In chemistry, a catalyst support is the material, usually a solid with a high

surface area The surface area of a solid object is a measure of the total area that the surface of the object occupies. The mathematical definition of surface area in the presence of curved surfaces is considerably more involved than the definition of arc ...

, to which a

catalyst Catalysis () is the process of increasing the rate of a chemical reaction by adding a substance known as a catalyst (). Catalysts are not consumed in the reaction and remain unchanged after it. If the reaction is rapid and the catalyst recyc ...

is affixed. The activity of

heterogeneous catalysts In chemistry, heterogeneous catalysis is catalysis where the phase of catalysts differs from that of the reactants or products. The process contrasts with homogeneous catalysis where the reactants, products and catalyst exist in the same phase. P ...

is mainly promoted by atoms present at the accessible surface of the material. Consequently, great effort is made to maximize the

specific surface area Specific surface area (SSA) is a property of solids defined as the total surface area of a material per unit of mass, (with units of m2/kg or m2/g) or solid or bulk volume (units of m2/m3 or m−1). It is a physical value that can be used to det ...

of a catalyst. One popular method for increasing surface area involves distributing the catalyst over the surface of the support. The support may be inert or participate in the catalytic reactions. Typical supports include various kinds of

carbon Carbon () is a chemical element with the symbol C and atomic number 6. It is nonmetallic and tetravalent In chemistry, the valence (US spelling) or valency (British spelling) of an element is the measure of its combining capacity with o ...

, alumina, and

silica Silicon dioxide, also known as silica, is an oxide of silicon with the chemical formula , most commonly found in nature as quartz and in various living organisms. In many parts of the world, silica is the major constituent of sand. Silica is one ...

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 In chemistry, coprecipitation (CPT) or co-precipitation is the carrying down by a precipitate of substances normally soluble under the conditions employed. Analogously, in medicine, coprecipitation is specifically the precipitation of an unbound " ...

. For example, an acidic solution of aluminium salts and precatalyst are treated with base to precipitate the mixed hydroxide, which is subsequently

calcined Calcination refers to thermal treatment of a solid chemical compound (e.g. mixed carbonate ores) whereby the compound is raised to high temperature without melting under restricted supply of ambient oxygen (i.e. gaseous O2 fraction of air), gener ...

. 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 Hydrogen is the chemical element with the symbol H and atomic number 1. Hydrogen is the lightest element. At standard conditions hydrogen is a gas of diatomic molecules having the formula . It is colorless, odorless, tasteless, non-toxic, an ...

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 The Phillips catalyst, or the Phillips supported chromium catalyst, is the catalyst used to produce approximately half of the world's polyethylene. A heterogeneous catalyst, it consists of a chromium oxide supported on silica gel. Polyethylene, ...

, consisting of chromium oxide supported on silica, is activated by a stream of hot air.

Spillover

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 often oversimplified. It is known for example that adsorbates, such as

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.

Catalyst leaching

For insufficient interaction between catalyst and support, leaching of the catalyst may occur over time and after extended use of a supported catalyst. Leaching is detrimental for environmental and commercial reasons. For

electrophilic In chemistry, an electrophile is a chemical species that forms bonds with nucleophiles by accepting an electron pair. Because electrophiles accept electrons, they are Lewis acids. Most electrophiles are positively charged, have an atom that carri ...

catalysts this issue may be addressed by choosing a more basic support. This strategy may negatively affect the catalyst activity, therefore a subtle balance between leaching and activity is required.

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 Platinum is a chemical element with the symbol Pt and atomic number 78. It is a dense, malleable, ductile, highly unreactive, precious, silverish-white transition metal. Its name originates from Spanish , a diminutive of "silver". Platinu ...

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 Titanium dioxide, also known as titanium(IV) oxide or titania , is the inorganic compound with the chemical formula . When used as a pigment, it is called titanium white, Pigment White 6 (PW6), or CI 77891. It is a white solid that is insolubl ...

, Pt no longer binds with H₂ with the same

stoichiometry Stoichiometry refers to the relationship between the quantities of reactants and products before, during, and following chemical reactions. Stoichiometry is founded on the law of conservation of mass where the total mass of the reactants equal ...

. This difference is attributed to the electronic influence of the titania on the platinum, otherwise called strong metal-support interaction.

Heterogenized molecular catalysis

Molecular catalysts, consisting of transition metal complexes, have been immobilized on catalyst supports. The resulting material in principle combines features of both homogeneous catalysts – well defined metal complex structures – with the advantages of heterogeneous catalysts – recoverability and ease of handling. Many modalities have been developed for attaching metal complex catalysts to a support. However, the technique has not proven commercially viable, usually because the heterogenized transition metal complexes are leached from, or deactivated by, the support.

Supports for electrocatalysis

Supports are used to give mechanical stability to catalyst nanoparticles or powders. Supports immobilize the particle reducing its mobility and favouring the chemical stabilization: they can be considered as solid capping agents. Supports also allow the nanoparticles to be easily recycled. One of the most promising supports is

graphene Graphene () is an allotrope of carbon consisting of a single layer of atoms arranged in a hexagonal lattice nanostructure.

for its porosity, electronic properties, thermal stability and active surface area.

Examples

Almost all major heterogeneous catalysts are supported as illustrated in the table hereafter.

References

{{reflist Catalysis,