The reaction occurs in two stages.
Hydrogenation of the alkene can not be prevented by using one molar equivalent of H2. (If one molar equivalent of H2 is used, some alkene and some alkyne are obtained as products, and, since there are not enough H2 molecules to react with all the alkyne molecules, some unreacted alkyne is recovered.) To prevent hydrogenation of the alkene, a less active catalyst must be employed. Lindlar catalyst, which is a mixture of Pd, CaCO3, and lead salts, is the catalyst of choice in catalytic hydrogenation of alkynes to give alkenes.
see also dissolving-metal reduction