Which of the following metals cannot be used to absorb H_2 gas on its…

The correct answer is option 1. Iron.

The metal that cannot be used to absorb H₂ gas on its surface is Iron

Here is why:

Iron: Unlike the other options, iron readily reacts with hydrogen gas to form iron hydrides (\(FeH_2\) and \(Fe_2H_3\)). This reaction effectively consumes the hydrogen and prevents its absorption on the iron surface.

Platinum, Palladium, and Nickel: These metals belong to the group of noble metals, known for their low chemical reactivity. They exhibit a phenomenon called hydrogen absorption or chemisorption, where hydrogen molecules dissociate and bond with the metal atoms without forming true chemical compounds. This allows them to "hold" a significant amount of hydrogen on their surfaces.

Therefore, while iron reacts with hydrogen, forming hydrides, platinum, palladium, and nickel can absorb it due to their chemisorption ability.