In ammonia (NH3) synthesis over promoted iron catalysts (Haber–Bosch), which substance acts as a key promoter for the active iron phase?

Introduction / Context:
Industrial ammonia synthesis relies on magnetite-derived iron catalysts modified by structural and electronic promoters. These promoters enhance activity, selectivity, and stability under high pressure and moderate temperature. Recognizing the correct promoter system is a common exam theme in fertilizer and catalysis modules.

Given Data / Assumptions:

• Catalyst base: Fe (from Fe3O4 pre-cursor reduced in situ).
• Operating conditions: high pressure with H2/N2 feed; strong poisons (e.g., CO) removed upstream.
• Promoters include oxides that tune basicity/electronic properties and mechanical robustness.

Concept / Approach:
Potassium oxide (K2O) is a classic electronic promoter for ammonia synthesis catalysts. It increases surface basicity, improves nitrogen adsorption/activation, and enhances electron density at iron active sites. Structural promoters like Al2O3 and CaO improve dispersion and sintering resistance, but K2O’s role as an electronic promoter is particularly emphasized in textbooks.

Step-by-Step Solution:
Identify the ammonia catalyst family: promoted iron.List common promoters: K2O (electronic), Al2O3 and CaO (structural), sometimes SiO2 as support/structural aid.Check options: only K2O fits the standard promoter profile for NH3 synthesis iron catalysts.Select K2O as the correct answer.

Verification / Alternative check:
Process literature consistently cites K2O-promoted iron (with Al2O3, CaO) as the workhorse system in Haber–Bosch loops; ruthenium catalysts are used in some modern plants but the question targets the conventional iron system.

Why Other Options Are Wrong:
SiO3: not a standard promoter; SiO2 may act as support/structural component, not the primary promoter cited.V2O5 and U2O3: unrelated to conventional NH3 iron catalyst promotion; V2O5 is associated with SO2 oxidation, not ammonia synthesis.

Common Pitfalls:
Confusing structural supports with electronic promoters; mixing catalyst systems from unrelated processes (e.g., vanadia for sulfuric acid plants).

Final Answer:
K2O