Platinum induced crosslinking of polycarbosilanes for the formation of highly porous CeO2/silicon oxycarbide catalysts

A new synthesis scheme for the formation of porous CeO2/Pt-polycarbosilane composites using inverse microemulsions is presented. Aqueous hexachloroplatinic acid was used as a hydrosilylation catalyst causing crosslinking of allyl groups in a liquid polycarbosilane (PCS). The resulting polymers are temperature stable and highly porous. The Pt catalyst content and post-treatment of the polymer can be used to adjust the porosity. For the first time hydrophobic polymers with specific surface areas up to 896 m(2)/g were obtained by catalytic crosslinking of polycarbosilanes. Ceria nanoparticles 2-3 nm in diameter are well dispersed in the PCS matrix as proven using high resolution electron microscopy. Porosity of the hydrophobic materials could be increased up to 992 m(2)/g by adding divinylbenzene in the oil phase. Pyrolyses at 1200-1500 degrees C and post-oxidative treatment at various temperatures produce porous ceramic structures with surface areas up to 423 m(2)/g. X-Ray diffration investigations show that the crystallinity of the SiC matrix can be controlled by the pyrolysis temperature. Post-oxidative treatments cause silicon oxycarbide formation. Structure and morphology of the polymeric and ceramic composites were investigated using Si-29 MAS NMR, FESEM, FT-IR and EDX techniques. The temperature programmed oxidation (TPO) of methane shows a high catalytic activity of CeO2/Pt-SiC(O) composites lowering the onset in the TPO to 400-500 degrees C.