Jerome ANDRIEUX a, Laetitia LAVERSENNE b, Raphael FILLON b, Laurence LYARD b, Thomas BUSLAPS a, Veijo HONKIMÄKI a.
a ID15, ESRF, 6 rue Jules Horowitz, 38043 Grenoble, France
b CNRS-Institut Néel, 38042 Grenoble, France
Recent improvements of ID15B beamline at ESRF allow in-situ study of gas/solid or liquid/solid reactions taking advantages of both combined techniques (SXRD/MS) and hard X rays (high volume gauge, high Q range, short acquisition time) (high pressure - high temperature gas loading system). The high temperature (700 °C) / high pressure (200 bars) gas loading system has been used to probe the dehydrogenation properties of LiBH4-MgH2 composite catalyzed by a transition metal catalyst (TM).
2LiBH4+MgH2 composites containing 0, 5, 10 and 15 wt% of TM catalyst were heated from 25 to 600 °C at 5 °C.min-1 under 1 bar dynamic Argon atmosphere (100 mL.min-1). Then a 5 h - 600 °C dwell was applied to the samples. During the heating process, 2D XRD patterns were collected each 30 s using the PIXIUM 4700 detector.
Phases identification during non-catalyzed dehydrogenation of 2LiBH4-MgH2 (0 wt% TM) is presented in Figure 1. The dehydrogenation steps were clearly observed and are in accordance with previous studies [1,2]. LiBH4 structure change (orthorhombic to hexagonal) was observed at ~130 °C. Thus, ß-LiBH4 melted at ~300 °C followed by MgH2 decomposition into Mg occurring at ~350 °C. Reaction of Mg and liquid LiBH4 lead to the formation of MgB2 at ~460 °C. Above this step, two unidentified phases were observed. Phase 1 and 2 are neither borides, lithium hydrides nor MgxLiy alloys as suggested by Yu et al. .
Catalyzing 2LiBH4+MgH2 did not change the dehydrogenation pathway until the decomposition of MgH2 (Figure 2). However, TM catalyzed one particular step toward the composite decomposition: the borides formation from Mg. Compared to non catalyzed dehydrogenation, a decrease from ~460 °C to ~415 °C was observed for this step from 0 to 15 wt% TM catalyst respectively (dashed red line, Figure 2). Thus, the nature of the phases formed after MgH2 decomposition differs from un-catalyzed decomposition and seems to vary accordingly to the quantity of catalyst. According to Vajo et al. , MgB2 formation leads to a destabilization of LiBH4. Increasing the kinetics of MgB2 formation by the use of TM catalyst is, as a consequence, one of the possible ways to decrease the overall temperature of the dehydrogenation steps, leading to a release of H2 at lower temperature. Moreover, all of the chemical transformations and phase formations occurred during the heat treatment. Isothermal dwell has no influence on the stability of the phases formed.
In conclusion, dehydrogenation steps of non-catalyzed and TM-catalyzed LiBH4-MgH2 composite have been characterized using the high pressure high temperature gas loading system available on ID15. Contrary to literature, high temperature steps lead to the formation of other phases than MgB2 and LiH. Thus the TM catalyst used acts only on one step which is the MgB2 formation, leading to a decrease of ~50 °C. Mechanism of such a transformation has to be studied in detail and need further investigations as well as the hydrogenation properties related to the use of this catalyst.
Figure1. Uncatalyzed LiBH4+MgH2 dehydrogenation.
Figure 2. Catalyzed LiBH4+MgH2 dehydrogenation
 Price et al. J. Alloys Compnds (2009) 472, 559-564.
 Swanson et al. Acta Metallurgica (1959) 7, 769-773.
 Yu et al. Chem. Commun. (2006) 3906-3908.
 Vajo et al. J. Alloys and Compds (2007) 446–447, 409–414.