Supports colloidal

In order to evaluate the catalytic characteristics of colloidal platinum, a comparison of the efficiency of Pt nanoparticles in the quasi-homogeneous reaction shown in Equation 3.7, with that of supported colloids of the same charge and of a conventional heterogeneous platinum catalyst was performed. The quasi-homogeneous colloidal system surpassed the conventional catalyst in turnover frequency by a factor of 3 [157], Enantioselectivity of the reaction (Equation 3.7) in the presence of polyvinyl-pyrrolidone as stabilizer has been studied by Bradley et al. [158,159], who observed that the presence of HC1 in as-prepared cinchona alkaloids modified Pt sols had a marked effect on the rate and reproducibility [158], Removal of HC1 by dialysis improved the performance of the catalysts in both rate and reproducibility. These purified colloidal catalysts can serve as reliable... 

Why is heat treatment chosen to better effect the stripping off the protecting shell of a supported colloidal catalyst precursor ... 

Attempts to support colloidal platinum particles with surfactant zinc porphyrin micelles allow hydrogen production at lower platinum concentrations than when the porphyrin and the particle are separate but an irreversible electron donor is still necessary for hydrogen production.116... 

Colloidal palladium or platinum supported on chelate resin beads were employed for the stereoselective hydrogenation of olefins 86). Colloidal palladium supported on iminodiacetic acid type chelate resin beads was prepared by refluxing the palladium chloride and the chelate resin beads in methanol-water. Using the resin-supported colloidal palladium as a catalyst, cyclopentadiene is hydrogenated to cyclopentene with 97.1% selectivity at 100 % conversion of cyclopentadiene under 1 atm of hydrogen in methanol at 30 °C. Finely dispersed metal particles ranging from 1 to 6 nm in diameter are the active species in the catalyst. 

Apart from the massive form, the element is most often encountered as platinum black , a finely divided and highly reactive material that has some catalytic activity. It can be prepared by reducing chloroplatinic acid with any of a variety of reductants, such as alkaline formaldehyde. For practical applications, the metal is often deposited in colloidal form on an oxide, graphite, or other support, so that it better retains its high surface area and therefore its catalytic activity. Supported colloidal Pt has some advantages as a catalyst in organic synthesis. ... 

These catalysts are prepared by adding a suspension of a supporting colloid in an appropriate solvent to a solution of a noble metal salt dissolved in the same solvent. The resulting mixture is then reduced with hydrogen at room temperature and atmospheric pressure to convert the metal salt to the metal. Frequently (5,6) it has been found necessary, in order to obtain reduction, to convert the noble metal salt to the corresponding hydroxide prior to the reduction. 

It has been found that the activity of these catalysts is dependent upon the concentration of the supporting colloid. In Figure 3 are reported the results of experiments carried out with a constant weight of platinum and palladium and varying amounts of polyvinyl alcohol. It can be seen that the maximum activity for 10 mg. of palladium is attained with 250 mg. of polyvinyl alcohol and with 100 mg. of the colloid for the same... 

The use of naked but supported colloids and clusters is well established in heterogeneous catalysis [6]. Normally, the metal particles are directly generated on the support from metal salts. Such catalysts usually possess high activities however, compared with homogeneously working catalysts, they show considerably reduced selectivities. This is due to a broad particle size distribution as well as to the lack of ligand molecules influencing the selectivity decisively. 

Ermakova, L.E. et al., Electrochemical characteristics of sihcon and titanium oxides and titanium-oxygen nanostructures on silica supports. Colloid J., 61, 714,1999. 

Zacharaki, 1. et al.. The influence of calcination on the size of nanocrystals, porous structure and acid-base properties of mesoporous anatase used as catalytic support. Colloids Surf. A, 324. 208, 2008. 

Limousy, H. et al.. Determination by zetametry and streaming induced potential measurements of the amounts of catalytic precursors necessary to saturate a support. Colloids Surf. A, 181, 91, 2001. 

Also briefly mentioned earlier is the fact that the physical properties of the interface of nanoparticles in solution/solvent or electrolytes may lead not only to colloidal behavior but also to particle-particle interaction or particle-solvent interaction. Self-supporting colloid network structures allow for the coexistence of high conductivity with mechanical stability, enabling colloidal gels to be used as electrolytes [76-78]. 

In order to remove excess protecting shell the supported colloidal pre-catalysts - as obtained in step 2 - are activated via heat treatment at 300 °C in an intermittent stream of oxidizing and reducing gases. We have coined the term conditioning (i.e. reactive annealing) for this process [126]. 

The example shows that the three-step preparation procedure described above produces true nanocatalysts having naked metal particles of defined size deposited on the support surface. Generally, carbon-supported colloidal pre-catalysts are conditioned at 300 C. However, individual heating and gas flow conditions may be optimized for every catalyst system on the basis of TGA-MS analysis data. For example, the optimum temperatures for conditioning supported nanometaUic pre-catalysts having tetraoclylammonium or aluminum-organic protective shells are 280 °C and 250 G respectively [96, 126]. 

It was found that for enantioselective hydrogenation over modified platinum catalysts the most suitable SC-solvents are ethane and propane as indicated in Table 5.15. Application of SC-CO2 in hydrogenation on chiral modified Pt-catalysts proved to be less suitable because CO2 is partly hydrogenated into CO, which poisons the catalyst But polymer-supported colloidal Pd nanoparticles as catalysts in supercritical CO2 (SCCO2) revealed top s as high as 4 x 10 h" at 15 bar hydrogen, and 50°C (Niessen et al. 

The advantage of the supported colloid-catalyst is demonstrated by comparison of the activity of three Rh-catalysts (5 % on charcoal) in the butyronitrile-hydrogenation-test ... 

Qamli ST, Send S, Tuncel A. Nucleotide isolation by boronic acid functionalized hydrophilic supports. Colloid Surf. A Physicochem. Eng. Aspects 2002 207 127-137. 

E. Chibowski, L. Holysz, K. Terpilowski and M. Jurak, Investigation of superhydrophobic effect of PMMA layers with different fillers deposited on glass support. Colloids Surfaces. A. 291,181-190 (2006). 

Kim, S., Jung, Y. and Park, S.J. (2008). Preparation and electrochemical behaviors of platinum nanocluster catalysts deposited on plasma-treated carbon nanotube supports. Colloid. Surface A, 313, pp. 189-192. 

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