Platinum nanoparticle catalysts, polymer

Catalytic Hydrogenations. Table IV shows some results for the catalytic hydrogenation of cyclohexene, and a selection of results obtained for the hydrogenation of cis-cyclooctene and 1-hexene is given in Table V. The results show that a variety of polymer-protected platinum nanoparticles are catalytically active, and conversions of 100 % were obtained in most cases. The catalysts could be employed either directly as a colloidal dispersion or as a solid after the evaporation of the solvent and the subsequent redissolving in the mixture for liquid-phase hydrogoiation. Unlike most catalysts systems, they could be stored in air for several weeks/months and still showed very good catalytic activity. 

In a typical Pt/C catalyst, there are several modes of degradation that can occur through the oxidation of the carbon support. Figure 3.1 illustrates many of the mechanisms of performance loss in a PEMFC which are mainly due to the loss of TPB active sites. The first method (Figure 3.1a) in which TPB sites become inactive is from the loss of contact between the catalyst particles and the polymer electrolyte membrane. Detachment of the membrane, also known as membrane delamination, can be caused by the corrosion of the carbon support and can result in protons being unable to reach the platinum nanoparticles. This inevitably leads to lessening of the Pt utilization and catalytic activity of the MEA. 

State-of-the-art catalyst in low and intermediate temperature polymer electrolyte membrane fuel cells (PEMFC) is a powdered material consisting of platinum nanoparticles between 1 and 5nm in size that are supported—preferably in high dispersion—on a carbon-based support. 

Carbon-supported platinum (Pt) and platinum-rathenium (Pt-Ru) alloy are one of the most popular electrocatalysts in polymer electrolyte fuel cells (PEFC). Pt supported on electrically conducting carbons, preferably carbon black, is being increasingly used as an electrocatalyst in fuel cell applications (Parker et al., 2004). Carbon-supported Pt could be prepared at loadings as high as 70 wt.% without a noticeable increase of particle size. Unsupported and carbon-supported nanoparticle Pt-Ru, ,t m catalysts prepared using the surface reductive deposition... 

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. 

Dendrimer-encapstdated catalysts are another area of active research for polymer-supported catalysts. The nanoparticles are stabilized by the dendrimers preventing precipitation and a omeration. Bimetallic nanoparticles with encapsulated metals (dendrimer-encapsulated catalyst DEC) from commercially available fourth-generation PAMAM dendrimers and palladium and platinum metal salts were prepared via reduction by Crooks and co-workers [34], following previous work in this area [35], The simultaneous incorporation of Pt and Pd reflects the concentrations in solution. The bimetallic DECs are more active than the physical mixture of single-metal DEC [35, 36] in the case of the hydrogenahon of allyl alcohol in water, with a maximum TOP of 230 h compared to TOP = 190 h obtained for monometallic palladium nanoparticles (platinum TOP = 50 h ). 

The use of polymer-stabilised colloidal platinum clusters (containing cinchoni-dine) in acetic acid provides up to 97.6% ee in the reduction of methyl pyruvate (MeCOCOzMe). Polyvinylpyrrohdine is used as the polymer, and the particle size is small. The use of nanoparticles as a support for catalysts is an attractive concept owing to the large surface areas of these materials. However, such supported catalysts of this type are often difficult to recover. Hu and coworkers have achieved the immobilisation of ruthenium(phosphine)( 1,2-diamine) catalysts onto... 

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