The kinetics of ammonia synthesis over Ru-based catalysts: 1. The dissociative chemisorption and associative desorption of N2

O. Hinrichsen, F. Rosowski, A. Hornung, M. Muhler, G. Ertl

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Abstract

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 Al2O3 (Ru/Al2O3) 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 (kads) and associative desorption (kdes). 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 kads and kdes 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.

Original languageEnglish
Article numberCA971447
Pages (from-to)33-44
Number of pages12
JournalJournal of Catalysis
Volume165
Issue number1
DOIs
StatePublished - 1997
Externally publishedYes

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