Abstract
The dissociative chemisorption of N2is generally accepted to be the rate-determining step of ammonia synthesis over Ru-based catalysts. The interaction of N2with the following three Ru catalysts has been studied: Ru supported on Al2O3(Ru/Al2O3) and on MgO (Ru/MgO), and Ru/MgO promoted with cesium (Cs–Ru/MgO). Temperature-programmed N2adsorption and desorption experiments and the isotopic exchange reaction14N14N+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 (kads) and associative desorption (kdes). The dissociation of N2was indeed found to be a slow and activated process. Ru/Al2O3was found to be rather inactive for N2dissociation. Ru/MgO turned out to be a heterogeneous system with respect to the interaction with N2due to the presence of promoted active sites which dominate the rate of N2dissociation. Promotion by cesium was observed to enhance bothkadsandkdessignificantly and rendered the Ru metal surfaces uniform toward the interaction with N2. The initial sticking coefficient and the rate of desorption of N2derived from the microkinetic models are in good agreement with results obtained with Ru single crystal surfaces under ultra-high vacuum conditions.
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