Sonochemical doping of Ti-catalyzed sodium aluminum hydride

A new and very effective sonochemical technique has been developed for doping NaAlH4 with metal catalyst prior to high energy ball milling. When NaAlH4 was sonochemically doped with 2 mol% TiCl3 in a decalin slurry using THF as a co-solvent and then ball milled, the dehydrogenation temperature in the 90–150 °C range decreased by about 30 °C during temperature programmed desorption (5 °C/min) compared to a conventionally wet doped and ball milled sample. Similarly, during constant temperature desorption the dehydrogenation kinetics of sonochemically doped and ball milled samples of NaAlH4 increased by factors of 9.0, 5.1 and 3.1, respectively, at 90, 110 and 130 °C over those exhibited by conventionally wet doped and ball milled samples. These marked kinetic enhancements persisted through several dehydrogenation/hydrogenation cycles, now with corresponding increases by factors of 16.1, 4.5 and 3.5, and with a striking factor of four improvement in the hydrogenation kinetics also realized at 1250 psia and 125 °C. The observed kinetic effect was interpreted in terms of the complementary mechanochemical effects imparted to the sample by high-intensity ultrasound followed by high energy ball milling. It was surmised that sonochemical doping induced superior mixing of the titanium(III) chloride and sodium aluminum hydride reagents, thereby fostering the formation of smaller catalyst and NaAlH4 particles. This ensured a finer dispersion of the catalyst particles over the surfaces of the NaAlH4 crystals prior to and during the subsequent high energy ball milling process, which greatly improved the mechanochemical effectiveness of ball milling. Strong evidence in support of this supposition stemmed from sonochemical doping being only marginally more effective than conventional wet doping without subsequent ball milling, and scanning electron microscopy images revealing the formation of the factor of ten smaller NaAlH4 particles for the sonochemically doped and ball milled samples.