Decoding technical multi-promoted ammonia synthesis catalysts

Area of Science:

• Chemical Engineering
• Materials Science
• Catalysis

Background:

• Industrial ammonia production relies on the Haber-Bosch process using iron-based catalysts.
• Understanding structure-activity relationships in complex industrial catalysts has been limited due to simplified model systems.
• A detailed understanding of technical catalyst evolution and promoter roles is crucial for process optimization.

Purpose of the Study:

• To investigate the structural evolution of complex, multi-promoted ammonia synthesis catalysts under operando conditions.
• To elucidate the role of promoters in catalyst activation, stability, and performance.
• To establish clear structure-activity correlations for industrial ammonia synthesis catalysts.

Main Methods:

• Operando scanning electron microscopy (SEM) to visualize structural changes during catalysis.
• Near-ambient pressure X-ray photoelectron spectroscopy (NAP-XPS) to probe surface composition and electronic states.
• Analysis of complex, multi-promoted industrial catalyst formulations.

Main Results:

• Identified catalyst activation as a critical step involving the formation of the active structure.
• Discovered the active structure comprises a nanodispersion of iron (Fe) decorated with mobile potassium (K)-containing adsorbates, termed "ammonia K".
• Revealed that mineral cementitious phases containing Al, Si, Ca, and Fe oxides stabilize the porous catalyst structure.

Conclusions:

• The synergistic action of multiple promoters is key to the superior performance of industrial ammonia synthesis catalysts.
• Promoters contribute simultaneously to structural stability, hierarchical architecture, catalytic activity, and resistance to poisoning.
• The identified "ammonia K" structure and promoter synergism provide critical insights for designing next-generation ammonia synthesis catalysts.