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Structure-Activity Relationships in RuCs/MgO Catalysts During Ammonia Synthesis
Linus Biffar1, Niklas Martin Brinker1, Peter Pfeifer1,2
1Institute for Micro Process Engineering, Karlsruhe Institute of Technology, Hermann-von-Helmholtz-Platz 1, 76344, Eggenstein-Leopoldshafen, Germany.
Chemsuschem
|July 24, 2025
Summary
Cesium promotion enhances ruthenium catalysts for ammonia synthesis by increasing metallic ruthenium content. However, catalyst deactivation is linked to cesium leaching, especially in the presence of oxygen impurities.
Area of Science:
- Catalysis
- Materials Science
- Chemical Engineering
Background:
- Industrial ammonia synthesis requires high temperatures and pressures for efficient reactant conversion.
- Ruthenium-based catalysts promoted with alkali metals offer potential for lowering energy requirements by facilitating nitrogen dissociation.
- Understanding the behavior of ruthenium (Ru) and cesium (Cs) species is crucial for optimizing catalyst performance.
Purpose of the Study:
- To investigate the structural changes of Ru and Cs species in RuCs/MgO and Ru/MgO catalysts.
- To analyze catalyst behavior during reduction and ammonia synthesis under varying pressures.
- To examine catalyst deactivation mechanisms when exposed to oxygen impurities.
Main Methods:
- Operando X-ray absorption spectroscopy (XAS) was employed to probe catalyst structure.
- Experiments were conducted during catalyst reduction and ammonia synthesis.
- Catalyst performance was evaluated under pure gas feed and with 25 ppm oxygen impurity at pressures up to 19 bar(a).
Main Results:
- Interconversion between RuO2, dispersed RuOx, and metallic Ru species was observed in both catalysts.
- Cesium promotion increased metallic Ru content, reducing dispersed RuOx, and leading to higher ammonia output.
- Oxygen exposure transformed Cs+ cations to hydrated species, and irreversible deactivation was attributed to cesium leaching.
Conclusions:
- Cesium promotion is effective in enhancing metallic Ru content and ammonia synthesis activity.
- Catalyst deactivation is primarily caused by cesium leaching, exacerbated by oxygen impurities.
- The study provides insights into the structure-activity-deactivation relationship of Ru-based ammonia synthesis catalysts.
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