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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 supportsEdit

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 precatalystsEdit

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 supportEdit

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.

Catalyst leachingEdit

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 catalysts this issue may be addressed by choosing a more basic support.[5] This strategy may negatively affect the catalsts activity, therefore a subtle balance between leaching and activity is required.[6]

SpilloverEdit

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.[7]

Strong metal-support interactionEdit

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.[8]

Heterogenized molecular catalysisEdit

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.[9]

Supports for electrocatalysisEdit

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.[10]

One of the most promising supports is graphene for its porosity, electronic properties, thermal stability and active surface area.

ExamplesEdit

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

Almost all major heterogeneous catalysts are supported.

Process Reactants, product(s) Catalyst Support
Ammonia synthesis (Haber–Bosch process) N2 + H2, NH3 iron oxides alumina
Hydrogen production by Steam reforming CH4 + H2O, H2 + CO nickel K2O
Ethylene oxide synthesis C2H4 + O2, C2H4O silver with many promoters alumina
Ziegler–Natta polymerization of ethylene propylene, polypropylene; ethylene, polyethylene TiCl3 MgCl2
Desulfurization of petroleum (hydrodesulfurization) H2 + organosulfur compounds, RH + H2S Mo-Co alumina

See alsoEdit

ReferencesEdit

  1. ^ http://goldbook.iupac.org/S06147.html
  2. ^ Zhen Ma, Francisco Zaera "Heterogeneous Catalysis by Metals" in Encyclopedia of Inorganic Chemistry, 2006, John Wiley. doi:10.1002/0470862106.ia084
  3. ^ L. Keith Hudson, Chanakya Misra, Anthony J. Perrotta, Karl Wefers, F. S. Williams “Aluminum Oxide” in Ullmann's Encyclopedia of Industrial Chemistry 2002, Wiley-VCH, Weinheim. doi:10.1002/14356007.a01_557.
  4. ^ Max P. McDaniel "A Review of the Phillips Supported Chromium Catalyst and Its Commercial Use for Ethylene Polymerization" Advances in Catalysis, 2010, Volume 53, p. 123. doi:10.1016/S0360-0564(10)53003-7
  5. ^ Aboelfetoh, E. F.; Fechtelkord, M.; Pietschnig, R., "Structure and catalytic properties of MgO supported vanadium oxide in the selective oxidation of cyclohexane", J. Mol. Cat. A 2010, volume 318, pp. 51. doi:10.1016/j.molcata.2009.11.007
  6. ^ Aboelfetoh, E. F.; Pietschnig, R., "Preparation, Characterization and Catalytic Activity of MgO/SiO2 supported Vanadium Oxide Based Catalysts",Catal. Lett. 2014, volume 144, pp. 97. doi:10.1007/s10562-013-1098-z
  7. ^ Conner, W. C.; Falconer, J. L., "Spillover in Heterogeneous Catalysis", Chemical Reviews 1995, volume 95, pp. 759. doi:10.1021/cr00035a014
  8. ^ S. J. Tauster "Strong metal-support interactions" Accounts of Chemical Research, 1987, volume 20, pp 389–394. doi:10.1021/ar00143a001
  9. ^ Sandra Hübner, Johannes G. de Vries, Vittorio Farina "Why Does Industry Not Use Immobilized Transition Metal Complexes as Catalysts?" Advanced Synthesis and Catalysis 2016, Volume 358, pp. 3–25. doi:10.1002/adsc.201500846
  10. ^ Hu, H.; Xin, J.H.; Hu, H.; Wang, X.; Miao, D.; Liu, Y. (2015). "Synthesis and stabilization of metal nanocatalysts for reduction reactions – a review". Journal of Materials Chemistry. 3: 11157–11182. doi:10.1039/C5TA00753D.