Kinetic Insights into Ammonia Decomposition over Rhodium: A Combined Experimental, Modeling, and Theoretical Study

Ammonia decomposition over rhodium (Rh) catalysts was investigated across a wide range of pressures (0.5–5 bar) to gain kinetic insights relevant to hydrogen generation applications. Microcalorimetry experiments were conducted to quantify the catalytic reaction rates, from which global kinetic parameters were extracted under kinetics-controlled conditions (T ≤ 750 K). The results demonstrated that reaction rates initially increased with pressure but subsequently exhibited a slight decrease at higher pressures. A thermodynamically consistent microkinetic model consisting of 12 elementary steps was developed, incorporating pressure-dependent adsorption behavior of ammonia. Validation against a DFT-calculated energy diagram for Rh(111) confirmed the reliability of the derived kinetic parameters in the final mechanism. The microkinetic model successfully reproduced the experimental results across all tested pressures. Recombination of nitrogen adatoms and their subsequent desorption into gaseous N₂ were identified as the rate-limiting steps, with nitrogen adatoms dominating the surface coverage. Furthermore, Rh catalysts showed excellent chemical stability without nitridation under the reaction conditions. This study provides a robust framework for the design and optimization of ammonia-fed hydrogen production systems under practical operating conditions.