A catalyst carrier or support is typically a solid having a high surface area on which the active catalytic material is immobilized, deposited, or loaded. It is common knowledge that the activity of heterogeneous catalysts is dependent on atoms of the catalytic material that are accessible to the reactants, and therefore significant effort is made to maximize the surface area of the catalyst. In addition to stabilizing the catalyst particles, the carrier or support helps in maximizing the surface area of the catalyst. The most common catalyst supports, or carriers are activated carbon, alumina, silica, calcium carbonate, strontium carbonate, barium sulphate, titania, zirconia, zeolites, and any other material suitable for the process.

To understand what a catalyst support or carrier is, let us consider a heterogeneous catalytic hydrogenation reaction. For this reaction to proceed to completion, both the hydrogen gas and the substrate must be adsorbed on the catalyst surface, activated, and then react. This adsorption process is dependent upon the type of catalyst used and is often affected by the support for the catalyst. The function of the support with respect to the catalyst varies for every individual process and needs to be optimized to meet the process or product requirements. The support provides a larger contact surface, and it may influence the structural and physical properties of the catalyst (metal or oxide) deposited on it. The selectivity of the catalyst as well as its activity may also be influenced by the nature of the support. The support could also protect the catalyst from poisons and hinder aggregation to preserve the fine dispersion of the catalyst on its surface. The support materials are usually highly thermally stable and are designed to withstand the reaction/experimental/process conditions. The carrier or support can be inert or participate in the catalytic reaction, and therefore, broadly, the following factors are considered while designing/selecting a catalyst for a process

• Product economics
• Process conditions
• Type and amount of catalyst
• Type and surface area of support or carrier
• Promotors or additives required to achieve optimal selectivity to the desired products

At ACPL, activated carbon, alumina, silica, calcium carbonate, strontium carbonate, and barium sulphate are the most commonly available supports, however, catalyst with any support desired by the customer can be manufactured.

1. Activated Carbon: Activated carbon materials are the most popular supports used in the manufacture of heterogeneous catalysts. The use of activated carbon as a catalyst support is significantly advantageous as compared to other support materials like alumina or silica because they provide a higher internal surface area leading to higher reaction rates at a lower cost per cubic meter of the catalyst. The carbon surface is inert, especially in strongly acidic and basic conditions, the pore size distribution and the chemical properties on the surface can be tuned (polarity and hydrophobicity) according to the requirements of the envisaged process. Additionally, the metal particles can be recovered simply by burning the carbon support. There are several activated carbon catalysts available and are chosen based on the product and process requirements.
2. Alumina: Alumina, aloxide, aloxite, or alundum are some of the names used to refer to aluminium oxide having the Al2O3. There are several types of alumina available. It is a highly versatile material having several applications. As a catalyst, Alumina is used in several industrial applications, the largest scale of which is the Claus process used in refineries to convert the waste hydrogen sulphide gas into elemental sulphur. It is also useful for dehydration of alcohols to alkenes. As a catalyst carrier or support, alumina is used in several important industrial processes like:
   • The Haber Bosch Process for the manufacture of ammonia.
   • Hydrodesulfurization of petroleum products.
   • Ethylene oxide synthesis.
3. Silica: Silica or silicon dioxide (SiO2) is a highly versatile material having several industrial applications. Silica supports with extremely high surface areas can be manufactured and silica-based supports, adsorbents and catalysts are widely used in the manufacture of process intermediates & fine chemicals, elastomers & polymers, alternative energy water treatment & environmental pollution control. For example, the Phillips catalyst, used for about half of the industrial production of polyethylene, is a silica supported chromium catalyst.
4. Calcium Carbonate / Strontium Carbonate: It is a low surface area support and typically used when low surface area or basicity of catalyst supports are required.
5. Barium Sulphate: This is another low surface area support. For example, palladium supported on barium sulphate is used as a catalyst in the Rosenmund reduction reaction in which an acyl chloride is selectively reduced to an aldehyde in presence of hydrogen.

Examples of industrial processes using supported metal catalysts.