Mechanochemical synthesis and XPS analysis of sodium alanate with different additives

NaAlH4 was mechanochemically synthesized under high hydrogen pressure. Additives can be added to NaAlH4 directly during the milling process. In situ monitoring of the pressure and temperature was used to provide an insight into the reactions occurring in the vial during synthesis—a process so far based purely on empirical considerations. This method was then applied to directly compare different additives during synthesis. The following trend was obtained for the effectiveness of the additives: TiCl3 > CeCl3 > ScCl3 ≫ Ti. This trend was also verified for desorption by differential scanning calorimetry and by monitoring decomposition at room temperature during long-term storage. In addition, the direct comparison by X-ray photoelectron spectroscopy of the chemical state of the additive after milling illustrated indirectly the importance of the reduction of the chloride—and the formation of NaCl—during synthesis. These reactions can be linked to the formation of vacancies necessary for fast diffusion of hydrogenated species within the material.