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.