Metallic particles, encapsulation

Several studies have been reported in which model TiO thin films were used as supports, which clearly demonstrated the metal particle encapsulation [59, 60]. Linsmcicr ct al. [61] and Taglaucr and Knozingcr [62] have studied the behavior of Rh which was evaporated onto an clcctrochcmically produced TiO (anatase) film using low energy ion scattering (LEIS sec Section... 

Summarizing, the H2 ehemisorption studies here reported show a partial suppression of the H2 adsorption after a high temperature reduction suggesting that, in addition to a metal sintering, significant metal particle encapsulation occurs. A possible methodology for the determination of the true H/Pd ratios is also disclosed. 

The present work reports a promoting effect of the CeQ gZro 4O2 mixed oxide on the catalytic reduction of NO by CO at moderate temperatures. This effect is attributed to the Ce4+/Ce3+ redox couple which efficiently reduces NO. The interaction of the Pd/Ceo.gZro 4O2-AI2O3 catalysts with H2 reveals that considerable metal particle encapsulation occurs after a high temperature reduction. This suggests that the choice of an appropriate metal particle size may be an important factor to avoid the catalyst deactivation. Finally, a methodology for the determination of the H/Pd ratios is discussed. 

FIGURE 10.10 Schematic showing the stages of synthesis of a metal particle encapsulated in graphitic shells during the CVD process. 

In general, encapsulated metal particles were observed on all graphite-supported catalysts. According to Ref. [4] it can be the result of a rather weak metal-graphite interaction. We mention the existence of two types of encapsulated metal particles those enclosed in filaments (Fig. 1) and those encapsulated by graphite. It is interesting to note that graphite layers were parallel to the surface of the encapsulated particles. 

As in the case of graphite-supported catalysts, some metal particles were also encapsulated by the deposited carbon (Fig. 4). However, the amount of encapsulated metal was much less. Differences in the nature of encapsulation were observed. Almost all encapsulated metal particles on silica-supported catalysts were found inside the tubules (Fig. 4(a)). The probable mechanism of this encapsulation was precisely described elsewhere[21 ]. We supposed that they were catalytic particles that became inactive after introduction into the tubules during the growth process. On the other hand, the formation of graphite layers around the metal in the case of graphite-supported catalysts can be explained on the basis of... 

Fig. 5. Acetylene decomposition on Co-HY (973 K, 30 minutes) (a) encapsulated metal particle (b) carbon filaments (A) and tubules of small diameters (B) on the surface of the catalyst.

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]. 

Common to all encapsulation methods is the provision for the passage of reagents and products through or past the walls of the compartment. In zeolites and mesoporous materials, this is enabled by their open porous structure. It is not surprising, then, that porous silica has been used as a material for encapsulation processes, which has already been seen in LbL methods [43], Moreover, ship-in-a-bottle approaches have been well documented, whereby the encapsulation of individual molecules, molecular clusters, and small metal particles is achieved within zeolites [67]. There is a wealth of literature on the immobilization of catalysts on silica or other inorganic materials [68-72], but this is beyond the scope of this chapter. However, these methods potentially provide another method to avoid a situation where one catalyst interferes with another, or to allow the use of a catalyst in a system limited by the reaction conditions. For example, the increased stability of a catalyst may allow a reaction to run at a desired higher temperature, or allow for the use of an otherwise insoluble catalyst [73]. 

However, for the dendrimer nanocomposite metallic systems this change in shape was not observed. Again, due to the high stability to intense laser pulses, the anisotropy value of the gold dendrimer nanocomposite, which can be viewed as a measure of the symmetry of the particle, did not change after several repeated cycles of measurements. It is possible that the initial optical pumping of the electron-phonon modes of the metal particles is partially absorbed by the encapsulating PAMAM dendrimer. 

The emission of the metal particles may thus originate from a band-to-band transition in the metal particle, which occurs at about 516 nm for gold [60, 119]. As stated above, the nature of the interaction of the dendrimer (PAMAM) host is still uncertain, there could be very strong electrostatic interactions that may play a part in the enhancement of the metal particles quantum efficiency for emission. However, one would expect that this enhancement would result in slightly distorted emission spectra, different from what was observed for the gold dendrimer nanocomposite. Further work is necessary to completely characterize the manner in which the dendrimer encapsulation enhances the emission of the metal nanoparticles. With further synthetic work in preparation of different size nanoparticles (in other words elongated and nonspherical shape particles, including nanorods) it may be possible to develop the accurate description of a... 

Tab. 1.2 Summary of typical purification techniques for CNTs. a Treatment can remove metal catalyst residues. b Carbon residues (e.g. amorphous or organic aromatic debris).c Purification introduces covalently bonded functional groups. d Only if not covered with carbon or encapsulated within CNT. e Only amorphous carbon around metal particles. From [39] with kind permission from ACS Publications.

This section briefly describes dendrimer-encapsulated metal particles, a new family of composite materials, and their applications to catalysis. 

The approach for preparing dendrimer-encapsulated Pt metal particles is similar to that used for preparation of the Cu composites chemical reduction of an aqueous solution of G4-OH(Pt +)n yields dendrimer-encapsulated Pt nanoparticles (G4-OH(Ptn)). A spectrum of G4-OH(Pt6o) is shown in Fig. 12 a it displays a much higher absorbance than G4-OH(Pt +)6o throughout the wavelength range displayed. This change results from the interband transition of the encapsulated zero-valent Pt metal particles. 

---