Adsorption of Pt(cod)me(2) onto organic aerogels from supercritical solutions for the synthesis of supported platinum nanoparticles

Abstract

The thermodynamics and kinetics of adsorption of Pt(cod)me(2) onto resorcinol-formaldehyde aerogel (RFA) from supercritical carbon dioxide (scCO(2)) was investigated by using high performance liquid chromatography (HPLC) to measure Pt(cod)me(2) concentrations in the fluid phase. It was found that the adsorption isotherms of Pt(cod)me(2) at 35 degrees C for different CO2 pressures could be represented by modified Langmuir isotherms. The kinetics of adsorption was determined by following the Pt(cod)me(2) 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)me(2) molecules were reduced at atmospheric pressure under flowing hydrogen at 200 degrees 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)me(2) 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)me(2) onto RFA at 80 degrees C led to concurrent reduction and Pt nanoparticle growth.