Nucleation-promoting effect of mixed Zn and ZnO particles on tetra-n-butylammonium bromide hydrate

Abstract

Tetra-n-butylammonium bromide (TBAB) hydrate is an ionic semi-clathrate hydrate, and it can be used as an energy-storage and gas-separation medium. Nucleation of TBAB hydrate is unlikely to occur and this characteristic causes problems in the refrigeration system due to the need for excessive cooling and delays in gas separation due to the slow formation. Therefore, a technique for rapid formation of TBAB hydrate is required. In a previous study, it was found that addition of mixed and ground Zn and ZnO particles (Zn–ZnO) to TBAB aqueous solution significantly promoted nucleation of TBAB hydrate. Since the nucleation of TBAB hydrates was effectively promoted by the very simple addition of Zn–ZnO, this technique can be easily applied to conventional systems. However, the nucleation-promoting mechanism have not been clarified. The clarification of the nucleation-promoting effect of Zn–ZnO on TBAB hydrates is important to elucidate the nucleation-promoting mechanism of clathrate hydrates by the addition of substances. Therefore, the objective of the present study was to investigate the nucleation-promoting mechanism of Zn–ZnO on TBAB hydrate. Experimental results showed that immersion of Zn–ZnO in TBAB aqueous solution reduced the nucleation-promoting effect, but a decrease in the nucleation-promoting effect was not observed when Zn–ZnO was immersed in water. Microscopy observation confirmed that immersion of Zn–ZnO in TBAB aqueous solution had little effect on its surface morphology, but X-ray diffraction and X-ray photoelectron spectroscopy indicated formation of solid TBAB on Zn–ZnO. Zn–ZnO did not show a significant nucleation-promoting effect on tetrahydrofuran hydrate, which is a non-ionic clathrate hydrate. This suggest that Zn–ZnO has properties that attract tetra-n-butylammonium and Br ions, and that the proximity of these ions may promote formation of TBAB hydrate. It was also suggested that the mechanism of nucleation promotion by the addition of substances is different for ionic clathrate hydrates and non-ionic clathrate hydrates.