Solid-State NMR Studies of the Local Structure of NaAlH4/C Nanocomposites at Different Stages of Hydrogen Desorption and Rehydrogenation

Structural properties of NaALH(4)/C nanocomposites were Studied using Na-23 and Al-27 solid-state NMR. The samples were synthesized by melt infiltration Of it highly Porous carbon support, with typical pore sizes of 2-3 rim. Physical Mixtures of high surface carbon with alanates in different stages of hydrogen desorption show somewhat broadened resonances and a small negative chemical shift compared to pure alanates. This is most likely caused by a Susceptibility effect of the carbon Support material, which shields and distorts the applied magnetic field. After melt infiltration Na-23 and Al-27 spectra are broadened with a small downfield average shift, which is mainly Caused by a chemical shift distribution and is explained by a larger disorder in the nanoconfined materials and a possible charge transfer to the carbon. Our measurements show that the local Structure of the nanoconfined alanate is the similar to bulk alanate because a comparable chemical shift and average quadrupolar Coupling Constant is found. In contrast to bulk alanates, in partly desorbed nanocomposite samples no Na3AlH6 is detected. Together with a single release peak observed by dehydrogenation experiments, this points toward a desorption in one single Step. Na-23 spectra of completely desorbed NaAlH4/C and NaH/C nanocomposites confirm the formation of metallic sodium at lower temperatures than those observed for bulk alanates. The Structural properties observed with solid-state NMR of the nanoconfined alanate are restored after a rehydrovenation cycle. This demonstrates that the dehydrogenation of the NaAlH4/C nailocomposite is reversible, even Without a Ti-based catalyst.