Catalysts, general supporting colloids

In this chapter, we discussed some general results obtained using a new route of preparation method of metallic supported catalysts by using colloidal oxide chemistry... 

The example shows that the three-step preparation procedure described above produces true nanocatalysts having naked metal particles of defined size deposited on the support surface. Generally, carbon-supported colloidal pre-catalysts are conditioned at 300 C. However, individual heating and gas flow conditions may be optimized for every catalyst system on the basis of TGA-MS analysis data. For example, the optimum temperatures for conditioning supported nanometaUic pre-catalysts having tetraoclylammonium or aluminum-organic protective shells are 280 °C and 250 G respectively [96, 126]. 

The first step in the development of an anode catalyst is preparation. Several approaches have been used for the production of catalysts, both supported and unsupported. It is generally agreed that preparation has an important influence on catalyst performance [380]. Several techniques have been used to prepare the catalysts, such as colloidal chemistry methods [381-385], the impregnation method [386-390], and the reverse micelles method [391, 392]. Although the colloidal chemistry methods and the reverse micelles method produce very promising results, they are very complex compared to the impregnation method. The... 

Recently, Chaudhari compared the activity of dispersed nanosized metal particles prepared by chemical or radiolytic reduction and stabilized by various polymers (PVP, PVA or poly(methylvinyl ether)) with the one of conventional supported metal catalysts in the partial hydrogenation of 2-butyne-l,4-diol. Several transition metals (e.g., Pd, Pt, Rh, Ru, Ni) were prepared according to conventional methods and subsequently investigated [89]. In general, the catalysts prepared by chemical reduction methods were more active than those prepared by radiolysis, and in all cases aqueous colloids showed a higher catalytic activity (up to 40-fold) in comparison with corresponding conventional catalysts. The best results were obtained with cubic Pd nanosized particles obtained by chemical reduction (Table 9.13). 

Our refined models allow a good description of the concentration dependence of rate and e.e. with different Pt-catalysts and additives in various solvents. This is well in line with results reported for various modifiers [6a, 6c, 6d] and supports like Pt/Al203 [4], Pt/Si02 [6b], Pt-zeolite [6e] or Pt-colloids [1]. With etpy as substrate, a qualitatively similar behavior is observed in all cases. Therefore, the concept of reversible adsorption of the modifier on the catalyst seems to be generally applicable. 

This universally known [66] reaction was discovered independently by Heck and Mizoroki about 30 years ago. Basically it consists of the arylation or vinylation of alkenes and is generally catalyzed in solution by palladium species (Scheme 5.7). One of the major problems of the early homogeneous systems was the precipitation of palladium black. Addition of phosphanes improves the stabiUty however oxidation of this Ugand is a drawback for easy purification of the products. Consequently, development of heterogeneous catalysis [67] through supported palladium or stabilized colloidal palladium catalysts is an area of great interest... 

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