Fabrication And Electroactivity Of Nanoporous Pd,Pd-Au And Pd-Ru Electrocatalysts

The direct liquid fuel cells have been paid high attention and investigated widely, due to the high energy efficiency, low-pollution, abundant sources, easy storage and transportation of fuels. The direct liquid fuel cells have broadly application fields in the mobile phones, laptops, music players and other portable power sources. The core issues of their commercialization are how to reduce the consumption of precious metal, or to enhance the catalytic activity and to search for high efficient and low-cost non-platium catalyst. Since the 20th century, electrocatalytic oxidation of small molecules has been highly attented by foreign research institutions, and it has important scientific and practical significance to understand the electrochemical phenomena, develop electrode kinetics and improve electro-oxidation mechanism. Platium, palladium, gold, silver and other precious metals on the electro-catalytic properties of small molecules have been studied extensively to explore their excellent electro-catalytic activity. This is a much challenging work.In this work, we have prepared the novel electro-catalysts of nanoPd, Au/nanoPd and PdRu by the hydrothermal process using different reduction agents in the presence of EDTA. Then we have researched their electro- catalytic acfivity towards the oxidation of methanol, ethanol, formic acid, and formaldehyde and the reduction of hydrogen peroxide. The electro-activity has been investigated by the conventional electrochemical techniques including cyclic voltammetry(CV), chronoamperometric(CA), linear scanning voltammetry(LSV), electrochemical impedance spectroscopy(EIS), et al.The morphlogical stracture of the samples has been characterized by using the scanning electron micronscope(SEM) and energy scattering spectra(EDS).The principal contents and research conclusions of this thesis are as follows:1. The application, research progress and preparation of the Pd-coutaining catalysts are summarized, it is expounded the mechanism of small organic molecules oxidation such as methanol, ethanol, formic acid, formaldehyde and reduction activity like hydrogen peroxide. To develop nano catalysts is the main concept. 2. Make sure the preparation process of palladium series nanometer catalysts. In this thesis, different nanometer palladium electrodes are fabricated by the hydrothermal process with adding EDTA through formaldehyde, glycol or polyethylene. The gold modified nanometer palladium catalyst is preparated by the constant potentioal deposition. The binary palladium ruthenium metal electrodes are obtained with EDTA as a complexing agent and formaldehyde as a reducing agent. Results show that nano-metal catalysts particles are directly deposited on the surface of titanium substrate, and the catalysts have high stability.3. Scanning electron microscopy(SEM), energy disperse spectroscopy (EDS), are employed to investigate the morphology and element compositions of nanoPd, Au/nanoPd, andPdRu.The main conclusions are as follows:(1) Morphologies of the prepared samples have been examined by scanning electro microscopy(SEM) and characterized by nanoporous network structures. The sizes of the Pd nanoparticles for the nanoPd/PEG, nanoPd-EDTA/PEG, nanoPd-EDTA/HCHO, nanoPd/EG, and nanoPd-HCHO are around 230, 130, 60, 80, and 150 nm, respectively. Their SEM images show that the Pd nanoparticles are connected with each other to form a three-dimensional texture which provides a considerably large real surface area.(2) For the preparation of the Au-modified nanoPd electrode: The porous structure of the nanoPd electrode offers plenty of sites and provides stable immobilization of the gold particles. Energy dispersed spectra of these samples indicate characteristic energy peaks of gold at ca. 2.1 and 9.7 keV, and palladium at ca. 3.1 and 3.25 keV.(3) All samples present a similar three-dimensional texture which is formed by the interconnection of the particles on the Ti surface. A typical SEM image of samples shows that a porous network structure is observed and the size of the particles(PdRu) is ca. 100 ¹50 nm. This porous structure provides a considerably large real surface area of the samples. Energy dispersed spectra of these samples indicate characteristic energy peaks of palladium at ca. 3.1 and 3.25 keV, and ruthenium at ca. 2.2 keV. In addition, the energy peak at around 4.5 keV is ascribed to the substrate Ti.4 Electro-catalytic activity of the nanoPd for the oxidation of methanol, ethanol, formic acid, formaldehyde and the reduction of hydrogen peroxide in alkaline solution has been studied using CV, CA, LSV and EIS, and the catalytic activiyu of the electrode kinetic process is analyzed. In addition electro-oxidation of formic acid on the gold modified palladium and ethanol oxidation on palladium ruthenium electrode have bee n investi gated. The mai n results are as follows:(1) Cyclic voltammetry (CV) and electrochemical impedance spectra (EIS) have been applied to evaluate the electrocatalytic acitivity of the nanoPd/Ti electrode towards methanol oxidation in alkaline solution. CV results show that the nanoPd /Ti electrode exhibits high anodic peak densities and a low onset potential for methanol oxidation. Also nanoPd /Ti electrode shows excellent CO tolerance during the oxidation of methanol. Nyquist and Bode plots of electrochemical impedance show that methanol electro-oxidation on the nanoPd /Ti exhibits low impedance values, and that with the increase of methanol concentrations, the impedance value for methanol electrooxidation decreases. The prepared nanoporous Pd electrode shows the significantly high electroactivity for methanol oxidation.(2) The oxidation of ethanol on the nanoPd-EDTA/HCHO exhibits a high anodic peak(151mA cm^-2) and a low onset potential of -0.788V. According to the analysis for elecreochemial impedance spectra(EIS), the nanoPd-EDTA/HCHO shows very low charge transfer resistances in 1.0 mol L^-1NaOH cantaining various concentrations of ethanol.(3) The electro-oxidation of formic acid on these Pd electrodes has been studied with cyclic voltammograms(CV) in the 1.0 mol L^-1NaOH+0.5 mol L^-1HCOOH solution. During the positive potential scan formic acid oxidation on the Pd-EDTA/HCHO exhibits a high anodic peak (132.0 mA cm-2) and a low onset potential of -0.98V , showing better electrocatalytic activity for formic acid oxidation than others. Effect of formic acid concentration on electrochemical characteristics of the nanoPd-EDTA/HCHO electrode is also investigated.(4) Electrocatalytic activity of the nanoPd towards formaldehyde oxidation in alkaline media has been evaluated by cyclic voltammetry (CV), chronoamperometry (CA) and electrochemical impedance spectroscopy (EIS). Electrooxidation of formaldehyde on the nanoPd electrode takes place at a low onset potential of ca. -0.85 V (vs SCE) and large anodic current densities. Chronoamperometric data of the nanoPd electrode show high and stable anodic currents for formaldehyde oxidation. It is also observed that the steady-state current density shows a well linear increment with formaldehyde concentration in the range of 0 to 20 mmol L^-1formaldehyde. EIS investigation at different formaldehyde concentrations presents very low values of charge transfer resistances for formaldehyde oxidation on the nanoPd. Results show that the prepared nanoPd electrode is a highly efficient catalyst for formaldehyde oxidation in alkaline media.(5) Electroactivity of the nanoPd-EDTA/HCHO catalyst towards the electroreduction of hydrogen peroxide in 1.0 mol L^-1NaOH solution has been evaluated by voltammetric techniques. Both linear scan voltammetric and chronoamperometric data present significantly large steady-state reduction current density with regard to the hydrogen peroxide electroreduction on the prepared nanoPd-EDTA/HCHO catalyst. Results show that the prepared nanoPd-EDTA/HCHO catalyst is a much more effective electrocatalyst for the electroreduction of hydrogen peroxide in alkaline media.(6) Electrocatalytic activity of the nanoPd and Au/nanoPd towards formic acid in alkaline solution is evaluated by cyclic voltammetry(CV) and electrochemical impedance spectroscopy(EIS). CV results reveal that Au/nanoPd presents a low onset potential and high anodic peak densities, showing that the deposited gold on nanoPd electrode can enhance palladium catalyst for formic acid electrooxidation activity. Also Nyquist plots indicate that formic acid electrooxidation on the Au/nanoPd exhibits low impedance values. Results show that the prepared Au/nanoPd electrode is an effective electrocatalyst towards formic acid oxidation in alkaline media.(7) CVs of the prepared electrocatalysts (nanoPd, Pd99Ru1, Pd96Ru4, Pd87Ru13 and Pd35Ru75) in alkaline media present extremely high anodic peaks towards ethanol oxidation. Among the prepared catalysts, Pd87Ru13 displays the (relatively) largest anodic peak current density, a wide peak current plateau and a most negative onset potential for ethanol oxidation. It is further found that the peak current plateau on the Pd87Ru13 catalyst becomes wider with the increase of ethanol concentration in the range of 0.1 to 2.0 mol L^-1. This wide peak current plateau is also observed at higher potential scan rates. This would be ascribed to the different reaction rates between the oxidation of Pd itself and ethanol oxidation on the Pd87Ru13 catalyst. Nyquist plots of the Pd87Ru13 catalyst present low impedance values, showing that ethanol oxidation on the Pd87Ru13 catalyst has a low charge transfer resistance. The study demonstrates that the addition of an appropriate amount of Ru to Pd improves the electrocatalytic activity of the Pd catalyst for ethanol oxidation. The prepared porous Pd87Ru13 catalyst could be a good choice for the ethanol oxidation in the promising alcohol fuel cell.Although the electro-catalytic activity of the small organic molecules has been extensively investigated, the new catalyst preparation is still an important work in this field. In the thesis the novel structure of porous palladium, palladium gold, pallaium ruthenium catalysts have been successfully prepared. Our study will be of signifacance in the development of direct liquid fuel cells and in the electrochemical study on the electro-oxidation of small molecules.