Rosenmund reduction: Acid chlorides can be reduced to aldehydes when halogenated in the presence of a catalyst such as palladium deposited on barium sulphate. 

In the Rosenmund reduction, BaSO₄ is taken with the catalyst Palladium (Pd) acts as which of the following?

Catalytic poison 

The Pd catalyst be poisoned, for example with BaSO₄, because the untreated catalyst is too reactive and will give some overreduction. Some of the side products can be avoided if the reaction is conducted inn strictly anhydrous solvents.