Article pubs.acs.org/JPCC
New Aspects on the Decomposition of Sodium Alanate Revealed by Small-Angle X-ray Scattering Sabrina Sartori,* Kenneth D. Knudsen, and Bjørn C. Hauback Institute for Energy Technology (IFE), P.O. Box 40, NO-2027 Kjeller, Norway S Supporting Information *
ABSTRACT: Transition metals added to sodium aluminum hydride by high-energy ball milling have been shown to significantly enhance its absorption and desorption properties. In the present study, we have used small-angle X-ray scattering to elucidate how TiCl3 affects the nanostructure of NaAlH4 particles. Scattering data from as-purchased and ball milled NaAlH4 for 6 and 15 min are compared with NaAlH4 ball milled for the same time with 4 mol % TiCl3. Drastic differences were noticed in the two systems which cast a new light on the decomposition of NaAlH4, in particular on the effect of ball milling and of TiCl3 on the morphology, grain size, and distribution of the phases. The particle morphology of pure NaAlH4 showed significant evolution/changes during heating from room temperature to 290 °C, as evidenced by the variations in the power-low scattering parameter, α. Drastic changes were noticed in the particle surface structure during the phase transformation from NaAlH4 to Na3AlH6 + Al, when the system becomes less compact and the particle surface rougher. The addition of TiCl3 induces a different effect on both surface and mass structure, at least in the nanometer length scale considered in this study: the particles retain their surface morphology at all temperatures. Furthermore, even after short ball milling times the addition of TiCl3 increases the system compactness with reduction of internal voids.
1. INTRODUCTION The growing interest toward a hydrogen-based energy economy lies in its clean combustion and the prospect of reduced dependence on fossil fuels. However, there are significant challenges and requirements for on-board storage of H2, such as high reversible storage capacity, fast kinetics, ambient operating temperature, and acceptable safety. Solid state hydrogen materials in which hydrogen is absorbed (and released in a controlled manner) inside the solids are considered to be the most attractive solutions. A large number of hydrides with high theoretical hydrogen content (such as 18.4 wt % H for LiBH4 and 14.9 wt % H for Mg(BH4)2) have been studied during the last years.1The sodium alanate (NaAlH4) system remains one of the most promising, since Bogdanović et al.2 reported that the kinetics and reversibility of NaAlH4 were greatly improved by a titanium-based additive, enabling H absorption/desorption at temperatures around 120−140 °C. The decomposition of NaAlH4 occurs in two distinct steps 3NaAlH 4 ↔ Na3AlH6 + 2Al + 3H2 (1) Na3AlH6 ↔ 3NaH + Al + (3/2)H2
such as LiBH4 and Mg(BH4)2. A wide variety of techniques have been employed to study the effect of Ti additives on NaAlH4,3−41 such as Raman spectroscopy, nuclear magnetic resonance, X-ray absorption, and X-ray photoelectron spectroscopy, among others. Nevertheless, despite years of intense research, the role of the additives remains uncertain. Small-angle scattering techniques42−44 are useful to obtain information on nanoparticles and clusters at length scales in the range of 1 to several hundred nanometers. Variations in the scattering length density, i.e., the electron density for smallangle X-ray scattering (SAXS), give rise to scattering intensity which is increasingly more dominant at low angles as the inhomogeneities become larger. Mass fractal (Dm) and surface fractal (Ds) dimensions can be extracted from the power-law scattering regime of the intensity (I) versus the scattering wave vector (q), where q = |q| = (4π/λ)(sin θ) and 2θ is the scattering angle. When the scattering data satisfy the condition of qR ≫ 1, where R is the particle size, the scattered intensity I(q) can be well approximated as proportional to 1/qα. The power-law scattering exponent α can thus be evaluated from a plot of log (I) versus log q. At low/intermediate q, values of α between 1 and 3 (also called Dm in this range) correspond to a mass fractal (i.e., aggregates of particles). At higher q, the power-law data are related to surface properties through the surface fractal dimension Ds, where α = 6 − Ds. For rough
(2)
giving the theoretical reversible hydrogen storage capacity of 5.6 wt % H. The decomposition of NaH into Na and hydrogen occurs at higher temperatures (>400 °C) and is thus considered too high for applications. Understanding the mechanism of the enhanced kinetics with the addition of transition metals such as Ti is a potential key to destabilizing more stable compounds © 2012 American Chemical Society
Received: July 8, 2011 Revised: January 16, 2012 Published: January 23, 2012 3875
dx.doi.org/10.1021/jp206479m | J. Phys. Chem. C 2012, 116, 3875−3881
The Journal of Physical Chemistry C
Article
particles, the slope parameter α varies between 3 and 4 corresponding to Ds of 3 and 2, respectively. Particles having fully smooth surfaces are characterized by the slope parameter α = 4, and those having a highly rough surface area are characterized by α being close to 3. The effect of several additives on the morphology of NaAlH4 has been measured at room temperature with ultrasmall-angle X-ray scattering.35 In their study, Dobbins et al.35 concluded that among TiCl2, TiCl3 (which give the best kinetic enhancement of NaAlH4), and VCl3 and ZrCl4 (which do not perform as well) the one which provides the highest powder particle surface area at room temperature is TiCl3. These data suggest a correlation between the kinetic activity and the morphological changes on sodium alanate induced by additives. Among other techniques, SAXS has been used in the present work to explore the possible variations upon heating of the mass filling properties and surface structure of the NaAlH4 alone as well as with the addition of TiCl3.
2. EXPERIMENTAL DETAILS Pure NaAlH4 (Aldrich, purity 96%) was ball milled for 6 and 15 min with or without 4 mol % TiCl3 in a Fritsch Pulverisette 7 at 720 rpm and a ball:powder ratio of 20:1. The samples were always kept under a protective argon atmosphere and handled in a glovebox with
