Monometallic nanocluster

The reduction of metal hydroxides or oxides powder by polyol was first reported by Figlarz and co-workers, which gave rise to fine powders of Cu, Ni, Co and some noble metals with micrometer sizes (polyol process) [32,33]. The polyol process was first modified for the preparation of PVP-protected bimetallic and monometallic nanoclusters such as Pt/Cu, Pd/Pd, Pt/Co, Pt, Pd, etc. [34-38]. The previous results definitely revealed that Pt, Pd, Cu and Co in these PVP-protected metal or alloy nanoclusters were in a zero-valent metallic state. 

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. 

Coreduction of mixed ions is the simplest but smart method to synthesize bimetallic nanoclusters. However, this method is not always successful. Au/Pt bimetallic nanoclusters were prepared by citrate reduction by Miner et al, from the corresponding two metal salts, such as tetrachloroauric(III) acid and hexachloropla-tinic(IV) acid. Reduction of the metal ions is completed within 4 h after the addition of citrate. They studied the formation of colloidal dispersion by ultraviolet-visible (UV-Vis) spectroscopy, which showed that the spectrum is not the simple sum of those of the two monometallic nanoclusters, indicating that the bimetallic nanoclusters have an alloy structure. 

In 1970, Turkevich and Kim tried to grow gold on Pd nanoclusters to obtain goldlayered Pd nanoclusters. " The deposition of one metal element on preformed monometallic nanoclusters of another metal seems to be very effective. For this purpose, however, the second element must be deposited on the surface of preformed nanoclusters. If the second element cannot be deposited on the preformed nanoclusters, mixtures of two kinds of nanoclusters will be produced. 

An attempt to prepare PVP-capped Au-core/Pd-shell bimetallic nanoclusters was made by the successive reduction procedure in ethanol/water. When the preparation of Au nanoclusters preceded the reduction of Pd ion, only mixtures of Pd and Au monometallic nanoclusters were produced (Fig. 3.22(a)). On the other hand, when the preparation of Pd nanoclusters preceded the reduction of Au ions, some bimetallic nanoclusters were found. The Au/Pd bimetallic nanoclusters thus obtained did not have a core/shell structure but a cluster-in-cluster structure (Fig. 3.22(b)), although the bimetallic nanoclusters obtained by the coreduction have a core/shell structure. This strange phenomenon can be understood by considering... 

In an earlier study, Turkevich and Kim proposed gold-layered Pd nanoclusters.Three types of Au/Pd bimetallic nanoclusters, such as Au-core/Pd-shell, Pd-core/Au-shell and random alloyed particles, were prepared by successive reduction. Two kinds of layered Pd/Pt bimetallic nanoclusters were also reported by successive reduction. However, detailed analyses of the structure of these bimetallic nanoclusters were not carried out at that time. Only the difference in UV-Vis spectra between the bimetallic nanoclusters and the physical mixtures of the corresponding monometallic nanoclusters was discussed. 

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. 

It is seen from Fig. 21.4 that our nanocluster catalyst, RUj Pt is exceptional in its selectivity to other bimetallic nanocatalysts, and it is also superior to industrially used monometallic supported catalysts such as Pt and Rh. This augurs well for the future use of high-area, thermally stable, nanoparticle catalysts in a wide range of hydrogenations to yield desirable chemical products from plant-crop sources. 

In another example, nanodustered Pt(0) catalysts based on cross-linked macro-molecular matrixes were evaluated in the hydrogenation of an a,(i-unsaturated aldehyde, citral. The monometallic catalysts exhibit remarkable selectivity for gera-niol/nerol when 2-3 nm, regularly shaped, spherical metal nanoclusters are deposited on the supports from solutions of solvated platinum atoms prepared by metal vapor synthesis (MVS). The immobilization in the polymer framework of ions of a second metal such as Fe(II), Co(II), or Zn(II) enhances the selectivity of the Pt catalysts by up to more than 90% [18],... 

Mono- and Bimetallic Supported Catalysts. - The key factor in designing supported metal catalysts is the knowledge about the reaction mechanisms and information about the role of different types of active sites in a given step of the catalytic reaction. The performance of supported mono-functional monometallic catalysts is governed by the metal particle size, metal dispersion, overall morphology of the metal nanocluster, the character of metal-support interaction, and the electronic properties of the metal. In bifunctional supported metal catalysts in addition to the above listed factors the metal/acid balance, and the type and strength of the acid function play a key role in the overall performance. 

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. 

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