Synthesis and pyrolysis of platinum containing polyacrylonitrile latex particles

The incorporation of highly-dispersed platinum (Pt) nanoparticles into proton exchange membrane fuel cells (PEMFC) as a possible catalyst has gained tremendous attention in the past decade. The major obstacle to fully commercialize PEMFCs is the high cost of the Pt as the catalyst. In this project, the incorporation of highly-dispersed platinum into poly(acrylonitrile) (PAN) latex particles is being carried out to form a possible catalyst precursor for fuel cell applications. In this work, the encapsulation of Pt salt into PAN particles via 3 different synthesis routes: (1) dispersion polymerization, (2) miniemulsion polymerization, and (3) miniemulsification were evaluated.;Dispersion polymerization is the usual method for producing particles in a single-step with sizes ranging from 0.1 ∼ 15 &mgr;m. The dispersion polymerization of acrylonitrile (AN) in the presence of Pt salt as was carried out initially. However, after elemental analysis, the results indicated that platinum was not successfully incorporated in the PAN particles. After measuring the solubility of the Pt salt in AN-saturated deionized (DI) water by UV spectrometry, it was found that only 3.6 wt% of the Pt salt was dissolved in the monomer mixture, this is then believed to be the main reason for the unsuccessful incorporation of Pt salt into the PAN particles.;Miniemulsion technology offers many possible applications such as encapsulation of inorganic particles, oils, and polymers. Pt-containing PAN particles were then prepared via miniemulsion polymerization using 2,2'-azobis(2-methylbutyronitrile) (V59) and 2,2'-azobis(4-methoxy-2,4-dimethylvaleronitrile) (V70) as the initiator. However, a large amount of coagulum was obtained after polymerization due to the high water solubility of acrylonitrile (AN). Thus, the encapsulation of Platinum (II) acetylacetonate (Pt salt) into PAN/Polystyrene(PS) copolymer particles was carried out and results indicated that 98.7 wt% of the Pt salt was encapsulated when the PAN/PS ratio is equal to 8:2.;Pt-containing PAN/PS particles are then pyrolyzed under an inert atmosphere at 800 ºC via a one-step process. Because the PAN/PS particles are attached to each other, pyrolyzed PAN/PS particles were all fused together after the one-step pyrolysis process. In order to increase the carbon yield and avoid the thermal fusion between PAN/PS particles during the thermal degradation process, a two-step pyrolyzation process was introduced. In addition, SiO2 particles with a particle size of 20 nm were mixed with PAN/PS particles and formed a monolayer using a controlled rapid deposition process by drawing a meniscus of a suspension across a silica wafer. TEM images showed that Pt nanoparticles with a particle size of 5∼20 nm could be observed, although thermal fusion between PAN/PS particles could still be observed. Thus, Iridium-coated Pt-containing PAN/PS particles were pyrolyzed using a two-step pyrolysis process and the results indicated the successful separation of PAN/PS particles after pyrolysis, although this technique may not useful for PEMFC application.;The miniemulsification process involves the use of an ionic surfactant, such as sodium lauryl sulfate, and a costabilizer, such as hexadecane. Synthesis of Pt-containing PAN particles by miniemulsification was also carried out. However, the coalescence of PAN particles during the solvent removed process will make the incorporation of platinum salt into PAN particles more complex and uncontrollable.