Vanadium pentoxide is used as a catalyst in the preparation of:

Sulphur trioxide gas

Reason — Vanadium pentoxide is used during contact process for the preparation of sulphur trioxide gas.

$2\text{SO}_2 + \text{O}_2 \xrightleftharpoons [450-500\text{^{\circ} C }/1 - 2 \text{ atmos}]{\text{V}_2\text{O}_5} 2\text{SO}_3 + \text{45 K cals}$