Synthesis, structure and NH₃ sorption properties of mixed Mg_1-xMn_x(NH₃)₆Cl₂ ammines

This paper describes the synthesis, crystal structure, and NH₃ sorption properties of Mg_{1- x}Mn_x(NH₃)₆Cl₂ (x = 0-1) mixed metal halide ammines, with reversible NH₃ storage capacity in the temperature range 20-350 °C. The stoichiometry (x) dependent NH₃ desorption temperatures were monitored using in situ synchrotron radiation powder X-ray diffraction, thermogravimetric analysis, and differential scanning calorimetry. The thermal analyses reveal that the NH₃ release temperatures decrease in the mixed metal halide ammines in comparison to pure Mg(NH₃)₆Cl₂, approaching the values of Mn(NH₃)₆Cl₂. Desorption occurs in three steps of four, one and one NH₃ moles, with the corresponding activation energies of 54.8 kJ∙mol^-1, 73.2 kJ∙mol^-1 and 91.0 kJ∙mol^-1 in Mg_0.5Mn_0.5(NH₃)₆Cl₂, which is significantly lower than the NH₃ release activation energies of Mg(NH₃)₆Cl₂ (Ea = 60.8 kJ∙mol^-1, 74.8 kJ∙mol^-1 and 91.8 kJ∙mol^-1). This work shows that Mg_{1- x}Mn_x(NH₃)_yCl₂ (x = 0 to 1, y = 0 to 6) is stable within the investigated temperature range (20-350 °C) and also upon NH₃ cycling.