Normal activated carbon removes many odorous substances from the air, including organic solvents or poisons.
However, some common gaseous chemicals such as cyanides or ammonia are adsorbed poorly, or not at all. The activated carbon must be impregnated to eliminate these poisons. To do this, the large internal surface is coated with reactants for these poisons = chemisorption.
Another group of chemicals can be removed with catalysts (for example phosphines, etc.). In this case, the activated carbon is coated with these catalysts. A catalyst promotes a chemical reaction without its own properties changing. Its sole function is to facilitate the reaction.