In Haber's process of manufacture of ammonia.
\(N_2 (g) + 3H_2(g) \underset{Mo(s)}{\overset{Fe(s)}{\longrightarrow}} 2NH_3 (g)\)
molybdenum acts as:
Answer & explanation
Correct answer: option 4
The correct answer is Option (4) → Promoter of catalyst
Promoters are substances that enhance the activity of a catalyst, while poisons decrease it. In the Haber process finely divided iron is the catalyst and molybdenum is the promoter that makes the iron more active:
N2(g) + 3H2(g) → 2NH3(g) (Fe(s) catalyst, Mo(s) promoter, about 200 bar, 700 K)
Molybdenum on its own is not the catalyst; it improves the working of the iron.
Why the other options are incorrect
Option (1) Homogeneous catalyst — Incorrect. A homogeneous catalyst is in the same phase as the reactants (e.g. NO in the lead chamber process). Here the metals are solids and the reactants are gases.
Option (2) Heterogeneous catalyst — Incorrect. This describes the role of iron, the solid catalyst on whose surface \( \ce{N2} \) and \( \ce{H2} \) react; molybdenum only promotes it.
Option (3) Enzyme catalyst — Incorrect. Enzymes are protein catalysts of reactions in living systems (e.g. invertase, urease); no enzyme is used in the Haber process.
NCERT (Pre-rationalised edition, 2022-23):
Promoters and Poisons – “Promoters are substances that enhance the activity of a catalyst while poisons decrease the activity of a catalyst. For example, in Haber’s process for manufacture of ammonia, molybdenum acts as a promoter for iron which is used as a catalyst.”