アイテム詳細

要旨

The dissociative chemisorption of N₂ is generally accepted to be
the rate-determining step of ammonia synthesis over Ru-based catalysts.
The interaction of N₂ with the following three Ru catalysts
has been studied: Ru supported on Al ₂O₃ (Ru/Al ₂O₃) and on MgO
(Ru/MgO), and Ru/MgO promoted with cesium (Cs–Ru/MgO). Temperature-programmed N₂ adsorption and desorption experiments and the isotopic exchange reaction ¹⁴N¹⁴N+¹⁵N¹⁵N⇆2¹⁴N¹⁵N were performed in a microreactor flow system. A microkinetic analysis based on the Langmuir–Hinshelwood Hougen–Watson mechanism has been applied to these kinetic experiments yielding the rate constants of dissociative chemisorption (k_des) and associative desorption (k_des). The dissociation of N₂ was indeed
found to be a slow and activated process. Ru/Al₂O₃ was found to be rather inactive for N₂ dissociation. Ru/MgO turned out to be a heterogeneous system with respect to the interaction with N₂ due to the presence of promoted active sites which dominate the rate of N₂ dissociation. Promotion by cesium was observed to enhance both (k_ads) and (k_des) significantly and rendered the Ru metal surfaces
uniform toward the interaction with N₂. The initial sticking coefficient and the rate of desorption of N₂ derived from the microkinetic models are in good agreement with results obtained with Ru single
crystal surfaces under ultra-high vacuum conditions.