A century of data: Thermodynamics and kinetics for ammonia synthesis on various commercial iron-based catalysts

This work presents an enhanced thermodynamic model, an equilibrium model, and a unified kinetic model for ammonia synthesis. The thermodynamic model, based on the modified Soave-Redlich-Kwong Equation-of-State, introduces a polarity correction factor of −0.2368 for NH₃, accurately capturing the polarity effect at high pressures up to 1000 bar. The proposed Arrhenius-based equilibrium model is compatible with flowsheeting software, avoiding the need for custom routines. A Langmuir-Hinshelwood kinetic model is used for ammonia synthesis on iron-based catalysts, considering only N* and H* surface species. Including additional species, like NH*, did not improve data fitting. The inclusion of H* species enhances accuracy especially at lower temperatures (<400 °C). This kinetic model accurately describes over a century of ammonia synthesis data, covering various iron-based catalysts across a broad range of conditions: temperatures of 251–550 °C, pressures of 1–324 bar, H 2 /N 2 ratios of 0.33–8.5, and space velocities of 1–1800 Nm 3 ·kg cat −1 ·h −1 . Activity differences among catalysts, under identical conditions, are accounted for by a Relative Catalytic Activity factor, which reflects differences in the density of active sites and promoter distributions. The model's accuracy supports the hypothesis that, despite variations in catalyst precursor, promoter, and preparation methods, the reaction mechanism and active site remain similar under operating conditions. • Modified SRK EoS with NH₃ polarity correction captures non-ideality up to 1000 bar. • The Arrhenius equilibrium model can be easily implemented in flowsheeting software. • A single kinetic model with catalyst activity factors fits 12 datasets accurately. • Including N* and H* coverage is key; other surface intermediates are negligible.