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Anchoring Bond Between Ru and N Atoms of Ru/CaNH Catalyst: Crucial for the High Ammonia Synthesis Activity Hitoshi Abe, Yasuhiro Niwa, Masaaki Kitano, Yasunori Inoue, Masato Sasase, Takuya Nakao, Tomofumi Tada, Toshiharu Yokoyama, Michikazu Hara, and Hideo Hosono J. Phys. Chem. C, Just Accepted Manuscript • DOI: 10.1021/acs.jpcc.7b07268 • Publication Date (Web): 01 Sep 2017 Downloaded from http://pubs.acs.org on September 4, 2017

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The Journal of Physical Chemistry

Anchoring Bond between Ru and N Atoms of Ru/Ca2NH Catalyst: Crucial for the High Ammonia Synthesis Activity Hitoshi Abe,∗,†,‡,¶ Yasuhiro Niwa,† Masaaki Kitano,§ Yasunori Inoue,k Masato Sasase,§,¶ Takuya Nakao,k Tomofumi Tada,§ Toshiharu Yokoyama,§,¶ Michikazu Hara,k,¶ and Hideo Hosono§,k,¶ Institute of Materials Structure Science, High Energy Accelerator Research Organization, 1-1 Oho, Tsukuba, Ibaraki 305-0801, Japan., Department of Materials Structure Science, School of High Energy Accelerator Science, SOKENDAI (the Graduate University for Advanced Studies), 1-1 Oho, Tsukuba, Ibaraki 305-0801, Japan., ACCEL, Japan Science and Technology Agency, 4-1-8 Honcho, Kawaguchi, Saitama, 332-0012, Japan., Materials Research Center for Element Strategy, Tokyo Institute of Technology, 4259 Nagatsuta, Midori-ku, Yokohama 226-8503, Japan., and Laboratory for Materials and Structures, Tokyo Institute of Technology, 4259 Nagatsuta, Midori-ku, Yokohama 226-8503, Japan. E-mail: [email protected]

∗

To whom correspondence should be addressed Institute of Materials Structure Science, High Energy Accelerator Research Organization, 1-1 Oho, Tsukuba, Ibaraki 305-0801, Japan. ‡ Department of Materials Structure Science, School of High Energy Accelerator Science, SOKENDAI (the Graduate University for Advanced Studies), 1-1 Oho, Tsukuba, Ibaraki 305-0801, Japan. ¶ ACCEL, Japan Science and Technology Agency, 4-1-8 Honcho, Kawaguchi, Saitama, 332-0012, Japan. § Materials Research Center for Element Strategy, Tokyo Institute of Technology, 4259 Nagatsuta, Midoriku, Yokohama 226-8503, Japan. k Laboratory for Materials and Structures, Tokyo Institute of Technology, 4259 Nagatsuta, Midori-ku, Yokohama 226-8503, Japan. †

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Abstract Key functions for the stability and activity of Ru catalysts for ammonia synthesis were examined by x-ray absorption fine structure (XAFS) experiments. Ru K-edge XAFS measurements were carried out for two Ru catalysts, Ru/Ca2 NH and Ru/CaNH, which include N atoms in the supports. The stable Ru–N bond was observed in the Ru/Ca2 NH catalyst, but not in the Ru/CaNH catalyst, which was also supported by theoretical calculations in terms of Ru–anion bond strength. The bond observed in the Ru/Ca2 NH works to anchor the Ru nanoparticles to the Ca2 NH support, and the activity has been maintained. Formation of anchoring bonds between catalyst particles and supports is essential to create highly active and long lasting catalysts.

Introduction Industrial ammonia synthesis is essential for producing foods and plants since ammonia is used to manufacture synthetic fertilizers. In addition, ammonia has attracted much attention as a hydrogen source, which can be a type of fuel. The conventional industrial process known as Haber–Bosh process 1 is usually operated under high temperatures (673–773 K) and pressures (10–30 MPa), and is energy consuming. Many highly active catalysts have been developed for ammonia synthesis. 2,3 Efficient catalysts, which can work under moderate conditions, have been developed by utilizing Ru as catalyst particles. 4–7 Recently, a Ru-loaded electride [Ca24 Al28 O64 ]4 (e− )4 (Ru/C12A7:e− ) catalyst was reported to reduce the apparent activation energy of ammonia synthesis by half. 8,9 The stable electride C12A7:e− works as an efficient electron donor, 10 which provides electrons to Ru particles so as to dissociate the triple bond of N2 easily. Consequently, N2 dissociation process is no longer a rate-determining step, and the bottleneck is shifted to the step to form N–Hn species. 11 Ru/Ca(NH2 )2 catalyst also shows the high activity and excellent stability for ammonia synthesis. 12 The excellent stability results from formation of Ru–N bonds at the Ru–support interface, which was observed by XAFS 2


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measurements, although the surface composition of the Ru/Ca(NH2 )2 catalyst was found to be significantly changed from the stoichiometry of Ca(NH2 )2 . It is also important to handle hydrogen not to cause hydrogen poisoning. 13–16 In the Ru/C12A7:e− system, the C12A7:e− support can reversibly store hydrogen in its cage and prevent Ru from hydrogen poisoning. 8 A reversible exchange of electrons and hydride ions was reported for a Ru/Ca2 NH catalyst, which also exhibits a high activity for ammonia synthesis. 17 The Ca2 NH phase, which contains H− ions in the framework, is formed from Ca2 N ([Ca2 N]+ ·e− ) during ammonia synthesis. The reversible reaction of an anionic electron − with hydrogen ([Ca2 N]+ · e− + xH ↔ [Ca2 N]+ · e− 1−x Hx ) causes the formation of hydrogen

deficient Ca2 NH with a low work function (2.3 eV), which accounts for high electron donating ability. The high electron donating ability is also confirmed by XAFS analysis, 18 which revealed that Ca2 NH is close to the state of metallic Ca. In addition, the hydrides suppress H2 poisoning of the Ru surface due to their hydrogen storage properties. 17 The high activity of Ru/Ca2 NH for ammonia synthesis results from both the electron donation function and exchangeability of electrons and hydride ions. However, it is not clearly evident why the high activity stably goes on. We have investigated an essential factor in the long lasting stability of the activity. The local structure around Ru atoms in the Ru/Ca2 NH catalyst was examined by x-ray absorption fine structure (XAFS) experiments. Key functions for the stability and activity will be discussed by comparing with XAFS experiments on a Ru/CaNH catalyst, which contains NH2− ions (the formal charge of hydrogen is +1). Here we report that our XAFS study found Ru–N interactions in the Ru/Ca2 NH catalyst. The Ru–N bond, which is formed between Ru and N atoms of the Ca2 NH support, is one of the key factors to compose the highly active and long lasting catalyst, Ru/Ca2 NH. The bond works to anchor the Ru nanoparticles to the Ca2 NH support, and the activity has been maintained. The stable Ru–N bond was observed in the Ru/Ca2 NH catalyst, but not in the Ru/CaNH catalyst. Theoretical calculations also revealed that Ru–N bond in Ru/Ca2 NH

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is much stronger than that in Ru/CaNH. Formation of anchoring bonds between catalyst particles and supports is one of the most important factors to create highly active and long lasting catalysts.

Experimental The procedures of sample preparation were fully written in supporting information of a previous report, 17 and briefly mentioned here. Ca2 N powder was produced by solid-state reaction of Ca3 N2 powder and Ca metal shot at a molar ratio of 1:1. The mixture of them was pressed to form a pellet under pressure. The pellet covered with molybdenum foil was sealed in an evacuated silica tube. The silica tube was heated, and quenched into water. The obtained sample was ground into its powder form under an Ar atmosphere. CaNH was prepared by heating Ca3 N2 at 873 K under a flow of a H2 and N2 gas mixture. Ru particles were loaded on the Ca2 N and CaNH as follows. Ru3 (CO)12 and the powder of Ca2 N or CaNH were sealed in an evacuated silica tube. They were heated up to 523 K. Before STEM and XAFS measurements, Ru-loaded Ca2 N and CaNH were heated at 613 K for 10 h under N2 and H2 gas flow. During this pretreatment, Ca2 N was completely converted into Ca2 NH. Ammonia synthesis was conducted using 0.1 g of catalyst in a fixed bed flow system with a synthesis gas (H2 :N2 = 3:1) at a flow rate of 60 mL min−1 . The morphology and size of Ru particles were observed by scanning transmission electron microscope (STEM; JEM-2100F, JEOL) with an aberration corrector at the accelerating voltage of 200 kV. High-angle annular dark field imaging (HAADF) was used in the STEM mode. For sample preparation, the catalyst powder was dispersed into hexane solution, and then the solution was dropped to a copper grid with holey carbon film. The XAFS measurements were performed at the beamline AR-NW10A of PF-AR, the Photon Factory at the Institute of Materials Structure Science, High Energy Accelerator

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Research Organization, Tsukuba, Japan. The AR-NW10A has a Si(311) double-crystal monochromator, a focusing mirror, and two flat mirrors for higher order reduction. The slit aperture is 2 mm × 1 mm. The samples with Ru loading amounts of 0.1wt% were measured by fluorescence yield mode by using a Ge pixel array detector (Canberra). XAFS data were analyzed by using the softwares named ATHENA and ARTEMIS, 19 and the FEFF6 code 20 was employed to calculate theoretical XAFS spectra. Ru–Ru and Ru–N paths were obtained by calculations of Ru bulk metal and Ru–N6 cluster, respectively, and these paths were used in fitting procedures.

Computational Density functional theory (DFT) calculations were conducted to determine the adsorption energies of a Ru atom on Ca2 NH and CaNH supports. Vienna ab initio simulation package (VASP 5.2) 21,22 was used for structural relaxation and total energy calculations. The core electrons are handled in the projector augmented wave (PAW) method; 23 3s, 3p, 4s electrons of Ca, 2s, 2p electrons of N, 1s electrons of H and 5s, 4p, 4d electrons of Ru are represented with wave functions. The electron exchange correlation was described as generalized gradient approximation of Perdew-Burke-Ernzerhof type. 24 To simulate the adsorption energies of a Ru atom on Ca2 NH and CaNH, we adopted Ca2 NH(100) and CaNH(001) surfaces, which are the most stable surfaces among the low-index surfaces (See Supporting Information for details). The calculated models of Ru-loaded Ca2 NH and CaNH were constructed as slab models including sufficient vacuum regions. The vacuum width of 20 ˚ A was used in the slab models to avoid artificial interactions between slabs periodically repeated along the normal axis of the surfaces. The cutoff energy of plane wave basis set was 500 eV. Monkhorst-Pack k-point grid 25 for the first Brillouin zone sampling was 3×3×1. The convergence criteria of energy and force are respectively 1.0×10−6 eV and 1.0×10−2 eV/˚ A for all models.

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Results and discussion Catalytic properties of the Ru catalysts are listed on Table 1. The data of the Ru 1.8wt% loaded catalysts at 613 K are cited from the recent study. 17 Those of the Ru 0.1wt% loaded catalysts were measured by the same procedure. The Ru 1.8wt%/Ca2 NH catalyst exhibits over ten times higher ammonia synthetic rate, rNH3 , than the Ru 1.8wt%/CaNH catalyst. The activation energy of the Ru 1.8wt%/Ca2 NH catalyst is about half of that of the Ru 1.8wt%/CaNH catalyst. Even at the very low Ru loading amount of 0.1wt%, the Ru/Ca2 NH catalyst has a relatively large value of rNH3 = 0.587 mmol g−1 h−1 . Table 1: Catalytic properties of Ru catalysts on ammonia synthesis at 613 K and 0.1 MPa. Catalyst

Ru loading (wt%) Ru/Ca2 NH 1.8 0.1

Ns (m2 g−1 ) 3.3×1018 a 6.2×1018 b

Am (Ru) (mmol g−1 h−1 ) 11.3 a 411.9 b

rNH3 (kJ mol−1 ) 3.386 0.587

Ea 60

17

Ru/CaNH

4.7×1018 a 2.5×1018 b

16.0 a 155.5 b

0.308 0.011

110

17

1.8 0.1

Reference

Ns : The number of surface Ru sites. Am (Ru): Surface area of Ru per Ru weight. a These values were calculated on the basis of CO chemisorption, assuming spherical metal particles and a stoichiometry of Ru/CO = 1. b These values were determined by averaging the particle size distribution measured using STEM (Fig. 1).

The morphological and local structures of these catalysts have been investigated to clarify key factors for the high catalytic activity. HAADF-STEM images of the Ru 0.1wt%/Ca2 NH and Ru 0.1wt%/CaNH catalysts were shown in Fig. 1. Ru nanoparticles were observed as bright spots. The mean size of Ru nanoparticles on Ca2 NH was 1.17 nm, which was less than half that of Ru nanoparticles on CaNH of 3.10 nm. The size distribution of Ru 0.1wt%/Ca2 NH is sharp, and the standard deviation is 0.24. The Ru 0.1wt%/CaNH shows a broad distribution with a tail to bigger side with standard deviation of 0.87. 6


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Figure 1: HAADF-STEM images of (a) Ru 0.1wt%/Ca2 NH and (b) Ru 0.1wt%/CaNH. Ru nanoparticles are shown as bright spots. Particle size distributions of (c) Ru 0.1wt%/Ca2 NH and (d) Ru 0.1wt%/CaNH. The mean sizes of Ru 0.1wt%/Ca2 NH and Ru 0.1wt%/CaNH are 1.17 and 3.10 nm, respectively.

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As summarized in Table 1, the number of surface Ru sites, Ns , of Ru 0.1wt%/Ca2 NH is 2.65 times as much as that of Ru 0.1wt%/CaNH. However, there is more than 50 times difference in the catalytic activity between these two catalysts. Although the catalytic activity is related to the number of surface Ru sites, such a large activity difference is mainly attributed to electron donating ability of the support material. As we demonstrated in the previous study, 17 the work function of Ca2 NH (2.3–2.8 eV) is much smaller than that of CaNH (=3.6 eV). Accordingly, the electron transfer from Ca2 NH to Ru nanoparticles causes a substantial lowering of the work function of Ru, resulting that N2 dissociation over Ru catalyst is significantly enhanced. The activity difference between Ru 1.8wt%/Ca2 NH and Ru 1.8wt%/CaNH is much smaller (∼10 times) than the case of Ru 0.1wt% catalysts. It can be considered that the activity of small-sized Ru nanoparticles is strongly influenced by electron donation form Ca2 NH because the fraction of Ru atoms connected directly with Ca2 NH is much higher than the case of large Ru particles. A similar result was reported for Ru-loaded C12A7:e− with low work function (2.4 eV). 8 XAFS studies on the Ru 0.1wt%/Ca2 NH and Ru 0.1wt%/CaNH catalysts were carried out. These smaller loading amounts of Ru enable us to observe interactions at the interface of Ru particles and supports. XANES spectra of Ru 0.1wt%/Ca2 NH and Ru 0.1wt%/CaNH, which were sampled after ammonia synthesis reactions at 613K, are shown in Fig. 2 together with those of Ru and RuO2 standards. The absorption edge positions of Ru 0.1wt%/Ca2 NH and Ru 0.1wt%/CaNH are at the same as that of Ru metal. These Ru nanoparticles were in their metallic states. The spectral features of Ru 0.1wt%/CaNH match those of Ru bulk metal. The Ru 0.1wt%/Ca2 NH catalyst has a broad and featureless spectrum, which differs from Ru bulk metal, showing that it would be very small nanoparticles. The 5p electronic structure was altered by the particle size effects. A similar feature was observed for Ni nanoparticles 26 and calculated for Pd nanoparticles. 27 The absorption spectra, EXAFS oscillations k 2 χ(k), and Fourier transforms (FTs) are shown in Fig. 3. The EXAFS oscillations k 2 χ(k) of Ru 0.1wt%/Ca2 NH and Ru 0.1wt%/CaNH

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1.2

1.0 ytisnetnI dezilamroN

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0.8

0.6

0.4

0.2

0.0 22080

22120

22160

22200

Photon Energy (eV)

Figure 2: XANES spectra of Ru 0.1wt%/Ca2 NH (blue) and Ru 0.1wt%/CaNH (red). Those of Ru metal (solid black) and RuO2 (dashed black) are also plotted. The samples are in the metallic state since the edge positions are the same as Ru metal.

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1.2

(a)

1.0 ytisnetnI dezilamroN

0.8

0.6

0.4

0.2

0.0 22000

22200

22400

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Photon Energy (eV)

3

(b)

2

c

)k( k ,noitallicsO SFAXE

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0

-1

-2

-3

0

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Wave Number k(

5

10 -1

12

14

)

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3

2

1

0 0

1

2

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Distance ()

Figure 3: (a) The absorption spectra, (b) EXAFS oscillations k 2 χ(k), and (c) FTs of Ru 0.1wt%/Ca2 NH (blue) and Ru 0.1wt%/CaNH (red). The spectra of Ru metal (solid black) and RuO2 (dashed black) are also plotted. 10


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oscillate in almost the same phase as that of Ru metal. The amplitude of Ru 0.1wt%/Ca2 NH is significantly suppressed, showing the smallness of the particles. A peak appeared at ∼ 2.3–2.4 ˚ A in each FT of Ru 0.1wt%/Ca2 NH and Ru 0.1wt%/CaNH samples results from a Ru–Ru bond, since the peak position is consistent with Ru metal. The FT of Ru 0.1wt%/CaNH was well fitted by Ru metal. The bond length and coordination number (CN) were obtained to be 2.67 ˚ A and 8.6 for the Ru–Ru interaction. The distance of 2.67 ˚ A perfectly corresponds to the bulk value, and the CN of 8.6 indicates small but not very tiny particles. 0.8

0.6 edutingaM TF

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Experiment Fitting curve Ru-Ru

0.4

Ru-N

0.2

0.0 0

1

2

3

4

5

6

Distance ()

Figure 4: The EXAFS curve fitting result for Ru 0.1wt%/Ca2 NH. The FT of the sample is plotted as filled circle. Red, blue, and green lines denote fitting curve, Ru–Ru path, and Ru–N path, respectively. The Ru particles are rigidly bonded to the N atoms of the support. The peak height of the Ru 0.1wt%/Ca2 NH is significantly small showing that the particles are very tiny. The EXAFS fitting spectra of the Ru 0.1wt%/Ca2 NH are shown in Fig. 11



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4. There is another peak at around 1–2 ˚ A, which was not recognized in the FT of Ru 0.1wt%/CaNH. One might think it Ru–O interaction, but it is not. As shown in Fig. 2, the Ru nanoparticles of Ru 0.1wt%/Ca2 NH were in their metallic state. That is, it was not oxidized, and a possibility of Ru–O interaction should be excluded. The shorter interaction was assigned to Ru–N interaction by careful EXAFS analysis, which was similar to the Ru–N bond reported for Ru/Ca(NH2 )2 catalysts. 12 These two peaks were fitted well by Ru–Ru and Ru–N interactions. The bond length of 2.64 ˚ A and CN of 3.0 were obtained for the Ru–Ru path, and 1.99˚ A and 2.2 for the Ru–N path (for fitting details, see Supporting Information). The small CN of Ru–Ru interaction denotes that the particles are very tiny, which is in good agreement to the HAADF-STEM image showing the mean diameter of 1.17 nm on Fig. 1(a). The CN of 2.2 for Ru–N interaction reflects strong and stable bonding between Ru nanoparticles and N atoms of the support. We suggest that the Ru–N bond works as an anchor, which prevents the Ru nanoparticles from moving and gathering together to be large particles. As a result, Ru nanoparticles on Ca2 NH stably stayed at their anchored spots, where they had been originally created during the catalyst preparation process. Ru nanoparticles on CaNH, however, aggregated together into larger particles, probably because of lack of anchoring bonds between Ru nanoparticles and CaNH support. The wider size distribution with tail shown in Fig. 1(d) implies that the aggregation of nanoparticles occurred during ammonia synthesis reaction. To understand the large difference on the Ru particle sizes on the two supports, a simple model based on Ru-anion bond strength (i.e., adsorption energy) was adopted, which was already succeeded in the explanation of the trend of Ru particle sizes on nitride, oxide, fluoride, and silicide. 28 Calculated adsorption energies of single Ru atom on Ca2 NH and CaNH were -4.59 and -2.64 eV, respectively, for the ontop site of N in the supports. In addition, the adsorption using the ontop site of H is acceptable only for Ca2 NH in terms of the adsorption structures determined by the DFT calculations (See Table 2). Actually, when structural optimization was performed for Ru/CaNH system with a Ru atom positioned at

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the ontop site of H as a starting structure, the Ru atom moved to the ontop site of N. The different behaviors of Ru on the ontop site between Ca2 NH and CaNH are explained as follows: H in Ca2 NH is anion (H− ) and Ru–H bond can be formed, however H in CaNH is cation (H+ ) and Ru–H bond is not able to be formed. That is, the strength of Ru sticking on the supports is clearly stronger on Ca2 NH than on CaNH in terms of both adsorption energies and variation of adsorption sites. Both N and H in Ca2 NH are anions as N3− and H− , which can be used as the adsorption centers for Ru, whereas the adsorption centers in CaNH come only from N of the NH units. Because of the chemical bond between N and H in the NH unit, the number of valence electrons of N in the NH unit for bond formation with Ru is decreased from N3− in Ca2 NH. That’s why we obtained the large difference of Ru–N bond strengths between Ru/Ca2 NH and Ru/CaNH. Since a strong bond of Ru–anion could lead to small Ru particles on supports, the calculated trend on Ru-anion bond strength corresponds well to the experimental trend of Ru particle sizes on Ca2 NH and CaNH. Table 2: Adsorption energy of single Ru atom on Ca2 NH and on CaNH. System Site Ead (eV) Ru on Ca2 NH N ontop -4.59 H ontop -2.87 Ru on CaNH

N ontop -2.64 H ontop N/A

Conclusions An essential factor to create highly active and long lasting Ru catalysts for ammonia synthesis was investigated. The interactions between Ru nanoparticles and supports were examined by XAFS experiments for Ru/Ca2 NH and Ru/CaNH catalysts. The highly active Ru/Ca2 NH catalyst, in which both N and H are anions, forms the anchoring bond between Ru and

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N atoms of Ca2 NH. No anchoring bond was observed in the Ru/CaNH catalyst, where H exists as proton. Theoretical calculations also revealed that Ru–N bond in Ru/Ca2 NH is much stronger than that in Ru/CaNH. The stable Ru–N bond found in the Ru/Ca2 NH catalyst works to anchor the Ru nanoparticles to the Ca2 NH support, and the ammonia synthesis activity has been maintained. Formation of anchoring bonds between catalyst particles and supports is essential to create highly active and long lasting catalysts.

Acknowledgement This work was supported by a fund from Accelerated Innovation Research Initiative Turning Top Science and Ideas into High-Impact Values (ACCEL) of Japan Science and Technology Agency in Japan. The XAFS study was carried out under the approvals of PF-PAC Nos. 2013S2-002 and 2016S2-004. Prof. Y. Murakami and Prof. J. Yamaura are appreciated for fruitful discussions.

Supporting Information Available Details on EXAFS fittings and DFT calculations are written in Supporting Information. This material is available free of charge via the Internet at http://pubs.acs.org/.

References (1) Mittasch, A.; Frankenburg, W. Early Studies of Multicomponent Catalysts. Adv. Catal. 1950, 2, 81–104. (2) Aika, K.; Hori, H.; Ozaki, A. Activation of Nitrogen by Alkali Metal Promoted Transition Metal I. Ammonia Synthesis over Ruthenium Promoted by Alkali Metal. J. Catal. 1972, 27, 424–431.

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(3) Ozaki, A. Development of Alkali-Promoted Ruthenium as a Novel Catalyst for Ammonia Synthesis. Acc. Chem. Res. 1981, 14, 16–21. (4) Aika, K.; Takano, T.; Murata, S. Preparation and Characterization of Chlorine-Free Ruthenium Catalysts and the Promoter Effect in Ammonia Synthesis. J. Catal. 1992, 136, 126–140. (5) Zhong, Z.; Aika, K. Effect of Hydrogen Treatment of Active Carbon as a Support for Promoted Ruthenium Catalysts for Ammonia Synthesis. Chem. Commun. 1997, 1223–1224. (6) Zeng, H. S.; Inazu, K.; Aika, K. The Working State of the Barium Promoter in Ammonia Synthesis over an Active-Carbon-Supported Ruthenium Catalyst Using Barium Nitrate as the Promoter Precursor. J. Catal. 2002, 211, 33–41. (7) You, Z.; Inazu, K.; Aika, K.; Baba, T. Electronic and Structural Promotion of Barium Hexaaluminate as a Ruthenium Catalyst Support for Ammonia Synthesis. J. Catal. 2007, 251, 321–331. (8) Kitano, M.; Inoue, Y.; Yamazaki, Y.; Hayashi, F.; Kanbara, S.; Matsuishi, S.; Yokoyama, T.; Kim, S.-W.; Hara, M.; Hosono, H. Ammonia Synthesis Using a Stable Electride as an Electron Donor and Reversible Hydrogen Store. Nat. Chem. 2012, 4, 934–940. (9) Kanbara, S.; Kitano, M.; Inoue, Y.; Yokoyama, T.; Hara, M.; Hosono, H. Mechanism Switching of Ammonia Synthesis Over Ru-Loaded Electride Catalyst at Metal-Insulator Transition. J. Am. Chem. Soc. 2015, 137, 14517–14524. (10) Matsuishi, S.; Toda, Y.; Miyakawa, M.; Hayashi, K.; Kamiya, T.; Hirano, M.; Tanaka, I.; Hosono, H. High-Density Electron Anions in a Nanoporous Single Crystal: [Ca24 Al28 O64 ]4+ (4e− ). Science 2003, 301, 626–629.

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Crucial for the High Ammonia Synthesis Activity - ACS Publications

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