Circular Low-Carbon Ammonia Synthesis Enabled by Microwave-Induced CH Bond Activation Reactions

This article presents an ammonia synthesis strategy that utilizes H₂ produced in-situ as a byproduct of the methane coupling reaction. This approach eliminates the need for CO₂-intensive hydrogen production via steam methane reforming while lowering the operating temperature required for CH bond activation through microwave-assisted (MW) catalysis. The process demonstrates strong potential for decarbonizing process heat via MW-driven electrification. A two-stage MW-thermal hybrid system is employed for the simultaneous activation of methane and nitrogen. In the first stage, methane undergoes nonoxidative coupling (NOCM) over a 1Cs-3Mo/CeO₂ catalyst under MW irradiation, enabling stable CH activation and selective formation of C₂ hydrocarbons with an apparent activation energy of 136 kJ·mol^-1. Ethylene is the dominant product below 750°C. The hydrogen-rich effluent from the NOCM reactor is routed directly to a downstream Ru-based catalytic reactor for ammonia synthesis, thereby eliminating the need for externally supplied hydrogen. Among the catalysts evaluated, 2Cs-4Ru/MgO exhibits the highest NH₃ formation rate (0.60 mmol·g^-1·h^-1). The influence of cofed ethylene on NH₃ productivity relative to conventional H₂/N₂ feed is examined to assess competitive adsorption between olefins and ammonia. Catalyst characterization using chemisorption, NH₃-TPD, and operando Raman spectroscopy provides insights into surface chemistry relevant to NH₃ formation. Operando Raman results indicate that the reduction of Ce⁴⁺ to Ce³⁺ generates oxygen vacancies that modify the electronic environment of adjacent Ru species, enhancing electron donation and strengthening back-donation into the antibonding π* orbitals of adsorbed N₂. For the nitrogen circular economy, MW-driven ammonia decomposition is demonstrated using monolithic catalyst. Overall, the MW-thermal hybrid platform offers a promising pathway toward decentralized and sustainable chemical manufacturing. Integration with renewable electricity and biomethane resources could further reduce the carbon footprint and enhance the overall sustainability of the system.