Synthesis of Platinum/Ceria Catalyst Using Supercritical Fluid Deposition

Catalysts containing platinum and ceria on alumina support are very important in environmental and energy applications, as they are very efficient at converting CO, hydrocarbons, and nitrogen oxides (NO_x) to less harmful CO₂, H₂O, and N₂. They are useful for the water-gas shift reaction, the goal of which is to oxidize CO in the presence of water to form CO₂ and H₂, which can be used in fuel cells for power generation. Dispersion of the active metal sites is key in evaluating catalyst activity, as it measures the efficiency of use of metal atoms. Dispersion is defined as the fraction of the number of metal atoms exposed over the total number of metal atoms. Highly dispersed catalysts are the most effective for the water-gas shift reaction. A promising synthesis for these catalysts is supercritical deposition. This technique leaves metal precursors on the surface, which are reduced to highly dispersed Pt in post-adsorption treatments, leaving homogeneous distributions of surface nanoparticles. We have found that exposure to CO₂ causes supported ceria to become highly dispersed, but, after calcination, the ceria accumulates around platinum. The effect of exposure to supercritical CO₂ on the nanostructure of deposited materials was studied. Catalysts were synthesized using incipient wetness impregnation, supercritical deposition, and incipient wetness followed by exposure of adsorbed precursors to supercritical CO₂. Dispersion measurements using CO pulse chemisorption at 195 K indicate that supercritical fluid deposition produced the most highly dispersed catalysts, followed by incipient wetness impregnation, with subsequent CO₂ exposure yielding the lowest dispersions.