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

What is the role of quinoline and sulphur in Rosenmund reduction? 

Quinoline and sulphur acts as a catalytic poison 

Quinoline and sulphur is frequently added in the Rosenmund reduction to moderate the activity of the catalyst and to prevent further reduction of the product aldehyde into alcohols. Quinoline and sulphur acts as a catalytic poison in this case.