Support or carrier
Support or carrier provides large surface area for dispersion of small amount of catalytically active agent. This is particularly important when expensive metals, such as platinum, ruthenium, palladium or silver are used as the active agent. Supports give the catalysts its physical form, texture, mechanical resistance and certain activity particularly for bifunctional catalysts. Area of the support can range from 1 - 1000 m²/gm. Common supports are alumina, silica, silica-alumina, molecular sieves etc. The surface area of  α - alumina is in the range  1-10 m²/gm whereas the surface area for  γ or η - alumina  can be in the range 100 – 300 m²/gm.

Support may be inert or interact with the active component. This interaction may result in change in surface structure of the active agent and thereby affect the catalyst activity and selectivity. The support may also exhibit ability to adsorb reactant and contribute to the reaction process.

Promoters:

Promoters are generally defined as substances added during preparation of catalysts that improve the activity or selectivity or stabilize the catalytic agents. The promoter is present in a small amount and by itself has little or no activity.

Promoters are termed as physical or chemical promoter depending on the manner they improve the catalyst performance.

The additives that maintain physical integrity of the support and/or deposited catalytic agents are termed as physical promoters. For example, addition of small quantities of alumina to an iron catalyst employed in ammonia synthesis prevents sintering of the iron crystallites. Thus, for this catalyst, alumina is a physical promoter. The addition of K₂O to the same catalyst increases the intrinsic activity of the iron crystallites and therefore acts as a chemical promoter. The promoter can be added during catalyst preparation or during reaction.

Negative promoters or  inhibitors: Inhibitors act opposite to promoters. When added in small amount it can reduce catalysts activity, selectivity or stability. Inhibitor is particular useful to prevent or reduce the activity of a catalyst for undesirable side reactions. In oxidation of ethylene, ethylene dichloride is added to inhibit CO₂ formation thus acting as an inhibitor.