In presence of acetanilide, the rate of decomposition of $H_2{O}_2$ reduced. Acetanilide is called a positive catalyst.

There are few reactions in which catalyst can slow down the chemical reaction. These types of catalyst are called as negative catalyst.

Example: Decomposition of hydrogen peroxide can be slowed down if some substances like glycerin, urea, acetanilide, sodium pyrophosphate etc are added to H₂O₂