Q.What is Rosenmund's reduction ? What is the purpose of adding $BaSO_4$ in this reaction ?

Rosenmund reduction is a selective catalytic hydrogenation that converts acid chlorides into aldehydes, and the $BaSO_4$ support is what deliberately weakens (poisons) the palladium catalyst so the reduction stops exactly at the aldehyde.

Reaction: An acyl (acid) chloride is treated with hydrogen gas in the presence of palladium supported on barium sulphate ($Pd/BaSO_4$), with a small amount of sulfur or quinoline added as an additional catalyst poison:

$R-COCl + H_2 \xrightarrow{Pd/BaSO_4,\ (S \text{ or quinoline})} R-CHO + HCl$

Example: benzoyl chloride is reduced to benzaldehyde:

$C_6H_5COCl + H_2 \xrightarrow{Pd/BaSO_4} C_6H_5CHO + HCl$

Purpose of $BaSO_4$ (and the sulfur/quinoline poison): Pure/unpoisoned palladium is such an active hydrogenation catalyst that it would reduce the acid chloride all the way past the aldehyde stage to the primary alcohol (and even further). $BaSO_4$ acts as a catalyst poison — it reduces (moderates) the surface activity of the Pd metal just enough that the reaction is selectively halted at the intermediate aldehyde stage, preventing over-reduction: …