Thermocatalytic ammonia synthesis beyond conventional Haber-Bosch: Principles, advances, challenges and opportunities

Transforming ammonia (NH 3 ) synthesis from the energy-intensive, fossil-fuel-dependent conventional Haber-Bosch (HB) process to a flexible, green hydrogen-based process is pivotal for decarbonization and enabling NH 3 utilization in the energy sector. The conventional HB process, operating under high temperature and pressure, is incompatible with green hydrogen systems and economically unviable for downscaled NH 3 production integrated with intermittent renewable energies . Therefore, developing alternatives capable of synthesizing NH 3 under moderate conditions is crucial for achieving green NH 3 production. This necessity has driven the development of a range of emerging technologies, including thermocatalytic, electrocatalytic, photocatalytic, and plasma-assisted processes, amongst which thermocatalysis stands out in terms of production rate, technology readiness, and economic feasibility, demonstrating the greatest potential for NH 3 synthesis transformation. This review provides a comprehensive overview of advanced thermocatalytic NH 3 synthesis beyond conventional HB process and the system integration with renewable sources. It highlights key limitations and advances in implementing new materials and auxiliary techniques, outlining the challenges and mitigation strategies for achieving high NH 3 productivity under mild conditions. Alongside multiscale modeling studies, the review covers catalyst development, reactor intensification, process integration, and system evaluation, examining progress and conducting meta-analysis in reaction mechanisms, emerging separation technologies, and system integration. Scientific obstacles, economic analysis, and environmental impacts are thoroughly discussed, offering state-of-the-art insights into mild NH 3 synthesis from fundamental research to practical applications. Additionally, recent industrial projects of green NH 3 production are summarized, showcasing scalability and commercial viability. Finally, the remaining challenges and opportunities in advanced thermocatalytic NH 3 synthesis are outlined, identifying future research frontiers.