Universal kinetic model for ammonia synthesis on various ruthenium-based catalysts

A Langmuir-Hinshelwood type kinetic model is proposed for ammonia synthesis on ruthenium-based catalysts. The model assumes the nitrogen dissociation step as the rate-limiting step and considers N* and H* species on the catalyst surface. The improved kinetic model effectively describes ammonia synthesis on ruthenium-based catalysts over a wide range of conditions, namely temperatures of 247–465 °C, pressures of 1–200 bar, H₂/N₂ ratios of 0.05–5, and space velocities of 6.6–4247 Nm³·kg_cat⁻¹·h⁻¹. The effects of temperature, pressure, H₂/N₂ ratio, and space velocity are all well captured. Differences in activity between the various catalysts, at the same operating conditions, are correlated via a Relative Catalytic Activity factor. A single correction factor per catalyst is introduced for the binding strength for the N* surface species, to account for effects of promoters and different support materials on the binding strength for the N* surface species. A novel kinetic model fitting framework was developed that integrates multiple literature datasets from different ruthenium-based catalysts with varying promoter compositions. The framework employs Covariance Matrix Adaptation Evolution Strategy with 5-fold cross-validation to achieve robust parameter estimation for high-dimensional kinetic models. A comparison with previous work on iron-based catalysts demonstrates that ruthenium-based catalysts are still significantly more expensive due to the high cost of ruthenium.