Supporting Information:

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1 Supporting Information: Synthesis of Colloidal Magnesium: A Near Room Temperature Store for Hydrogen Kondo-Francois Aguey-Zinsou* and José-Ramón Ares-Fernández Department of Materials, Queen Mary, University of London, London E1 4NS, UK Laboratorio de Materiales de Interés Energético, Department of Materials, Madrid University, Cantoblanco 28049, Madrid, Spain Fax: (+) f.aguey@qmul.ac.uk or f.aguey@ucl.ac.uk 1

2 Materials and Methods All experiments were performed under inert atmosphere. Tetrabutylammonium bromide (99 %, Sigma-Aldrich) and magnesium acetate (99 %, Sigma-Aldrich) were dried at 70 C for 72 h at high vacuum prior use. THF was distilled over sodium and benzophenone, and stored under Ar over a 4 Å molecular sieve. A Mg ribbon (3.5 mm wide, 0.35 mm thick, 99 + %, Sigma-Aldrich) and a Pt/Rd gauze (90:10 wt %, 80 mesh, 99.9 %, Alfa Aesar) were used as anode and cathode respectively. Before use, the Mg ribbon was polished with a fine grinding paper, then sonicated in ethanol and rinsed with acetone. The Pt/Rh gauze was washed with water, ethanol and acetone. Both electrodes were dried under vacuum at room temperature. The electrochemical synthesis of the Mg nanoparticles was done in a home made electrolysis cell. A 50 ml solution of tetrabutylammonium bromide (0.1 M) in THF was placed in the cell and magnesium acetate (70 mg) was dissolved in the electrolyte by stirring. A Mg ribbon (99 + %, Sigma-Aldrich) and a Pt/Rd gauze (90:10 wt %, 99.9 %, Alfa Aesar) were used as anode and cathode respectively and cleaned prior use. A current of 0.8 ma was applied and the electrolysis was stopped after reaching 210 C. The precipitate was separated by centrifugation, washed with THF and dried at 50 C under a high vacuum. The content of magnesium in the light gray powder obtained was determined by Atomic Absorption Spectroscopy (AAS). Prior analysis, the colloid (84 mg) was dissolved into 10 ml of nitric acid. According to AAS, the colloid contains 18 mass % of magnesium. Hydrogen was absorbed in Mg colloid by using a home made Sievert apparatus. High purity hydrogen ( %) was used and MgH 2 was formed after 5 h. 2

3 TEM characterizations were carried out on a JEOL 2010 microscope operated at 200 kv. The thermal stability of the colloid was characterized with a Thermogravimetric Analyser (Setaram) equipped with a DTA probe. The instrument was coupled with a Mass Spectrometer (Pfeiffer) for in-situ gas analysis. All measurements were performed under a 40 ml.min -1 flow of high purity Ar (99.999%) with a heating rate of 5 C.min -1. 3

4 DTA (µv) Endothermic 100 C 125 C 210 C 270 C 326 C Temperature ( C) TG (%) Figure S1. Thermogram (TG) of tetrabutylammonium bromide (TBA) and associated Differential Thermal Analysis curve (DTA). The DTA curve shows five main endothermic peaks. The first peak at 100 C corresponds to the melting of TBA. The second one at 125 C is a phase transition 1. The other peaks are related to the decomposition of TBA. 4

5 Transmittance (arbitrary units) (CH) from tetrabutylammonium Wavenuumbers (cm -1 ) Figure S2. Infrared spectrum of the Mg colloid. The spectrum was obtained with a Digilab Excalibur infrared spectrophotometer, equipped with a Diffuse Reflectance Infrared Fourier Transform (DRIFT) accessory. The scan resolution was 4 cm -1. 5

6 a) b) Mg colloid at 345 C Intensity (arbritary units) Mg colloid at 175 C Tetrabutylammonium bromide at 225 C Intensity (arbitrary units) CH 4 N 2 C 2 H 6 C 3 H m/z Temperature ( C) Figure S3. Mass spectral data for the decomposition of TBA and the Mg colloid. a) The mass spectra were recorded during the main decomposition steps. b) Total ion current of some of the gases resulting from the decomposition of the MgH 2 colloid. The colloid decomposes in two steps during which it losses 3.3 mass % between 150 and 300 C, and 42.3 mass % above 300 C (Figure 2a). Both mass losses correspond to the decomposition of TBA, as proven by mass spectrometry analysis of the evolved gases (Figure S3). The first decomposition is most likely due to some TBA salt left within the colloidal structure, while the second decomposition is related to the TBA shell stabilising the magnesium nanoparticles. This is highlighted by the Differential Thermal Analysis (DTA) of the colloid (Figure 2b). At 165 C, a small endothermic peak corresponding to the first decomposition is observed. This peak is followed by a double endothermic peak 6

7 and an additional exothermic peak at 285 C. While the two endothermic peaks are due to the melting and phase transition of the TBA 1 composing the shell of the magnesium core; the exothermic peak (not observed for pristine TBA, Figure S1) indicates the reaction of the magnesium nanoparticles with the TBA. Finally, the decomposition of the TBA shell is evidenced by a large endothermic peak at 340 C; a higher temperature compared to pristine TBA. 7

8 2 1 ln( -ln(1-f()) ) slope n = ln (time) Figure S4. Fit of the reacted fraction F() according to equation (1). The nucleation and growth of a new phase obeys to the following rate expression: n 1/ or [ 1 F( ))] n = kt F ( ) = 1 exp( kt) ln( (1) with n depending on the dimensionality of the growth process and its type 2. For a constant number of nuclei during the dehydriding process, n = d/m; and if the number of nuclei is increasing during the process, n = 1 + d/m. d is related to the dimensionality of the process (d = 1, 2 or 3), and m corresponds to the type of growth. For m = 1, the process is controlled by the reaction at the Mg/MgH 2 interface. For m = 2, the process is controlled by the diffusion of the hydrogen atoms. In the present investigation, the reacted fraction curved could be fitted by the rate expression (1) as shown Figure S4. Therefore, the rate limiting step of the dehydriding 8

9 reaction of the MgH 2 colloid is the nucleation and growth of the -phase. In addition, the exponent value of n = 2 obtained, suggest the followings: (a) a continuous nucleation and growth with a number of nuclei increasing with time, and a one dimensional interface process, or (b) a constant number of nuclei with an interface or diffusion controlled reaction. From the sole result of the exponent value, we cannot totally define the characteristics of the nucleation process, i.e. constant nucleation rate or constant number of nuclei. However, the number of nuclei would probably be constant unless a high driving force such as a higher equilibrium pressure for the MgH 2 colloid as compared to bulk MgH 2 exists. In addition, it is more likely that the process would be controlled by the reaction at the Mg/MgH 2 interface because hydrogen diffusion should be fast. References. (1) Wyrzykowskia, D.; Manieckib, T.; Pattek-Janczykc, A.; Stanekd, J.; Warnkea, Z. Thermochim. Acta 2005, 435, (2) Mintz, M. H.; Zeiri, Y. J. Alloys Compd. 1994, 216,