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

Abstract

Palladium (II) hexafluoroacetylacetonate (Pd(hfac)(2)) dissolved in liquid carbon dioxide (L-CO2) was used to deposit Pd nanoparticles onto low-surface-area alpha-alumina (13 m(2)/g) and high-surface-area gamma-alumina (207 m(2)/g). These nanoparticles were prepared by contacting Pd(hfac)(2) dissolved in L-CO2 with the alumina at 6.9 MPa and 28.5 degrees 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 degrees C. The adsorption isotherm of Pd(hfac)(2) on gamma-alumina suggests a weak interaction between the organometallic compound and the support. The average Pd particle size on the low-surface-area alpha-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 gamma-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 %.