Final answer:
Aluminum is not used as a catalyst for reactions involving hydrogen gas, unlike nickel, palladium, and platinum which are commonly used due to their ability to catalyze such reactions.
Step-by-step explanation:
The student asked which of the following metals is not used as a catalyst for reactions involving hydrogen gas: aluminum, nickel, palladium, or platinum. In the context of catalytic reactions with hydrogen, metals like nickel (Ni), palladium (Pd), and platinum (Pt) are frequently used due to their ability to facilitate the breaking of the H-H bond.
For example, during hydrogenation reactions, where hydrogen is added across a multiple bond, a platinum or palladium catalyst is usually employed. These metals, along with others like ruthenium and rhodium, are capable of forming adsorbed hydrogen atoms on their surface that are more reactive than hydrogen gas, which facilitates various chemical reactions.