Advantages of high surface area niobium oxide catalysts on MgH2 sorption properties
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0927-EE04-02
Advantages of high surface area niobium oxide catalysts on MgH2 sorption properties Vinay V Bhat1, Aline Rougier1, Luc Aymard1, Gholam A Nazri2, and Jean-Marie Tarascon1 1 LRCS, U-Picardie, 33 rue St Leu, Amiens, Picardie, 80039, France 2 GM, Warren, Michigan, 48090-9055 ABSTRACT We report the synthesis by ‘chimie douce’ route of high surface area (200 m2/g) nano crystalline Nb2O5 (so called p-Nb2O5) and the importance of its addition to enhance the hydrogen sorption properties of MgH2. All of the prepared Nb2O5 catalysts induce faster kinetics, up to twice the desorption rate, than commonly used commercial Nb2O5. Among them, both p-Nb2O5 and Nb2O5:350 (p-Nb2O5 heated to 350 °C) exhibit the best catalytic activity, since a 5.2 wt% hydrogen desorption was achieved at 300 °C for (MgH2)p-Nb2O5, as compared to less than 4 wt.% for commercial Nb2O5 added MgH2, (MgH2)c-Nb2O5, within 12 min. Furthermore, due to the addition of high surface area Nb2O5, the desorption temperature was successfully lowered down to 200 °C, with a significant amount of desorbed hydrogen (4.5 wt%). In contrast at this “low” temperature, (MgH2)c-Nb2O5 shows no desorption. Keywords: Magnesium hydride, Hydrogen storage, catalysis, BET specific surface area, Niobium oxide
INTRODUCTION Due to its high hydrogen capacity (7.6 wt. %), Magnesium is one of the most attractive hydrogen storage materials [1-3]. However, the slow kinetics of the sorption reaction associated with a high desorption temperature limit its use for practical applications. In the past ten years, several routes to improve the sorption properties were investigated. Among them, enhanced kinetics were reported for nano-crystalline magnesium hydride prepared by mechanochemistry in combination with various additives, including transition metals and oxides [4-13]. In the class of oxides, niobium pentoxide, Nb2O5, was identified as the “best” catalyst [14-17]. Since this work, numerous studies have debated on the mechanism occuring during the sorption and the role of transition metal oxides as catalyst [17-18]. Aiming at understanding the role of the chemical nature of the catalyst, we embarked in a very large screening of materials from oxides, halides and metals. In a first attempt at classifying, influence of acidic (V2O5, Nb2O5…) and basic (SnO2, Bi2O3…) additives were compared. None of the basic additives enhanced the kinetics, that instead deteriorated the MgH2 sorption properties. Among acidic oxides, Nb2O5 remains by far the “best”. However, interestingly the catalytic activity of the latter was smaller than the one of niobium halides except when prepared as high surface area oxides [13]. Inspired from our earlier work on the synthesis of nano-oxides using “chimie douce” techniques, high surface area Nb2O5 was successfully synthesized by a precipitation route. Its specific surface area was then simply tuned by annealing at various temperatures. Herein, the influence of the specific surface area of the Nb2O5 catalyst on the sorption properties of MgH2 will be discussed
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