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