Preparation and Structure of Polymer-capped Bimetallic Nanoclusters

2 Preparation and Structure of Polymer-capped Bimetallic Nanoclusters 

A polymer-capped metal nanocluster catalyst has not only the advantages similar to those of particles dispersed on an inorganic support but also new benefits in that 

In the case of bimetallic nanoclusters, their catalytic activities are usually much higher than those of the corresponding monometallic nanoclusters. Thus, the bimetallic nanoclusters are much more interesting not only for scientists in academia but also for researchers in industry. However, the bimetallic nanoclusters consisting of even the same elements can have various structures, on which the catalytic activities may depend. 

The structure of bimetallic nanoclusters depends on the preparation method and the kind of elements composing the bimetallic nanoclusters. Typical examples of structures are shown in Fig. 3.18. The random alloy indicates a structure in which the atoms of both elements are located in a completely random way, resulting in the formation of a kind of solid solution. The cluster-in-cluster indicates a structure in which the atoms of one element forms clusters and these clusters aggregate to form a larger cluster by including atoms of the other element which may form small clusters as well. In the core/shell structure, atoms of one element form a core and atoms of the other element surround the core to form the shell. The inverted core/shell is similar to the core/shell structure. The difference is the kind of element that forms the core and shell. The element forming a core in the core/shell structure now forms the shell in the inverted core/shell structure and vice versa. Thermodynamically, the core/shell structure is stable, while the inverted core/shell is metastable. 

The preparation methods of the colloidal dispersions of bimetallic nanoclusters are summarized in Fig. 3.19. Method no.l is the coreduction of the bimetallic complex which is produced by the complexation of two kinds of metal ions. Method no.2 is the coreduction of two kinds of metal ions in solution. It is sometimes rather difficult to distinguish method no. 1 from method no. 2. Method no. 3 shows a successive reduction method in which the second metal ions can be added to the dispersions after the first metal ions are reduced to form nanoclusters. No. 4 is a new method in which the dispersions of two kinds of metal nanoclusters are mixed in solution to form the dispersion of bimetallic nanoclusters. 

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