Additives can be classified into several types based on their functions: 1. Promoters: These are added to increase the catalytic activity. For example, alkali metals like potassium can act as promoters in ammonia synthesis. 2. Modifiers: These can alter the properties of the catalyst to enhance selectivity. Ligands are commonly used as modifiers in homogeneous catalysis. 3. Inhibitors: These are used to slow down unwanted side reactions. For instance, carbon monoxide can act as an inhibitor in methanation reactions. 4. Stabilizers: These additives help in maintaining the structural integrity of the catalyst. Silica and alumina are often used as stabilizers in heterogeneous catalysis.
