Audrey Hertz

High specific surface area YSZ powders from a supercritical CO2 process as catalytic supports for NOx storage–reduction reaction

Nanometric yttria-stabilized zirconia (3 mol% yttria, SC-3YSZ) was synthesized using an innovative supercritical carbon dioxide-aided sol–gel process and used as a support for NOx storage catalysts. Wash-coats composed of noble metals (Pt and Rh) dispersed on 3YSZ were deposited inside the porosity of mini-diesel particulate filters. Commercial and SC-3YSZ powders were used as supports independently and mixed. NOx storage/reduction performances were compared under cycling conditions according to the YSZ support surface area, the washcoat localization inside the DPF and the metallic dispersions. All the results emphasize that the presence of high specific surface area SC-3YSZ inside the DPF improves the NOx conversion, the N2 selectivity and the NOx storage capacity. This enhancement of the catalytic properties was linked with the increase in both the metallic dispersion and the number of storage sites on the catalyst surface.

Sol-Gel Synthesis Assisted by Supercritical CO2 - A Flexible Process for Ceramic Powder and Membrane Preparation

The increasing interest in supercritical fluids for synthesizing inorganic materials stimulated the development of new processes for production of nanophased powders/layers with controlled characteristics. Original processes based on sol-gel synthesis assisted by supercritical CO2 have been developed. Unusual powder structures/morphologies have been evidenced and cosolvent characteristics have been proved to influence powder specific surface area and crystallite sizes. The development of a new semi-continuous process also enabled to tailor powder morphology by controlling hydrodynamic parameters (injection speed and stirring rate). Especially, yttria-stabilized zirconia prepared with this method are composed of 5-7 nm crystallites aggregated in spherical nanoparticles (<100nm) and yielding particularly high specific surface areas (up to 250 m2/g). These YSZ powders can be compacted/deposited and sintered to prepare nanosized dense membranes for ionic conduction. Finally, SC-CO2 sol-gel processes can also be adapted to directly prepare gas separation membranes. In particular, an innovative process has been developed to coat tubular porous ceramic substrates with porous silica-based layer.