Explanation:

- Barium: This is not typically used as a catalyst in hydrogenation. It does serve other roles in chemical processes but not widely in hydrogenation.

- Iron: Commonly employed in industry, especially for ammonia synthesis, but not a classic choice for hydrogenation reactions.

- Tungsten: Known more for high-temperature applications and other catalytic processes, not standard for hydrogenation.

- Palladium: Frequently used as a catalyst in hydrogenation. It's highly effective for adding hydrogen to various unsaturated compounds.