Heterogeneous bulk catalysts

In order to investigate how the support material affects heterogenous gold catalysts, we sought a synthetic technique that could be easily applied to a variety of realistic bulk support materials, regardless of the surface... 

Among the large bulk of work regarding titanium(IV) silsesquioxanes, specific studies have been focused on synthesizing close molecular models of the active sites of titanium heterogeneous epoxidation catalysts in siUcaUte and in mesostruc-tured MCM-41 silica (Figure 14.3 gives some selected examples) [54, 61-66]. 

Heterogeneous catalysis deals with reactions between species that are adsorbed onto the surface of a catalyst. The role of the catalytic surface is to provide an energetically favorable pathway for the reaction. In order to find an explanation for the catalytic activity of substances, it is essential that we examine the properties of the surface, rather than at a collective property of the bulk catalyst. 

In heterogeneous catalysis, the first tests on UPD were performed on bulk catalysts which allows, for the preparation of the bimetallic catalyst, easy control of the electrochemical potential by an external device (potentiostat). In the same way all electrochemical techniques, particularly the control of catalyst potential required for submonolayer deposition, can be extrapolated to metallic catalysts supported on conductive materials such as carbon or carbides [8]. 

The processes are more complicated for heterogeneous systems, where the surfaces of bulk catalysts and carriers of the supported catalysts have a decisive influence on the catalyst properties. Knowledge of the number of active centers (C ) can also elucidate the role of mass transfer processes in Z-N polymerizations. 

This paper is primarily concerned with systems based on both homogeneous and heterogeneous (bulk and supported) Z-N catalysts. Transition metal oxide catalysts are not covered due to a different mechanism of their action [e.g. 3>], although some of methods have a universal applicability. 

Katz A, Davis ME (2000) Molecular imprinting of bulk, microporous silica. Nature 403 286 Bass JD, Anderson SL, Katz A (2003) The effect of outer-sphere acidity on chemical reactivity in a synthetic heterogeneous base catalyst. Angew Chem Int ed 42 5219... 

For catalytic hydrogenations we usually are concerned with three phases gaseous hydrogen, an, often dissolved, component to be hydrogenated in the liquid phase and a heterogeneous solid catalyst. Homogeneous catalysis is not very widespread. A number of good three phase reactors is available in process industries. Therefore we will restrict ourselves to the classic three phase reactors which already have proven their value in bulk chemicals processes. However, for fine chemicals applications a number of special requirements have to be met, which will be discussed in detail below. 

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