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:

Promoter of catalyst

The correct answer is option 4. Promoter of catalyst.

The Haber process is an industrial method for synthesizing ammonia from nitrogen and hydrogen gases:

\(N_2 (g) + 3H_2 (g) \underset{Fe(s)}{\longrightarrow} 2NH_3 (g)\)

Primary Catalyst: Iron (\(Fe\)) is the main catalyst used in the process. It facilitates the reaction between nitrogen and hydrogen gases to form ammonia.

Promoter: Molybdenum (\(Mo\)) is used as a promoter in this process.

Catalysts vs. Promoters:

Catalysts:

Function: Catalysts increase the rate of a chemical reaction by providing an alternative reaction pathway with a lower activation energy. They are not consumed in the reaction and remain unchanged after the reaction.

In the Haber Process: Iron acts as the primary catalyst. It provides a surface where the nitrogen and hydrogen gases can react to form ammonia.

Promoters:

Function: Promoters enhance the effectiveness of a catalyst. They do not have catalytic activity themselves but improve the performance of the primary catalyst by increasing its efficiency, stability, or selectivity.

In the Haber Process: Molybdenum is used as a promoter to improve the performance of the iron catalyst.

Role of Molybdenum in the Haber Process:

Enhancing Catalyst Performance:

Increased Activity: Molybdenum improves the activity of the iron catalyst by enhancing its ability to dissociate nitrogen and hydrogen molecules, which is a crucial step in the ammonia synthesis process.

Improved Stability: It helps stabilize the iron catalyst, reducing its deactivation over time and increasing the longevity of the catalyst.

Mechanism:

Molybdenum interacts with iron to form a more efficient catalyst system. This interaction often involves modifying the surface properties of iron, such as increasing its surface area or improving its ability to adsorb reactant gases.

Practical Benefits:

Higher Yield: The presence of molybdenum as a promoter helps achieve higher yields of ammonia by making the catalytic process more efficient.

Reduced Costs: By improving catalyst performance and stability, molybdenum helps reduce the frequency of catalyst replacement and overall operational costs.

Clarifying the Options:

1. Homogeneous Catalyst:

This would imply the catalyst and reactants are in the same phase, typically a liquid or gas phase. In the Haber process, the catalyst (iron) is a solid, and the reactants (nitrogen and hydrogen) are gases, so this does not apply.

2. Heterogeneous Catalyst:

This refers to a catalyst that is in a different phase from the reactants. Iron is a solid catalyst, and nitrogen and hydrogen are gases, making iron a heterogeneous catalyst. Molybdenum is not the main catalyst but supports the iron catalyst.

3. Enzyme Catalyst:

Enzymes are biological catalysts found in living organisms. The Haber process is an industrial chemical process, not involving biological enzymes.

4. Promoter of Catalyst:

Molybdenum acts as a promoter by enhancing the performance of the iron catalyst, which is the correct role of molybdenum in this context.

Conclusion:

Molybdenum’s role in the Haber process is to enhance the effectiveness of the iron catalyst by acting as a promoter. It improves the efficiency and stability of the primary iron catalyst, leading to better performance in ammonia synthesis. Therefore, the correct answer is option 4: Promoter of catalyst.