Adsorption Properties of Supported Platinum Catalysts Prepared using Dendrimers

The effect of different oxidation and reducing pretreatments on dendrimer-encapsulated platinum nanoparticles (Pt-DENs) dispersed on a high-surface-area sol-gel-made silica support was assessed by evaluating the capacity of the resulting catalysts to adsorb CO, NO, and acetylene using IR absorption spectroscopy. The untreated catalysts are themselves capable of CO uptake, but only slowly, in a diffusion-controlled process, and into a weak adsorption state. Either oxygen or hydrogen pretreatments are required for stronger adsorption. Under similar temperature and pressure conditions, O-2 pretreatments result in higher uptakes, but also lead to partial oxidation and sintering of the Pt nanoparticles, and still do not fully eliminate the dendrimer matter. Hydrogen pretreatments alone at 525 K proved sufficient to expose the metal nanoparticles to the gas adsorbents and to activate the catalyst for hydrocarbon conversion reactions. NO adsorption is also seen in the H-2-activated catalysts, much more extensive if adsorption is initiated at 125 K. Acetylene adsorption is via the pi bonding favored on surfaces partially covered with carbonaceous deposits, suggesting that the dendrimer moieties that remain on the Pt surface of these catalysts may temper the dehydrogenation activity of the metal and favor hydrogenation and isomerization steps.