Abstract: The decomposition of ammonia for hydrogen production is a promising method, with the key challenge being the development of low-cost, highly active, and selective catalysts that operate at moderate temperatures. In this study, carbon nano-onions (CNOs), a byproduct of methane pyrolysis at 850 ℃, were used as a support for loading active metal cobalt (Co) via a uniform deposition-precipitation method. Additionally, magnesium oxide (MgO) was introduced as a promoter to successfully prepare a high-performance ammonia decomposition catalyst. A systematic investigation was conducted on the effects of acid washing and potassium (K) activation treatments on the morphology of the CNOs support, leading to an in-depth exploration of their influence mechanisms on catalyst performance. Various characterization and chemical adsorption experiments confirmed a positive correlation between the basicity strength of the catalyst and its ammonia decomposition performance. It was revealed that the incorporation of CNOs significantly enhanced the electronic conductivity of the catalyst and facilitated the uniform dispersion of Co₂MgO₄ nanoparticles on the support. This uniform dispersion increased the exposure of basic active sites, thereby enhancing the catalyst's ability to adsorb ammonia molecules. The acid washing treatment introduced more oxygen-containing functional groups on the surface of CNOs, which acted as anchoring sites and formed strong chemical bonds (coordination or ionic bonds) with Co²⁺ or Mg²⁺, thus stabilizing Co₂MgO₄ particles. This strong interaction increased the reduction difficulty of the metal oxides, leading to an elevated reduction temperature. Catalytic performance tests demonstrated that the synergistic effect of CNOs, MgO, K, and Co significantly optimized the structural characteristics, metal particle size, and catalytic performance of the catalyst. Among a series of synthesized catalysts, Co₂Mg/K-CNO’ exhibited the best catalytic activity for ammonia decomposition, achieving a conversion rate of 99.6% at 550 ℃ and a space-time yield of 12000 mL·g_cat^-1·h^-1.

Key words: ammonia decomposition, catalyst, carbon nano-onion, acid washing, alkalinity, metal-support interaction