Modulating the Catalytic Activity and Selectivity of Palladium-Decorated Gold Nanoparticles With Pd Surface Coverage for Glycerol Oxidation

Glycerol, a by-product from biodiesel manufacture, is a highly functionalized renewable raw material. Supported precious metal catalysts have been studied extensively for selective oxidation as a means to upgrade glycerol. Bimetallic PdAu catalysts are shown to be more active than monometallic ones, but the reasons for the enhancement are not well understood. A bimetallic catalyst in which Pd metal coats the surface of Au nanoparticles (Pd-on-Au NPs) can serve as a model structure for studying nanostructural effects on glycerol oxidation. In this study, Pd-on-Au NPs with variable Pd surface coverages (sc%) ranging from 10 to 300 sc% were synthesized and immobilized onto carbon (Pd-on-Au/C). Tested for glycerol oxidation at 60 °C, pH 13.5, and 1 atm under flowing oxygen, Pd-on-Au/C (1.0 wt% Au loading, ~4-nm Au particles) showed significantly higher catalytic activities than monometallic Au/C (1.0 wt% Au loading, ~4-nm Au particles) and Pd/C (1.0 wt% Pd loading, ~4-nm Pd particles), both of which had TOF values of ~400 h^-1. Catalytic activity and selectivity of bimetallic Pd-on-Au catalysts were also found to be a function of surface coverage of the shell Pd atoms on the core Au atoms, with the highest activity and selectivity detected at ~80 sc% and ~60 sc% respectively. XAFS (x-ray absorption fine structure) spectroscopy results indicated that, while the metal oxidation states of Pd-on-Au/C with 60 sc% and 150 sc% and of Au/C were unaffected after glycerol oxidation, Pd/C became more oxidized post-reaction. Au appears to suppress Pd oxidation, which improves catalytic activity for glycerol oxidation. This catalyst material provides a potential way to convert glycerol into more economically valuable products.