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Previous studies reveal that combining non-thermal plasma with cheap metal catalysts achieved a significant synergy of enhancing performance of NH₃ decomposition, and this synergy strongly depended on the properties of the catalyst used. In this study, techniques of vacuum-freeze drying and plasma calcination were employed to improve the conventional preparation method of catalyst, aiming to enhance the activity of plasma-catalytic NH₃ decomposition. Compared with the activity of the catalyst prepared by a conventional method, the conversion of NH₃ significantly increased by 47% when Co/fumed SiO₂ was prepared by the improved method, and the energy efficiency of H₂ production increased from 2.3 to 5.7 mol(kW·h)^−1 as well. So far, the highest energy efficiency of H₂ formation of 15.9 mol(kW·h)^−1 was achieved on improved prepared Co/fumed SiO₂ with 98.0% ammonia conversion at the optimal conditions. The improved preparation method enables cobalt species to be highly dispersed on fumed SiO₂ support, which creates more active sites. Besides, interaction of Co with fumed SiO₂ and acidity of the catalyst were strengthened according to results of H₂-TPR and NH₃-probe experiments, respectively. These results demonstrate that employing vacuum-freeze drying and plasma calcination during catalyst preparation is an effective approach to manipulate the properties of catalyst, and enables the catalyst to display high activity towards plasma-catalytic NH₃ decomposition to produce H₂.