Metal particle preparation

Recently, Chaudhari compared the activity of dispersed nanosized metal particles prepared by chemical or radiolytic reduction and stabilized by various polymers (PVP, PVA or poly(methylvinyl ether)) with the one of conventional supported metal catalysts in the partial hydrogenation of 2-butyne-l,4-diol. Several transition metals (e.g., Pd, Pt, Rh, Ru, Ni) were prepared according to conventional methods and subsequently investigated [89]. In general, the catalysts prepared by chemical reduction methods were more active than those prepared by radiolysis, and in all cases aqueous colloids showed a higher catalytic activity (up to 40-fold) in comparison with corresponding conventional catalysts. The best results were obtained with cubic Pd nanosized particles obtained by chemical reduction (Table 9.13). 

To answer these questions requires some understanding of the properties of small metal particles, both structural and electronic. In this review we shall examine first the evidence relating to metal particles prepared by direct methods, e.g., vapour deposition or condensation in the gas phase. Then we shall consider whether this information can be applied to the case of supported metals where both precursor decomposition and support effects may add to the complexity of the total system. We shall then consider whether further changes in catalytic properties occur after preparation, i.e., during the catalytic reaction. Finally, we shall summarize some of the more recent evidence concerning the nature of structure sensitivity. 

Sometimes HRTEM reveals graphitelike structures around metal particles prepared below the temperature of carbon ad-atom aggregation at metal surfaces. Thus, encapsulation of some platinum particles ( 4 nm) into the support matrix has been observed in Pt/Vulcan XC-72 catalysts [34]. The carbon networks were oriented parallel to the metal surface in the vicinity of the metal particle but more randomly farther away from it. 

Figure 3 Procedure of metal particle preparation from microemulsion. The obtained Pd particles displayed an average size of 7 nm...

In the case of metal particle preparation the choice of the metal precursor is of paramount importance. Obviously, water-soluble precursors are desired, generally transition metal salts, but even then different behaviours may be expected from different precursors. From Table 4 it can be observed that the solubility of chloroplatinic acid in a microemulsion is seven times higher than that of rhodium, iridium and palladium chlorides l... 

In this section we will focus on the description of several aspects relevant to the preparation of both catalytically active metal particles and metal-supported catalysts via the microemulsion technique. Regarding the metal supported catalysts, in some cases both the metallic particles and the support were synthesized by microemulsions. However, in general metallic particles prepared from microemulsions were deposited on commercial supports. The catalytic behaviour of these microemulsion-derived materials will be commented and, when possible, compared to catalysts obtained from traditional techniques under similar reaction conditions. Selected results concerning the study of the strong metal-support interaction effect (SMSI) obtained with catalysts prepared by microemulsion will be detailed . Several papers dealing with the preparation of immobilized metal particles on supports have been described although the catalytic behaviour of the solids was not studied. However, their potential catalytic ability led us to include those papers within this chapter. 

Pt, Ir, Rh, Ru, Pd Noble metal particle preparation Hydrocarbons Berol 050 Na2PdCl4 119... 

Figure 16.1.7 Scanning electron micrographs of metal particles prepared using the slow growth method. The composition of the plating solutions employed for the electrodeposition of these metal particles is listed in Table 16.1.1. The deposition current density observed in each experiment was as follows (MoOj) 180-140 pA cm , (Cd) 40-60 pA cm , (Cu) 40-60 pA cm , (Ni) 240-260 pAcm , (Au) 30-40 pAcm, and (Pt) 5-100 pAcm". Reprinted with permission of the American Chemical Society and Elsevier.

Sahiner, N., 2006. In situ metal particle preparation in cross-linked poly(2-acrylamido-2-methyl-1-propansulfonic acid) hydrogel networks. Colloid Polymer Science 285, 283—292. 

Ishizuki, N., Torigoe, K., Esumi, K., and Meguro, K. (1991). Characterization of precious metal particles prepared using chitosan as a protective agent. Colloids Surf. 55, 15-21. 

Vapor-grown carbon fibers have been prepared by catalyzed carbonization of aromatic carbon species using ultra-fine metal particles, such as iron. The particles, with diameters less than 10 nm may be dispersed on a substrate (substrate method), or allowed to float in the reaction chamber (fluidized method). Both... 

Sample Pore 0 (A) Method of preparation Metal (wt%) Metal particle diameter (nm)... 

The segregation process of graphite on the surface of a metal particle is similar to that proposed by Ober-lin and Endo[35] for carbon fibers prepared by thermal decomposition of hydrocarbons. Flowever, the... 

The data obtained up to the present time show that the method of catalyst preparation by the reaction of organometallic compounds with surface reactive groups may be applied to generate both isolated ions of transition metals (in various valent states) or superfine metal particles on the surface of the support. 

Barium and strontium salts of polystyrene with two active end-groups per chain were prepared by Francois et al.82). Direct electron transfer from tiny metal particles deposited on a filter through which a THF solution of the monomer was percolated yields the required polymers 82). The A.max of the resulting solution depends on the DPn of the formed oligomers, being identical with that of the salt of polymers with one active end-group per chain for DPn > 10, but is red-shifted at lower DPn. Moreover, for low DPn, (<5), the absorption peak splits due to chromophor-chromophor interaction caused by the vicinity of the reactive benzyl type anions. 

Active heterogeneous catalysts have been obtained. Examples include titania-, vanadia-, silica-, and ceria-based catalysts. A survey of catalytic materials prepared in flames can be found in [20]. Recent advances include nanocrystalline Ti02 [24], one-step synthesis of noble metal Ti02 [25], Ru-doped cobalt-zirconia [26], vanadia-titania [27], Rh-Al203 for chemoselective hydrogenations [28], and alumina-supported noble metal particles via high-throughput experimentation [29]. 

Both PtRu/MgO catalysts prepared from cluster precursor and organometallic mixture were active for ethylene hydrogenation. The apparent activation energy of the former catalyst obtained from the Arrhenius plot during -40 to -25°C was 5.2 kcal/mol and that of the latter catalyst obtained during -50 to -30°C was 6.0 kcal/mol. The catalytic activity in terms of turn over frequency (TOP) was calculated on the assumption that all metal particles were accessible for reactant gas. Lower TOP of catalyst prepared from cluster A at -40°C, 57.3 x lO" s" was observed probably due to Pt-Ru contribution compared to that prepared from acac precursors. 

A real catalyst is now prepared and it is assumed that, as described above, all the metal particles consist of Ru covered by Ni, which exposes solely the Ni(lll) surface. We now want to estimate the area of the metal by adsorbing CO at 300 K. It is found that the surface is saturated with CO at this temperature when 3.4 mL CO (measured at 1 bar and 300 K) has been adsorbed on 1 g of the catalyst. 

As the metal particle size decreases the filament diameter should also decrease. It has been shown that the surface energy of thirmer filaments is larger and hence the filaments are less stable (11,17-18). Also the proportion of the Ni(l 11) planes, which readily cause carbon formation, is lower in smaller Ni particles (19). Therefore, even though the reasons are diverse, in practice the carbon filament formation ceases with catalysts containing smaller Ni particles. Consequently, well dispersed Ni catalysts prepared by deposition precipitation of Ni (average metal particle size below 2-3 nm) were stable for 50 hours on stream and exhibited no filamentous coke [16]. 

We have reviewed experiments on two classes of systems, namely small metal particles and atoms on oxide surfaces, and Ziegler-Natta model catalysts. We have shown that metal carbonyls prepared in situ by reaction of deposited metal atoms with CO from the gas phase are suitable probes for the environment of the adsorbed metal atoms and thus for the properties of the nucleation site. In addition, examples of the distinct chemical and physical properties of low coordinated metal atoms as compared to regular metal adsorption sites were demonstrated. For the Ziegler-Natta model catalysts it was demonstrated how combination of different surface science methods can help to gain insight into a variety of microscopic properties of surface sites involved in the polymerization reaction. 

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