Ceria prepared using supercritical antisolvent precipitation: a green support for gold–palladium nanoparticles for the selective catalytic oxidation of alcohols

Abstract

CeO₂ has been prepared from an acetate precursor by an antisolvent precipitation technique using supercritical CO₂. The supercritically synthesized ceria support was used to prepare Au–Pd based catalysts for the selective oxidation of alcohols in solvent-free conditions using molecular oxygen as oxidant. The supercritically precipitated catalyst demonstrated high activity for alcohol oxidation, and it was much more active than catalysts prepared using a CeO₂ support derived from the acetate through a non-supercritical synthesis route. The bimetallic Au–Pd supported catalyst was considerably more active than monometallic catalysts containing Au and Pd only. HAADF imaging and STEM–XEDS mapping showed that both Au and Pd metallic components were intimately mixed and uniformly highly dispersed over the supercritical nanocrystalline CeO₂ support spheres. In contrast, the Au–Pd catalyst on the non-supercritical CeO₂ support showed discrete uniform Au–Pd alloy particles with a size range of 50 to 150 nm. The homogeneous alloy particles were Au-rich and Pd-deficient as compared with the preparation ratio and a low number of highly dispersed Pd was also associated with the support. XPS data for the ceria supported catalysts confirmed the differences of metal dispersion and identified that in both cases the surface species present were Au⁰ and Pd²⁺. On extended re-use the non-supercritical Au–Pd/CeO₂catalyst showed sequential deactivation. On the contrary, the supercritical Au–Pd/CeO₂catalyst showed a significant increase of activity, and it was only during the third re-use that the activity was marginally lower than the fresh catalyst. After use the spherical morphology of nanocrystalline supercritical CeO₂ started to increasingly break down, until the morphology started to resemble the CeO₂ prepared by the non-supercritical route. Simultaneously there was an increase of the metal particle size on the supercritically prepared CeO₂ support, as discrete Au-rich and Pd-rich bimetallic particles were formed, and there was a decrease of the metal content. XPS confirmed the loss of metal on use and showed that there was reduction of the ceria surface during use. The increase of activity with a corresponding loss of metal components indicates that the fresh catalyst contains Au and Pd in inactive forms, whilst the active sites have extremely high turnover frequencies.