A. Heel, G. Kasper

Aerosol Science and Technology, 39 (2005), 1027-1037

A continuous 2-step aerosol process is described for the generation of SiO2 supported palladium (Pd/SiO2) catalyst particles from metal-organic (MO) precursors. In a first flow reactor, submicron SiO2 support particles are generated by chemical vapor synthesis (CVS) from TEOS [tetraethyl(ortho)silicate]. These silica particles are then coated with palladium in a second flow reactor by chemical vapor deposition (CVD) from (η3-allyl)(η5-cyclopentadienyl)palladium [Pd(allyl)Cp]. The sublimation and decomposition behavior of both metalorganic precursors was measured by thermo-gravimetric analysis (TGA) and FTIR; the vapor concentration of Pd(allyl)Cp was determined for the range of process conditions used. Each process step was characterized both with regard to aerosol properties as well as morphology and composition of individual particles. This was done with a variety of on-line and off-line techniques including electrical mobility analysis, TEM, energydispersive X-ray analysis, and physisorption methods likeBET. The coating thickness was also measured on line by a high-resolution single-stage impactor (SS-LPI) technique. It is shown that the continuous CVS process can be set to generate constant concentrations and sizes of silica support particles with a specific surface area of 350 m2 g−1, which are carbon free and non-porous. The silica particles can be restructured to spheres if desired. The continuous palladium CVD process was able to generate variable and defined coatings of narrowly distributed Pd nanoparticles with mean sizes between 0.75 and about 3.5 nm. The on-line measurements by SS-LPI showed equivalent coating thicknesses from 0.3 nm up to 3 nm, which were stable over several hours.