Investigation of the supercritical deposition of platinum nanoparticles into carbon aerogels

The preparation of platinum/carbon aerogel (CA) nanocomposites by the supercritical deposition method was investigated. CAs were impregnated with dimethyl(cyclooctadiene)platinum, CODPtMe2, from supercritical carbon dioxide (scCO2) solutions and the resulting CODPtMe2/CA composites were converted to Pt/CA composites. The adsorption isotherms of CODPtMe2 on CAs were measured and could be represented by the Langmuir model. The results indicated a strong interaction between CODPtMe2 molecules and the CA surface and that a substantial fraction of the surface of the CAs was covered with CODPtMe2 molecules at relatively low concentrations. Four different reduction methods were used to convert the CODPtMe2 impregnated CAs which were: (1) thermal reduction at atmospheric pressure in an inert atmosphere; (2) thermal reduction in scCO2; (3) chemical reduction in scCO2 with hydrogen; and (4) chemical reduction at atmospheric pressure with hydrogen. Method 1 gave highly dispersed Pt nanoparticles (1-3 nm) at loadings ranging from 10 to 40 wt.%. The use of hydrogen in Method 4 increased the average particle size by a factor of 2 over Method 1 at the same Pt loading, but the particles still had a narrow unimodal size distribution. When the thermal reduction was carried out in scCO2, loadings as high as 73% could be obtained. Method 3 generated a composite having a disordered columnar Pt coating and equiaxed particles ≈1 μm in diameter on the external surface of the monolith and dispersed Pt nanoparticles in the interior. The analysis of the reaction products in scCO2 indicated an autocatalytic reaction. Increasing the Pt loading was found to decrease the surface area of the CA, primarily through blockage of the micropores.