Hydrogen sorption kinetics of MgH2 catalyzed with titanium compounds
Introduction
A key technical challenge in the commercialization of hydrogen-powered vehicles is the lack of safe and efficient hydrogen storage means for on-board application [1]. Hydrogen storage in solid hydrides has emerged as a promising option because it stores hydrogen in a safe and compact way. Magnesium hydride (MgH2) has been considered a promising hydrogen storage material as it offers high gravimetric hydrogen capacity (7.6 wt.%) and excellent reversibility. Furthermore, magnesium is light and abundant. However, the practical availability of MgH2 for high capacity hydrogen storage is greatly hampered by thermodynamic and kinetic limitations [2]. Numerous studies have been carried out to develop suitable Mg-based systems that show fast sorption kinetics at moderate temperatures, which is of practical interest for on-board application. After decades of extensive research efforts, partial success has been achieved in improving the kinetics by incorporating appropriate catalysts (or additives), yet within the thermodynamic limit. It is found that the addition of hydrogen storage alloys, transition metals and their compounds, and carbon additives by milling all accelerate the de-/rehydrogenation reactions of MgH2 [3], [4], [5], [6], [7], [8], [9], [10], [11]. Among them, transition-metal compounds recently attracted considerable interest as high effective catalysts, generally due to the high affinity of transition-metal cation toward hydrogen. In particular, small addition of Nb2O5 significantly improves both the absorption and desorption kinetics of MgH2 [8], [12]. In the mesoporous form, Nb2O5 even led to significant hydrogen uptake at room temperature, which is one of the best results ever reported [10]. Interestingly, comparative studies based on various transition-metal oxides emphasize that the appropriate chemical interaction between catalyst and host hydride is essential for realizing high catalytic activity, suggesting the functionality of anion in tailoring the activity of transition-metal cation [12].
Titanium compounds have been successfully used to improve the sorption kinetics in several typical hydrogen storage systems, exhibiting a high affinity toward hydrogen even at moderate temperatures. For example, titanium halides are exceptionally effective in lowering the kinetic barrier of dehydrogenation reactions for alkaline alanates [13], [14], [15], [16]. In addition, TiN was found to be among the few candidates that are active in accelerating the amide/imide conversion [17]. With respect to MgH2, our preliminary study found that upon milling with TiF3, absorption can be completed within 30 s even at 100 °C [18]. Of particular interest, significant difference in activity was observed among titanium compounds with varying anions, yet the catalytic mechanism involved has not been well established [15], [16], [17]. Motivated by this, we performed a systematic investigation on the hydrogen sorption kinetics of MgH2 catalyzed with various titanium compounds, with attention paid to their catalytic enhancements at moderate temperatures. Comparative studies on the activities of transition-metal compounds with varying anions may provide new insight into the function of anion in tailoring the catalysis reaction.
Section snippets
Experimental
The starting material MgH2 was prepared by mechanical milling of magnesium powder (purity >99.9%, ∼300 mesh) under hydrogen atmosphere with an initial pressure of ∼1 MPa, followed by hydrogenation at 350 °C for 12 h. The process was repeated three times to achieve a hydrogenation ratio of ∼80%, as determined using the volumetric method [11]. The TiF3, TiCl3 (99.999%), TiO2 (99.8%), and TiH2 (98%) powders were all purchased from Sigma–Aldrich Corp. and were used as received. The TiN (99.8%) and MgCl
Results and discussion
Fig. 1 presents the hydrogen absorption kinetics of Mg catalyzed with various titanium compounds at 150 °C. Overall, all titanium compounds are effective in improving the absorption kinetics of Mg as compared to the neat sample. The absorption rate in the order of highest to lowest corresponds to the catalysts TiF3, TiCl3, TiO2, TiN and TiH2, as summarized in Table 1. Clearly, TiF3 exhibits superior activity over other compounds at the moderate temperature. At 150 °C, 3.8 wt.% hydrogen can be
Conclusions
We systematically investigated the effect of various titanium compounds on the hydrogen sorption kinetics of MgH2, focusing on their different catalytic activities upon varying anions. Among them, TiF3 addition leads to the most significant improvement on both absorption and desorption rates. Comparative studies indicate that the reaction products TiH2 and MgF2 resulted from adding TiF3 fail to explain the superior catalytic activity. However, a positive interaction between TiH2 and MgF2 is
Acknowledgements
The financial support for this research from the National Basic Research Program of China (973 Program, Grant No. 2010CB631305) and the Hundred Talents Project of Chinese Academy of Sciences is gratefully acknowledged.
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