Catalyst support

In the Phillips catalyst, used for about half of the industrial production of polyethylene, the chromium catalyst is supported on silica.

In chemistry, a catalyst support is the material, usually a solid with a high surface area, to which a catalyst is affixed.^[1] The activity of heterogeneous catalysts and nanomaterial-based catalysts occurs at the surface atoms. Consequently, great effort is made to maximize the surface area of a catalyst by distributing it over the support. The support may be inert or participate in the catalytic reactions. Typical supports include various kinds of carbon, alumina, and silica.^[2]

Applying catalysts to supports

Two main methods are used to prepare supported catalysts. In the impregnation method, a suspension of the solid support is treated with a solution of a precatalyst, and the resulting material is then activated under conditions that will convert the precatalyst (often a metal salt) to a more active state, perhaps the metal itself. In such cases, the catalyst support is usually in the form of pellets. Alternatively, supported catalysts can be prepared from homogeneous solution by co-precipitation. For example, an acidic solution of aluminium salts and precatalyst are treated with base to precipitate the mixed hydroxide, which is subsequently calcined.^[3]

Activation of precatalysts

Supports are usually thermally very stable and withstand processes required to activate precatalysts. For example, many precatalysts are activated by exposure to a stream of hydrogen at high temperatures. Similarly, catalysts become fouled after extended use, and in such cases they are sometimes re-activated by oxidation-reduction cycles, again at high temperatures. The Phillips catalyst, consisting of chromium oxide supported on silica, is activated by a stream of hot air.^[4]

Interactions of catalyst and support

Supports are often viewed as inert: catalysis occurs at the catalytic "islands" and the support exists to provide high surface areas. Various experiments indicate that this model is oversimplified or even wrong.

Spillover

It is known for example that adsorbates, such as hydrogen and oxygen, can interact with and even migrate from island to island across the support without re-entering the gas phase. This process where adsorbates migrate to and from the support is called spillover. It is envisaged, for example, that hydrogen can "spill" onto oxidic support perhaps as hydroxy groups.^[5]

Strong metal-support interaction

Strong metal-support interaction is another case highlighting the oversimplification that heterogeneous catalysts are merely supported on an inert substance. The original evidence was provided by the finding that particles of platinum bind H₂ with the stoichiometry PtH₂ for each surface atom regardless of whether the platinum is supported or not. When, however, supported on titanium dioxide, Pt no longer binds with H₂ with the same stoichiometry. This difference is attributed to the electronic influence of the titania on the platinum, otherwise called strong metal-support interaction.^[6]

Heterogenized molecular catalysis

Molecular catalysts have been immobilized catalyst supports. The resulting material in principle combines features of both homogeneous catalysts - well defined structures - with the advantages of heterogeneous catalysts - recoverability and ease of handling. Many modalities have been invented for attaching molecular catalysts to the support. The technology has not proven commercially viable, usually because the heterogenized molecular species a leached from or deactivated by the support.^[7]

Examples

Ceramic-core converter of the type found in many automotive catalytic converters.

Almost all major heterogeneous catalysts are supported.

See also

• fluidized bed reactor

References