Metallic particles as catalysts

Lee C-L, Tsujino K, Kanda Y, Ikeda S, Matsumura M (2008) Pore formation in silicon by wet etching using micrometre-sized metal particles as catalysts. J Mater Chem 18 1015 Li X, Bohn PW (2000) Metal-assisted chemical etching in HF/H[sub 2]0[sub 2] produces porous silicon. Appl Phys Lett 77 2572-2574... 

Latex or emulsion polymers are prepared by emulsification of monomers in water by adding a surfactant. A water-soluble initiator is added, e.g., persulfate or hydrogen peroxide (with a metallic ion as catalyst), that polymerises the monomer yielding polymer particles, which have diameters of about 0.1 pm. The higher the concentration of surfactant added, the smaller the polymer particles. 

Recently, Liew et al. reported the use of chitosan-stabilized Pt and Pd colloidal particles as catalysts for olefin hydrogenation [51]. The nanocatalysts with a diameter ca. 2 nm were produced from PdCl2 and K2PtCl4 upon reduction with sodium borohydride in the presence of chitosan, a commercial biopolymer, under various molar ratios. These colloids were used for the hydrogenation of oct-1-ene and cyclooctene in methanol at atmospheric pressure and 30 °C. The catalytic activities in term of turnover frequency (TOF mol. product mol. metal-1 h-1)... 

Figure 6.18 Left first shell coordination numbers from EXAFS versus H/M values from selective hydrogen chemisorption for a number of supported Ni, Rh, Ir and Pt catalysts. Right, relative diameter of half-spherical metal particles as a function of H/M. The curves (right) correspond to the straight lines in the left part of the Figure (adapted from [41,42[).

When catalysts are recycled as solid residues, it is important to exclude impurities that may piggyback —such as metal particles—as the active species. This was probed in two ways. First, the tape was removed after a first cycle, rinsed, and transferred to a new vessel. A second charge of 17 and dibutyl ether was added, but not the PhMe2SiH. The sample was warmed to 55 °C, the now off-white tape was fished out , and PhMe2SiH was added. The rate profile was similar to the first cycle (ca. 20% slower at higher conversions), consistent with predominant homogeneous catalysis by desorbed fiuorous species. Second, the second cycle of a sequence was conducted in the presence of elemental mercury, which inhibits catalysis by metal particles [57]. However, the rate profile was the same as a sequence in the absence of mercury. 

As the catalytic reaction proceeds at the surface of the metal particles, the catalysts have been prepared to expose a large metal area, typically 10 - 100 m per gram of catalyst. 

Electron transfer from the adsorbed particle toward the metal is to be expected especially with transition metals serving as catalysts and adsorbed particles having unpaired or tt electrons. Electron transfer from the metal toward the adsorbate requires only that the metal be able to deliver electrons coming from s or p bands. 

The dipolar MSSR applies strictly to a flat metal surface. However, the consideration by Pearce and Sheppard (93) that adsorbed layers are typically a few angstroms (tenths of nanometers) thick in relation to the diameters of larger metal particles in catalysts (up to tens of nanometers) led to the consideration that the MSSR could have substantial effects on the intensities of infrared absorptions from adsorbed species on metal catalysts with large particles. It has been estimated that parallel modes of vibration will have their infrared absorption bands substantially attenuated at metal particle diameters of greater than 2 nm (94). This is proving to be a very important consideration in the interpretation of the infrared spectra from adsorbed hydrocarbon species on metal catalysts (20, 95, 96) and has recently become widely accepted as valid (52, 54, 57, 62, 97). 

In conclusion, the doping of oxide materials opens promising new routes to change the morphology and electronic properties of supported metal particles as used in heterogeneous catalysis. Thin oxide films are ideally suited to elucidate such doping effects, as they can be explored by means of conventional surface science techniques at a fundamental level. The identified mechanisms can be transferred to real catalysts later, as the doping approach is not based on specific thin-film effects. 

Metal nanoparticles embedded in thermosensitive core-shell microgel particles can also work efficiently as catalyst for this reaction. Figure 13 shows the oxidation reaction of benzyl alcohol to benzaldehyde in aqueous media by using microgel-metal nanocomposite particles as catalyst. All reactions were carried out at room temperature using aerobic conditions. It is worth noting that the reaction conditions are very mild and no phase transfer catalyst is needed. It has been found that microgel-metal nanocomposites efficiently catalyze the aerobic oxidation of benzyl alcohol at room temperature. No byproducts have been detected by GC after the reaction, and water is the only product formed besides the aldehyde. 

Catalytic reactions can be run over large, massive metal particles as well as the much smaller, dispersed metal crystallites. The massive metal catalysts can be the single crystal catalysts such as those shown in Fig. 3.2 or polycrystalline forms of bulk metal such as wires, foils or ribbons. These latter materials were used somewhat routinely in the early catalytic research efforts that were involved with developing the mechanisms of vapor phase catalytic processes. These materials were considered to be analogs of the supported catalysts in which the effect of the support, if any, was eliminated. 

The support porous structure and the rate of solvent removal from the pores as well as the nature of solvent and metal compound dissolved can considerably influence both the distribution of the active component through the support grain and the catalyst dispersion [163,170-173]. As a rule, the resulting particles size of the active component will be smaller, the more liquid-phase ruptures caused by evaporation of the solvent from the support pores are attained before the solution saturation. Therefore, supports with an optimal porous structure are needed to prepare impregnated Me/C catalysts with the finest metal particles. As a result, carbon supports appropriate for synthesis of such catalysts are very limited in number. Besides, these catalysts will strongly suffer from the blocking effect (see Section 12.1.2) because some of the metal particles are localized in fine pores. 

Water and isopropyl alcohol serve as electron donors in the 1 and 2 reaction, respectively. In this way traces of gold [56], silver [57], mercury [58], platinum [59] and other metals can be removed from solutions. Same procedure was used to purposely deposit noble metal "islands" as catalyst onto semiconductor particles (see above), e.g., to prepare platinized Ti02 suspensions by illuminating Ti02 particles in the H2PtCl6 solution. 

J4.10 Metal clusters and particles as catalyst precursors and catalysts... 

Metal particles have received considerable recent attention as catalysts. Metal catalysts adopt several different types of configurations, including nanoclusters, metal particle-supported catalysts, and enzymatic metal clusters [1-8]. A common characteristic of these catalysts is the existence of nanoscale active sites. Moreover, it is well known that the boundary between the materials composing these catalysts... 

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