Electrochemical Oxidation of Glycerol on Bimetallic PtCu/C in Alkaline Medium and Tuning the Product Selectivity to C3 Products

Abstract

For more than a decade, since 2009, biodiesel production has led to excessive production of its by-product, glycerol, consequently decreasing its market value and creating waste issues for the biodiesel industry. Valorization of glycerol is a promising strategy to enhance the sustainability of the biodiesel industry. Electrochemical oxidation of glycerol stands out among the other methods (i.e., hydrogenolysis, dehydration, and catalytic oxidation) due to its simplicity, eco-friendliness, and cost-effectiveness. Glycerol electrooxidation reaction has a wide range of products including C3 (i.e., glyceric acid), C2 (i.e., glycolic acid) and C1 (i.e., formic acid) products, and a complex reaction pathway. Moreover, some of the products spontaneously convert to each other or form decomposition products in the strong alkaline medium, making the product analysis challenging. Therefore, this thesis started by establishing a foundation for the quantitative technical analysis method of glycerol electrooxidation products. Proton Nuclear Magnetic Resonance (H-NMR) was developed as an alternative to High-Performance Liquid Chromatography (HPLC) by providing the capability to assess the products in their medium and detect the chemical conversions in alkaline medium. Additionally, H-NMR is a highly sensitive technique with a low detection limit of 0.01mM for the GOR product, and its accuracy was confirmed with less than 8% error by using a sample product mixture with known concentrations. Most importantly, the proposed chemical pathways were determined by using H-NMR, assisting in a deeper understanding of the glycerol electrooxidation mechanism in an alkaline medium. Subsequently, this thesis explores an efficient catalyst for the glycerol electrooxidation reaction since the state-of-the-art catalysts developed for the glycerol electrooxidation reaction mostly include noble metals hindering their commercialization due to their high price and low stability resulting from susceptibility to CO poisoning. Specifically, catalysts that can hinder the C-C cleavage provide more economic advantages since C3 products have higher market prices than the C2 and C1 products. Thus, the primary aim of this thesis is to develop a cost-effective catalyst with high selectivity towards C3 products, by demonstrating high activity, and stability to make the glycerol electrooxidation reaction economically feasible. This thesis uses the catalyst development strategy of alloying the noble metal with the transition metal, where Pt and Cu were chosen, respectively. PtxCu100-x/C bimetallic alloy catalysts were prepared using the chemical reduction method. The effect of Pt:Cu ratio on the electrochemical performance was studied by using a three-electrode cell in an alkaline medium, revealing the Pt31Cu69/C as the best-performing catalyst with the highest Pt-mass normalized current density (5.9 mA μgPt-1), highest geometrical current density (75.3 mA cm-2), and low onset potential (~0.38V vs RHE) among Pt/C and other PtxCu100-x/C catalysts. Conducting parametric studies on the electrolyte concentrations and applied potential, the C3 selectivity of Pt31Cu69/C catalyst demonstrated the highest selectivity to GLY (75 %) and C3 (86 %) after 10 h chronoamperometry at optimal conditions. Subsequently, the impact of the metal oxide effect on the Pt-based catalyst, PtCu/C-CeO2, and Pt/C-CeO2 were prepared using the ball milling method. The CeO2 impact on the selectivity towards C3 products was investigated. PtCu/C-CeO2 showed the highest selectivity and concentration for C3 products compared to Pt/C-CeO2, Pt/C and PtCu/C. Additionally, temperature impact on the performance towards glycerol electrooxidation reaction was studied, revealing temperature increase further increases the selectivity and concentration for C3 products (90%, and 18.4 mM). All the catalysts prepared in this thesis were characterized physically using analytical techniques of transmission electron microscopy (TEM), scanning electron microscopy (SEM), X-ray diffraction (XRD), and inductively coupled plasma optical emission spectroscopy (ICP-OES).