1. Poisons: These are substances that strongly adsorb to the active sites of a catalyst, rendering them inactive. Common examples include sulfur compounds, phosphorus, and heavy metals. 2. Fouling Agents: These substances can physically block active sites or pores of the catalyst. Examples are coke deposits in hydrocarbon processing or dust particles. 3. Dilutants: Materials that dilute the active sites of the catalyst, reducing its overall activity, such as excess reactants or byproducts. 4. Moisture and Oxygen: In certain catalytic systems, especially in heterogeneous catalysis, moisture and oxygen can be detrimental, leading to oxidation or hydrolysis of the catalyst.
