Preparation metal particle formation

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]. 

Pt/Rh bimetallic nanoclusters were similarly prepared by this alkaline EG method [12]. The particle sizes of bimetallic Pt/Rh nanoclusters (0.37 g/1 in total metal concentration) ranged from 0.9 to 2.1 nm with an average diameter of 1.3 nm. A combined EDX analysis, using an electron beam of 1.0 nm in diameter, revealed that both signals of Pt and Rh existed in each individual particle and the average ratio of Pt to Rh (1.4 1) was close to the charged ratio (1.33 1) in the preparation, proving the formation of bimetallic nanoclusters. 

These routes involve the formation of (usually) prereduced metal particles that are then adsorbed or deposited onto the support. They have the advantage that the particle size of the particles is predetermined by the chemistry of the colloids and that resulting catalysts have narrow particle size distributions. However, the colloidal particles often are surface stabilized by surfactant molecules, which can be difficult to remove once the particles are adsorbed onto the support. One further disadvantage is that the colloidal particles are prepared at high dilution (typically millimolar concentrations— for example, 0.2 g Ft 1 ), which is a disadvantage in terms of scale-up. 

The major disadvantages of colloidal catalysts studied so far can be attributed to problems in controlling the metal colloid formation (control of particle size, particle size distribution, structure of metal colloids) and stabilization of the prepared particles, which are not yet completely solved. But it is exactly the stability of the nanoparticles, that is decisive for long-term usage during catalytic processes. Moreover for catalytic application, it is extremely important to preserve the large surface of such colloidal systems. 

Application of amphiphilic block copolymers for nanoparticle formation has been developed by several research groups. R. Schrock et al. prepared nanoparticles in segregated block copolymers in the sohd state [39] A. Eisenberg et al. used ionomer block copolymers and prepared semiconductor particles (PdS, CdS) [40] M. Moller et al. studied gold colloidals in thin films of block copolymers [41]. M. Antonietti et al. studied noble metal nanoparticle stabilized in block copolymer micelles for the purpose of catalysis [36]. Initial studies were focused on the use of poly(styrene)-folock-poly(4-vinylpyridine) (PS-b-P4VP) copolymers prepared by anionic polymerization and its application for noble metal colloid formation and stabilization in solvents such as toluene, THF or cyclohexane (Fig. 6.4) [42]. 

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