| NaAlH4 is one of the most popular light metal complex hydrides because of the advantages of high hydrogen storage capacity and moderate temperature conditions. Without catalyst, dehydriding speed of NaAlH4 is rather slow, and it needs high hydrogen pressure to rehydriding. While doped with catalyst, hydriding/dehydriding performance increase effectively, therefore, it is an important aspects in the study of NaAlH4 hydrogen storage materials to find suitable catalyst. What is more, processing method has important influence on the catalytic effect of catalyst, thus, processing methods are also important aspect.In this paper, the catalytic effects of coupling of TiAl3 with Al as well as CeAl4 with Al on hydriding and dehydriding of NaAlH4 were studied. It was shown in the experiment results that synthesis degree of NaAlH4 during ball milling, and the hydriding/dehydriding kinetic properties increase due to the coupling between catalyst and Al which was formed during sintering. The hydriding/dehydriding kinetics in CeAl4-doped situation was better than that in TiAl3-doped situation, the possible reasons may lie in the 4f electronic sturcture and lower melting point of CeAl4.Secondly, Na-based raw materials (NaH and Na) were compared in the preparation of NaAlH4. It was found that in the same in-situ milling conditions, many hydrogen storage properties of the as-milled sample based on metal Na was better than that based on NaH:1) higher reversible absorption and desorption capacities; 2) faster dehydriding speed; 3) better cycling stability. This may be the reason that the chemical potential of Na in elemental Na was higher; besides, it may also be due to better affinity between Na and Al, for they are both metal.Thirdly, influence of coupling among CeTi2Al20-TiAl3-Al on the hydriding/dehydriding performance of NaAlH4 were explored. CeTi2Al2o-3TiAl3-91Al composite with interface between any two phases were obtained by arc melting plug anneal method, which were ball milled after mixed with Na in hydrogen atmosphere. It was shown in experiments that in CeTi5Al20:Na=1:91 situation, the productivity of NaAlH4 during ball milling is obvious higher than that in CeTi5Al120:Na=1:120 situation, which lie in the reason that CeTi2Al20 and TiAl3 only act as catalyst. While the hydriding/dehydriding kinetic of the as-milling CeTi2Al120-3TiAl3-91Al-91Na decrease, when CeTi2Al20-3TiAl3-91Al was just mixed. Besides, in situation with catalyst-reaction coupling, the hydriding/dehydriding performance of the sample based on CeTi2Al20-TiAl3-Al was better than the sample based on CeTi2Al20-Al. Thus, CeTi2Al20-TiAl3 have cooperative catalytic effect on hydriding/dehydriding of NaAlH4, and tight coupling between catalyst and reaction was recommend.Finally, dehydriding kinetics of CeAl4-doped NaAlH4 sample and CeTi2Al20-TiAl3 co-doped NaAlH4 sample were studied comparatively. Dehydrogenation activation energy of samples was studied by multiple rate DSC experiments; and the grain size of the related phases was studied by XRD experiments. The samples here involve the as-milled samples and samples after several hydriding/dehydriding cycles. It was found that the decomposition activation energies of the CeTi2Al20-TiAl3 co-doped sample after ball-milling were 72.55kJ·mol-1and 73.42kJ·mol-1 in the first and second steps respectively, as for the CeAl4 co-doped sample the values were bigger (146.64kJ·mol-1 and 105.06kJ·mol-1 respectively). The decomposition activation energy of the two samples after repeated hydrogenation in both dehydriding steps was increased, which was found by XRD tests to be related to the grain size incensement of the samples due to hydrogenation. In contrast, the grain size of CeTi2Al20-TiAl3 co-doped sample was not easy to grow up. This shows that milling method is beneficial to refine the particle size, and to increases the contact between the catalyst and the reactants. Moreover, the growth of reactants grain can be slow down, and nucleation of reactants can be promote by suitable catalyst at the same time. |
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