Co-Mg Synergistic Carbon Nano Onions Catalyst: Preparation and Low-temperature Catalytic Hydrogen Production from Ammonia Decomposition

Decomposition of ammonia for hydrogen production is a promising method, but still needs developing low-cost, highly active and selective catalysts which can 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 prepare a high-performance ammonia decomposition catalyst. An investigation was conducted on the effects of acid washing and potassium (K) activation treatments on morphology of the CNOs support and catalyst performance with in-depth exploration of their influence mechanisms. Various characterization and chemical adsorption experiments confirmed a positive correlation between basicity strength of the catalyst and its ammonia decomposition performance. It was revealed that incorporation of CNOs significantly enhanced electronic conductivity of the catalyst and facilitated 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 CNOs surface which acted as anchoring sites to form strong chemical bonds (coordination or ionic bonds) with Co2+ or Mg2+, thus stabilizing Co2MgO4 particles. These strong chemical bonds 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 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·gcat-1·h-1.