ABSTRACT
Zirconium phosphate-absorbed ammonia gas and the ammonia concentration (pressure) decreased to 2 ppm (ca. 20 Pa). However, it has not been clarified what the equilibrium pressure of zirconium phosphate is during ammonia gas ab/desorption. In this study, the equilibrium pressure of zirconium phosphate during ammonia ab/desorption was measured using cavity ring-down spectroscopy (CRDS). For ammonia-absorbed zirconium phosphate, a two-step equilibrium plateau pressure was observed during the ammonia desorption in gas. The value of the higher equilibrium plateau pressure at the desorption process was about 25 mPa at room temperature. If the standard entropy change (ΔS0) of the desorption process is assumed to be equal to the standard molar entropy of ammonia gas (192.77 J/mol(NH3)/K), the standard enthalpy change (ΔH0) is about -95 kJ/mol(NH3). In addition, we observed hysteresis in zirconium phosphate at different equilibrium pressures during ammonia desorption and absorption. Finally, the CRDS system allows the ammonia equilibrium pressure of a material in the presence of water vapor equilibrium pressure, which cannot be measured by the Sievert-type method.
ABSTRACT
Zirconium phosphate [Zr(HPO4)2·H2O] absorbs 2 mol(NH3)/mol[Zr(HPO4)2·H2O] with a low equilibrium plateau ammonia concentration of around 1 ppm in water. In this study, in order to investigate the regeneration process of ammonia-absorbed zirconium phosphate [Zr(NH4PO4)2·H2O], Zr(NH4PO4)2·H2O was heat-treated above 353 K under an inert gas. Then, the structures of the heat-treated samples were evaluated using powder X-ray diffraction and thermogravimetry-mass spectrometry measurements. Zr(NH4PO4)2·H2O started to desorb ammonia and the crystal water at 353 K. Then, Zr(NH4PO4)2·H2O was changed to the anhydrous monoammoniate [Zr(NH4PO4)(HPO4)] at 473 K and formed anhydrous zirconium phosphate [Zr(HPO4)2] at 673 K. The anhydrous zirconium phosphate and the anhydrous monoammoniate reabsorbed ammonia in ammonia water. Those initial absorption rates were small compared with Zr(HPO4)2·H2O. The slow kinetics of the anhydrous zirconium phosphate corresponded to the small interlayer distances. The ammonia concentration composition isotherms indicated that the anhydrous zirconium phosphate and anhydrous monoammoniate have a low ammonia equilibrium plateau concentration of around 1 ppm in ammonia water. Zr(NH4PO4)2·H2O is formed from Zr(NH4PO4)(HPO4) by the reabsorption of ammonia and water after 1-10 cycles. We found that zirconium phosphate is an ammonia remover which can be used repeatedly at 473 K.
ABSTRACT
The capacitances of porous carbon anodes were determined using a Ni(OH)2 cathode. We found that the capacitances were 300-700 F g-1 and above 3 times those of the carbon anodes prepared by electrical double layer formation, revealing the large capacitances based on protium H adsorption in the presence of highly concentrated KOH solution.
ABSTRACT
The temperature rise of AB5-type alloys by hydrogen adsorption was limited by their critical temperatures (Tc). We found the relation between the H2 desorption temperatures of metal hydrides at atmospheric pressure (Ts) and their Tc followed the Guldberg rule (Tc = 3/2 Ts), revealing a simple method to estimate Tc.
ABSTRACT
The standard entropy differences between hydrides and other elements (metals, liquid N2, toluene) ΔS were increased with the volume differences ΔV. It was found that ΔS is roughly expressed by the following equation, |ΔS|âRln|ΔV|, in which R is the gas constant.
ABSTRACT
The ammonia absorption process of zirconium phosphate has been studied using the concentration-composition-isotherm (CCI), X-ray diffraction and thermogravimetry-mass spectrometry (TG-MS). It was clarified that the equilibrium plateau concentration appeared due to two phase coexistence.
