Supported Pd catalyst preparation using liquid carbon dioxide

The use of liquid carbon dioxide(LCO 2) for synthesis of supported metal catalysts was investigated. The extremely low surface tension of LCO 2 makes it an excellent wetting agent, even on very low surface energy substrates. This low surface tension may help to reduce the redistribution of metal compounds caused by the high surface tension of typical liquid solvents. In addition the low surface tension of LCO 2 is advantageous in preventing capillary collapse of sol-gels and high-aspect-ratio structures during drying and facilitates the permeation of organomettallic compounds into small pores. The low viscosity of LCO 2 can result in faster diffusion and can help reduce solute concentration gradients. Particle diameters in the range 20-60 nm were obtained on an α alumina support by adjusting the solution concentration and on a γ-alumina support, Pd diameters in the range 2-7 nm can also be controlled by varying the concentration.