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Zusammenfassung:
Supercritical fluid reactive deposition (SFRD) using supercritical carbon dioxide (scCO2) as solvent and reaction medium was employed to generate mono- and bimetallic Pt and Pd nanoparticles (NPs) on SBA–15 as mesoporous support material. Highly uniform particles in the 5 nm range were deposited preferentially inside the pores according to transmission electron microscopy (TEM) and further characterization including X–ray diffraction (XRD), energy–dispersive X–ray spectroscopy (EDX), and X–ray photoelectron spectroscopy (XPS). Coupling of Scanning Transmission Electron Microscopy (STEM) with EDX demonstrated the formation of alloy-type structures for the bimetallic Pt/Pd particles. Comparison of the SFRD-process using scCO2 with the impregnation using conventional liquid solvents showed that the materials characteristics were similar to deposition from n–pentane, but significantly different from toluene or THF. The differences could be rationalized on basis of characterstic physico-chemical solvent properties. Notably, the choice of solvent during preparation was also reflected in the catalytic performance of the materials for hydrogenation of levulinic acid (LA) to γ–valerolactone (GVL), whereby the nature of the support and the Pt/Pd-ratio were additional parameters to control the activity with the SFRD approach. Bimetallic Pt/Pd particles were found to result in superior activity as compared to either of the individual monometallic species. Thus, the use of SFRD allows to avoid organic solvents during the catalyst preparation leading even to improved catalytic properties in favourbale cases.
