Nanoscale palladium-on-gold catalysis for glycerol oxidation

This thesis covers the work carried out during an exploratory study into the use of a supported palladium-covered gold catalyst for the aqueous phase oxidation of glycerol under mild reaction conditions. The study was initiated by combining the following triggers: 1. glycerol is experiencing a global glut due to the massive production of biodiesel and the price drop of crude and refined glycerol. There is a high need to actuate practical avenues for converting glycerol into value-added products, 2. Available knowledge regarding the use of palladium, platinum or gold-based catalysts for the oxidation of glycerol, and 3. Positive experiences with a palladium-covered gold-based catalyst in the conversion of perchloroethylene and trichloroethene present in groundwater. This palladium-on-gold catalyst offers a potential to convert glycerol into more economically valuable products like glyceric acid, lactic acid and tartronic acid. Also, the feasibility of this catalytic technology can be improved by re-designing the catalyst in order to increase catalytic activity. Insight in the nanoscale palladium-on-gold catalyst and the mechanism of reaction is gained. Research is performed in two parts; Part I describes a Pd surface coverage study and part II describes a catalyst size study. In part I monometallic Au and Pd supported catalysts and a series of carbon supported Pd on Au (Pd/Au) catalysts with different Pd surface coverages ranging from 10% to 300% were prepared and used in the liquid phase oxidation of glycerol in water using oxygen as the oxidant. Systematic investigation of catalytic activity with alternate Pd surface coverages on support in the selective oxidation of glycerol has not yet been reported. Catalytic activity and selectivity were 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 ~60% and ~100% respectively. A 98.1% conversion of glycerol was observed at a 60% Pd surface coverage, in just 3 hours time. It is hypothesized that the reactivity order for metal species is Pd0 > Au0 > Pd2+ and that Au has a unique ability to stabilize surface Pd atoms in metallic form that is resistant to experimental conditions (pH 13.6, 60 °C, and O2 flow). A reaction mechanism has been proposed for the glycerol oxidation using Pd/Au catalysts to explain the formation of all observed compounds. In part II Pd/Au nanoparticles with a core diameter of ~3, ~7 and ~10 nm with a Pd loading of 60% were prepared and characterized by EXAFS. The composition and atomic-scale structure of the catalysts before, during and after the liquid phase oxidation of glycerol in water using oxygen as the oxidant, are reported. From the small change in Fourier transforms, the corresponding bond distances, the Debye-Waller Factor of 0.0 and the almost completely constant coordination numbers, it can be concluded that the structure of Pd/Au nanoparticles holds for different core diameters and remains intact during the aqueous-phase selective oxidation of glycerol. This catalyst provides a potential way to convert glycerol into more economically valuable products.