Carbon Supported PD And PD-AU Catalysts For Formic Acid Electrochemical Oxidation

In this thesis the preparation of Pt/C and Pd/C catalysts for electrocatalytic oxidation of formic acid were investigated. The X-ray diffraction (XRD) spectroscopy, transmission electron microscope (TEM), energy disperse spectroscopy (EDS) and the measurement of the electrochemically active surface area (ESA) were used to characterize the catalysts. The cyclic voltammetry (CV), linear sweeping curves (LSV) and chronoamperometry (CA) were used to measure the electrocatalytic performances of the catalysts for the oxidation of formic acid. Electrochemical impedance Spectroscopy (EIS), normal pules voltmmetry and inverse pules voltmmetry were used to investigate the reaction mechanism of formic acid electrooxidation.Three systems were used to prepare Pd/C catalysts. The results indicate that the Pd/C catalyst which prepared in ethylene glycol (EG) system exhibited greater eletrocatalytic activity and stability for the oxidation of formic acid. The particle size of the Pd/C catalyst which prepared in glycol system is about 4-5 nm.The ESA of the Pd/C catalysts were increased by adding P123 into the mixture during the preparation of Pd/C catalysts and the electrocatalytic behavior is improved to some extent.The reaction mechnism for formic acid electrooxidation is different on Pd/C and Pt/C electrode. At lower potential, formic acid oxidation on Pd/C is through direct pathway, or to produce the CO2 directly. However formic acid oxidation on Pt/C is produce CO only. At higher potential, the oxidation rate of formic acid on Pd/C controlled by both the rate of adsorbtion of formic acid on the surface of Pd/C and the oxidation of adsorbed formic acid. Formic acid oxidation on Pt/C at higher potential is through indirect pathway, or first oxidized to CO and CO final oxidation to CO₂.Carbon supported Pd-Au catalysts were prepared by liquid phase reduction method. The results indicate that Au/C without obviously eletro-catalytic activity for formic acid electrochemical oxidation, however, the eletro-catalytic activity and stability of Pd/C catalyst for formic acid oxidation were improved evidently by Au add into the Pd/C catalyst. The electrocatalytic performance of the Pd-Au/C catalyst with the atomic ratio of 3:1 is best.Kinetic study of formic acid oxidation on Carbon supported Pd-Au catalysts. It was found that the oxidation of formic acid on Pd-Au/C electrode is a completely irreversible kinetic process; at the same time some kinetic parameters were determined: the charge transfer coefficient is 0.21,apparent diffusion coefficient is 1.0×10^-7 cm²Â·s^-1.The oxidation current densities increased with the rising of the temperature in the range of 20～60°C showed that the activity for the oxidation of formic acid on Pd-Au/C electrode enhanced with the elevating temperature. The activation energy of the oxidation of formic acid is significantly increased as the potential rose. It means formic acid oxidation is more sensitive to temperature at relatively higher potential.Besides, preliminary study of Pd-Co/C catalysts for electrocatalytic oxidation of formic acid, but the preparation methods of Pd-Co/C catalysts and the best atomic ratio are forward to futher exploring.