The Haber Bosch Catalyst from Solid state Chemistry to Mesotechnology

A comprehensive, multi-scale, multi-modal study of the structural diversity of a technical multi-promoted Haber Bosch catalyst for ammonia synthesis is provided. The different contributions of ammonia Fe are revealed and  the presence of cement phases that have yet not been incorporated into discussions on the function of this catalyst system is shown. The structure of this system resembles a masterpiece of mesotechnology.

Abstract

Ammonia is industrially synthesized over multi-promoted Fe-based catalysts for more than a century. Although ammonia synthesis reflects a prototypical catalytic reaction, rational catalyst design is still impossible as the full structural complexity of this catalyst system often referred to as ammonia iron and its structural entanglement is barely understood. Here, the mesoscopic structure of a technical, multi-promoted ammonia synthesis catalyst is uncovered using a scale-bridging electron microscopy approach complemented by X-ray diffraction and spectroscopy to explore the structural integrity of ammonia iron. Amorphous contributions and structures of the melilite type and tricalcium aluminate as additional phases are identified. Furthermore, the understanding of the ammonia iron family by unveiling the role of the platelet-Fe perimeter, framework Fe, thin film Fe, and refractory Fe is extended. Their interconnectedness is highlighted, suggesting that each component has to be present to fulfill a specific task. The study demonstrates that catalysis science can only proceed if it openly explores the full complexity of catalytic systems.