Haber–Bosch ammonia catalyst: which substance is classically used as a promoter for the iron-based ammonia synthesis catalyst?

Introduction / Context:
Industrial ammonia synthesis typically employs magnetite-derived iron catalysts. To achieve high activity and stability under demanding conditions (high pressure, moderate-to-high temperature, hydrogen-rich gas), promoters are added. Knowing the difference between electronic and structural promoters is a common exam topic in chemical/fertilizer technology.

Given Data / Assumptions:

• Base catalyst: iron (Fe) derived from Fe3O4 (magnetite).
• Promoters can be electronic (altering adsorption characteristics) or structural (enhancing surface area and sintering resistance).
• Single best answer is requested.

Concept / Approach:
Potassium oxide (K2O) is a classical electronic promoter for the iron catalyst, improving nitrogen adsorption/activation by modifying the electronic environment of the Fe surface. Other common additives include Al2O3 and CaO as structural promoters that help maintain pore structure and limit sintering. Noble metals like Pt or base metals like Ni are not standard promoters for the traditional iron system (Ni is instead used for methanation or steam reforming catalysts).

Step-by-Step Solution:
Identify the classic promoter set for Fe-based NH3 synthesis.Electronic promoter: K2O → enhances surface properties for N2 activation.Therefore, select K2O as the correct answer.

Verification / Alternative check:
Standard catalyst compositions list Fe with K2O (electronic) and Al2O3/CaO (structural). This division appears in most ammonia technology references.

Why Other Options Are Wrong:
Pt and Ni are not used as promoters in the conventional Fe Haber–Bosch catalyst; Al2O3 is a promoter but is typically categorized as structural rather than the classic single “promoter” sought in such MCQs; CuO is associated with low-temperature shift catalysts, not NH3 synthesis.

Common Pitfalls:
Thinking “any oxide additive” is interchangeable; promoter roles are distinct. Also, mixing up NH3 catalyst formulation with reforming/shift/methanation catalysts elsewhere in the ammonia–urea complex.

Final Answer:
K2O