What if alkene is an reactant?

Catalytic hydrogenation -> Two hydrogens are added one to each carbon in the double bond, via a syn addition to an alkene. Exothermic reaction. Happens the same way with alkynes.

If the double bonded carbons are attached to two different functional groups, either the erythro/threo products might be produced.

Erythro — similar function groups are on the same side in a fischer projection.

Threo — similar function groups are on different sides.

When the catalyst is in a different phrase than the reactants = heterogeneous catalyst, catalyst and reactants are in the same phrase = homogeneous catalyst