Ruthenium catalysts for ammonia synthesis at high pressures: Preparation, 
characterization, and power-law kinetics

Rosowski, Frank; Hornung, Alessandra; Hinrichsen, Olaf; Herein, Daniel; Muhler, Martin; Ertl, Gerhard

doi:10.1016/S0926-860X(96)00304-3

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Ruthenium catalysts for ammonia synthesis at high pressures: Preparation, characterization, and power-law kinetics

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Rosowski, Frank
Physical Chemistry, Fritz Haber Institute, Max Planck Society;

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Hornung, Alessandra
Physical Chemistry, Fritz Haber Institute, Max Planck Society;

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Hinrichsen, Olaf
Physical Chemistry, Fritz Haber Institute, Max Planck Society;

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Herein, Daniel
Inorganic Chemistry, Fritz Haber Institute, Max Planck Society;

/persons/resource/persons21898

Muhler, Martin
Physical Chemistry, Fritz Haber Institute, Max Planck Society;

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Ertl, Gerhard
Physical Chemistry, Fritz Haber Institute, Max Planck Society;

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Citation

Rosowski, F., Hornung, A., Hinrichsen, O., Herein, D., Muhler, M., & Ertl, G. (1997). Ruthenium catalysts for ammonia synthesis at high pressures: Preparation, characterization, and power-law kinetics. Applied Catalysis A, 151(2), 443-460. doi:10.1016/S0926-860X(96)00304-3.

Cite as: https://hdl.handle.net/21.11116/0000-0008-B4C4-F

Abstract

Supported Ru catalysts for NH₃ synthesis were prepared from Ru₃(CO)_I2 and high-purity MgO and AI₂O₃. In addition to aqueous impregnation with alkali nitrates, two non-aqueous methods based on alkali carbonates were used to achieve alkali promotion resulting in long-term and high-temperature stable catalysts. For the reliable determination of the Ru particle size, the combined application of H₂ chemisorption, TEM and XRD was found to be necessary. The power-law rate expressions were derived at atmospheric pressure and at 20 bar which were shown
to be efficient tools to investigate the degree of interaction of the alkali promoter with the Ru metal particles. The following sequence with respect to the turnover frequency (TOF) of NH₃ formation was found: Cs₂CO₃-Ru/MgO>CsNO₃-Ru/MgO>Ru/MgO>/K-AI₂0₃>Ru/AI₂0₃. The Cs-promoted Ru/MgO catalysts turned out to be more active than a multiply-promoted Fe catalyst at atmospheric pressure with an initial TOF of about 10^-2 s^-1 for the non-aqueously prepared Cs₂CO₃-Ru/MgO catalyst at 588 K. The strong inhibition by H₂ was found to require a lower molar H₂:N₂ ratio in the feed gas than 3:1 in order to achieve a high effluent NH₃ mole fraction. The optimum ratio for Cs₂CO₃-Ru/MgO at 50 bar was determined to be about 3:2, resulting in an effluent NH₃ mole fraction which was just a few percent lower
than that of a multiply-promoted Fe catalyst operated at 107 bar and at roughly the same
temperature and space velocity. Thus, alkali-promoted Ru catalysts are an alternative to the conventionally used Fe catalysts for NH₃ synthesis also at high pressure.