In ammonia synthesis (Haber–Bosch), plain iron is not used alone as the catalyst. What is the principal reason additional promoters are required rather than relying on iron by itself?

Introduction / Context:
Ammonia synthesis uses a promoted iron catalyst (typically magnetite-derived iron with potassium, alumina, and calcium promoters). This question probes why unpromoted, plain iron is not used alone, highlighting the need for structural and electronic promoters to sustain activity under severe temperature–pressure conditions.

Given Data / Assumptions:

• Operating window roughly 400–500 °C and high pressure.
• Reaction is exothermic; temperature control is essential.
• Catalyst must resist sintering and maintain active surface area.

Concept / Approach:
Plain iron tends to sinter and lose dispersion at elevated temperatures, especially above about 520 °C. Promoters such as K2O (electronic promoter) and Al2O3/CaO (structural promoters) stabilize the iron, inhibit crystal growth, and maintain active sites for N2 dissociation, thereby preserving activity over long runs.

Step-by-Step Solution:

Verification / Alternative check:
Historical and modern licensor data emphasize promoted iron formulations; deactivation by sintering at excessive temperature is a well-documented risk, managed by promoters and temperature control.

Why Other Options Are Wrong:

• Decomposes ammonia under all conditions: iron can catalyze decomposition at high T, but this does not preclude its use; promoters manage performance.
• Oxidised above 500 °C: the converter atmosphere is highly reducing (H2-rich), not oxidising.
• None of these / iron liquid: iron is solid at these temperatures, and the main issue is sintering/structural stability.

Common Pitfalls:
Confusing catalyst decomposition of ammonia at very high temperature with normal synthesis conditions where equilibrium and kinetics are balanced using promoted catalysts.

Final Answer:
Its activity declines rapidly if heated above 520 °C due to sintering and structural changes