Thermodynamic and Spectroscopic Analyses of Zirconium Phosphate-Absorbed Ammonia

Zirconium phosphate is insoluble in water and has a large ammonia storage capacity with a low ammonia plateau concentration in water. In this study, the thermodynamics of ammonia on zirconium phosphate (ZrP) and the local structure such as the interaction between ammonia and ZrP have been investigated using the temperature dependence of ammonia concentration composition isotherm (CCI) measurements, ³¹P magic angle spinning nuclear magnetic resonance (³¹P MAS NMR), and Fourier transform infrared (FT-IR) spectroscopy. The standard enthalpy changes ΔH⁰ of ZrP-absorbed ammonia (1 mol(NH₃) mol–¹ (ZrP):ZrP-NH₃ and 2 mol(NH₃) mol–¹ (ZrP):ZrP-2NH₃) were calculated to be ≤ −111 and −64 kJ mol⁻¹(NH₃), respectively. ³¹P MAS NMR and FT-IR indicated that the local structures of ZrP-NH₃ and ZrP-2NH₃ were different. It was suggested that nitrogen in ammonia reacts with hydrogen in the phosphate group to form a N–H covalent bond in NH₄⁺ having hydrogen bonds, although the number of hydrogen bonds in ZrP-NH₃ was smaller than that in ZrP-2NH₃.