Characterization of Pd/γ-Al2O3 Catalysts Prepared Using [Pd(hfac)2] in Liquid CO2

Abstract

A series of Pd/γ-Al2O3 catalysts was prepared from [Pd(hfac)2] (hfac = hexafluoroacetylacetonate) in liquid carbon dioxide using the method reported by Kim et al. [Chem Mater 18:4710 (2006)]. The catalysts were characterized using CO pulse chemisorption, diffuse-reflectance infrared Fourier transform spectroscopy (DRIFTS), X-ray absorption fine structure (XAFS) spectroscopy, X-ray photoelectron spectroscopy (XPS), and electron microscopy. The catalysts were reduced initially in the high-pressure CO2 reaction cell using H2 at 75 °C. Samples were removed, stored in a desiccator, and re-reduced in situ at 250 °C prior to pulse chemisorption, DRIFTS and XAFS. CO pulse chemisorption evidenced that the Pd dispersion decreased from 55% to 5% as the Pd loading increased from 0.58 to 3.94 wt.%. The as-prepared 0.58 and 1.77 wt.% Pd/γ-Al2O3 catalysts (after air exposure) contained oxidized Pd species that were converted after in situ reduction to supported Pd particles. The average Pd particle sizes of these two catalysts (16 and 23 A, respectively) estimated from the first-shell Pd–Pd coordination numbers are in good agreement with the CO chemisorption results. DRIFTS evidenced a prevalence of weakly bound linear CO (νCO = 2083 cm−1) adsorbed on the 0.58 wt.% Pd catalyst. A 2.95 wt% Pd catalyst (49 A average particle size) also exhibited a strong linear CO band (νCO = 2093 cm−1). In contrast, CO chemisorption on a commercial 1 wt.% Pd/Al2O3 catalyst (37 A average particle size) gave predominantly 2-fold bridging CO species. We infer that the supported Pd particles prepared from [Pd(hfac)2] are rougher on the atomic scale (with a higher percentage of edge and corner atoms) than equivalently sized particles in conventionally prepared Pd/γ-Al2O3 catalysts.