Preparation of SBA-15 supported Pt/Pd bimetallic catalysts using supercritical fluid reactive deposition: how do solvent effects during material synthesis affect catalytic properties?

Abstract

Supercritical fluid reactive deposition (SFRD) using supercritical carbon dioxide (scCO₂) as the solvent and the reaction medium was employed to generate mono- and bi-metallic Pt and Pd nanoparticles (NPs) on SBA-15 as a mesoporous support material. Highly uniform particles in the 5 nm range were deposited preferentially inside the pores detected by transmission electron microscopy (TEM). The materials were further characterized by 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 method using conventional liquid solvents showed that the material characteristics were similar to deposition from n-pentane, but significantly different from deposition from toluene or THF. The differences could be rationalized on the basis of characteristic physico-chemical solvent properties. Notably, the choice of solvent during the 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 controlling 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 one to avoid organic solvents during the catalyst preparation, leading even to improved catalytic properties in favourable cases.