Dispersion reduction procedure effect

With special techniques for the activation of the metal—e.g. for removal of the oxide layer, and the preparation of finely dispersed metal—the scope of the Refor-matsky reaction has been broadened, and yields have been markedly improved." The attempted activation of zinc by treatment with iodine or dibromomethane, or washing with dilute hydrochloric acid prior to use, often is only moderately successful. Much more effective is the use of special alloys—e.g. zinc-copper couple, or the reduction of zinc halides using potassium (the so-called Rieke procedure ) or potassium graphite. The application of ultrasound has also been reported. ... 

The preparation of an activated metal for use in an organometallic synthesis can be effected in a separate sonication step. An early example of this was the preparation of finely dispersed mercury for the reduction of a,a -dibromoketone to a mixture of a-acetoxyketones which favours the tertiary product (Eq. 3.6) [71]. The procedure is simple in that the dibromocompound, dissolved in acetic acid is subjected to sonication in a bath in the presence of a small amount of mercury. In this particular case... 

The process of cation exchange is that by which protons and other cations on the surface or within the structure of the support are replaced by cations of the active metal and this leads, with the noble elements of Groups 8-10, first to atomically dispersed species and then, after an optimal calcination and careful reduction with hydrogen, to extremely small metal particles [18], The procedure is especially effective with zeolites. But the introduction of catal Tically active species into the cavities of these materials, as opposed to placing them on their external surface, presents certain difficulties, namely the lack of suitable cations or cationic complexes [18], Nevertheless, different kinds of Au/zeoiite systems have been prepared this way [78-89],... 

Because of the cost of noble metal, the inetal loading should be low. and the inetal must be well dispersed to make the exposed metal surface as high as possible. Each type of metal complex, the conditions for ion exchange, the calcination procedure, and the reduction conditions have profound effects on the dispersion of the metal. 

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