TOF of Ru/C12A7:e2 is enhanced significantly by an increase in the total pressure and reaches a maximum (0.25 s21 ) at 1.0 MPa, whereas the TOF of Ru-Cs/MgO is almost independent of pressure. Table 1 summarizes the catalytic performance of various Ru-loaded catalysts under high-pressure conditions (1.0 MPa) at 400 8C. The amount of formed NH3 (see Table 1) is much smaller than that at thermodynamic equilibrium (about 4 vol%). Under these conditions, Ru/C12A7:e2 is substantially superior to Ru-Cs/MgO and Ru-Ba/AC with respect to TOF/surface Ru atoms and TOF/total Ru atoms. The maximum TOF of Ru/C12A7:e2 at 1.0 MPa reaches 0.98 s21 , which is still larger than those (0.05–0.28 s21 ) reported for Ru-Ba/AC and Ru-Cs/MgO at higher pressures (2.0–6.3 MPa)11,34–36. Such an enhancement of catalytic activity with pressure is not observed for conventional oxide-supported Ru catalysts, but instead the catalytic performance is independent of total pressure or decreases with increasing total pressure, because the dissociative adsorption of H2 on Ru prevents the adsorption and cleavage of N2 at high pressure20,37. Figure 3b demonstrates that the Ru/C12A7:e2 catalyst continu￾ously produces ammonia without any decrease in activity for 75 hours, even under high pressure. The total amount of ammonia pro￾duced during the reaction reached 35 mmol. If all the electrons entrapped in the crystallographic cages of the whole C12A7:e2 sample were consumed in the ammonia synthesis reaction (that is, one electron used for the cleavage of one N2 molecule), then the ammonia yield would be only 452 mmol (per 0.2 g catalyst), which is smaller by two orders of magnitude than the amount of ammonia produced. This result indicates that Ru/C12A7:e2 func￾tions as a stable catalyst for the ammonia synthesis reaction and electrons encapsulated in the Ru/C12A7:e2 catalyst can be used repeatedly during the reaction. Reaction mechanism. To understand the high catalytic performance of Ru/C12A7:e2, the adsorbed state of N2 on various Ru-loaded catalysts was examined using Fourier transform infrared (FT-IR) spectroscopy. Figure 4a shows that each catalyst exhibits broad peaks around 2,300–2,100 cm21 , caused by the stretching vibrational mode of chemisorbed N2 on the Ru surface with an end-on orientation10,38–40. The peak frequency (2,194 cm21 ) of N2 adsorbed on Ru/C12A7:O22 is lower than that (2,245 cm21 ) on Ru/g-Al2O3, which suggests that C12A7:O2 has alkaline characteristics similar to those of MgO (refs 38,39). In contrast, the spectrum for Ru/C12A7:e2 has three peaks at 2,234, Pressure (MPa) 0 0.2 0.4 0.6 0.8 1.0 0 0.30 0.25 0.20 0.15 0.10 0.05 a TOF (molecule site–1 s–1) Time (h) Ammonia production (mmol) 40 30 20 10 0 0 20 40 60 80 100 b Ru/C12A7:e– Ru-Cs/MgO Figure 3 | Catalytic performance of Ru/C12A7:e2 under high-pressure conditions. a, TOFs for high-pressure ammonia synthesis over 1 wt% Ru/C12A7:e2 and 6 wt% Ru-Cs/MgO. Reaction conditions: catalyst, 0.2 g; synthesis gas, H2/N2 ¼ 3 with a flow rate of 60 ml min21 ; temperature, 360 8C. The error range defined as the standard deviation for a set of experimental runs was+5% for the red and+7% for the black data points. b, Time course of ammonia formation over 1 wt% Ru/C12A7:e2. Reaction conditions: catalyst, 0.2 g; synthesis gas, H2/N2 ¼ 3 with a flow rate of 60 ml min21 ; pressure, 1.0 MPa, reaction temperature (360 8C). The error range was+5%. Table 1 | Catalytic performance of Ru catalysts on various supports at 400 8C under a pressure of 1.0 MPa. Catalyst Surface area (m2 g21 ) Ru loading (wt%)* Dispersion (%)† Particle size (nm)† NH3 synthesis rate (mmol g21 h21 ) ‡ TOF (s21 ) § TOF (3103 Ru atom21 s 21 ) NH3 (vol%)} Ru-Ba/AC 310 9.1 14.3 9.3 8,285 0.02 2.6 1.14 Ru-Cs/MgO 12 6.0 18.6 7.2 12,117 0.03 5.7 1.67 Ru/C12A7:O22 1–2 1.2 3.4 39.2 888 0.06 2.1 0.12 Ru/C12A7:e2 1–2 0.1 15.6 8.5 994 0.22 34.9 0.13 1–2 0.3 4.1 32.9 3,686 0.98 39.8 0.50 1–2 1.2 3.2 41.3 8,245 0.59 18.8 1.13 1–2 4.0 2.0 68.5 6,089 0.22 4.3 0.83 *Ru content was determined by inductively coupled plasma atomic emission spectroscopy. † Dispersion and particle size were calculated on the basis of CO chemisorption values, assuming spherical metal particles and that the stoichiometry of Ru/CO ¼ 1 (ref. 19). ‡ NH3 synthesis conditions: catalyst, 0.2 g; synthesis gas, H2/N2 ¼ 3 with a flow rate of 60 ml min21 ; temperature, 400 8C; pressure, 1.0 MPa. § TOF was calculated from the rate of ammonia synthesis divided by the number of CO atoms chemisorbed on the Ru surfaces. TOF was also calculated from the rate of ammonia synthesis divided by the number of Ru atoms deposited on the catalysts. } NH3 mole fraction in the reactor effluent. ARTICLES NATURE CHEMISTRY DOI: 10.1038/NCHEM.1476 4 NATURE CHEMISTRY | ADVANCE ONLINE PUBLICATION | www.nature.com/naturechemistry © 2012 Macmillan Publishers Limited. All rights reserved.

NAtURe CHEMISTRY DOL:10.1038/NCHEM1476 ARTICLES b3.5 224 Abs .00 B ourt)( aN2)=046 BH)=0.9 400 29 0604 -02 Log PH, 2A7e-t360℃under atmo and a total tlo rate of 60 ml min.The error range was +6%for the C12A7:e ￥9 a,日 sited or s on the R C12A7 aptured in the pC1262e-intef In the spectrumshift of 71-72 cm species are chemisorbed N2 on the Rucates that t to posit NATURE CHEMISTRY I ADVANCE ONLINE PUBLICATION www.nature.com/naturechemistr 012 Macmiln Publishers Limiled Allrights reserved

2,176 and 2,124 cm21 . In the spectrum of 15N2 adsorbed on Ru/C12A7:e2, these three bands show a red shift of 71–72 cm21 compared to the peak positions for 14N2 adsorbed on Ru/C12A7:e2, which is in good agreement with the value estimated according to the isotope effect (that is, 2,124 cm21 × (14/15)1/2 ¼ 2,051 cm21 ) (refs 38,39), which indicates that these species are chemisorbed N2 on the Ru surfaces. The vibrational peaks caused by N2 adsorbed on Ru/C12A7:e2 are observed at lower wavenumbers than those of the other samples, which implies that the N;N bond of N2 on Ru/C12A7:e2 is weakened 2,400 2,200 2,000 1,800 Abs 0.004 2,124 2,176 2,194 2,245 2,104 2,053 a Log (NH3 synthesis rate) (μmol g–1 h–1) b 2.9 3.0 3.1 3.2 3.3 3.4 3.5 –1.2 –0.8 –0.6 –0.4 –0.2 0 –1 Wavenumber (cm–1) Log PN2 , PH2 α(N2) = 0.46 β(H2) = 0.97 Figure 4 | FT-IR and kinetic analyses. a, Difference FT-IR spectra of N2 molecules before and after N2 adsorption on Ru/Al2O3 ( 14N2, black), Ru/C12A7:O22 ( 14N2, green), Ru/C12A7:e2 ( 14N2, red) and Ru/C12A7:e2 ( 15N2, blue) at 2170 8C under 5 kPa of N2. b, Dependence of NH3 synthesis rate on the partial pressures of N2 and H2 on Ru/C12A7:e2 at 360 8C under atmospheric pressure and a total flow rate of 60 ml min21 . The error range was+6% for the black and+3% for the blue data points. Ru e– e– e– e– e– e– e– e– e– N N H H C12A7:e− Ru Ru H– H– H– H– H– e– e– e– e– N–N bond weakening Spill over N N N N N H N H H H H H H H H H H Ru e– N N H H Figure 5 | Possible pathway for the ammonia synthesis reaction over Ru/C12A7:e2. a, Electron transfer from C12A7:e2 to Ru metal particles deposited on the C12A7:e2 surface. b, N–N bond weakening as a result of back donation from Ru d-orbitals to the p*–antibonding orbitals of N2. Dissociative adsorption of H2 forms hydrogen adatoms on the Ru surface and hydrogen adatoms spill over onto C12A7:e2. c, Spill-over hydrogen adatoms are captured as H2 ions in the nanocages via the reaction with electrons in the cages. Nitrogen species adsorbed on Ru near the Ru-C12A7:e2 interface react with hydrogen species from C12A7 nanocages and the resulting electrons are recovered in the nanocages. d, Formation of an ammonia molecule from intermediate species (N, H, NH, NH2) and the restitution of electrons to nanocages of C12A7:e2. NATURE CHEMISTRY DOI: 10.1038/NCHEM.1476 ARTICLES NATURE CHEMISTRY | ADVANCE ONLINE PUBLICATION | www.nature.com/naturechemistry 5 © 2012 Macmillan Publishers Limited. All rights reserved

ARTICLES NATURE CHEMISTRY DOL:10.1038/NCHEM.1476 from and nea ar to the Ru-C respective oxides Ru/C12A7:s s this in a be rega own that the (ig ection t the N.re orde that titute the Mgo of the reaction 12A7 the N propert Ku-l s&Tsndingcancatrbntedoncftaigntcectondo on order for +1,a valu rate-limiting step. H Method ad on Ru gu catalts for am in an tube 0.25℃m ion d by A7:6 d in a rated up to 1.5 MPa and w as H 140 Ru/C12AZy- dy-state c 0 Cs/ .1 to 1.3 MPa).T cid solutio ofN N and H med that am nia is n /C12A TEM (M- The pof the 12A in Cl with d over Ru ible Mgo.T g He (50 ml ine on ru duri h Ru/C12A7e (Fig.5)is of the of Ru h back specia RU-C a es are al ly on Ru es react ions ed as the bac ence spectra ad the 27 during the reaction erences NATURE CHEMISTRY ADVANCE ONLINE PUBLICATION wwwnature.com/naturechemistry 2012 Macmillan Publishers Limited All rights reserved

further by the electrons encaged in C12A7:e2. The peaks at 2,176 and 2,124 cm21 can be attributed to N2 species on Ru atoms far from and near to the Ru-C12A7:e2 interface, respectively39. To clarify the reaction mechanism for ammonia synthesis over Ru/C12A7:e2, the kinetics of ammonia production over the catalyst were investigated (Fig. 4b and Supplementary Table S3)41. It is well known that the reaction order for N2 is almost unity in the rate equation for ammonia synthesis over most conventional catalysts (Supplementary Table S3)11,20,41,42. Although the N2 reaction order of Ru/MgO shown in Supplementary Table S3 is in good agreement with the reported results, it was estimated to be 0.5 in the case of the Ru/C12A7:e2 catalyst, which indicates that the N2 dissociation over Ru/C12A7:e2 is much more efficient than those of conventional cat￾alysts. This finding can be attributed to an efficient electron donation from C12A7:e2, as shown in Fig. 4a. Figure 4b also shows that the reaction order for H2 is approximately þ1, a value distinct from other conventional Ru catalysts, which have reaction orders for H2 that are, in general, negative because H2-dissociative adsorption pre￾cedes that of N2 dissociation and hydrogen adatoms resist the effi- cient dissociative adsorption of N2 on a Ru surface11,20,21. This poisoning is enhanced with increasing total pressure, which limits Ru catalysts for ammonia synthesis, as mentioned above. The posi￾tive reaction order with Ru/C12A7:e2 means that Ru/C12A7:e2 is not poisoned by hydrogen. A plausible explanation for these unique characteristics of Ru/C12A7:e2 is attributed to the nano￾cages of C12A7:e2, which can capture hydrogen species as H2 ions26. Hydrogen adatoms on Ru readily spill over onto C12A7:e2 and are incorporated into the positively charged nanocages of C12A7:e2 as H2 ions, which prevents hydrogen adatoms from occu￾pying the Ru surface. Supplementary Table S4 summarizes the ammonia evolution from Ru/C12A7:e2 and Ru-Cs/MgO when heated in a pure N2 atmosphere after ammonia synthesis in the pres￾ence of N2 and H2. It was confirmed that ammonia is not adsorbed on both catalysts before exposure to pure N2 gas. Apparently, Ru/C12A7:e2 evolves ammonia, which means that hydrogen is incorporated into C12A7:e2 as H2 ions during ammonia synthesis and the H2 ions in C12A7:e2 react with nitrogen species formed by N2 dissociation, which results in ammonia formation. The amount of H2 ions encaged in the Ru/C12A7:e2 catalyst was estimated to be 7.6 × 1019 cm23 (that is, about 4% of the electrons in C12A7:e2 are replaced with H2 ions). However, no ammonia evolution was observed over Ru-Cs/MgO. These results verify that C12A7:e2 func￾tions as a reversible hydrogen-storage material and prevents hydro￾gen poisoning on Ru during ammonia synthesis. A possible mechanism for the ammonia synthesis reaction over the Ru/C12A7:e2 catalyst (Fig. 5) is proposed as follows. Electrons encaged in C12A7:e2 are transferred to the Ru metal, which causes a substantial lowering of the work function of Ru by raising the Fermi level. As a consequence, back donation from Ru with an excess electron density to the N2 p*-antibonding orbital is enhanced, especially near the Ru-C12A7:e2 interface, which leads to a weakening of the N;N bond of adsorbed N2. Although H2 molecules are also adsorbed dissociatively on Ru surfaces, hydro￾gen adatoms readily spill over onto C12A7:e2 and are incorporated into the nanocages as H2 ions. This incorporation prevents hydro￾gen adatoms from poisoning the Ru surface. The activated nitrogen species react with H2 ions entrapped by C12A7 nanocages or with hydrogen adatoms on Ru to form ammonia. In the case of the former, H2 ions leave electrons in the cages and the resulting hydro￾gen species react with the activated nitrogen near the Ru-C12A7:e2 interface. The electrons encaged in C12A7:e2 are used repeatedly during the reaction. Conclusions We found that the stable electride C12A7:e2 works as an excellent electron donor and reversible hydrogen-storage material to enhance the catalytic activity for ammonia synthesis, with a TOF an order of magnitude greater than those reported so far. This material is com￾posed of the abundant oxides CaO and Al2O3, and has the unique properties of a very low work function and high chemical and thermal stabilities. As this material may, in a sense, be regarded as the bulk crystal of Fþ centres of alkaline earth oxides, the surface concentration of electrons energetically available for injection to Ru is much higher than that from a conventional Fþ centre and the subnanometre-sized cages that constitute the crystal structure include the surface layer as a H2 reservoir, which suppresses the lowering of the reaction rate at high pressure. These characteristic properties make Ru-loaded C12A7:e2 an efficient catalyst for ammonia synthesis. These findings imply the potential for catalysts of chemical reactions in which the activation of an inert molecule, such as N2 and CO2, by electron injection is the rate-limiting step. Methods Preparation of Ru/C12A7:e2. C12A7:e2 powders were prepared by the reaction of C12A7, CaO.Al2O3 (CA) and Ca metal at 700–1100 8C. Ru loading was conducted using a solvent-free preparation method. Typically, C12A7:e2 powder and Ru3(CO)12 were sealed in an evacuated silica tube and heated under a temperature programme of 2 8C min21 up to 40 8C, held for one hour, 0.25 8C min21 up to 70 8C, held for one hour, 0.4 8C min21 up to 120 8C, held for one hour, 0.9 8C min21 up to 250 8C, hold for two hours and then cooled to ambient temperature. Ammonia synthesis reactions. These were conducted in a silica glass or a stainless￾steel flow set-up that operated up to 1.5 MPa and were supplied with an extra pure (.99.99995%) mixture of H2:N2 (3:1). Before the reactions, all the catalysts were treated in a stream of N2 þ 3H2 under 0.1 MPa using a temperature programme of heating to 400 8C in two hours and then standing at 400 8C for one hour. The concentration of ammonia in the stream that left the catalyst bed (0.2 g) was monitored under steady-state conditions of temperature (320–400 8C), gas flow rate (60 ml min21 ) and pressure (0.1 to 1.3 MPa). The ammonia produced was trapped in 5 mM sulfuric acid solution and the amount of NH4 þ generated in the solution was determined using an ion chromatograph (LC-2000 plus, Jasco) equipped with a conductivity detector. Characterization. TEM (JEM-2010F, Jeol) images of the samples were obtained to determine the microstructural characteristics. The Brunauer–Emmett–Teller specific surface areas of the samples were determined by the measurement of nitrogen adsorption–desorption isotherms at 2196 8C using an automatic gas-adsorption instrument (NOVA 4200e, Quantachrome) after evacuation of the samples at 150 8C. Ru content was determined by inductively coupled plasma atomic emission spectroscopy (ICP-AES; ICPS-8100, Shimadzu). Ru dispersion was determined by CO-pulse chemisorption (BELCAT-A, BEL, Japan) at 50 8C using a He flow of 30 ml min21 and pulses of 0.09 ml (9.88% CO in He). Prior to the dispersion analysis, the catalyst was treated with flowing He (50 ml min21 ) at 400 8C for 15 minutes and then flowing H2 (50 ml min21 ) at 400 8C for 15 minutes. Adsorbed hydrogen atoms on the reduced catalysts were removed by purging with He (50 ml min21 ) at 400 8C for 15 minutes. To calculate the metal dispersion a stoichiometry of Ru/CO ¼ 1 was assumed. FT-IR spectra of adsorbed N2 were measured using a spectrometer (FT/IR-6100, Jasco) equipped with a mercury– cadmium–tellurium detector at a resolution of 4 cm21 . Samples were pressed into self-supported disks (20 mm diameter, about 20 mg). A disk was placed in a silica-glass cell equipped with NaCl windows and connected to a closed gas-circulation system to allow thermal adsorption–desorption experiments. The disk was heated under vacuum at 500 8C for one hour, and then treated with circulated H2 (27 kPa) with a liquid-nitrogen trap for one hour. The disk was then evacuated at the same temperature for one hour to remove the hydrogen, then exposed to 5 kPa of N2 to obtain infrared spectra for adsorbed N2. After the pretreatment, the disk was cooled to 2170 8C under vacuum to obtain a background spectrum. Pure N2 (99.99995%) and H2 (99.99999%) were supplied to the system through a liquid-nitrogen trap. Isotopic nitrogen (15N2, 98%) was used without purification. The infrared spectrum of the sample at 2170 8C prior to N2 adsorption was used as the background for difference spectra obtained by subtracting the backgrounds from the spectra of N2-adsorbed samples. In this experiment, 2 wt% Ru/C12A7:e2 with an electron concentration of 5.0 ×1020 cm23 , 2 wt% Ru/C12A7:O22 and 3 wt% Ru/Al2O3 were used for FT-IR measurements. Received 24 May 2012; accepted 6 September 2012; published online 21 October 2012 References 1. Mittasch, A. Early studies of multicomponent catalysts. Adv. Catal. 2, 81–104 (1950). ARTICLES NATURE CHEMISTRY DOI: 10.1038/NCHEM.1476 6 NATURE CHEMISTRY | ADVANCE ONLINE PUBLICATION | www.nature.com/naturechemistry © 2012 Macmillan Publishers Limited. All rights reserved.

NATURE CHEMISTRY DOL:10.1038/NCHEM.1476 ARTICLES Hon ia synthesis from first-principles calculations. 27. m 1) .Ed 307,403 Chem,Int E 5298-5308( tive activation of N, 427,527- .N.D.Sel 2002)- R.C.S repara山 32 33 a M.C .14,16 81 01 10 35 (195 g-Pile suppo 239.313-3252006 Sm,O,on Ra/ Birker,A&Muler.M.The ynthes 178-185(19% .Ea40.1061-1063 13. 1.A 8112 27,424-431(197 16 41 ecn by alkali metal-n 2 430-43419 ted o carbon.(2003) 报120127 Acknowledgements 368(2004 1.M.EA ed by rt Author contributions A n a nanoporous single crystal & gas in nanopon Additional information le in the nof the no,M,H aptatsand Competing financial interests NATURE CHEMISTRY I ADVANCE ONUINE PUBLICATION I wwwnafure.com/naturechemistn 012 Macmll Publishers Limited All righis reserved

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Discussions with K. Nakajima and Y. Toda are acknowledged. We thank T. Yoshizumi, S. Nakamura and D. Lu for their technical assistance. This work was supported by a Funding Program for World-Leading Innovative R&D on Science and Technology from the Japan Society for the Promotion of Science. A part of this work was supported by a fund from the Element Strategy Initiative Project of the Ministry of Education, Culture, Sports and Science for Technology in Japan. Author contributions H.H. proposed the idea behind this research and M.H. and H.H. directed the entire project. M.K., Y.I., Y.Y., F.H., S.M., S.K., T.Y. and S-W.K. performed the synthesis, characterization and catalytic testing of Ru/C12A7:e2. All the authors discussed the results and commented on the study. M.K., M.H. and H.H. co-wrote the manuscript. Additional information Supplementary information is available in the online version of the paper. Reprints and permission information is available online at http://www.nature.com/reprints. Correspondence and requests for materials should be addressed to H.H. for C12A7:e2 and M.H. for reactions. Competing financial interests The authors declare no competing financial interests. NATURE CHEMISTRY DOI: 10.1038/NCHEM.1476 ARTICLES NATURE CHEMISTRY | ADVANCE ONLINE PUBLICATION | www.nature.com/naturechemistry 7 © 2012 Macmillan Publishers Limited. All rights reserved