Absorption and Desorption Characteristics of NH<sub>3</sub> with Metal Chlorides for Ammonia Storage

Ammonia has attracted great attention as a hydrogen carrier owing to its high hydrogen content of 17.8 wt%. However, a high pressure of approximately 1 MPa is required to store NH3 in its liquid state at room temperature. The toxicity of ammonia has raised a great deal of concern about leakage from the storage vessel to the ambient atmosphere. To solve this problem, we have focused on absorption and desorption reactions of ammonia with metal chlorides. Some metal chlorides can store NH3 in the form of a stable chemical substance even at room temperature and low NH3 concentrations. Moreover, metal ammine complexes decompose into metal chlorides and ammonia by heating at relatively low temperatures below 473 K. In this study, the reaction characteristics of MeCl2·lNH3 (Me=Mn, Ni, Mg, Sr, and Ca) complexes were investigated by using a thermogravimetric analyzer. A detailed study was carried out on the NH3 absorption/desorption reactions using MnCl2 and NiCl2 because of their considerably high NH3 absorption capacity and fast reaction rates. A rate expression was determined as a function of the reaction temperature and NH3 pressure for the reaction-controlled period. It was also demonstrated that the reactivity of the metal chlorides was maintained after ten cycles of absorption and desorption.