Catalysis of Polymer-capped Bimetallic Nanoclusters

The effect of the additive on the catalytic activity can be attributed to two kinds of effects. One is a ligand effect (an electronic effect) and the other is an ensemble effect (a steric effect). Let us consider the case in which the original metal works as the catalyst and provides a catalytic site(s). If the additive has the former effect on the catalysis of the original metal, the additive can influence the electronic density and/or electronic structure of the original metal. In this case the additive atom must be located near the catalyst metal. Since the catalytic site is located on the original metal, it is not necessary for the additive to be located on the surface. Nevertheless the additive must be located near the active site of the catalyst metal. The effect is the most effective when the additive atom is adjacent to the catalyst metal. 

In the case of an ensemble effect the catalytic sites of the catalyst metal are located on the surface and the additive metal atom must be attacked by the substrate together with the catalyst metal. Thus, both catalyst and additive metals must be located on the surface of the metal particles. Although it is not necessary for the atom of the additive metal to be adjacent to the catalyst metal, it is better for the additive atom to be adjacent to the catalytic site. Then both additive atom 

When atoms of both the additive metal and the catalyst metal form a bimetallic nanocluster composed of the two elements, it is considered to have a variety of bimetallic structures. These have been discussed in the previous section and shown in Fig. 3.18. Here, some examples of the catalytic reactions in which bimetallic nanoclusters have higher activity than the corresponding monometallic nanoclusters are described. 

Hydrogenation reactions have been most extensively studied for evaluating the catalytic activity of nanoclusters. Precious monometallic (Pd, Pt, Rh) nanoclusters capped by PVP or polyvinylalcohol have high catalytic activities for hydrogenation of olefins. We applied PVP-capped Pd/Pt and Au/Pd bimetallic nanoclusters prepared by the coreduction method to the selective hydrogenation of 1,3-cyclooctadiene to cyclooctene. In both cases the bimetallic nanoclusters with a Pd content of 80 % showed the highest activity. As shown in the previous section, such bimetallic nanoclusters were found to have Pt- or Au-core/Pd-shell structures and Pt- or Au-cores are completely covered with Pd monoatomic layers. This improvement in catalytic activity can be interpreted only by a ligand effect of the core elements. 

Ni/Pd alloy bimetallic nanoclusters prepared by the improved alcohol reduction method under high temperature were applied to the catalysis for hydrogena- 

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