Large amount of water is a NH3 absorbent in the plant facilities for emergency. NH3 and NH4+ coexist in ammonia water. For example, potential of hydrogen (pH) is 11 in 2500 ppm ammonia water and the ratio of NH3 and NH4+ are 98% and 2%, respectively. The aqueous solution releases NH3 due to the high equilibrium vapor pressure, resulting in increase of the negative effects on the environment. Therefore, in order to reduce ammonia released to the atmosphere, sulfuric acid is added in the aqueous solution. In this case, ammonium sulfate is formed and dissolves in the water. Therefore complicated process for ammonia removal from the solution is required. So, we focused on proton-based solid acids as ammonia storage materials which are insoluble in water. In ammonia water, the proton of the solid acid reacts with NH3 to form NH4+, and the storage capacity is proportional to the proton concentration.
In this study, we used zirconium phosphate, zeolite and impregnated activated carbon as proton-based solid acids. We also used activated carbon allophane and sepiolite as references. Ammonia storage properties of these samples were evaluated in the water solution. The ammonia storage capacity of zirconium phosphate was 10.2 wt%, which is the highest value among the studied samples.
The proton concentration in the samples was determined by titration experiments using NH4HCO3. The proton concentration corresponds to the ammonia storage capacity. This indicates that NH3 reacts with H+ to form NH4+ in the solid acid.
X-ray diffraction was used to estimate the structure of the zirconium phosphate before and after ammonia absorption. We confirmed that the interlayer spacing spreads from 0.77 nm to 0.96 nm by ammonia absorption. Temperature desorption scans of NH3 absorbed zirconium phosphate indicates that NH3 is desorbed below 673 K.
Then, zirconium phosphate was heat-treated at 673 K under vacuum and hydrothermally treated at 443 K. XRD measurement showed that the interlayer spacing was shrunk from 0.96 nm to 0.77 nm by the heat and hydrothermal treatments. Therefore, NH3 absorbed zirconium phosphate can be converted back to zirconium phosphate.
This work was partially supported by Council for Science, Technology and Innovation (CSTI), Cross-ministerial Strategic Innovation Promotion Program (SIP), “energy carrier” (funding agency : JST).
