Preparation of highly dispersed silica-supported palladium catalysts by a complexing agent-assisted sol-gel method and their characteristics

Pd/SiO2 catalysts were prepared by a complexing agent-assisted sol-gel method, and impregnation methods using an organic complexing agent (imp-org) or water (imp-w) as the impregnation solvent. The Pd/SiO2 catalysts were characterized by TG-DTA, Fourier transform-infrared (FT-IR) spectroscopy, C-13 NMR, X-ray powder diffraction (XRD), transmission electron microscope (TEM), X-ray photoelectron spectroscopy (XPS) and CO adsorption, and the difference between the preparation methods was discussed. The sol-gel catalysts usually contained some organics after the heat treatment in hydrogen. Moreover, it was found that trace amounts of organics remain even in the sol-get catalyst activated at 873 K in hydrogen. In the sol-gel catalyst at 673 K in hydrogen, the palladium was somewhat positively charged, On the other hand, the palladium in the sol-gel catalyst activated at 873 K and both impregnation catalysts activated at 673 K in hydrogen were completely metallic. The palladium particles in the sot-gel catalysts were much smaller and more uniform in size than those in the corresponding impregnation catalysts. Furthermore, on the TEM images, the metal particles in the sol-gel catalysts in comparison with those in both impregnation ones seemed to be somewhat covered with the silica or embedded in the silica. Reflecting this, with an increase in the activation temperature, the particles did not grow in size as rapidly in the sol-gel catalysts as in both impregnation ones. (C) 2002 Elsevier Science B.V. All rights reserved.