Synthesis of mesoporous SiO2–Al2O3 hollow spheres using ultrasonic irradiation and their activity for hydrolysis of ammonia borane

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In this study, mesoporous silica–alumina (SiO 2 –Al 2 O 3 ) hollow spheres (HSs) were synthesized through ultrasonic irradiation and evaluated in terms of their activity for the hydrolysis of ammonia borane (NH 3 BH 3 ). The mesoporous SiO 2 –Al 2 O 3 HSs were formed using a sol–gel method under ultrasonic irradiation using polystyrene particles as templates. We synthesized the samples at ultrasonic irradiation times for 0, 3, and 6 h. The morphologies of the samples were observed using transmission electron microscopy. The shell thickness of the samples increased with increasing ultrasonic irradiation time. In 27 Al solid-state nuclear magnetic resonance spectra, the sample with ultrasonic irradiation for 6 h showed the highest proportion of 4-coordinated aluminum species attributed to aluminum in the silica network and the lowest proportion of 5-coordinated aluminum species attributed to amorphous aluminum. The activity for hydrolysis of NH 3 BH 3 was determined at room temperature in the presence of the samples with ultrasonic irradiation for 0, 3, and 6 h. According to the results, 7.7-, 10.6-, and 12.3-mL hydrogen were finally generated in the presence of the samples with ultrasonic irradiation for 0, 3, and 6 h, respectively. These results indicate that the amount of hydrogen evolution increased with increasing ultrasonic irradiation time. Furthermore, the amount of hydrogen evolution depended on the number of acid sites of the samples measured using the neutralization titration method. Meanwhile, the reaction time to complete hydrogen generation increased as the ultrasonic irradiation time of the samples increased. Therefore, we measured the pore size distribution of the samples using N 2 adsorption–desorption measurement. The samples with ultrasonic irradiation for 0, 3, and 6 h had peaks at approximately 2.4, 2.1, and a value smaller than 1.4 nm nm, respectively. From these results, the mesopore size of the samples decreased with increasing ultrasonic irradiation time. This result might be related to the number of mesopores formed on the shell of the HSs.

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