Which compound of transition metal is used as a catalyst in the Contact Process?

Vanadium (V) oxide

The correct answer is Option (4) → Vanadium (V) oxide

Vanadium (V) oxide, also known as vanadium pentoxide ($V_2O_5$), is the essential catalyst used in the Contact Process for the industrial manufacture of sulfuric acid ($H_2SO_4$).

Role in the Contact Process

The Contact Process involves the oxidation of sulfur dioxide ($SO_2$) to sulfur trioxide ($SO_3$). This reaction is highly exothermic and reversible:

$2SO_2(g) + O_2(g) \rightleftharpoons 2SO_3(g) \quad \Delta H = -196 \text{ kJ/mol}$

In the presence of the $V_2O_5$ catalyst at temperatures between 400°C and 450°C, the reaction reaches equilibrium much faster.

Mechanism of Catalysis

The catalytic activity of $V_2O_5$ is a perfect example of how transition metals use their variable oxidation states to facilitate a reaction:

Oxidation of $SO_2$: The sulfur dioxide reacts with the vanadium (V) oxide. The vanadium is reduced from its +5 oxidation state to +4, while the $SO_2$ is oxidized to $SO_3$:

$V_2O_5 + SO_2 \rightarrow V_2O_4 + SO_3$

Regeneration of Catalyst: The vanadium (IV) oxide then reacts with oxygen from the air to be oxidized back to vanadium (V) oxide, completing the catalytic cycle:

$2V_2O_4 + O_2 \rightarrow 2V_2O_5$

Comparison with other options

• Copper (II) oxide: Not used in the Contact Process; occasionally used in the oxidation of alcohols.
• Manganese (IV) oxide: Frequently used as a catalyst for the decomposition of hydrogen peroxide.
• Iron (III) oxide: Finely divided iron is the catalyst for the Haber Process (ammonia synthesis), not the Contact Process.