Synergistic catalysis of Au-Co@SiO 2 nanospheres in hydrolytic dehydrogenation of ammonia borane for chemical hydrogen storage† Zhang-Hui Lu, a Hai-Long Jiang, a Mahendra Yadav, a Kengo Aranishi ab and Qiang Xu * abc Received 26th September 2011, Accepted 19th January 2012 DOI: 10.1039/c2jm14787d Core–shell structured Au-Co@SiO 2 nanospheres have been synthesized using a reverse-micelle method. During heat treatment in vacuum, multiple Au-Co nanoparticles (NPs) embedded in SiO 2 nanospheres (Au-Co@SiO 2 -RT) merged into single Au-Co NPs in SiO 2 (Au-Co@SiO 2 -HT), resulting in a size increase of the Au-Co NPs. The Au-Co@SiO 2 -HT nanospheres showed better catalytic activity than that of Au-Co@SiO 2 -RT. The higher catalytic activity of Au-Co@SiO 2 -HT could be attributed to the decrease in the content of basic ammine by the decomposition of metal ammine complexes during the heat treatment. Compared with their monometallic counterparts, the bimetallic Au-Co NPs embedded in a SiO 2 nanosphere show higher catalytic activity for the hydrolytic dehydrogenation of NH 3 BH 3 to generate a stoichiometric amount of hydrogen at room temperature for chemical hydrogen storage. The synergistic effect between Au and Co inside the silica nanospheres plays an important role in the catalytic hydrolysis of NH 3 BH 3 . 1. Introduction Hydrogen has attracted considerable attention as a globally accepted clean energy carrier. Currently, the search for safe and efficient hydrogen storage materials is one of the most difficult challenges for the transformation to a hydrogen-powered society as a long-term solution for a secure energy future. To meet the US Department of Energy (DOE) target for on-board applica- tions, hydrogen storage materials must have a high gravimetric hydrogen capacity with a rapid hydrogen release rate. There have been a large number of reports on hydrogen storage materials. 1–9 However, big challenges still remain. Ammonia borane (NH 3 BH 3 , AB) has a hydrogen capacity as high as 19.6 wt%, exceeding that of gasoline and making it an attractive candidate for chemical hydrogen storage applications. 10–19 Intensive efforts have been made to enhance the kinetics of the hydrogen release from this compound from both solid and solution approaches. 20–27 In this laboratory, we have thus been exploring efficient and economical catalysts 24–27 and are systematically investigating cobalt-based catalysts that exhibit high efficiency for hydrogen generation. 28–31 Nanoparticles (NPs) coated on spherical silica or within silica hollow spheres have attracted increasing attention in the fields of catalysis, 32,33 absorbents, 34,35 photonics, 36,37 and magnetics. 38 Different approaches have been developed to synthesize such materials, most of which were micrometer sized particles. 39,40 Due to the possibility of obtaining monodisperse NPs, the synthesis of ultrafine metal NPs in a reverse micelle system has received much attention. 32 Some ultrafine metal NPs within silica nanospheres have been successfully synthesized by using a crystal template method in a reverse micelle system. 34,35,41–43 Herein, for the first time, we report the synthesis of two core– shell structured Au-Co@SiO 2 -RT and Au-Co@SiO 2 -HT nano- spheres, of which the latter is prepared by heat treatment of the former, with small Au-Co NPs (<3 nm diameter) embedded in the SiO 2 nanospheres. Compared with monometallic Au@SiO 2 and Co@SiO 2 , both of the Au-Co@SiO 2 catalysts show higher catalytic activity for the hydrolysis of ammonia borane, gener- ating a stoichiometric amount of hydrogen. Unexpectedly, Au- Co@SiO 2 -HT with relatively large single Au-Co NPs embedded in SiO 2 nanospheres presents a better catalytic activity than Au- Co@SiO 2 -RT with multiple small Au-Co NPs embedded in SiO 2 nanospheres, which is due to the decrease in the content of basic ammine by the decomposition of metal ammine complexes during the heat treatment. 2. Experimental 2.1 Chemicals Ammonia-borane (NH 3 BH 3 , Aviabor, 97%), sodium borohyride (NaBH 4 , Aldrich, 99%), hexamminecobalt(III) chloride (Co(NH 3 ) 6 Cl 3 , Mitsuwa Chem. Co., >99.0%), hydrogen tetra- chloroaurate(III) tetrahydrate (HAuCl 4 $4H 2 O, Wako Pure a National Institute of Advanced Industrial Science and Technology (AIST), Ikeda, Osaka, 563-8577, Japan b Graduate School of Engineering, Kobe University, Nada Ku, Kobe, Hyogo, 657-8501, Japan c CREST, Japan Science and Technology Agency (JST), Kawaguchi, Saitama, 332-0012, Japan. E-mail: q.xu@aist.go.jp † Electronic supplementary information (ESI) available. See DOI: 10.1039/c2jm14787d This journal is ª The Royal Society of Chemistry 2012 J. Mater. Chem., 2012, 22, 5065–5071 | 5065 Dynamic Article Links C < Journal of Materials Chemistry Cite this: J. Mater. Chem., 2012, 22, 5065 www.rsc.org/materials PAPER Downloaded by University of Science and Technology of China on 06/05/2013 15:51:59. Published on 03 February 2012 on http://pubs.rsc.org | doi:10.1039/C2JM14787D View Article Online / Journal Homepage / Table of Contents for this issue