Low-temperature ammonia adsorption-desorption performance of Al2O3-supported, graphite-casted metal chloride pellets

Ammonia (NH 3 ) is widely recognized as a carbon-free hydrogen carrier with high energy density, making it a promising energy vector for sustainable energy systems. However, its efficient storage and release remain technical challenges. In this study, the low-temperature NH 3 adsorption-desorption performance of four pure metal chlorides (CaCl 2 , MgCl 2 , MnCl 2 , and CuCl 2 ) was systematically evaluated. Among them, CuCl 2 and MnCl 2 exhibited superior NH 3 uptake capacities, but also suffered from volume expansion and particle agglomeration during repeated cycling. To overcome these limitations, composite sorbent pellets were developed by supporting metal chlorides on Al 2 O 3 via a graphite-casting method. The resulting CuCl 2 -based composite pellet achieved an optimized balance of high NH 3 adsorption capacity (∼ 0.36  g NH 3 /g), excellent regeneration efficiency (> 96 %), and adequate mechanical strength. Excessive CuCl 2 loading reduced both regeneration efficiency and mechanical stability, while lower loading enhanced active phase dispersion and sorbent performance. 30 wt% CuCl 2 offered the best performance in terms of adsorption, regeneration, and structural durability. Additionally, the optimized Cu 30 -Al-C sorbent maintained stable NH 3 adsorption capacity over 20 cycles, confirming excellent reusability. The enhanced performance was attributed to the synergistic effects of Al 2 O 3 support and the hydrophobic nature of the graphite-casting process, which enabled uniform distribution of active compound and mitigated structural deterioration during cycling. These results demonstrate the feasibility of graphite-casted, Al 2 O 3 -supported metal chlorides as structurally stable and regenerable NH 3 sorbents for low-temperature ammonia storage applications.