Researchers at the at the Max Planck Institute for Coal Research (Max-Planck-Institut für Kohlenforschung) report the mechanocatalytic synthesis of ammonia over a cesium-promoted iron catalyst under mild conditions—room temperature and pressures down to 1 bar.

Ammonia synthesis via the Haber-Bosch process normally requires temperatures of 400-500 °C and pressures of 150-300 bar. An open-access paper on their work is published in the journal Angewandte Chemie International Edition.

Ammonia is the basis of all nitrogen-containing fertilizers and is under discussion as transport and storage molecule for hydrogen in future energy systems. It is estimated that currently about 2% of the global energy demand is used for ammonia synthesis.

The grinding process in a ball mill activates a catalyst in such a way that it facilitates the synthesis of ammonia at a much lower temperature and pressure than is necessary in the well-established Haber-Bosch process. © Frank Vinken / MPG

Although optimized for more than 100 years, the Haber-Bosch process still requires essentially the conditions applied at its invention: high temperatures of up to 500 °C and reaction pressures of up to 200 bar to achieve reasonable ammonia concentrations at sufficiently high rates. The most significant innovation was the introduction of ruthenium-based catalysts, showing high activity also at somewhat lower pressures. Nonetheless, ammonia production still is dominated by the iron-catalyzed Haber-Bosch process, with the ruthenium catalyst used in some plants in a last reactor downstream of the main, iron-catalyzed reaction.

… In recent years, mechanochemistry has evolved as a promising alternative strategy for the activation and transformation of molecules. Also heterogeneously catalyzed gas-phase reactions can be mechanically activated, resulting in significant enhancements of activity. Due to the high importance, mechanocatalytic ammonia synthesis has also been attempted, albeit in less than a handful of reports with partly unclear validity of the claims.

… we have developed a system for the mechanocatalytic synthesis of ammonia from its elements working at room-temperature and down to atmospheric pressure. While several systems were identified that led to ammonia formation, the most promising systems consists of a mixture of iron with small amounts of elemental cesium. This catalytic system operates both under batch conditions and in a continuous process for more than 60 h, leading to continuous formation of ammonia at values up to 0.26 vol. %.

This study demonstrates the continuous catalytic synthesis of ammonia from the elements in a manner probably similar to the Haber-Bosch process, but at ambient temperature and pressure conditions due to the influence of mechanical forces, something which has remained elusive in spite of more than hundred years of efforts via different approaches.

—Reichle et al.

Resources

• S. Reichle, M. Felderhoff, F. Schüth. (2021) "Mechanocatalytic Room-Temperature Synthesis of Ammonia from Its Elements Down to Atmospheric Pressure", Angew. Chem. Int. Ed., doi: 10.1002/anie.202112095