Characterization of Palladium (Pd) on alumina catalysts prepared using liquid carbon dioxide

Palladium (II) hexafluoroacetylacetonate (Pd(hfac)₂) dissolved in liquid carbon dioxide (L-CO₂) was used to deposit Pd nanoparticles onto low-surface-area α-alumina (13 m²/g) and high-surface-area γ-alumina (207 m²/g). These nanoparticles were prepared by contacting Pd(hfac)₂ dissolved in L-CO₂ with the alumina at 6.9 MPa and 28.5 °C and then slowly venting gaseous CO₂ until L-CO₂ was completely evaporated. After depressurization to remove the CO₂ and unabsorbed Pd(hfac) ₂, the impregnated Pd(hfac)₂ was reduced in hydrogen at a relatively low temperature of 75 °C. The adsorption isotherm of Pd(hfac)₂ on γ-alumina suggests a weak interaction between the organometallic compound and the support. The average Pd particle size on the low-surface-area α-alumina, measured by scanning electron microscopy, increased from 13.1± 3.5 to 59.9 ±11.3 nm, and the metal dispersion, measured by pulsed CO chemisorption, decreased from 11 to 3%, as the Pd loading on the alumina was increased from 0.15 to 1.54 wt %. With the high-surface-area γ-alumina, Pd particle size, measured by scanning transmission electron microscopy, increased from 3.1 ± 1.9 to 7.0 ± 5.9 nm, and the metal dispersion decreased from 56 to 5%, as Pd loading was increased from 0.58 to 3.94 wt %.