Metal-Support Effects and Promotion Relation to Catalyst Synthesis

1 Metal-Support Effects and Promotion Relation to Catalyst Synthesis 

A very different system is used, however, in preparing a catalyst supported by silica, which contains only weakly acidic silanol groups. In preparing a Pt on silica catalyst, Pt(NH3)4 is typically used to carry out the ion-exchange reaction with silanol protons. 

In subsequent catalyst activation steps, a complex set of reactions can take place, and depend on catalyst loading as well as the chemistry. For surface complexes that are not easily reduced, such as for Co + or Co + reacted with alumina or silica, the support-metal particle interface may exist as a Co-aluminate or -silicate layer. Under reducing conditions, this interface is covered with reduced Co. In addition, subsequent catalyst preparation steps carried out on a reactive support such as TiOg can increase the interface with a reduced metal particle by partially covering the reduced metal particle with the oxide or by an increased wetting of the particle surface which will increase the interfacial area[ l. 

The chemical reactivity of the catalyst support may make important contributions to the catalytic chemistry of the material. We noted earlier that the catalyst support contains acidic and basic hydroxyls. The chemical nature of these hydroxyls will be described in detail in Chapter 5. Whereas the number of basic hydroxyls dominates in alumina, the few highly acidic hydroxyl groups also present on the alumina surface can also dramatically affect catalytic reactions. An example is the selective oxidation of ethylene catalyzed by silver supported by alumina. The epoxide, which is produced by the catalytic reaction of oxygen and ethylene over Ag, can be isomerized to acetaldehyde via the acidic protons present on the surface of the alumina support. The acetaldehyde can then be rapidly oxidized over Ag to COg and H2O. This total combustion reaction system is an example of bifunctional catalysis. This example provides an opportunity to describe the role of promoting compounds added in small amounts to a catalyst to enhance its selectivity or activity by altering the properties of the catalyst support. To suppress the total combustion reaction of ethylene, alkali metal ions such as Cs+ or K+ are typically added to the catalyst support. The alkali metal ions can exchange with the acidic support protons, thus suppressing the isomerization reaction of epoxide to acetaldehyde. This decreases the total combustion and improves the overall catalytic selectivity. 

The chemistry at the interface of a transition metal and a reactive metal oxide, such as TiOg, can be quite different to that carried out over large metal particles alone. Reducible oxide supports such as Ti02 or V2O5 can help to promote the chemistry on the metal and behave quite differently to than oxides such as alumina. The dissociation of CO, for example, is usually considerably enhanced at the interface of the metal and a reactive oxide, where it can dissociate leaving a carbon atom attached to the metal and the oxygen at a cation site such as Tl or V on the metal oxide supprort. 

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