Porous CeOx/SiC nanocomposites prepared from reverse polycarbosilane-based microemulsions

Reverse microemulsions consisting of aqueous cerium nitrate, solution as the internal phase and polycarbosilane dissolved in heptane as the continuous phase were used as a precursor for the controlled synthesis of dispersed cerium oxide particles inside a porous SiC matrix. According to dynamic light scattering experiments, the effective diameter of the cerium hydroxide particles obtained after ammonia addition is effectively controlled in a range of 2-10 nm with the molar water/surfactant ratio (R-w = 6-16). Pyrolysis at 1200 to 1500 degrees C produces materials with specific surface areas up to 240 m(2) g(-1) Whereas crystallization of the matrix is achieved only at higher temperature, cerium oxide particles form agglomerates composed of smaller nanoparticles that tend to dissolve into the ceramic matrix at 1500 degrees C leaving macropores behind. The high specific surface area is attributed to the presence of mesopores with a broad size distribution. Excess carbon present after pyrolysis is removed by oxidation at 900 degrees C causing a significant decrease of the surface area for high surface area materials, whereas intermediate surface area materials (50-100 m(2) g(-1)) show a high textural stability.