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

Abstract

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 m²/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 m²/g by adding divinylbenzene in the oil phase. Pyrolyses at 1200–1500 °C and post-oxidative treatment at various temperatures produce porous ceramic structures with surface areas up to 423 m²/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 ²⁹Si 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 °C.