Impregnation, Adsorption and Ion-exchange

Filling the pores of the support with a solution of the catalytically active element, after which the solvent is removed by drying, is a straightforward way to load a support with active material. However, in this process various interactions are possible between the dissolved catalyst precursor and the surface of the support, which can be used to obtain a good dispersion of the active component over the support. To appreciate the importance of such interactions we need to take a closer look at the surface chemistry of hydroxylated oxides in solution. 

In water, the hydroxyls react with protons and OH groups, giving the surface ionic character. The following equilibrium reactions occur  

To an extent the surface charges are determined by the pH of the solution, and by the isoelectric point of the oxide, i.e. the pH at which the oxide surface is neutral. The surface is negative at pH values below the isoelectric point and positive above it. Obviously, the charged state of the surface enables one to bind catalyst precursors of opposite charge to the ionic sites of the support. 

For example, Pt/Si02 catalysts are conveniently made by impregnating a silica support vith a basic solution (pH 8-9) of platinum tetraammine ions, Pt(NH3)4 (dissolved as chloride). As the silica surface is negatively charged, the Pt-containing ions attach to the SiO entities and disperse over the surface. The pH should be kept belo v 9 because other vise the silica surface starts to dissolve. 

In nonpolar solvents, for example alcohols, the hydroxyls of the support can also be used to anchor alkoxy compounds to the surface in a condensation reaction, in which one alkoxy ligand reacts with the proton of the surface OH to give the corresponding alcohol, and the complex binds to the support. An example is the anchoring of zirconium ethoxide, Zr(OC2H5)4, to silica by means of the reaction 

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For the deposition of active phase(s), impregnation, adsorption and ion exchange, (co)precipitation, deposition precipitation, and in situ crystallization methods can be used. Moreover, it is possible to mix the active phase in the mixture for extrusion or to deposit the active phase by using a mixed sol containing both the precursors of the oxidic species and the active phase or a slurry with the precursor powder of the active phase. Other coating techniques, e.g., CVD or CVI techniques, can also be used. The dispersion of the active phase depends strongly on the method and conditions used, its precursors form as well as on the history of the active phase. 

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