(299d) Highly Dispersed Metals on Metal Oxide Supports Via Reactive Deposition from Supercritical CO2

Formation of dispersed metal nanoparticles from precursors deposited on supports from supercritical carbon dioxide has been a topic of interest over the past decade. Materials made by this method have shown promise as active catalysts that employ noble metals efficiently, and they also exhibit potential for adsorptive separations. Examples of such materials include a variety of transition metals deposited on carbon-based supports and metal oxides. For carbon-based materials, including nanotubes and aerogels, the adsorption of organometallic precursors has been shown to follow thermodynamically-limited adsorption isotherms, and this behavior has even been used to control the composition of bimetallic nanoparticles. Our work in adsorption of similar precursors on metal oxide supports indicates a more complex deposition mechanism in these systems.

We have investigated the deposition of platinum and copper acetylacetonates from supercritical CO₂ on both γ-alumina and anatase catalyst supports. A comparison of these materials with others of similar composition prepared by wet impregnation from methanol led to the observation that metal adsorption from CO₂ is irreversible, while adsorption from methanol can be reversed. A potential explanation for this behavior is that a specific interaction between CO₂ and protonated surface oxides results in surface bicarbonate groups, which in turn undergo ion exchange with the organometallic precursors, leaving the metal ions bound to carbonate sites on the surface. This mechanism should result in highly dispersed metal ions having strong interactions with the surface. During calcination and reduction, the carbonates decompose, leaving metal nanoparticles on the surface. The effect of reduction method and of temperature during the calcination and reduction processes on the both size and morphology of the particles formed will be discussed.