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 reservedsurface 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.