Effect of crystallite size of Al on the reversible hydrogen storage of NaAlH4 and few aspects of catalysts and catalysis

NaAlH4 has been catalyzed by MWCNT, Ce rich mischmetal (Mm), MmNi5 and TiO2 catalysts. The general aspect which relates every catalyst with the hydrogen storage capacity of NaAlH4 system has been verified through XRD analysis. Interesting features like chemical reduction, grain size variation, hydrogenation/dehydrogenation and phase transformation of the catalytic species are noticed. In the case of reversible hydrogen uptake, an interesting relationship exists between the restored hydrogen capacity, crystallite size of Al (desorbed in the dehydrogenation reaction) and the applied hydrogen pressure. Thus, as far as the reversible hydrogen storage in NaAlH4 is concerned, the mysterious role of catalyst seems to be a process of restricting the size of Al in a narrow range. The factors considered for analyzing this claim are discussed in detail.

► Catalyst itself undergoes physical and/or chemical transformation during recycling. ► Reduction of TiO2, formation & phase transformation of TixAly phases are observed. ► Hydrogen diffusion possibility through catalysts is proved by Mm & MmNi5 catalysts. ► Infiltration possibility of hydride in CNT bundles is discussed. ► Relation of Al size with reversible H storage capacity is demonstrated.