A catalyst support is a material with a large specific surface on which catalytically active metal particles are fixed. The activity of heterogeneous catalysts is largely determined by the accessibility of the active metals on the support material. The support can either remain inert or actively participate in the catalytic reactions. Typical materials for catalyst supports are various forms of activated carbon, aluminum oxide and silicon dioxide.

There are two main methods used to prepare supported catalysts. In the impregnation method, a suspension of the solid support material is treated with a precursor solution. The resulting material is then activated, for example by calcination, which converts the precursor (often a metal salt) into a more active state, such as the metal itself. In these cases, the catalyst support is often used in pellet form. Alternatively, supported catalysts can be prepared by co-precipitation from a homogeneous solution. An example of this is treating an acidic solution of aluminum salts and precursors with a base to precipitate a mixed hydroxide, which is then calcined.

Catalyst support materials are generally very thermally stable and can easily withstand the processes required to activate the precursors. For example, many precursors are activated by treatment with a stream of hydrogen at high temperatures. Likewise, catalysts can be reactivated after long-term use by oxidation-reduction cycles, which are also carried out at high temperatures. One example of this is the Phillips catalyst, which consists of chromium oxide on silica and is activated by a stream of hot air.

Catalyst supports are often considered to be inert, with the catalytic activity occurring exclusively at the catalytic “islands” (nanoparticles), while the support merely provides a large specific surface area on which these are finely distributed to achieve high dispersion. However, experiments have shown that this model is often oversimplified. For example, it is known that adsorbates such as hydrogen and oxygen can interact with the support material and even migrate from one catalytic island to the next across the surface of the support without entering the gas phase. This process, in which adsorbates transfer to or return from the support, is called spillover. A well-known example is the transfer of hydrogen to oxide support materials, where it “spills over” in the form of hydroxy groups.