Catalyst-driven pathways to novel reactors for green ammonia synthesis.

Load-flexibility requirements of emerging P2A concepts and recent advances in ammonia synthesis catalysts challenge the established design paradigms of conventional adiabatic Haber-Bosch reactors (e.g., AICR). In particular, external direct-cooled polytropic fixed-bed reactors (EDCRs) appear to exploit modern, highly active catalysts more effectively. As catalyst activity is expected to increase further, it remains to be clarified to what extent existing reactor concepts can translate these improvements into reactor-level performance gains, or whether the reactor design itself must evolve. To address this question, a detailed model-based analysis of AICR and EDCR operation is carried out, in which catalyst improvements are represented by systematic scaling of the apparent activation energy. In this way, promising directions for future catalyst development can also be identified. The results show that increased catalyst activity improves single-pass conversion within a limited operating window; outside this range, equilibrium limitations dominate, and higher activity mainly reduces the required catalyst volume. While the operating trajectory of an AICR is fundamentally constrained by the adiabatic temperature rise and reaction ignition, the EDCR provides additional degrees of freedom and direct coupling of the operating trajectory to the reaction rate. This allows reactor operation to be aligned more closely with kinetically favorable conditions. As a result, the EDCR consistently outperforms the AICR, and even with current catalysts, single-pass conversions of up to 57% can be achieved. This indicates that reducing ammonia inhibition becomes increasingly important, since high ammonia concentrations strongly reduce reaction rates and thus limit high conversions to low space velocities. The study underscores that future catalyst development for ammonia synthesis should be coupled to reactor design, with particular emphasis on low-temperature activity, thermal catalyst stability, and reduced ammonia inhibition.