Bimetallic catalyst systems cluster formation

Thus, it would be expected that at least a small fraction of Ni-Cu intermetallic compound may be formed. In fact, theoretical studies carried out on Ni-Cu systems indicate that the formation of bimetallic Ni-Cu clusters with an important surface nickel enrichment is very likely [14]. Taking into account the peculiar characteristics of the starting oxide precursor, in which a very high interaction between Ni and Cu cations exists, and the TPR results, we suggest that the formation of some Ni-Cu bimetallic clusters in the Ni-Cu-Zn-Al system could be possible In other words, since the catalytic behaviour of the Ni-Zn-Al system is significantly modified by the addition of Cu, it is very likely that some kind of interaction between Ni and Cu exists. It is probable, then, that the Ni-Cu-Zn-Al catalyst is constituted by monometallic nickel and copper clusters strongly interacting, and/or bimetallic Ni-Cu clusters. 

A study in this field has revealed an essential difference between the morphologies of the Pt colloid formed under particular conditions in the hydrosilylation of various olefins with EtsSiH in the presence of Karstedt s catalyst (132). The clear conclusion is that the catalytic hydrosilylation under such conditions (silane, rich olefin, poor) is a molecular process proceeding via metal clusters and that the formation of colloids is associated with deactivation of the system. However, the latter process is avoided by a direct use of prefabricated colloids. The size of such colloids can be controlled and the process can be efficiently performed heterogeneously. A combination of bimetallic colloids consisting of a ligand-stabilized Pt shell on Pd cores supported on alumina improves markedly catalytic activity in the test reaction—hydrosilylation of 1-octene (133). 

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