Surface Coverage Controls the Apparent Activation Energy and Reaction Orders in Ru-Catalyzed Ammonia Synthesis: A Microkinetic Study.

A reversible Langmuir-Hinshelwood microkinetic framework is developed to clarify coverage effects in ammonia synthesis over Ru. Kinetic parameters are obtained by least-squares fitting of the modeled rate to experimental data measured on Ru powder over 380-460 °C. Across all temperatures, the model with N2 dissociation (step 3) as the rate-determining step provides the best agreement with the experiment and enables computation of steady-state coverages. The calculated reaction orders for N2, H2, and NH3 capture the main experimental trends over 400-460 °C, although some deviations remain, particularly for the H2 order. The framework also explains the strong flow-rate dependence of the apparent activation energy: changes in residence time alter the in situ NH3 level, shift NHx coverages and the vacancy fraction, and thereby modulate both reaction orders and the apparent activation energy without changing the intrinsic barrier of N2 dissociation. These results emphasize that coverage and space velocity must be considered when interpreting kinetic indices and comparing catalytic performance across different experimental conditions.