A catalyst support is a material with a large specific surface area 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 include various forms of activated carbon, aluminum oxide, and silicon dioxide.
Two main methods are used to produce 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 frequently used in pellet form. Alternatively, supported catalysts can be produced from a homogeneous solution by co-precipitation. An example of this is the treatment of 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 withstand the processes required to activate the precursors. For example, many precursors are activated by treatment with a hydrogen stream at high temperatures. Similarly, catalysts can be reactivated after prolonged use by oxidation-reduction cycles, also carried out at high temperatures. An example of this is the Phillips catalyst, which consists of chromium oxide on silicon dioxide and is activated by a hot air stream.
Catalyst supports are often considered inert, with catalytic activity occurring exclusively at the catalytic “islands” (nanoparticles), while the support merely provides a large specific surface area on which these nanoparticles are finely dispersed to achieve high dispersion. However, experiments have shown that this model is frequently 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 support surface without transitioning into the gas phase. This process, in which adsorbates transfer to or 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 hydroxyl groups.
