Hydrogen Generation from the Hydrolysis of Ammonia-borane and Sodium Borohydride Using Water-soluble Polymer-stabilized Cobalt(0) Nanoclusters Catalyst O ¨ nder Metin and Saim O ¨ zkar* Department of Chemistry, Middle East Technical UniVersity, 06531, Ankara, Turkey ReceiVed February 27, 2009. ReVised Manuscript ReceiVed May 12, 2009 Polymer-stabilized cobalt(0) nanoclusters were prepared from the reduction of cobalt(II) chloride in the presence of poly(N-vinyl-2-pyrrolidone) (PVP) stabilizer in methanol solution. PVP-stabilized cobalt(0) nanoclusters were found to be stable in solution and could be isolated as solid material and characterized by TEM, XPS, FT-IR, and UV-visible electronic absorption spectroscopy. PVP-stabilized cobalt(0) nanoclusters were employed as catalyst in the hydrolysis of sodium borohydride and ammonia-borane, which have been considered as solid-state hydrogen storage materials for portable fuel cell applications. PVP-stabilized cobalt(0) nanoclusters were found to be highly active catalyst in both hydrolysis reactions, even at room temperature. Kinetic studies show that the catalytic hydrolyses of sodium borohydride and ammonia-borane are both first order with respect to catalyst and substrate concentration in aqueous medium. The effect of the NaOH concentration on the catalytic activity of the PVP-stabilized cobalt(0) nanoclusters in the hydrolysis of sodium borohydride was also studied. The activation parameters of these hydrolysis reactions were determined from the evaluation of the kinetic data. The PVP-stabilized cobalt(0) nanoclusters provide a lower activation energy for the hydrolysis of sodium borohydride both in aqueous medium (E a ) 63 ( 2 kJ · mol -1 ) and in basic solution (E a ) 37 ( 2 kJ · mol -1 ) compared to the value reported for bulk cobalt (E a ) 75 kJ · mol -1 ). 1. Introduction In the past few years, many efforts have been given for the exploration of high-capacity hydrogen storage materials 1 to solve one of the most important problems in the hydrogen economy toward sustainable energy future. 2-5 Among the chemical hydrides tested as solid hydrogen storage materials, 6 sodium borohydride, NaBH 4 , appears to be a suitable hydrogen source, particularly for portable fuel cell applications, 7 due to a number of advantageous properties such as relatively high hydrogen content of 10.7 wt %, which meets the US Department of Energy criteria for hydrogen storage materials (6 wt %), 2 stability under ordinary conditions in solid state and solution, and liberating hydrogen in controllable way upon hydrolysis, yielding just environmentally benign products. Recent studies have shown 8 that ammonia-borane, H 3 NBH 3 , needs to be considered as hydrogen storage material as it has high hydrogen content 19.6 wt % and high stability under the ambient conditions. 9 Sodium borohydride and ammonia-borane both liberate hydrogen upon hydrolysis as given in eqs 1 and 2, respectively, only in the presence of suitable catalysts. Our recent studies 10 have shown that using water-dispersible transition metal(0) nanoclusters is a promising way to increase the catalytic activity in the hydrolysis of sodium borohydride, as the activity of heterogeneous catalyst is directly related to its surface area. Very recently we have also shown that zeolite- framework-stabilized rhodium(0) nanoclusters are highly active catalysts for hydrogen generation from the hydrolysis of ammonia-borane. 11 Here, we report for the first time the synthesis, characterization, and employment of PVP-stabilized cobalt(0) nanoclusters as highly active catalyst in hydrogen generation from the hydrolysis of sodium borohydride and ammonia-borane. PVP-stabilized cobalt(0) nanoclusters were prepared from the reduction of cobalt(II) chloride by sodium borohydride in the presence of PVP in methanol solution. PVP- stabilized cobalt(0) nanoclusters were found to be stable in solution and could be isolated as solid material and characterized by TEM, XPS, FT-IR, and UV-visible electronic absorption spectroscopy. The effect of the NaOH concentration on the catalytic activity of the PVP-stabilized cobalt(0) nanoclusters in the hydrolysis of sodium borohydride was also examined. The kinetics of the hydrolysis reactions were studied by * To whom correspondence should be addressed: E-mail: sozkar@ metu.edu.tr; phone: +90 312 210 3212; fax: +90 312 210 3200. (1) Schlapbach, L.; Zu ¨ttel, A. Nature 2001, 414, 353–358. (2) Basic Research Needs For the Hydrogen Economy, Report of the Basic Energy Sciences Workshop on Hydrogen Production, Storage and Use, May 13-15, 2003; Office of Science, U. S. Department of Energy; URI: http://www.sc.doe.gov/bes/hydrogen.pdf. (3) Annual Energy Outlook 2005 With Projections to 2025; Energy Information Administration: February 2005; URI: http://www.eia.doe.gov/ oiaf/aeo/pdf/0383(2005).pdf (4) Turner, J.; Sverdrup, G.; Mann, K.; Maness, P. G.; Kroposki, B.; Ghirardi, M.; Evans, R. J.; Blake, D. Int. J.Energy Res. 2008, 32, 379– 407. (5) IAC Report, Lighting the Way Towards a Sustainable Energy Futures; Interacademy Council: Amsterdam, 2007. (6) Hydrogen, Fuel Cells & Infrastructure Technologies Program; US DOE; URI: http://www.eere.energy.gov/hydrogenandfuelcells/storage. NaBH 4 (aq) + 2H 2 O(l) 9 8 catalyst NaBO 2 (aq) + 4H 2 (g) (1) H 3 NBH 3 (aq) + 2H 2 O(l) 9 8 catalyst (NH 4 )BO 2 (aq) + 3H 2 (g) (2) Energy & Fuels 2009, 23, 3517–3526 3517 10.1021/ef900171t CCC: $40.75  2009 American Chemical Society Published on Web 06/15/2009 Downloaded by COLORADO ST UNIV FT COLLINS on July 16, 2009 Published on June 15, 2009 on http://pubs.acs.org | doi: 10.1021/ef900171t