Supported metal nanoclusters preparation

A limitation of supported metal nanoclusters prepared from molecular metal carbonyl clusters is that, so far, clusters of only several metals (Ru, Rh, Ir, and Os) have been made in high yields (80 to 90%, with the likely impurity species being mononuclear metal complexes). However, this disadvantage is offset by the advantage of the characterizations, which show that some clusters are stable even during catalysis, at least under mild conditions. 

This section of this chapter includes a brief review of methods of preparation and properties of supported metal nanoclusters only catalysts that have been relatively well characterized and found to be nearly uniform are considered. The nanoclusters described here lack the structural definition... 

Ir4(CO)i2 and Ir6(CO)i6, supported metal nanoclusters, 68-69 Ir4 in zeolite NaX supported metal nanoclusters, 69 theoretical investigation, 70 Iron oxide support, preparation of gold particles on, 6-7... 

In the preparation and activation of a catalyst, it is often the case that the chemical form of the active element used in the synthesis differs from the final active form. For example, in the preparation of supported metal nanoclusters, a solution of a metal salt is often used to impregnate the oxide support. The catalyst is then typically dried, calcined, and finally reduced in H2 to generate the active phase highly dispersed metal clusters on the oxide support. If the catalyst contains two or more metals, then bimetallic clusters may form. The activity of the catalyst may depend on the metal loading, the calcination temperature, and the reduction temperature, among others. 

Similarly, Pd, Ag, and Pd-Ag nanoclusters on alumina have been prepared by the polyol method [230]. Dend-rimer encapsulated metal nanoclusters can be obtained by the thermal degradation of the organic dendrimers [368]. If salts of different metals are reduced one after the other in the presence of a support, core-shell type metallic particles are produced. In this case the presence of the support is vital for the success of the preparation. For example, the stepwise reduction of Cu and Pt salts in the presence of a conductive carbon support (Vulcan XC 72) generates copper nanoparticles (6-8 nm) that are coated with smaller particles of Pt (1-2 nm). This system has been found to be a powerful electrocatalyst which exhibits improved CO tolerance combined with high electrocatalytic efficiency. For details see Section 3.7 [53,369]. 

This argument is confirmed by the study of CO pulse chemisorption by Biffis at al., mentioned above. In this piece of investigation, the authors prepared a 2% (w/w) palladium catalyst supported by Lewatit UCP 118, a macroreticular resin (nominal cld = 18 %) from Bayer. Its TEM characterization showed a remarkably heterogeneous distribution of the metal nanoclusters, which are apparently located close to the surface of the polymer nodules [62] (Figure 9). 

Mao and Mao invented a method for synthesizing supported metal catalysts with small metal nanoparticles (1-3 nm) even at high metal loadings (30-50 wt.%) [25]. The obtained metal catalysts exhibited superior electrocatalytic performance in fuel cells. In this invention, the unprotected metal nanocluster colloids prepared according... 

The small metal particle size, large available surface area and homogeneous dispersion of the metal nanoclusters on the supports are key factors in improving the electrocatalytic activity and the anti-polarization ability of the Pt-based catalysts for fuel cells. The alkaline EG synthesis method proved to be of universal significance for preparing different electrocatalysts of supported metal and alloy nanoparticles with high metal loadings and excellent cell performances. 

The electronic structure, morphology, and chemical reactivity of metal nanoclusters have attracted considerable attention due to their extensive technological importance. Chemical reactions and their catalytic relevance have been investigated on a variety of well-characterized, supported model catalysts prepared by vapor deposition of catalytically relevant metals onto ultrathin oxide films in ultrahigh vacuum conditions. Such ultrathin film supports are usually prepared by vaporizing a parent metal onto a refractory metal substrate in an oxygen atmosphere at a high temperature. These unique model systems are particularly well suited for surface-... 

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],... 

Impregnation and reduction method can also be used to prepare noble metal nanoclusters. It was demonstrated that surface morphology, metal particle size and support material are important parameters on the catalytic activity of catalyst systems. Activation energies were 23,21 and 21 kJ mol for Ru, Rh and Pt catalyst on aliunina, respectively. Average particle sizes of Ru/Alumina, Rh/Alumina, Pd/Alumina, Pt/Alumina, Pt/C, Pt/SiO and Au/Alumina were 1.8, 2.5, 3.6, 1.5, 1.9, 5.1 and 2.6 nm, respectively. Moreover the BET surface areas were 40,41,37,38,239,396 and 35 m g" [99]. Interestingly a very different hydrogen generation performance was reported. The time required to complete the hydrolysis reaction of ammonia borane was reported. Hence, several reaction times have been demonstrated for comparison piuposes. However, it is not possible to convert this data to conventional rate equations. 

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