Preparation of Hetrogenous Catalyst Lec 6
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Transcript of Preparation of Hetrogenous Catalyst Lec 6
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Preparation of heterogeneous
catalyst
lec 6
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Industrial catalysts are generallyshaped bodies of various forms, e.g., rings, spheres, tablets, pellets.
The production of heterogeneouscatalysts consists of numerousphysical and chemical steps.
The conditions in each step have adecisive influence on the catalystproperties.
the main physical properties of acatalyst that are influenced by the
production Conditions are: Activesurface area; pore structure;mechanical strength.
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Components of a Typical
Heterogeneous Catalyst
A. Active phase - metal that provides active sites where the
chemical reaction takes place
B. Support or Carrier - high surface area oxide whichdisperses and stabilizes the active phase
(adds efficiency, physical strength, sometimes selectivity)
C. Promoter(s) - additive which improves catalyst properties, e.g. activity, selectivity, catalyst life
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Bulk catalysts
Bulk catalysts also known asprecipitated catalysts are
mainly produced when theactive components are cheap.
The preferred method ofproduction is precipitation.
One or more components inthe form of aqueous solutionsare mixed and then coprecipitated as hydroxides orcarbonates. An amorphous orcrystalline precipitate or a gel
is obtained, which is washedthoroughly until salt free. Thisis then followed by furthersteps: drying, shaping,calcination, and activation
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Preparation of bulk catalyst
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Typical examples of precipitated catalysts
are:
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Impregnated catalyst ( support catayst)
One of the best known methods for producingcatalysts is the impregnation of poroussupport materials with solutions of active
components. Especially catalysts with expensive active
components such as noble metals areemployed as supported catalysts.
A widely used support is Al2O3, Activatedcarbon and silica gel.
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Preparation of catalyst by
impregnation
Impregnation as a means of supported catalystpreparation is achieved by filling the pores of asupport with a solution of the metal salt.
The catalyst is prepared either by spraying thesupport with a solution of the metal compoundor by adding the support material to a solution ofa suitable metal salt, This is then followed by
drying and subsequent decomposition of the saltat an elevated temperature, either by thermaldecomposition or reduction.
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impregnation
The support is immersed ina solution of the activecomponent under preciselydefined conditions
(concentration, mixing,temperature, time).Depending on theproduction conditions,selective adsorption of the
active component occurs onthe surface or in the interiorof the support. The result isnon uniform distribution.
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Supported metal catalyst
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The advantages of impregnated catalysts
compared with precipitated catalysts
Pore structure and surface of the catalyst can
be controlled.
More economic, since the content of
expensive active components is often low.
The distribution and crystallite size of the
active components can generally be varied
over a wide range.
Multiple impregnation is possible.
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Examples of industrial impregnated
catalysts are:
Ethylene oxide catalysts in which a solution of a
silver salt is applied to Al2O3
Catalysts in the primary reformer of ammonia
synthesis, with 10
20 % Ni on Al2O3 Catalysts for the synthesis of vinyl chloride from
acetylene and HCl: HgCl2/activated carbon; HgCl2
is applied from aqueous solution
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Dry mixing
Physically mixed, grind, and calcination