The catalyst used in hydrogenation of oil is

Concept: Hydrogenation is the addition of hydrogen across a carbon-carbon double bond (C=C), and it is run as a surface (heterogeneous) catalysis. A transition metal adsorbs H2 on its surface and splits it into reactive hydrogen atoms, the unsaturated molecule is held on that same surface, and the two hydrogen atoms then add across the double bond. The metal is not consumed - it only provides the surface that lowers the activation energy.

Application - hardening of an edible oil:

1. Edible oils are triglycerides whose fatty-acid chains carry several C=C bonds; those kinks stop the chains from packing closely, so the fat stays liquid at room temperature.

2. Hydrogen gas is bubbled through the warm oil at about 450-475 K over finely divided nickel, which exposes a very large metal surface to both reactants.

3. Hydrogen atoms add across the C=C bonds, so the unsaturated chains become saturated; the straightened chains now pack closely, the melting point rises, and the oil sets into a semi-solid fat (vanaspati or margarine).

4. Nickel is the metal used for this on plant scale because it gives sufficient activity at a small fraction of the cost of the noble metals and is easily filtered out of the finished fat; the reaction is known as the Sabatier-Senderens reduction.

Cross-check - the reaction each of the other metals is normally used for:

• Fe (promoted with K2O and Al2O3): combination of N2 and H2 into ammonia in the Haber process.

• Mo (as MoS2 with cobalt on alumina): hydrodesulphurisation, which strips sulphur from petroleum fractions as H2S.

• Pt (as a gauze or finely divided metal): oxidation of ammonia to nitric oxide in the Ostwald process; platinum will hydrogenate C=C bonds in the laboratory, but its cost rules it out of bulk oil hardening.

Result: the catalyst used in the hydrogenation of oil is nickel (Ni).