Adsorption of Pt(cod)me2 onto organic aerogels from supercritical solutions for the synthesis of supported platinum nanoparticles

The thermodynamics and kinetics of adsorption of Pt(cod)me2 onto resorcinol–formaldehyde aerogel (RFA) from supercritical carbon dioxide (scCO2) was investigated by using high performance liquid chromatography (HPLC) to measure Pt(cod)me2 concentrations in the fluid phase. It was found that the adsorption isotherms of Pt(cod)me2 at 35 °C for different CO2 pressures could be represented by modified Langmuir isotherms. The kinetics of adsorption was determined by following the Pt(cod)me2 uptake of the RFA spheres; these data correspond closely to the behavior from a mass transfer model based on diffusion within the pore volume with the assumption of local equilibrium at the solid–fluid interface. The adsorbed Pt(cod)me2 molecules were reduced at atmospheric pressure under flowing hydrogen at 200 °C. The resultant Pt nanoparticles were distributed uniformly on the surface and had narrow size distributions. The average particle size of the nanoparticles increased with platinum loading from 2.0 nm at 10 wt.% to 3.3 nm at 34 wt.%. The Pt nanoparticles in an RFA pellet had a uniform radial size distribution, even though the pellet was impregnated with Pt(cod)me2 for too short a short period of time for the system to reach adsorption equilibrium. The high mobility of the atomic Pt evolved during the reduction process is believed to be responsible for this phenomenon. Performing the adsorption of Pt(cod)me2 onto RFA at 80 °C led to concurrent reduction and Pt nanoparticle growth.