| Summary: | This paper describes the synthesis, crystal structure, and NH<sub>3</sub> sorption properties of Mg<sub>1-x</sub>Mn<sub>x</sub>(NH<sub>3</sub>)<sub>6</sub>Cl<sub>2</sub> (<i>x</i> = 0–1) mixed metal halide ammines, with reversible NH<sub>3</sub> storage capacity in the temperature range 20–350 °C. The stoichiometry (<i>x</i>) dependent NH<sub>3</sub> 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 NH<sub>3</sub> release temperatures decrease in the mixed metal halide ammines in comparison to pure Mg(NH<sub>3</sub>)<sub>6</sub>Cl<sub>2</sub>, approaching the values of Mn(NH<sub>3</sub>)<sub>6</sub>Cl<sub>2</sub>. Desorption occurs in three steps of four, one and one NH<sub>3</sub> moles, with the corresponding activation energies of 54.8 kJ⋅mol<sup>-1</sup>, 73.2 kJ⋅mol<sup>-1</sup> and 91.0 kJ⋅mol<sup>-1</sup> in Mg<sub>0.5</sub>Mn<sub>0.5</sub>(NH<sub>3</sub>)<sub>6</sub>Cl<sub>2</sub>, which is significantly lower than the NH<sub>3</sub> release activation energies of Mg(NH<sub>3</sub>)<sub>6</sub>Cl<sub>2</sub> (E<sub>a</sub> = 60.8 kJ⋅mol<sup>-1</sup>, 74.8 kJ⋅mol<sup>-1</sup> and 91.8 kJ⋅mol<sup>-1</sup>). This work shows that Mg<sub>1-x</sub>Mn<sub>x</sub>(NH<sub>3</sub>)<sub>y</sub>Cl<sub>2</sub> (<i>x</i> = 0 to 1, <i>y</i> = 0 to 6) is stable within the investigated temperature range (20–350 °C) and also upon NH<sub>3</sub> cycling.
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