Ni/MWCNT-GNP for Clean Hydrogen Via Catalytic NH₃ Decomposition: The Importance of Desorption Kinetics Achieving High NH₃ Conversion

Ammonia decomposition has emerged as a promising approach to on-site hydrogen production. In this study, nickel supported on multiwalled carbon nanotube-graphene nanoplatelet (Ni/MWCNT-GNP) was investigated as a catalyst for low-temperature ammonia decomposition. The catalysts were prepared by impregnating 10 wt% Ni on to MWCNT-GNP supports with different GNP contents (0–40 wt%). Physicochemical properties were characterized using XRD, H₂-TPR, H₂/N₂/NH₃-TPD, CO chemisorption, TEM, and XRF. XRF confirmed the presence of NiO in the fresh catalysts, whereas XRD showed partial autoreduction at around 550 °C under inert conditions. CO chemisorption revealed that the 30 wt% GNP catalyst had the highest Ni dispersion (2.36%) and surface area (15.71 m²/g). The TPD analysis showed that this catalyst exhibited the lowest desorption onset temperatures for H₂, N₂, and NH₃, indicating lower desorption energies and enhanced reaction rates. TEM confirmed that a 70:30 MWCNT:GNP ratio resulted in the smallest average Ni particle size (50.44 nm) and the most uniform particle distribution among the studied catalysts. The catalytic test revealed that catalytic performance was highly dependant on the GNP content, in the following order: 30 wt% > 40 wt% > 20 wt% > 10 wt% > 0 wt%. Remarkably, the 30 wt% GNP catalyst achieved 98% NH₃ conversion at 600 °C, significantly higher than the other formulations. Overall, these results highlight the importance of support composition in enhancing metal–support interactions and promoting N₂ desorption, thereby contributing to the development of high-performance nickel-based catalysts for sustainable hydrogen production from ammonia.